oilwell drilling technology 1 wb1
TRANSCRIPT
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Oilwell Drilling Technology ~
Unit 1 : Basic Concepts
Contents Page
* Training Target 1.2
* Introduction 1.3
* Section 1 - Reservoirs and Reservoir Rocks 1.5
* Section 2 - Exploration Techniques 1.15
* Section 3 - Drilling Rig Types 1.21
* Section 4 - Drilling Personnel 1.33
* Check Yourself - Answers 1.37
Visual Cues
~~ training targets for you to achieve'1rt by the end of the unit
[1] test yourself questions to see how much you understand
III
[\Ii check yourself answers to let you see if you have been thinking along the right lines
activities for you to apply your new knowledge or find things out for yourself ~
[~ summaries for you to recap on the major steps In your progress
1.1
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\1~~~ Training Target The aim of this unit is to give you an insight into the basic concepts and techniques used in the exploration for oil and gas. We will also look briefly at the drilling process, design features of drilling rigs and the personnel involved in the drilling operation.
When you have completed the unit you will be able to:
• Name the two main types of sedimentary rock. D • Define the rock properties of porosity and permeability. D * Explain in broad terms the origin of petroleum. D * Identify the types of rock structure which can form a petroleum reservoir. D • Describe the large scale survey techniques used in petroleum exploration. D • Explain the principles of three small scale survey techniques used in petroleum exploration. D • Describe the basic concepts of the drilling process. D • List the main types of drilling rig in use and the key features of each. D * List the main personnel involved in the drilling operation and the functions of the drilling crew. D
Tick each box when you have met the target.
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Unit 1 : Basic Concepts f(~
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Introduction
Natural petroleum is contained in underground reservoirs. The aim is to get it from the reservoir to the surface in a safe and efficient manner.
There are two main stages involved in this:
* finding the petroleum * transporting it to the surface and processing
it for the next phase
The first of these stages is called exploration, and the second production.
It is worth noting here that natural petroleum is also referred to as:
* petroleum * oil and gas or * hydrocarbons
I will be using all of these expressions from time to time.
We are never absolutely sure whether oil or gas is present in a reservoir until we have:
* drilled into that reservoir * obtained a sample of the reservoir fluids at
the surface
Therefore, we can say that drilling a well is the last step in the exploration stage. Once we know that hydrocarbons are there, underground, it is necessary to decide whether the amount present is sufficient to justify the expense of installing production facilities - offshore production platforms for example.
So, a programme of appraisal drilling is planned, which aims to define:
* the size and shape of the reservoir
* how much hydrocarbon is in place there
* how much of this oil and gas can actually be brought to the surface
* what difficulties the operation is likely to encounter.
If, after carrying all this out, we still believe that it is worthwhile proceeding, we enter the production stage proper.
Production facilities need to be designed and installed - for example, how many wells or production platforms are required? There are usually a number of wells drilled into one reservoir.
As the number of wells drilled into a reservoir increases, so does the cost but there are benefits:
* the overall rate of petroleum extraction can usually be increased
* long term damage to the reservoir can be minimised by avoiding points of high production at a few isolated wells.
Development or production drilling is now carried out, giving the ideal distribution of wells over the reservoir to achieve the best economic return for the whole operation. That is - the most hydrocarbon, at the least cost, in the shortest time.
All these different types of drilling activity exploration, appraisal, or development - will use a wide range of skills contained within the area we call Drilling Technology.
1.3
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Before we look into the detail of the drilling operation, this unit on Basic Concepts provides you with some background on four related topics:
* Section 1 talks about the structure of a reservoir and the types of rock which will be found there.
* Section 2 examines the various exploration techniques which need to be carried through before the location of the first hole to be drilled is selected.
* Section 3 looks at the different drilling rig types and indicates why a particular design is selected for a particular purpose. It also covers some basic drilling concepts.
* Section 4 considers the people who work on a drilling rig and how they relate to each other.
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Unit 1 : Basic Concepts ~ I
Section 1 - Reservoirs and Reservoir Rocks
A reservoir is not a huge underground cavern filled with fluid, as many people still imagine.
It is actually a rock system and within the pores, cracks and channels of this system the reservoir fluid - gas, oil, water or, in many cases, a mixture of all three, is stored.
In this section you will find out about:
* the basic geology of reservoir rocks
* the types of reservoir fluid and how they got there
* the structure of reservoirs - why they act as reservoirs
Basic Geology of Reservoir Rocks
Most reservoirs are made up of sedimentary rocks. There are two principle types of sedimentary rock in which hydrocarbons are commonly found. These are:
* clastic (or detrital) rocks
* biochemical rocks
Clastic rocks are formed by the settling out and accumulation of solid particles such as sand. These particles are formed by the weathering of larger rocks.
They are carried (by rivers, etc.) to the point where they are deposited. Further layers of rock particles (many thousands of feet thick in some cases) may be laid down on top of this sediment layer which will eventually form the reservoir.
The force exerted by these further layers (known as the overburden) together with other chemical and physical changes result in the formation of typical clastic rocks such as sandstones and shales.
Biochemical rocks are formed by the accumulation of marine life remains - fragments of shells, coral, skeletons and so forth. Again, application of pressure and other changes result in the formation of typical biochemical rocks such as limestones, chalks and dolomites.
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Figure 1 shows photographs of a typical sandstone and dolomite, taken through an electron microscope and magnified many times. Details of individual rock particles and pores can clearly be seen.
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Properties of Reservoir Rocks
We now have a reservoir rock which is perhaps thousands of feet below the surface. You already know that the reservoir fluids (hydrocarbons and water) will reside in the pores, cracks and channels of this system.
However, our main interest in the reservoir rock is knowing:
• how much space is available for storage of these fluids
* how easily they will flow to the wellbore
from where they can be transported to the surface.
These characteristics are referred to as porosity and permeability.
Porosity
The space which is available between the rock particles (known as the pore space - see figure 2) is one important guide as to how much hydrocarbon may be present.
To get a better visual impression of this, think of a container full of marbles. The marbles represent individual rock particles which are greatly magnified. Between the marbles packed in the container you can see spaces. These are the pores. Added together they form the pore space.
Figure 2
The volume of pore space, expressed as a percentage of the total rock volume, is called the porosity of the rock. The storage capacity of our reservoir, then, depends on the total volume of the rock (how big the reservoir is) and its porosity.
@] Actlvlty
Find two empty containers - empty yoghurt cartons or plastic cups would be ideal - some dry sand and some water.
Fill one container with sand and the other with water. Now, pour some of the water, slowly, into the container of sand.
What happens? Note down here what you see.
You will need your yoghurt cartons for the next activity.
1.7
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I hope you saw that the water disappeared into the sand. You will recall that I explained to you what pore space was - the volume occupied by the pores, cracks and channels of the reservoir rock system. It is into this void space that our water is disappearing.
Incidentally, it is worth noting that you could estimate the storage capacity of this mini-reservoir in the yoghurt carton by measuring the volume of water necessary to fill to the top of the sand.
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u Test Yourself 1
Imagine a block of sandstone as a mini-reservoir with a total rock volume of one cubic metre (or 1 000 Iitres).
If the porosity of this reservoir is 18% of the total volume of the rock, what is the maximum volume of oil it could hold (in litres, say)?
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You will find the answer in Check Yourself 1 on page 1.37.
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Permeability to pass through in order to reach the well. This is Reservoir Fluids shown in figure 3.
You should now have a mental picture of how the Scientists do not agree on hawaii and gas were Permeability is a measure of how easy it is for the pore space in the reservoir provides storage originally formed. The most popular idea is called reservoir fluids to make this journey. The higher capacity for reservoir fluids - which can include the organic theory. This supposes that these the permeability (expressed in units called gas, oil and water. hydrocarbons were created from the remains of Darcies), the easier for these fluids to flow and, small plants and animals living mainly in the sea. other things being equal, the higher the production However, it is necessary that these fluids can flow, Their remains would be covered up by other rock rate from that particular well. at an economic rate, through the reservoir to the deposits washed down by rivers, sealed from the
wellbore, from where they are transported to the air and, over time, exposed to pressure and other It is worth noting that the permeability (and surface. The property which allows this flow is changes (in much the same way as the reservoir
called permeability. therefore ease of flow) is affected not only by the rocks themselves). type of rock but also by the nature of the fluid passing through it. Heavy oils will usually find it The pore spaces in the rock must be connected The oil and gas formed by this process, however, more difficult to move through the pores of a given together, providing a continuous channel for fluids did not usually stay in the same place. You know rock than water. from your own experience that oil floats on water.
t t f t~: A lot of sea water was trapped with the plant and -: , I +,\/. I .. '.1/ '. 1(" 7 I'I~ animal life and when petroleum was formed it
tended to float upwards, through the water-filled
.. ~-,.~- (~) 0 { -~,/" pores of the rock, until it could rise no higher. (We
paths taken by fluid flowing .. -(:/~ ~< -(" • -/." '--\~ ..") I ;(·---1··1~)I(~./tJ> will look at the reasons for this in the next section).
) {.)[~~/( !b\, \~lJ~ through the pore space I ( 0..-/ I \. ) ;'l ~-;,
/1'........ \, i )1 ." .. ···)~U·~,./ ,'\,-'" --....... ' The place where petroleum was first formed is "'" .... (-;.~) )/_ // ..J known as the source rock. The process of moving o '" ~ ~. / . ,
/' _.'.L_ f from there is called migration, and you will have ~ <); (,<-- .. O.'~ c::~ ....'c):" s-: },(,Ji\ guessed, the rock in which this petroleum comes to ~ .• _.~.' <, 'l'~-_4 ~' -~()\,1" L \~-o k' I rest is called the reservoir rock. Our task is to . '" . / / -- I~Q - .' :l --- / -, r , \ /,- ~~ ._Q . \. "'. l ~ find this petroleum reservoir. . ,,'" - I \ I ,,,,··'~d_Y -c·o-(~J;\A)/.
Figure 3
1.9
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~ Activity
If you can obtain some clean, coarse grit (free from mud and clay, that is) you can try this:
Take your two empty yoghurt cartons saved from the first Activity. You also need the grit and a small quantity of oil (light machine oil or even cooking oil will do).
Fill one container with water as before. Add about 1/4" of oil to the other container and then fill it with the grit. Pour some of the water into the container of grit until free water is visible at the surface. Leave overnight.
When you come back to look at it, note down here what you see.
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1.10
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You should see that a film of oil becomes clearly visible on the surface. This oil, therefore, must have migrated from the bottom of the container to the top.
It has floated upwards through the water which is filling the pores of the grit until it can go no higher. This is how natural petroleum migrates from the source rock ie. the rock where it was formed, to the reservoir rock.
This small reservoir contains larger rock particles than you would usually find in practice but the idea is exactly the same. It does indicate very clearly that your yoghurt-carton reservoir has both porosity (to hide the oil and water) and permeability (to allow the oil to reach the surface).
The Structure of Reservoirs
You now have a mental picture of the petroleum after it was formed, floating upwards through rock pores and channels filled with water. It will stop when it is no longer able to do this - when it encounters a rock layer which does not contain any pores and channels. This layer is called an impervious rock, which means not permeable.
The impervious rock layer must be shaped in a certain way, otherwise the petroleum would find its way round the edges and continue its upward migration. The impervious rock layer must form a trap, such as a cap rock.
Figures 4a, 4b and 4c illustrate various types of traps and how petroleum can accumulate under these to form the petroleum reservoir.
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You will notice from these diagrams that the reservoir fluids contained within the rock pores have separated (over millions of years) into distinct layers - gas at the top, then oil, then water as you would expect.
In practice, the boundaries between the layers are not as sharp as in the picture. This separation is, however, important, when we decide how far into the reservoir we should drill before completing the well. Unit 9 will look at this in more detail.
-- porous, permeable reservoir rock
Figure 4a
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water
porous, permeable reservoir rock
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Test Yourself 2
Have a look at the sketch below. It shows two subsurface geological structures. If oil migrates upwards through the porous and permeable reservoir rock then do you think that either of these structures could form an effective petroleum reservoir?
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permeable "NO;' rock.>: ""'" t / "Il0E C (\\, r II(/f ; '.' .'. u· ~< reservoir rock
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'<, ~ ,~,. ,~ ~, ;,And why?
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You will find the answer in Check Yourself 2 on page 1.38.
1.13
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The task is now to find the petroleum reservoir, and the next section looks at some of the more common exploration techniques which are used prior to the final test - drilling. ~ Summary
In this section we have looked at sedimentary rocks and how they were first deposited. You will have learnt that, in order to form an effective petroleum reservoir, this sedimentary rock must possess two properties: porosity and permeability.
I described how petroleum was first formed in the source rock and how it migrated upwards towards the reservoir.
The petroleum reservoir will then:
* consist of sedimentary rock having both porosity and permeability
* lie underneath an impermeable (impervious) layer (such as a cap rock), forming a trap past which the reservoir fluids cannot leak
* contain reservoir fluids which have separated, in many cases into a gas layer at the top, an oil layer in the middle and a water layer underneath
You now know what a petroleum reservoir is. In the next section you will learn how to find it.
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Unit 1 : Basic Concepts ~ I
Section 2 .. Exploration Techniques
In the previous section, I described how sedimentary roCKS are laid down and why the presence of these rocks would indicate the possibility of oil and gas being present.
We need to know the broad geographical areas in the world where these sedimentary rocks may be found. This gives us a first clue as to where we should drill.
These areas can be very large and techniques are required to pinpoint more accurately the likely subsurface structures where oil and gas could accumulate. You already know that these are called traps and will be familiar with the basic types.
In this section, you will find out about:
* sedimentary basins
* exploration techniques - large scale
* exploration techniques - small scale
Sedimentary Basins
We have seen, previously, that most scientists now accept the organic theory of petroleum production.
In other words, they believe that oil and gas were formed from plant and animal remains, deposited from rivers and seas, covered with further layers of rock sediment and subjected to high pressures, high temperatures and so on.
If this is the case, our sedimentary basins, or geographical areas where large quantities of sedimentary rock are found, will be located where old river systems have deposited large quantities of sediment into ancient seas. The locations of these seas are well known by geologists, based on a wide range of other evidence.
However, knowing that a petroleum reservoir could be contained within a sedimentary basin which may be hundreds of miles across is not much help to an oil company. They want to know where to drill the first well - precisely.
Therefore, we require other techniques which will pinpoint more accurately the most favourable location for drilling.
Exploration Techniques - large scale
Here, we must think of techniques which are suitable for land locations, and those which are applicable over water.
Land
Geologists have found that they can sometimes identify subsurface structures like faults and domes by viewing ground contours at the surface. You will see this more clearly by looking again at figures 4a, band c and noting that these subsurface shapes can, to some extent, be mirrored at ground level.
Aerial photographic surveys are a most effective means of gaining this broad impression. Domes and outcrops often stand out clearly, perhaps by changes in the vegetation. Figure 6 on the next page shows an aerial photograph which illustrates some of these characteristics.
1.15
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The use of 3-dimensional photography increases the ease with which surface characteristics can be picked out from aerial photographs.
Nowadays, in addition, photographs from orbiting satellites are increasingly used for this type of exploration work.
Aerial surveys are often accompanied by field surveys - geologists on the ground investigating in more detail some of the structures picked out from the air. They may be looking for the size and shape of a dome, or the slope and rock type of an outcrop. Again, figures 4a, band c demonstrate how this information might be useful in selecting a location to drill.
Water
Over water, the same type of information is needed, but other techniques are required.
Sonar (reflected sound wave) surveys can be used to plot the contours of the seabed, while divers are sometimes employed to carry out a field survey underwater.
!Figure 6
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Exploration Techniques ~ small scale
At this stage, explorers for oil would want some confirmation that what they saw on the surface actually indicated a trap-type structure underground.
They would also need some hint of the depth of any likely structure. It would be very difficult to plan an effective drilling programme if the target rock formation could be at 5 000 feet or 15 000 feet.
We will/oak at three common methods for obtaining this information:
* gravity surveys * magnetic surveys * seismic surveys
Gravity Surveys
Gravity surveys, as the name suggests, measure differences in the pull of gravity from one underground rock layer to another. Dense rocks will have a higher gravitational pull. Figure 7 shows, in simple form, how a gravity meter responds to layers of dense rock and how the results are recorded.
pull
~ - »>
; ; J
~ layer of
7~~"-"'-:dense roc~ ~~/~/~
~~2 ~==-=- - :==-----=:::;----=::t-= - - -- ~
A cap rock will be present in some petroleum reservoirs (look at figures 4a, band c again) and, as this may well be more dense than the underlying reservoir rock, a gravity survey could be a useful tool to detect it.
gravity meter record
gravity meter response
subsurface rock structure
maximum gravitational
-- ~ <,
<,
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1.17
~
Magnetic Surveys On land
In this type of survey, a magnetometer is used to * by explosive charge measure the strength and direction of the local * by thumper truck, which drops a heavy weight onto the ground magnetic force. The presence of underground , by Vibrator, where a vibrating surface is kept in contact with the ground rocks containing, particularly, iron will distort the normal pattern of the earth's magnetic field in that Figure 8a shows a seismic survey on land. area. This distortion can be measured and recorded to give a picture of structures containing magnetic minerals.
survey truckSeismic Surveys ~
exploSiv;.,.The word seismic means relating "to earthquakes charge
and this gives a clue to the principle of this technique.
A shock wave is set off at the surface and travels down through successive rock layers. At the boundary of each one, part of the wave energy is " reflected back to the surface where it is picked up by sensitive instruments called geophones (on =-~:~~\~" -,-~ =--~~~ \ \ "",,land) or hydrophones (on water). -~---~ \ \ " ,
c =~ \ \ \, _--The shock waves themselves can be generated in \ \ 'y' y' ";/ /
\ I /a variety of ways. For example: " / ->: --------
Figure 8a
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/
=-~~~-~~~~
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Over water
* by compressed air charge, which offers less threat to marine life than explosives
You can see this set-up in figure 8b.
compressed air
survey charge hydrophonesvessel
/ ~
/ /
/
/
""-,~"-~~~- ' .: ~~ //
==-~~~~~~~=~~=,/--~~§:-~==~~=-=-===;-:=;::~-=-==:=~
In both cases, the shock waves travel through different types of rock at different speeds and therefore will arrive back at the surface at different times. It obviously takes a computer to analyse the large number of signals being received at the surface, but a remarkably accurate picture can be built up of subsurface formations in terms of both shape and physical characteristics.
Looking back over this section, you will notice that all of the techniques described are carried out on or above the surface, although they do give us some good indications of what lies below.
When the drilling site is selected and the well is actually being drilled, however, we find ourselves directly in contact with deeper and deeper subsurface rock formations.
This gives us new opportunities for looking at, and getting information from, these rock layers. I will describe these opportunities for you in detail when we come to Unit 9 - Formation Evaluation.
Figure 8b
1.19
~ I
~ Summary
In this section I described for you how sedimentary basins were originally formed. You will have learnt that they can be very large and we need some other way of homing-in on the first drilling location.
First, a broad survey of the area can be carried out by:
* aerial photography land * satellite photography land * field geologists land * sonar techniques water
Then a precise location for drilling can be chosen with the help of three techniques:
* gravimetric surveys measures differences in the gravitational pull of various rocks
* magnetic surveys measures differences in the magnetic properties of various rocks
* seismic surveys measures differences in the speed of sound through various rocks
We now know where to drill. In the next section you will look at some basic steps in the drilling operation and the rigs used to do the job.
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Unit '1 : Basic Concepts
Section 3 - Drilling Rig Types
~ I
By using the appropriate exploration techniques described in the last section, it should now be possible to select a location to drill. A particular type of rig will be chosen to meet the location and depth requirements of this exploration well.
As you will have learnt from the introduction to this unit, different types of well will be drilled at different stages of the oilfield development. Exploration, fixed semiappraisal and development drilling were land jack-up platform submersible
drilfshipmentioned and each of these will influence the design of the rig to be used.
Location will also have a significant influence. Arctic, jungle or desert conditions? Drilling from land or water? And so on.
In this section we will look at the main types of drilling rig designs:
land rigs
or, to drill from water,
* jack-up rigs * platform rigs * semi-submersibles * drillships
Figure 9 shows the typical locations in which these various rig types operate. Figure 9
1.21
~ ".-'
c'
In Unit 2 you will look at Drilling Systems and Equipment. It will become clear, whatever the rig design, it must carry out the same basic function to make a hole in the ground.
It will also comprise these five basic systems:
* hoisting system
* rotating system
* circulating system
* power system
* blowout prevention system
I said that all drilling rigs have the same essential job - to make a hole in the ground.
Before we look at the different types of rig, therefore, I feel we should have a brief look at some basic drilling concepts. This will allow you to picture more clearly the various stages of the drilling operation.
Basic Drilling Concepts
The drilling of any well is carried out in a number of clearly defined stages. At each stage the actual hole is drilled using a drill bit. A typical drill bit is shown in figure 10,
Figure 10
The bit is connected to the bottom of an assembly of pipes called a drill string. As the hole is deepened more lengths of drill pipe are added until the total depth of that stage is reached.
1 The first section of hole is drilled using a large diameter bit (see figure 11), This hole is drilled to a relatively §fl?ILgw depth before drilling is stopped and the drill string and bit are removed from the hole.
drill stri~gsurface
,'.::': ,'.':':;:
',,:'
-'_'__ 1st section of . 'hole drilled with
large diameter bit
. '.': ..... >( '.:'
, 1 ~<.~S·
drill bit (say, 26" diameter)
.: .:..?
Figure 11
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2 Steel pipe called a casing string is lowered At a pre-determined depth, drilling is once again into the hole. Its function is to stop the drilled stopped and this section of hole is lined with hole from collapsing as it is deepened. casing. This casing is also cemented in place
as shown in figure 13 below. (You will look at other functions of casing in surface
more detail in Unit 5 • Casing and Cementing). ,'1 st section' I,~~·
of hole (26" dta.) 3 The diameter of the casing string is smaller
1st section than that of the drilled hole. This means that of hole there is a gap between the outside of the casing
I " cement filled
anriulusand the inside of the hole. This ring shaped
annulusspace is called the annulus. ~. .':- filled with " ','II" ,cement 11-. 'list casing
1:~ ~·.GTo secure the casing in place and isolate the string 1st casing string ~ .• t7·"·11.'" "
Cl .- to:. ,. ::JI, '.'formations behind the casing, the annulus is f> &. ,.'(20" dia.) -'filled with a cement slurry. i J.'
.' . • /"a:""11 ".: A r: ,.II'~Figure 12 shows the status of the hole at this ,~,~ :,.
.-.: 2nd section stage. of hole (171/2" dia.)
'....<\
" 111-41__ cement filled .~: ." annulus
Figure 12
2nd casing iAI sIring (13%" dia.)
4 When the cement has set, drilling can continue. A drill bit which will fit inside the casing is connected to the drill string and more hole is
Figure 13 drilled.
1.23
~ I
--'--~---
This sequence of drilling, then lining the hole with casing which is cemented in place, continues until the total depth of the well is reached. Figure 14 shows a well drilled to its total depth.
Bear in mind, when you look at drawings of wells, that they will be grossly out of proportion. If we tried to illustrate a 10 000 feet deep well which was 12 inches in diameter, using a scale of 1 in 12, we would require a sheet of paper the height of the Eiffel Tower!
""""
~t.·.1~~~ction e : of hole .·.·0
~:. ,,;1---- cement
"r"1st casing'
III ~I;; ::, ~:,II ;
~ +_~2nd section
of hole
f.:.: 'dll:~cement 'e -:111,
lit" 2nd casing b Ijl'.<\ I ,.14
lAlJ'f
,.;~Iast section of :.'.~ • hole drilled to
. " ~..
'. total depth (121// dia.l ~ .;,.. .~\~ ;f...--.-cement
lu-l'r;./ fin~1 ca~~~. .. II~~
Figure 14
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[] Test Yourself 3 Below I have given you a selection of bit sizes, casing sizes and hole depths which are out of order:
Bit Sizes 171/ " 26" 81/ " 121/ " 2 2 4
Casing Sizes 20" 7" 133/ 8" 95/ 8
"
Hole Depth 11 500' 4500' 9500' 1 500'
Use this information to fill in the missing details in the drilling programme below:
Drilling Programme
Bit Size Casing Size Hole Depth
lS (ij,Stage One 2.\:J' l
L!"I[ ) Stage Two
Stage Three
'\."
Stage Four
You will find the answer in Check Yourself 3 on page 1.39.
1.25
~ I
Land Rigs
Land rigs vary enormously in size - in their capacity to lift, circulate fluids and generate power.
Masts
For lighter work, cantilever masts (also known as jack-knife derricks) are common. Figure 15 shows a typical one.
Masts of this type are assembled on the ground from large welded sections fastened together with pins. They may then be raised to the vertical position by using the rig's own power unit and hoisting line. Small masts may be truck mounted, while some are telescopic. The rigging-up time for masts tends to be less than for conventional derricks.
Rigging-up time is the time it takes to assemble a mast into the vertical position, on-site from all its components. It also includes the time to install the power unit, all the cables and the piping.
\ pin
/connection
raising leg ,/
figure 15
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Derricks
There is no really clear-cut distinction between a mast and a derrick. Sometimes the words are used interchangeably.
To keep things simple, we will regard a derrick as the framework-tower type of support usually associated with oilwell drilling. Figure 16 shows a common arrangement.
Typically, derricks are assembled on-site by bolting individual pieces together. The rigging-up time for this method is, of course, longer than for a cantilever mast.
We will look at the differences between masts and derricks in more detail in Unit 2.
-- substructure
Figure 16
1.27
---------------_.
~
Jack-Up Rigs
A jack-up rig consists of a movable platform which can be jacked up and down the (usually) three supporting legs. Figure 17 shows one of the standard designs
These provide a common means of drilling in water, where the water depth is relatively shallow say, 50 to 350 feet
Jack-ups will be floated out to location and the legs then lowered independently until they are bedded securely and the platform is level and above wave height.
It is clear that their use would be restricted when there are strong currents or an unstable seabed.
Figure 17
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steel jacket
~
Figure 18
-------- »>
-
Platform Rigs
A platform is a fixed installation offshore from which development drilling and petroleum production is carried out. A steel platform design is shown in figure 18, as an example.
The deck, supported by a steel jacket in this case, carries equipment and accommodation modules and a helicopter pad. It will also support one or more drilling rigs and associated equipment.
~~
1.29
-- -- _.- -_.
Semi-Submersibles In this case, a deck is supported by a tubular structure, and by two hulls to provide buoyancy.
A semi-submersible is a floating drilling rig. A Again, the deck carries equipment and typical layout is shown in figure 19 below. accommodation modules, a helicopter pad and a drilling rig.
Semi-submersibles can move easily from one location to another either by being towed or under their own power.
They are mainly used, therefore, for exploration and appraisal drilling where this ease of movement is essential.
When on location, the semi-sub (as it is often helipad called) takes on water ballast (into the two hulls,
etc). This will lower the structure in the water and lower the centre of gravity.
In this position it is shielded from the effects of ~ rough water at the surface and achieves a high
'-'-----.~ degree of stability.
-----....
~~ ~~~-.
hulls _ -----
•m&
A semi-sub can operate in deeper water than a jack-up. Its maximum operating water depth depends on the type of mooring system employed. Some semi-subs use anchors with wire and chain to hold them on station. Others use dynamic positioning which is a system of computer controlled thrusters, to maintain their position. We will look at mooring systems in more detail in Unit 6. Modern semi-subs using anchors may, in exceptional circumstances, drill in water up to 3 000 feet deep.
Semi-subs using dynamic positioning systems are capable of drilling in even deeper waters, up to 6 000 feet deep.
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1.30
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Drillships
These are ships specially built or converted for deep sea drilling. Their main features are:
* the ability to drill in almost any depth of water * greater mobility than semi-subs or jack-ups * dynamic positioning equipment can be fitted, as with semi-subs * greater storage capacity than other rig types • not as stable as semi-subs or jack-ups while drilling
Figure 20 will give you an overall impression of the layout.
derrick _
~-
~. -
Figure 20
[] Test Yourself 4 You plan to drill three exploration wells in the following depths of water. What type of rig would you select to drill each of these wells.
* 4500 feet t ril
* 280 feet -) JV;" ",. U {? ~ i.~
* 1 500 feet t.efl": I S'';<>i'<J"' e( In bIer
You will find the answer in Check Yourself 4 on page 1.39.
1.31
I ~
[g Summary
To begin with, we looked at some basic drilling concepts. I will go into these in more detail in later units. For now, however, you have an overall impression.
The various types of drilling rig were then described. I talked about two types of land rig a mast and a derrick.
Then we considered drilling from water and looked at jack-ups, platform rigs, semi-submersibles and drillships. The differences between these rig types were highlighted.
We will now take a look at the personnel who operate these rigs.
1.32
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Unit 1 : Basic Concepts ~
Section 4 ... Drilling Personnel
The way in which rigs are crewed up will, of course, vary greatly from one situation to another. There are common features and these are the ones I wish to concentrate on.
Drilling Operations ~ Personnel
operating company
t I company rep.
•Ileurn engineer ling ,engineer site' geologistf_--I
Figure 21 gives a typical picture of: • operating (0;1) company personnel * drilling contractor personnel, and * the types of service companies
involved in the drilling operation, and how they relate to each other.
drilling contractor
t-«
1-- toolpusher
service companies
~--I r-
tourpusher
driller J
__ I
-.J asst. driller
derrickman n 19
roughneck v g
roustabout
fishing well servicing
Figure 21
OIl r--- ~ubsea engineering) I
chief mechanic rig mechanic motor man
responsible to
---- liaison with
1.33
~ I
The company representative on site (perhaps called the company man or drilling supervisor) ensures that the drilling programme is carried out in a safe and efficient manner.
He reports to, and is employed by, the operating company which holds the licence to drill the hole. He will have operating company specialists on site to assist him.
Service company personnel report to the company man, but must liaise very closely with the drilling team.
Members of this drilling team are listed opposite, together with their main tasks and reporting links.
ii ji,I :;1 :;1 l.;!
~ :1 j
Toolpusher:
Tourpusher:
Driller:
Assistant Driller:
Derrickman:
Roughneck:
Roustabout:
In overall charge of rig operations, implementing the drilling plan and compliance with all safety requirements. Reports to the company representative.
Assistant to the toolpusher. Will be in charge of a particular shift (often the night shift). Responsible to the toolpusher for implementing the drilling plan and for safety, and reports to him.
In charge of the drilling process and operations. Responsible for compliance with the drilling plan and for the drilling crew. Reports to the tool/tourpusher.
Assists the driller. Usually responsible to the driller for the operation of bulk storage equipment (for handling mud chemicals, etc.) and for the mud flowline system. Reports to the driller.
Responsible for the storage and movement of tubulars in the derrick and monitoring the mud systems. Reports to the driller.
Works on the rig floor. Responsible for general rig floor activities under the direction of the driller/assistant driller. Reports to the driller.
A member of the general workforce, assisting with the movement of materials, cleaning, painting, etc. Reports to the roustabout foreman (not shown on figure 21).
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- _.... .... _. = ==,~......---~
[1] Test Yourself 5 In this unit we have been through some of the major steps in a typical exploration programme. I would like you to think back over what you have learnt and list as many of the steps in the programme as you can.
You will find the answer in Check Yourself 5 on page 1.40.
1.35
~ I
. ----------------------------------------
Summaryl!1 In this section we have looked at a typical drilling operation and described the people and activities which make it happen. I have indicated how they link together into an efficient team.
Finally, I have listed the essential members of the drilling crew and detailed their main tasks.
You have now completed the first unit about Basic Drilling Concepts in this Drilling Technology open learning programme.
Go back to the Training Target at the beginning of this unit and check that you can tick all of the boxes. If you are unsure of anything, read over the relevant sections again and have a chat with your tutor if necessary.
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Unit 1 : Basic Concepts ~
Check Yourself .. Answers
Check Yourself 1
The maximum amount of oil which our mini-reservoir can hold is the pore volume of the reservoir.
We can calculate this by multiplying
total rock volume x porosity
or 1 cu. metre (1 000 Iitres) x 18% = 180litres
Our block of sandstone, therefore, could hold a maximum of 180 litres of oil.
1.37
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Oil has now migrated to our two possible reservoirs. You can see this below:
Check Yourself 2
In the first sketch, oil has accumulated under the caprock, which has prevented it migrating further. This could therefore be an effective petroleum reservoir. In the second drawing, the impervious rock layer has a break in it, due to faulting, and this has allowed oil to leak out and upwards. The structure would not, therefore form an effective petroleum reservoir. ../1
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~ Check Yourself 3
Drilling Programme
Bit Size Casing Size Hole Depth
Stage One 26" 20" 1 500'
Stage Two 17 1/ 2
" 133/ 8" 4500'
Stage Three 121/ 4
" 95/ 8
" 9500'
Stage Four 81/ 2
" 7" 11 500'
If your answers are not the same as these, try working through the section again.
Check Yourself 4
To drill in 280 feet of water a jack-up would be suitable, as long as the seabed provided a sound footing for the jack-up legs and local currents were not too strong.
To drill in 1 500 feet of water, a semi-submersible is likely to be used which uses either anchors or dynamic positioning systems to maintain its position.
To drill in 4 500 feet of water, a dynamically positioned drillshlp is the more likely choice.
1.39
~ I
~ Check Yourself 5
The major steps in a typical exploration programme are roughly as * On the basis of this evidence you can decide where to drill. follows: However, the drilling rig must be selected and to do that you need to
answer a few more questions: * By consultation with geologists, you would know where the major
sedimentary basins are located in the geographical area of - are you drilling on land? interest to you. - or over water?
- if over water, in what depth are you drilling? The next stage which we have not talked about is how to obtain - how deep does the hole need to be? permission for all the work we intend to carry out on the project. How this is done varies enormously from one country to another and is often a * Your rig is selected. You must ensure that it is crewed up properly matter for legal experts to handle. I suggest we leave it to them. and all the service company personnel you require will be on hand at
the correct time. * Assuming permissions have been granted, an aerial or satellite
survey, possibly combined with a ground survey will be carried out over land locations. Over water, a sonic survey may be called for. Let drilling commence.
* This work will narrow the area down and allow more detailed surveys using gravitational, magnetic or seismic techniques. The use of seismic methods is a high probability in any modern exploration programme.
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Training Target
Introduction
Section 1 - The Hoisting System
S~l<;;Hon 3 - T.ne Circulating System
* Section 5 - The Blowout Prevention (BOP) System
* Check Yourself - Answers
Page
2.2
2.3
2.4
2.12
2.28
2.38
2.41
2.54
2.1
~
r.f!tli!il\ ~ Training Target
When you have completed this unit, you will be able to:
* List the five basic drilling rig systems. D
* Summarise how each system operates. D
* Identify the components of each of the five systems. D
* Explain the function of the components identified. D
* Describe in simple terms the construction of the components. D
* Outline the relationship of each system in the overall drilling process. D
Tick each box when you have met the target.
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mud conditioning equipment
) - II I travelling block
fl" \I! , drilling hook
I l', F"i crown block
BOPs -I I If-7-.
Unit 2 : Drilling Systems and Equipment
Introduction
You saw in the first unit of this programme that In this unit we will go through each of the systems there are a number of different types of drilling rigs listed. We will look at the individual components in in common use. It doesn't matter what type of rig the system and see how they contribute to the you think about, it has one function only: overall drilling process.
to make a hole in the ground Although the equipment is basically the same on all rigs, floaters (semi-subs and drillships) have some
In order to perform this function safely and special equipment which is not found on land efficiently, the rig has a number of components installations. We will be looking at specialised which can be grouped together in four basic floating drilling rig systems and equipment in systems. These systems are the: Unit 6. In this unit we will concentrate on
conventional equipment such as you would find on * hoisting system a land rig or on a fixed platform offshore. power unit
* rotating system Figure 1 shows a simplified line drawing of a typical rig with its systems and components.
* circulating system As we go through the unit system by system, we
* power system will gradually build up the complete picture of the drilling installation which you see in figure 1.
In addition to the four I have listed above, there is a further system which must be considered in any discussion of rig components. This system, although not essential to the drilling process, is critical for rig safety. It is the:
* blowout prevention system
drill collars
Figure 1 drill bit
2.3
Unit 2 : Drilling Systems and Equipment
Section 1 - The Hoisting System
As you will shortly see, holes are drilled with long The Derrick lengths of pipe - with a drill bit at the end - which in a deep hole can weigh two hundred tons or more. The derrick is the tall, towerlike structure which This considerable weight has to be suspended in most people think of as an Oil Rig. the hole, raised and lowered. A hoisting system accomplishes this task. A standard (pyramid shaped) derrick consists of
four steel supporting legs standing on a square , If you think about it, the hoisting system performs base. This base is known as the substructure the same function as a crane. On a drilling rig, and the top of this substructure is the drilling rig however; the boom of the crane is fixed in the floor. The supporting legs are joined together with vertical position and is called the derrick (or mast). steel cross bracings which stiffen the structure and
give the necessary load bearing strength. This derrick is the first component in the hoisting system. Let's list all the components in the system The height of the derrick does not affect its load and then look at each of them in turn. bearing capacity but will limit the length of drill pipe
sections which may be removed. As you will see The Hoisting System Components later, it is necessary from time to time to pull all the
drill pipe out of the hole. The top of the derrick must be high enough above the rig floor for the* derrick pipe sections to be taken out of the hole and temporarily stored within the rig structure.* drawworks
Derrick heights vary from around 90 feet on small* drilling line land rigs, to over 150 feet on some large offshore installations. This height is measured from the rig* crown block floor.
* travelling assembly (travelling block and Figure 2 shows a simplified drawing of a derrick.hook)
~ ~ water table I
Ft:dJEl:sE--finger board
rig floor ,v" '" "Fl 1, iL/'lA
-- substructure
Figure 2
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You will see that there are two platforms incorporated in the structure. These are labelled:
* the water table
* the finger board (also called the monkey board)
The water table is a working platform at the top of the derrick which enables maintenance to be carried out on the equipment there.
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The finger board is another working platform which is located approximately 90 feet high in the derrick. This is where one of the rig crew (the derrickman) works when drill pipe is being pulled from the hole.
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You will recall at the start of this section I made reference to a derrick or mast.
- I
Figure 3 shows a simplified drawing of a mast.
A mast performs the same function as the derrick but is usually much lighter in construction. It is used on land drilling operations and is capable of being transported between locations as a complete unit or in a couple of sections.
mast rigged down
mast erected
Figure 3
2.5
_.._----
~ I
Drawworks
If we continue our analogy of a crane and hoisting system, the winch of the crane is equivalent to the drawworks of a drilling rig hoisting system.
The main purpose of this piece of equipment is to lift pipe out of and lower it back into the hole.
The drawworks is located in the middle of the back edge of the rig floor.
How it Works
Its principle feature is a spool, or drum, which can
driller's console (workstation)
Figure 4
be rotated using power from the rig power system. Wireline (drilling line) is reeled onto the drum and from there passes through a system of pulleys (crown and travelling blocks). A hook is fixed at the lower end of the pulley system. As with the winch of a crane, when the operator, in this case the driller, rotates the drum in one direction, the drilling line is reeled onto the drum raising the movable pulley and hook. If the drum is allowed to rotate in the other direction the hook will descend, pulled down by the suspended load. Since the drill pipe is connected to the moving pulley, the pipe can be raised or lowered.
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The drawworks usually has a second spool, known as a sand reel, fitted behind the main drum. The wire from this spool passes over a single pulley and can be used for running tools into and out of the hole. The sand reel is sometimes called a coring reel, however, it is not used much these days. Independent wireline units are used for this purpose.
Incorporated into the drawworks are the catheads. These are used together with large spanners or tongs to make or break the threaded connections between individual joints or pipe.
You will see in Unit 3 how these operations are actually carried out. We will also look at some alternative equipment used in these operations.
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Your tutor should be able to help you if you are unsure.
Make sure that you understand how this system works.
Activity
Look again at figure 4 and identify the main drum and catheads of the drawworks.
~ The Brake System
An important feature of the drawworks is the brake system. This allows the driller to control the tremendous load of the drill pipe or casing suspended from the pulley system. There are at least two brake systems on most rigs. One is a mechanical friction device which uses two bands passing over brake drums. The bands are connected to a large brake handle located at the side of the drawworks. This is operated by the driller to halt the descent of a loaded pulley system. The other brake is either hydraulic or, more commonly these days, electrically operated. This auxiliary brake is used to control the rate of descent of the load. It helps to reduce the wear on the primary friction system.
Figure 5 shows the friction type brake mechanism.
brake bands over
brake handle-
brake drums
Figure 5
2.7
~ I
Gear System
An integral part of the drawworks is the gear (transmission) system. A clutch, high and low speed gears together with a foot operated throttle, allow a wide range of hoisting speeds to be used.
Control
The driller's workstation is at the brake handle of the drawworks. From this position he must be able to control the rig and oversee the activities of his drilling crew. Instruments give him indications of the status of equipment and machinery, and operating controls are all within easy reach of the driller. We will look at most of this equipment when we consider the drilling operations in the next unit.
Look again at figure 4 and note the driller's workstation by the brake handle.
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The Drilling line, Crown Block and Travelling Assembly
The last three items in our list of hoisting system components together make up the pulley system which I have referred to on a number of occasions already.
The drilling line is a multistrand wire rope which is secured to the drum of the drawworks. Its diameter varies according to the type and size of rig, but on a large semi-submersible it might be as much as 13/ 4 " in diameter.
From the drum of the drawworks the line passes over one of the pulleys (sheaves) of the crown block which you can see in figure 6.
This fixed multi-sheaved assembly is located at the top of the derrick and is surrounded by the working platform which you will remember we called the water table.
The sheaves themselves are up to 5 feet in diameter and are free to rotate on heavy duty bearings.
crown block sheaves
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From the crown block the drilling line passes to and round a sheave contained within a moving set of sheaves known as the travelling block. Connected to the bottom of the travelling block is the drilling hook and these two items comprise the travelling assembly. The hook may be integral with the travelling block or may be removable.
Figure 7 shows a travelling assembly. Take a look at this now. Note the two ears on the hook. Two forged steel rods called links can be attached to the ears. The links support an item of equipment called an elevator which is used when all the pipe is being pulled from the well.
drilling line
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Figure 7
I will explain how the elevator is used in the next unit.
The drilling line then passes over another sheave crown
on the crown block and down again to the travelling block block. The number of passes the drilling line makes between crown and travelling blocks give an eight, ten or twelve line suspension.
After making its final pass over the crown block the drilling line goes down to the base of the rig where it is clamped on a drilling line anchor, sometimes called the deadline anchor. The drilling line is not terminated at the deadline anchor but continues on to a reel of spare line.
deadlineA careful record is kept of the work done by the anchor line. This is measured in units of ton-miles. (If a line has moved a one ton load a distance of one mile, it has received one ton-mile of usage). After a pre-determined number of ton-miles have been recorded, the line is slipped and cut. This means that the anchor is slackened and fresh line is slipped into the system from the spare line spool. A corresponding length of wire is then cut off at the drawworks end of the line.
Figure 8 shows the hoisting system in simplified form.
Figure 8
2.9
~ I
[] Test Yourself 1 In the following list of components some are part of the hoisting system and some are not. Tick either yes or no in the boxes provided.
Yes No
1. Travelling Block CJ 0 2. Drawworks 0 0 3. Drilling Line LJ lJ 4. Rotary Table 0 [J 5. Derrick D D 6. Drill Bit D U 7. Crown Block 0 D 8. Deadline Anchor D D 9. Drilling Hook D U 10. Drilling Collars 0 D 11. Shale Shaker D D 12. Drawworks Drum D U
You will find the answer in Check Yourself 1 on page 2.54.
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~ Summary
In this section we have looked at the components of a drilling rig hoisting system.
While working through this section you have learned what the function of the hoisting system is and the individual components which make up this system.
You saw that the components are:
" the derrick
* the drawworks
* the drilling line
* the crown block
* the travelling assembly
The drilling line is attached to the main drum of the drawworks and from there it passes to the top of the derrick. It then makes a number of passes between the crown block and travelling block below, to give an eight, ten or twelve line suspension. The line is then clamped by the dead line anchor at the base of the derrick.
You saw that the drilling hook is attached to the underside of the travelling block, forming the travelling assembly.
We likened this whole system to that of a crane, with the derrick representing the crane's boom in the vertical position. The driller lifts the drill pipe or other load by engaging a clutch and spooling line onto the drawworks drum. The load is lowered by releasing the brake on the drawworks drum, allowing the load to pull the travelling assembly down.
In the next section we will be looking at the equipment used to apply weight to and rotate the drilling bit.
2.11
Unit 2 : Drilling Systems and Equipment ~
Section 2 - The Rotating System
Just like any drilling operation whether it be in wood, steel or other material, some type of cutting tool has to be rotated whilst weight is applied to make the hole. In oilwell drilling operations the cutting tool is the drill bit and in this section we will look at the equipment used to turn the bit, i.e. the rotating system.
A rotating system can be thought of as having three main sub-systems:
* drill bit
* drill string
* rotating mechanism
The actual transmission of the rotating action to the drill bit can be carried out in a number of ways. I will first of all describe what we could call the conventional system. At the end of this section we will look at some alternative rotating mechanisms.
The Drill Bit
The drill bit is probably the most critical item of a drilling operation. It must be capable of making hole in rocks which vary from very soft clay-like material to extremely hard granites. Since one bit would not be suitable for these widely differing conditions, there are a number of different designs available.
They can be broadly classified into the following categories:
* drag bits
* tri-cone roller bits
* diamond bits
* polycrystalline diamond bits
Drag Bits
A drag bit is very rarely seen these days. It was one of the earliest types of bit and it cuts by the shovelling action of blades on the formation. I don't intend to say any more about drag bits but I have included an illustration of one, as figure 9.
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Tri-cone Roller Bits
Tri-cone roller bits are the most commonly used drill bits at present. A large variety of designs are available to cope with the different formations encountered. Figure 10 shows a typical tri-cone roller bit.
jet nozzle I /, (0
let's look at a typical bit arrangement.
As the name suggests, this type of bit has three cones which are free to roll on bearings. The cones incorporate the cutting structure. This consists of teeth cut into the cone, or inserts pressed into holes in the cone surface. For soft formations the teeth are long and widely spaced. This gives a digging or gouging action. Short stubby teeth which are closer together provide a chipping or crushing action. This is more suitable for hard formations.
Drilling fluid helps to lubricate the bit and carry away cuttings from the bottom of the hole. It exits the bit through holes called jet nozzles. The nozzles are replaceable so that the orifice size can be altered to match the fluid pressure and volume requirements. '
Diamond Bits
Diamond bits use industrial diamonds as their cutting structure. They drill by the scraping action of the diamonds which protrude from a metal matrix. The design of diamond bits varies greatly in the shape of the body, the size and setting of the diamonds. Figure 11 shows a diamond bit.
Figure 10 Figure 11
2.13
~
Polycrystalline Diamond Bits The drilling action of the bit is a shearing one. As each polycrystalline cutter rotates on a different path from its neighbour, it shears the rock ratherRecently, polycrystalline diamond bits have like the action of a lathe. This means that the bit isfound a lot of favour. These bits consist of a hard [1]
faced steel body in which are inserted cutters. more suitable for soft to medium non-brittle Test Yourself 2 formations.The cutters are discs of synthetic diamond and are
arranged in rows spiralling outwards and upwards The following 3 sentences describe Replaceable nozzles are fitted in the body of thefrom the bit centre to the outside which you can different drilling bits. Decide if the bit bit. Drilling fluid flows round the bit and past flowsee in figure 12. described is more suitable for a soft,channels cut in the bit's body. medium or hard formation.
When used under the right conditions, 1. A tri-cone bit with short cutting insertspolycrystalline diamond bits have greatly improved which are close together.penetration rates whilst reducing operating weight f-te'll.£;\on the bit. In general this also means longer bit life with a consequent reduction in overall drilling times. 2. A bit with a hard faced steel body and
synthetic diamond cutters inserted.
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3. A bit with three rollers where each roller has long widely spaced teeth cut into it.
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cutters of synthetic diamonds You will find the answers in
Check Yourself 2 on page 2.54.
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under-reamer
Other Drilling Tools
Before we leave the subject of bits, we should look at other special purpose drilling tools. These are under-reamers and hole openers.
These tools, when used, are placed immediately above a bit to enlarge or maintain the hole size. hole-opener
I have shown examples of these tools in figure 13.
collapsible arms
Figure 13
In the tools illustrated, the cutting action is by rotating cones (as with the tri-cone bit) which protrude out from the central stem.
Both these tools perform similar functions but the cones of the under-reamer are mounted on collapsible arms.
The arms are extended during the drilling operation by the pressure of drilling fluid circulating through the tool.
When no fluid is being circulated the arms retract allowing the tool to pass through a smaller section of hole.
Remember that hole size was discussed in Unit 1.
2.15
~•
The Drill String
During this programme you will come across a number of strings. The drill string is one. In Unit 6we will be looking at casing strings, and you will probably see the term tubing string. In this context a string consists of a number of individual lengths of pipe joined together.
The drill string is made up of lengths of drill pipe, plus the bottom hole assembly.
The bottom hole assembly consists of a number of items placed just above the bit. We will look at this shortly, but first let's concentrate on the drill pipe.
Drill Pipe
Drill pipe is tubular steel pipe with threaded end connections called tool joints. It is used as:
* a shaft to rotate the bit,
* a conduit to convey drilling fluid to the bottom of the hole and
* a tool to run in and pullout the bottom hole assembly and bit.
As the hole is deepened, extra lengths of drill pipe are added to the drill string as required.
Individual joints of drill pipe are about 32 feet long on average. The diameter varies but some frequently used pipe diameters are 4 '/2'" 5" and 5 1/z". These dimensions are always measured on the outside diameter. Figure 14 shows a length of drill pipe.
box end pin end
. :':":/:'.
Figure 14
The tool joints are screw threads used to join two lengths of pipe together. The threads themselves have a round profile with a pronounced taper for ease of connection and disconnection. One end of the pipe is a male thread usually called the pin end, the other is female and is referred to as the box end.
When drill pipe is being connected, the box end always points up so that the pin can be stabbed into it.
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Bottom Hole Assembly
The bottom hole assembly consists of a number of pieces of equipment which are placed just above the bit. These are:
drill collars
* stabilizers
* various subs
Drill collars
Drill collars are basically heavier weight drill pipes. They have larger outside diameters (up to 10") and smaller inside diameters. A string of drill collars has several tasks to perform, i.e.
* provide the necessary weight to the bit
hold the drill string in tension
help maintain hole direction
Let's consider these tasks. You may think the drill pipe can provide the weight on the bit and hold the string tension. Imagine a hole being drilled which is several thousand feet deep. If the weight of the drill string was allowed to rest on the bit, the whole drill string would be in compression. This means the string would buckle and twist, with the danger that the pipe might break (twist off).
drill string Having a string of heavy drill collars enables just in tension part of the weight of the collars to be applied to the bit. In this way the lower portion of the collars is in whole of compression with its weight resting on the bit. The drill string in
compressionupper portion plus the entire drill pipe section remains in tension, supported on the hook of the travelling assembly.
The number of drill collars required will depend on the weight on the bit necessary for optimum drilling rate under prevailing conditions. It is usual practice to have 10 to 30 percent excess drill collar weight over the amount applied to the bit.
The maintenance of hole direction in a vertical hole relies on the pendulum effect of the drill string. This effect is the tendency of the drill string to hang in a vertical position due to the force of gravity. Having heavy drill collars increases the pendulum effect and strengthens the tendency for the drill string to remain vertical.
In figure 15 I have shown how the drill collars keep the drill pipe in tension and provide weight to the bit for drilling.
part of weight of drill collars on bit with lower portion of collars in compression
Figure 15
2.17
~ I
Heavy Weight Drill Pipe (HWDP)
Before we leave the subject of drill pipe, I should mention heavier weight drill pipe. Joints of this pipe are placed between normal drill pipe and the drill collars. They act as a cross-over between the rigid drill collars and the flexible drill pipe. This helps to prevent failure at the cross-over point.
[!] Test Yourself 3
For a particular section of hole, the optimum weight on bit should be 50 000 Ibs. Drill collars of 8" diameter with a 3" bore are being used. These collars weigh 4 410 Ibs per joint.
The drilling fluid in the hole has a buoyancy factor of 0.833 which means that only 83.3% of the actual collar weight is available as weight on bit.
If 25% excess drill collar weight over weight on bit is used, how many drill collars are required in this bottom hole assembly?
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l~,ucO Lh, Jl You will find the answer in Check Yourself 3 on page 2.55.
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Stabilizers
Stabilizers are short lengths of pipe with fins or ribs which are the same size across as the bit diameter or slightly less.
The fins may be aluminium or rubber but more often are steel with tungsten carbide inserts on the edge.
They are located between the collars and also help to maintain a straight hole by keeping the collars centralized. Also, by a scraping action, they maintain a full hole diameter.
Figure 16 shows two stabilizers, one with straight ribs and one with spiral ribs.
spiral... ribs
.0 ... straight ribs
Figure 16
Subs
Subs - in the drilling industry, the word sub refers to any short length of pipe, collar and so on which has a specific function.
Crossover subs - are designed with different threaded ends to enable different sizes or types of drill pipe or collar to be connected together.
Shock sub- a shock sub may be placed just above the bit. It has a steel spring or rubber packing to absorb the impact of the bit bouncing on hard formation .
Bit sub - this is a short sub with a box on each end. It connects the bit to the drill collars and ensures that the collars and drill pipe are always run with the pin end facing down.
So the bottom hole assembly is the current arrangement of tools incorporated into the collar section of the drill string. It may consist of any arrangement of the items mentioned above, and possibly other specialised drilling tools.
2.19
~ I
[!] Test Yourself 4
The following statements relate to components in a bottom hole assembly. Tick which of these statements are true or false.
True False
1. A bit sub is used to connect the drill bit to the drill collars, it has a pin connection at each end. D ~
2. Stabilizers help maintain the,hole direction in a vertical hole.
3. Drill collars allow all the weight of the drill string to be applied to the bit.
4. Crossover subs are used to connect the bit to the drill pipe.
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You will find the answer in Check Yourself 4 on page 2.56.
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The Rotating Mechanism
This is the last sub-section in our rotating system. In a conventional system the rotating mechanism consists of the following components:
kelly and kelly bushing
* swivel
* rotary table and master bushing kelly
Kelly and Kelly Bushing
During normal drilling operations, the top of the drill string screws into a square or hexagonal sectioned pipe called the kelly. In fact, a small sub called the saver sub Is placed between the drill pipe and the kelly. This helps to prevent wear on the threads of the kelly which is screwed on and off more than any other joint in the whole string. You will see wily this is so in Unit 3.
kelly bushingThe kelly is a hollow forged steel rod approximately
40 feet in length. It's outer cross section is either square or hexagonal in shape over the greater part of it's length. At each end the kelly has a round shape of the same diameter as the tool joint of the drill pipe or saver sub. Look now at figure 17 which shows a kelly with its kelly bushing attached. Figure 17
2.21
~
Connected to the kelly but free to slide up and In the next section of this unit we will be looking at down over its whole length, is the kelly bushing. the drilling fluid circulating system. This piece of equipment has an internal profile the same as the outside of the kelly. Rollers are fitted You will see that the fluid enters the drill string to ensure that the kelly can move freely through the through the swivel. The inlet is referred to as the bushing even when the bushing is turning. The gooseneck because of its shape. The pressure of bottom of the kelly bushing has four drive pins or a the fluid at this point can be very high, so the square section which locate in corresponding holes swivel has high pressure seals built into it. in the rotary table bushing.
We will come back to the rotary table shortly. Rotary Table and Master Bushing
At the top of the kelly is fitted a valve called a kelly These are the final items in our rotating cock. This can be closed to prevent any backflow mechanism. The rotary table has two main of drilling fluid up the drill string. We will look functions: further at this item in the final section of this unit.
* to rotate the kelly and hence the drill string
The Swivel * to support the weight of the drill string when it is not supported by the hoisting
The kelly assembly is permanently attached to a system swivel. During drilling operations the swivel is suspended by a handle, or bail, from the hook of The second function we will be covering in detail the travelling block. The hook does not rotate but when we look at drilling operations in the next unit. the kelly does. The swivel therefore has two sections, one rotating and one non-rotating. The For now we will just look at the way the rotary table swivel has to be capable of supporting the total turns the kelly. weight of the drill string whilst the lower part rotates. This means that very heavy duty bearings are incorporated into its body. Figure 18 shows a
figure 18swivel.
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Rotary Table
The table itself consists basically of a disc which is located in the middle of the drilling rig floor. It is capable of being rotated from an electric motor connected to the rotary table by a shaft.
Alternatively, older units may be rotated by a drive mechanism consisting of a drive sprocket and chain. The drive sprocket is part of the drawworks.
Master Bushing
In the centre of the rotary table is a hole which accommodates a further bushing. This one is called the-master bushing. It is into the master bushing that the drive pins of the kelly bushing fit.
So, when the rotary table is spun, the master bushing transmits the rotary motion to the kelly bushing which in turn spins the kelly and drill string.
Figure 19 shows the rotary table and master bushing. It also shows the relationship between them and the kelly.
master bushing
holes for drive pins of~
kelly bushing
rotary table
Figure 19
2.23
~
[] Test Yourself 5
Match the items on the left with the correct section of the rotating system on the right by drawing connecting lines. I have done the first one for you.
swivel bail ...
tri-cone roller +----
drill collar +--__ ... drill bit
synthetic diamond insert +----~
kelly bushing .-~_.. .. drill string
gooseneck .-.-.
stabilizer .
master bushing ..
You will find the answer in Check Yourself 5 on page 2.56.
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Other Types Of Rotating Mechanism
At the start of this section I said that we would look at some alternative rotating mechanisms. We can do that now.
Although the kelly, kelly bushing and rotary table are by far the most common method used to turn the drill bit, two other systems are sometimes used. These are:
* downhole motors and turbines
* top drive systems
'Downhole Motors and Downhole Turbines
These are tools which allow the drill bit to be rotated without rotating the whole drill string. Drilling fluid being pumped down the drill string provides the energy to drive the motor or turbine. A drive shaft is connected from the motor to the bit so, when drilling fluid is being circulated, the motor or turbine is rotated and so is the bit.
These tools are used extensively in directional drilling operations, although their use is not limited to this application. We will be looking at directional drilling in Unit 8 where I will go a little deeper into the operation of the downhole motor and turbine.
Top Drive System
This is used instead of the rotary table, kelly and kelly bushing. Unlike the downhole motor however, it rotates the-whole drill string.
It turns the drill pipe from an electric motor assembly which is connected to the rig's conventional swivel. The system provides the rotating power of the rotary table up in the derrick.
Using a top drive unit enables drilling to be carried out using stands of drill pipe. A stand consists of three joints of pipe connected together, making a total length of ± 100 feet. This reduces the number of connections to be made during drilling.
2.25
~
·_----------------
[1] Test Yourself 6
Read through the following sentences and fill in the missing words from the list below.
A rotating system as tree main su systems, ten It, t e an t e rotating mec arusrn. · h h . b h drill bl h (frill U(iiNc,~~ d h . h'
There are a number of different drill bit designs available such as drag bits, J.~.l..~!;::;: ... :.,:.'... So.. , diamond bits and polycrystalline diamond bits.
\) f\~tr I,C {,III"!:' (( dhi' I d b bi I .. hi' ..................\. .' an a e openers are sometimes p ace a ave a It to en arge or maintain a a e size.
[ , ( II( ('. II (.) r .... Inand 'II' ). () ; -.'> are use d to h Id h t tri . tensi d main. tatn weiq . ioh It.stnng, 0 e s nng In ension an t on b'
· I t' hani h bl t h i,v- I! ;.' (( Hi.· hi h t . h . hIn a convl3n trona rota Ing mec arusm t e rotary ta e urns t e :..\.,.1. . .'.-:.. :.'.>... .. ,w IC ransrnits t e rotary motion tot e (\(,/\ :c;' •• bi.................... ;..: and from there to the drill stnng and It.
. .. ... (2, \0\-('1" . . Dnlling flUid enters the dnll stnng Via the : : In the swivel.
Choose the missing words from:
kelly bail top drive under reamers bit sub gooseneck tri-cone roller bits drill string stabilizers drill collars kelly bushing
You will find the answer in Check Yourself 6 on page 2.57.
2020
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~ Summary
In this section we have been looking at the equipment used to actually drill a hole. This consisted of a cutting tool, equipment to apply weight to the tool and equipment to rotate the tool. You saw that the cutting tool, which is called the drill bit, must be capable of making hole in a variety of different rocks. To do this, several types of bit are available, from a simple drag bit to sophisticated polycrystalline diamond bits.
We looked at the drill string next and I explained the function and construction of drill pipe and the components of the bottom hole assembly.
Finally, we considered the rotating mechanism. I pointed out that there are a number of ways of turning a bit but the most common system utilised a swivel, kelly, bushing and rotary table. We looked at the way this system operates to transmit a rotary motion from the rotary table through to the drill bit. At the end of the section we had a brief look at a couple of alternative methods of rotating the bit.
In the next section we will move on to the circulating system and you will see how drilling fluids are pumped and conditioned.
2.27
Unit 2 : Drilling Systems and Equipment ~•
Section 3 - The Circulating System
On a number of occasions already, I have mentioned the term drilling fluid, without saying much more about it. This term in fact covers a range of liquids (and sometimes gases) which perform a number of functions during the drilling operation.
Initially, the primary function of the drilling fluid was to clean, cool and lubricate the bit and to carry cuttings from the hole. Nowadays much more is expected of this fluid as you will see in Unit 4.
The drilling fluid is more commonly called drilling mud or simply mud and I will use this term during the rest of the section.
When drilling is in progress, mud is continuously pumped down through the drill string, and out of the jet nozzles in the bit. Since the diameter of the bit is larger than that of the drill string, an annular space is left around the drill string as drilling progresses.
The mud returns to the surface through this annulus carrying with it the cuttings from the bottom of the hole.
At the surface, the cuttings are sieved from the mud. The mud is further cleaned as necessary and then pumped back down the hole again.
In this section we will look at the equipment used to pump the mud and condition it at the surface. In other words the circulating system:
I have listed below the individual components of the circulating system:
* mud pits
* mud pumps
* standpipe and rotary hose
* swivel
* shale shaker
* mud conditioning equipment
Figure 20 on the next page shows the complete circulating system.
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I---l---- swivelpipe
Mud Pits
mud pumps rotary These are simply a series of interconnected tanks hose in which the mud is initially prepared and stored, The end tank from which the pumps take their
I kelly suction is known as the active pit. A mud mixing hopper is located by the active pit. This is used to add chemicals to the mud when its weight and mud mixing hopper \ consistency needs to be changed.
--.r-- suction line At the other end of the line of tanks is the pit which
receives the mud as it flows from the hole. This is known as the settling pit or sand trap. The underside of this tank is usually sloped. This means that any solid particles which settle to the bottom, gravitate towards valves. The valves are opened periodically to dump the accumulated solids. Between the active pit and the settling pit are other tanks in which mud is stored and conditioned. We will look at the conditioning equipment shortly.
Mud Pumps
At the heart of the circulating system are the mud pumps. Their function is to circulate the mud under pressure from the active pit, through the drill string, to the bit, and return it up the annulus to the
return settling pit. flow
Figure 20 line
2.29
~ I
action of a double acting pump
discharge discharge There are usually two pumps on a drilling rig. They are always of the positive displacement type. In other words plunger pumps rather like a bicycle pump.
The pumps are either: +={: Ao. n ... -., r: • t :.::, Af?-.~ I!; .. f: t * duplex, double-acting or * triplex, single-acting
suction suction A duplex pump has two cylinders. Each cylinder has two suction and two discharge valves. As the piston moves through the cylinder it is discharging mud in front at the same time as mud is filling the action of a single acting pump cylinder behind. discharge
A triplex pump has three cylinders with each . .,", ...... " .:.1 I'... . .• '_", .: '.• " ~: OJ." •••cylinder having only one suction and one discharge
valve. The cylinder is filled as the piston moves ;r~ back and is discharged as the piston moves
I'~ :.~ t:~ :.: ~ "';1forward. ' ~r:;For one complete cycle of each piston a triplex ,',. .' ./'"'I.i' ' .. '. '.. " pump discharges one cylinder full of mud. In a duplex pump however, because it is double acting, two cylinder volumes are discharged for every ... cycle of each piston. suction
Figure 21 shows the pump action for each type.
Figure 21
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Standpipe and Rotary Hose
After leaving the pumps the mud is piped to the gooseneck of the swivel. The connecting pipework consists of high pressure piping from the pump, the standpipe and the rotary hose. This pipework must be capable of handling large volumes of mud under high pressure but keep pressure losses to a minimum.
The pumps discharge mud to a manifold, an assembly of pipes and valves which permits isolation of pumps for maintenance and repair.
The extension of the piping in the derrick consists of a vertical pipe firmly clamped to the derrick. The pipe is known as the standpipe.
From the top of the standpipe the mud is passed to the swivel. During normal drilling operations the swivel will be slowly moving down while the standpipe is of course stationary. This means that the connection between the two must be flexible. The rotary hose provides this flexible link. If you look back to figure 20, you can see the relationship between the pumps, standpipe and rotary hose in the circulating system.
In my list of the components of the circulating system the swivel came next. However, as I have already described the swivel as part of the rotating mechanism, we will move on to the following component, the shale shaker.
Shale Shaker
Before looking at this item, think again about the path of the mud after the swivel. It is travelling down through the hollow kelly, the hollow drill pipe and collars and out through the jet nozzles in the bit. From there it is going to return to the surface via the annulus and flow into the settling pit.
On its return journey from the bottom of the hole, the mud will be carrying rock particles cut by the bit. Before the mud can be pumped back down the hole, these cuttings must be removed by the shale shaker.
The shale shaker which is shown in figure 22 is mounted above and at the rear end of the settling pit. It consists of a sloping, wire mesh screen which is made to vibrate. Mud returning from the hole flows through a pipe and passes over the screen. The liquid mud falls through the screen and into the settling tank. Larger particles are trapped on the screen from where they are shaken to the bottom edge to be collected for disposal.
Figure 22
Fine particles of sand and silt however will pass through the shale shaker. These must be removed in special desanders or desilters. We will look at some of this equipment now in the final part of this circulating system section.
2.31
~
Mud Conditioning Equipment
The properties of the mud must be very carefully controlled in order that it can do its job properly. Chemicals may be added as we have seen already. Unwanted substances such as sand and silt, or sometimes gas, may have to be removed.
Hydrocyclones are used as desanders and desilters. Mud is pumped into the hydrocyclone via a tangentially fitted inlet. This causes the mud to whirl round the cone shaped vessel creating high centrifugal forces. The suspended solids are driven towards the wall of the hydrocyclone and downwards in an accelerating spiral. The liquid moves inwards and upwards as a spiralling vortex.
The solids, i.e. sand or silt are discharged from the variable opening at the bottom of the unit, whilst' the liquid overflows from the top.
If you look at figure 23 you will see this action illustrated.
t solidsoutlet
Figure 23
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Another type of separation unit used for mud conditioning is the centrifuge. This is used for salvaging materials which are to be kept in the mud system. It consists of a rotating cone shaped drum
inlet
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discharge
which spins at a high speed. Inside is a screw conveyor which moves the coarse particles towards the discharge.
Figure 24 shows one of these units.
From time to time, high pressure low volume gas accumulations may be encountered whilst drilling. This gas can enter the mud causing it to become gas cut.
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Figure 24
2.33
~•
[1] Test Yourself 7
Think of two possible problems which may result from the mud becoming gas cut.
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You will find the answer in Check Yourself 7 on page 2.58.
Two types of degasser units are provided to separate gas from drilling mud.
mud gas separators
vacuum degassers
The first of these units usually consists of a vertical vessel through which the gas cut mud can be circulated.
Gas is released from the mud in the separator and is led away to be disposed of through a flare.
The gas free mud can then be returned to the pits. This type of unit is suitable for handling high pressure gas and mud which flows from a well when a kick takes place.
You will be introduced to kicks in Unit 7 when we cover well control.
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Vacuum degassers are more commonly used to separate entrained gas. This can be seen as foam bubbles on top of the mud in the pits.
vacuum pump
This type of degasser is a horizontal barrel which is located above a mud tank from which it takes the mud. The mud enters the vessel and overflows from a tray down a pair of inclined plates. A vacuum is created in the vessel which helps to release gas from the mud. The gas is withdrawn by the vacuum pump and vented to a safe place. The conditioned mud flows from the bottom of the degasser to be returned to the mud pits.
If you look at figure 25 you will see how the mud flows through one of these units.
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2.35
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----------_.------.
[1] Test Yourself 8 When drilling mud is being circulated it is taken from the active tank or pit and finally returns to the tank. In between, the mud passes a series of pieces of equipment. Number the following items in their correct sequence in the circulating path. I have done the first one for you.
Piece of Equipment Sequence
a Mud pits IT] b Stand pipe 0 c Mud pumps 0 d. Kelly UJ e Swivel [D f Drill pipe CJ 9 .'Drill bit [J h Annulus 0
Shale shaker LJ j Drill collars "CJk Rotary hose GJ
Mud conditioning equipment [IJ
You will find the answer in Check Yourself 8 on page 2.58.
2.36
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~ Summary
In this section we have concentrated on the equipment used to circulate drilling fluid (mud) through the drill string and back to the surface.
You saw that the individual components of the system consisted of:
* mud pits
mud pumps
* standpipe and rotary hose
* swivel
shale shaker
* mud conditioning equipment
I explained that the pumps take the mud from the pits and pass it to the swivel via the standpipe and rotary hose. The mud flows down through the hollow kelly, drill pipe and collars and jets out through nozzles in the bit. It returns to the surface up the annulus where it flows over a vibrating screen, the shale shaker. After cuttings have been removed at the shaker, mud conditioning equipment is used to remove sand, silt and gas, etc. Finally, the mud flows back into the active pit to be picked up by the pumps once more.
In the three sections we have looked at up to now we have covered the major items of equipment used to drill a hole. In the next section we will look at the system which provides the power necessary to operate the equipment.
2.37
Unit 2 : Drilling Systems and Equipment ~ I
Section 4 - The Power System
A number of the items of drilling equipment which we have looked at up to now require to be driven in During drilling, great weights have to be lifted, large some way. volumes of mud at high pressures have to be
pumped and the whole drill string has to be turned. This requires a great deal of power but power is
*
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[]] Test Yourself 9
Think about the systems we have covered, and write down the three major components that would require some kind of driving mechanism.
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You will find the answer in Check Yourself 9 on page 2.58.
also required for other machinery and equipment of the rig, such as the shale shaker, mud conditioning equipment, air compressors and so on.
On a jack-up rig, power is required for operating the jacking equipment. On a semi-submersible rig power is required for the ballasting system. In addition to that required for the operations, electrical power is needed for heating and ventilation, cooking, etc.
Altogether, we are looking at a considerable power demand.
In this section we are going to restrict ourselves to a discussion of the actual rig power requirements. The workings of different types of engines and electrical power generators are also beyond the scope of this unit. We will just look at the subject in fairly general terms.
During this section we will look at power requirements and types of power systems.
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2.38
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Power Requirements
Thinking again about the drawworks, rotary table and mud pumps, I have already said that they need a lot of power. But how much?
On a large offshore rig the total requirement for the three components could be easily 4 500 H.P. (Horse Power).
This is further divided up between the components as follows:
" drawworks and rotary 3 000 H.P.
* mud pumps 1 500 H.P.
I have lumped together the drawworks and rotary table because they are usually driven from the same power source.
To give you an idea of what this means, a drawworks with a 3 000 H.P. input would be capable of lifting a load of over one million pounds with 10 line suspension.
To provide this power, a number of different systems have been used over the years and this is what we will look at now.
Power Systems
In the early days of rotary drilling, steam power was used exclusively. It is hardly ever found these days and we can forget about it in this unit. Steam was succeeded by internal combustion engines using natural gas or Iiquified petroleum gas as fuel. More recently, diesel engines have become more popular as drilling engines and offshore these are almost invariably used.
The rig components can be driven directly from these engines using chains or belts to transmit power. Most offshore rigs, however, use a combination of diesel engines, generators and electric motors to drive the drawworks, etc.
For now, let us concentrate on these so-called diesel/electric systems.
On an electric rig, the drawworks, mud pumps, etc are driven by direct current (D.C.) electric motors. D.C. motors are used rather than alternating current (A.C.) motors because it is not practical to control the speed of A.C. motors. Some older electric rigs use a complete D.C. system. This means that the prime movers (the diesel engines) drive D.C. generators, with the D.C. electricity powering the motors. One drawback of this system is that for control
purposes each motor has to be powered by its own generator. This makes it necessary to run as many generators as the maximum number of motors running at anyone time.
An A.C.lD.C. system is more efficient. It uses standard machines to generate alternating current which is then fed into a common distribution system. A.C. power is then drawn from this distribution system. It has to be converted to controlled direct current for use with conventional D.C. motors. The devices which convert A.C. to D.C. are called S.C.R.s which is short for Silicon Controlled Rectifiers.
As I pointed out earlier, the generation, distribution and use of electricity is a vast subject and is beyond the scope of this programme. Therefore, I don't intend to try to go any deeper into the power systems on a rig in this unit.
2.39
._------_._-------------------------"-~=-------.,
~
cg. Summary
In this very brief section you have seen that modern drilling rigs require a great deal of power to drive the various components.
You have also seen that this power requirement can be met in a number of ways but most common these days is the diesel/electric system.
? 40
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Unit 2 : Drilling Systems and Equipment ~ I
Section 5 ... The Blowout Prevention (BOP) System
Most people associated with the oil industry have heard the term blowout. It conjures up pictures of a drilling rig with a column of oil shooting high above the derrick. Although blowouts do happen from time to time, they are relatively rare occurrences. However, the risk of a blowout is ever present during drilling operations. A drill crew must always be ready to take steps to combat the threat of such a hazard. Before we talk about the blowout prevention system we should be clear about what a blowout really is.
Blowouts
We could define a blowout. as being an uncontrolled escape of oil, gas or other well fluids to the atmosphere. It occurs when fluids under pressure are released during the drilling operation and which the various containment systems fail to check.
As you will see in later units, pressure is being exerted by fluids in the rock formations through which a drill bit makes a hole. Normally, the pressure being exerted by the column of drilling fluids is sufficient to contain the formation fluid pressures. If, however, for any reason the pressure of the drilling fluid column drops below that of the formation fluids, these fluids will enter the well bore.
An influx of formation fluids is called in drillers' terms, a kick. It is when a kick gets out of control that a blowout occurs.
In Unit 7 we will be looking much more closely at the subject of pressure control in a well. In this section I just want to describe the equipment used to contain and bring under control, a potential blowout. i.e. the blowout prevention system.
Blowout Prevention
The escaping fluids could flow up the annulus when drill pipe is in the hole. Or, if the drill string is out of the hole then the fluids could simply flow through the open hole. The blowout prevention system must be capable of making the well safe under any circumstances.
When the flow has been shut off, the well must be made ready to allow drilling operations to continue. This usually means releasing any fluids which have entered the well bore and pumping in new mud.
[] Test Yourself 10
What would you say were the three main functions of the BOP system.
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You will find the answer in Check Yourself 10 on page 2.59. _J
2.41
~ I
Sub-Systems
The system which performs these functions can be split into three sub-systems which are the:
* blowout preventer (BOP) stack
* BOP operating system
* choke and kill equipment
In this section I will take you through each of these sub-systems. You will see a typical equipment layout, how it is constructed and how it operates.
Once again let me emphasise that we will be looking at equipment that you are likely to see on a land based rig or fixed offshore platform type rig. Although a BOP system on a floater performs exactly the same function, there are differences in layout, etc. We will cover this in Unit 6 when we look at Floating Drilling in more detail.
The Blowout Preventer Stack
The BOP stack can be described as an assembly of valves and fittings. It is designed to close the top of a well and seal in any undesirable high pressures should a blowout threaten during drilling operations.
The valves themselves are known as blowout preventers or simply BOPs. I will use this abbreviation during the rest of this unit.
The stack is positioned beneath the rig floor directly under the rotary table. Since it must be capable of withstanding very high pressures it must have a very secure base. You will see in Unit 5 that one of the casing strings provides this foundation. For the time being, it is sufficient to say that the bottom item in the stack is bolted securely to a base unit.
Since a blowout could threaten at any time during the drilling operation no one BOP could cope with every situation. The stack therefore is built up of a variety of BOPs each of which has a specific function. The actual number of preventers and their arrangement depends on the degree of protection considered necessary. In figure 26 I have illustrated just-one of the many possible BOP stack arrangements
Starting at the bottom you will see a casing head. This is the base unit on which the first BOP in the stack is mounted. The casing head itself is attached to the top of a string of pipe called the surface casing. You will come across this again in Unit 5.
annular type
~, bell nipple
I
BOP
) ram type \. _ oJ BOP
J ram type t oJ BOP
____ drilling
spool
J ram type ~ oJ BOP
m casing head
Figure 26
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Types of BOP
Apart from the drilling spool and bell nipple, which I will discuss later, all the other items are BOPs.
You will notice that they are of different types. These are:
* Ram Type BOPs. and * Annular Type BOPs.
Let's look at each of these units, starting with the ram types.
Ram Type BOPs
Ram type BOPs consist of a body which houses a pair of rams. When these rams are retracted into the body cavity the BOP is open. In this position a vertical bore exists through which the drilling equipment can pass.
The rams are connected via piston rods to pistons acting in hydraulic cylinders. When hydraulic pressure is applied to the pistons, the rams are pushed towards each other. This makes the well safe.
The rams themselves are described as either:
* pipe rams or
* blind rams or
* shear rams
piston rod
Pipe rams are intended to close the top of the hole when drill pipe is in the well. They have a semi-circular cut out in the face of each ram and sealing rubbers built into these faces. When a pipe ram is operated the ram faces are pressed against each other. This forms a pressure tight seal around drill pipe in the hole and shuts off the annulus.
Figure 27 shows an outline bird's eye view of a pipe ram type BOP and indicates its operating principle.
pipe rams
drill pipe
Figure 27
2.43
~•
[] Test Yourself 11
Can you think of a potential problem which may occur if there is only one set of pipe ram BOPs in a stack.
Suggest a solution to the problem.
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Blind Rams
Supposing though there is no drill pipe in the hole at all. It would be pointless to close a pair of pipe rams, leaving a hole through the middle.
Blind rams can protect the well in this situation. Blind rams have no cutout in the faces. When they are operated a seal is made between the two ram faces and closes off the open hole.
Shear Rams
Some BOPs are fitted with rams which are capable of cutting the drill pipe and making a seal. These units are called shear rams. They would of course only be used in a dire emergency. Often, the blind and shear action is combined in one BOP which is then called a blind/shear ram BOP.
You will find the answer in Check Yourself 11 on page 2.59.
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Figure 28
Figure 28 shows a more detailed drawing of a ram type BOP. Take a look at this now and satisfy yourself that you can work out how the unit operates. Your tutor will be able to help you, if necessary.
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2.45
~ I
Annular Type BOPs
In any BOP stack the upper unit is an annular type BOP. These preventers use a ring of toughened rubber packing material to make a seal. In its relaxed state the hole through the packing ring is equal in diameter to the bore of the BOP When the preventer is operated, hydraulic pressure pushes a tapered piston upwards. This movement squeezes the packing ring in towards the centre. "" As the packing ring deforms it makes a seal around most sizes and shapes of pipe in the hole.
This type of preventer will not only close around any size of drill pipe, it will close around a kelly or even open hole.
If you look at figure 29 you will see how an annular preventer works.
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of material
hydraulic pressure admitted here will push piston up and squeeze packing to
seal
Figure 29
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Drilling Spool
There are two more items in the BOP stack which I haven't yet mentioned. The first of these is the drilling spool. This is a fitting placed between two of the preventers which has a through bore at least as large as the BOP bore. Two side outlets are incorporated to which are connected flowlines known as the choke and kill lines. We will look at these shortly.
Bell Nipple
Finally, on top of the stack is mounted the bell nipple, sometimes called the flow stack. It consists of a piece of pipe connected by a flange to the top of the annular BOP. It has the same bore as the drilling spool and the BOPs and at the top it is flared or tapered. During drilling operations the top of the bell nipple is the top of the well bore. When drilling tools are being lowered into the hole, the bell shape guides them and prevents them hanging up at this point.
A side outlet from the bell nipple diverts the returning drilling fluid through a return flowline to the shale shaker. A fill up line connected to the bell nipple allows the mud in the hole to be topped up. This has to be done when drill pipe is removed and the mud level in the hole drops.
@3 Activity
I suggest that you look again at figure 26. Identify all the components of the BOP stack and make sure that you know what each one does and how it works. If you are not sure how these elements work, your tutor or a colleague should be able to help you.
J
2.47
~
The BOP Operating System
You have seen that BOPs, both ram and annular ,. types are actuated by hydraulic pressure. This equipment is only of value if it can be operated quickly and conveniently in an emergency. Also, the preventers must be capable of being operated if the rig power is lost.
A typical operating system which can do this would consist of the following items:
* hydraulic oil reserve
* pump(s)
* accumulator(s)
* control console
* connecting pipework
The reservoir is simply a tank which contains a reserve of hydraulic oil used to close (or open) the preventers. It is part of a closed system, i.e. the oil returns to the tank when the preventers are re-opened.
From the tank, pumps take the fluid and develop the pressure necessary for preventer operation. The pumps are driven by either air or electrically powered motors.
The BOPs must be capable of being operated even if there is no power to drive the pumps.
Accumulators are used to store energy which can then be used to actuate the preventers when rig power is unavailable.
An accumulator is basically a pressure vessel. It is divided into two compartments which are separated by a diaphragm or a piston. The hydraulic oil occupies one compartment whilst the other is filled with an inert gas under pressure. Nitrogen is the most commonly used gas.
When a control valve is opened, the pressure of the compressed gas forces the hydraulic oil through the connecting pipework to the preventer piston.
The pump/accumulator units are usually designed so that the fluid charges are automatically maintained at the desired pressure. There should be sufficient fluid in the accumulators to close each preventer at least once.
The accumulators, pumps and oil reservoir are usually built into skid mounted assemblies. These also include piping and control valves which direct the flow of oil to each preventer. The complete assembly is located at some distance from the well. In order that the driller can react quickly to any emergency, a control console is placed close to hand on the rig floor.
The control console contains the operating levers for each preventer. It has a display of each BOP in its correct position relative to the actual stack. Each lever normally stands in a neutral position. To actuate a BOP the driller simply moves the lever to the close or open position. This action directs air pressure to actuating cylinders. These in turn operate the control valves on the main unit.
Figure 30 on the next page is a line diagram showing the layout of a basic BOP operating system.
2.48
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The Choke and Kill Equipment
In any BOP system there must be provision for allowing:
* controlled release of well pressure
* pumping into the annulus when the preventers are closed
The first of these provisions is catered for by the choke manifolding. Figure 31 shows you the basic layout of the choke manifold.
This consists of a connection to the side outlet of the drilling spool fitted with one or more valves. From there a flowline passes to a branched manifold. At the manifold, mud flowing from the well under pressure can be diverted through one of a number of chokes to the pits.
The chokes are orifice valves which are used to maintain back pressure on the well as fluid is released. The variable orifice (opening) in the choke is opened or closed to maintain the desired pressure.
Connected to the side outlet of the drilling spool opposite the choke, is the kill connection. From a valve (or valves) at this connection a flowline leads back to the rig mud pumps.
Having such a layout allows mud to be pumped into the annulus when preventers are closed above the drilling spool. I will discuss the operation of the choke and kill system in much more detail when we look at pressure control in Unit 7.
remote operated choke
adjustable choke
Figure 31
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[!J Test Yourself 12
Describe in your own words a BOP stack which has 3 preventers. Your answer should name each component and describe the function of these components.
You will find the answer in Check Yourself 12 on page 2.59.
J 2.51
~•
In this unit we have looked at 5 systems which together make up a complete drilling installation. I have listed a number of components from these systems. Fill in the table to match the components to the system they are part of. I have done the first one for you.
Test Yourself 13
rotating system
I hOi:',ng system
[!]
circulating system
I-powe, system
bit sub v (
master bushing J 1
SCR 'v >
saver sub v F accumulator ,i!:i deadline anchor d drilling spool' "l
centrifuge' -blind ram v
brake
shale shaker v C: swivel v
stand pipe (: drawworks v
kelly bushing ~
generators -c/
choke v'
links \ elevator desilter
You will find the answer in Check Yourself 13 on page 2.60.
blowout prevention system
drawworks
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~ Summary
In this section we have looked at the equipment used to contain and bring under control a potential blowout. This equipment is called the Blowout Prevention System.
You saw that the functions of the system can be listed as enabling the drilling crew to:
* close the top of the hole
* release any fluid under controlled conditions
* permit the pumping of new mud into the hole
In order to be able to perform these functions the system consists of:
a blowout preventer stack having a number of preventers, both ram type and annular preventers
an operating system which enables the driller to close and open the preventers remotely
a choke and kill system, used to release fluids under controlled conditions and allow the pumping of new drilling fluid into the well when preventers are closed
Throughout the section we have concentrated on conventional land or production platform type of equipment. In Unit 6 we will be looking at BOP equipment which is used in floating drilling applications.
We will also return to the subject of blowout prevention equipment in Unit 7 when we will cover pressure control in more detail.
You should now be able to go back to the Training Target set at the beginning of this unit and check that you can tick all of the boxes. If you have any problems, look at the appropriate section again or arrange a meeting with your tutor, who should be able to help you.
2.53
Unit 2 : Drilling Systems and Equipment ~
Check Yourself - Answers
~ Check Yourself 1 Check Yourself 2
1. Yes 9. Yes 1. hard formation
2. Yes 10. No. The collars are part of the drill string 2. soft to medium formation which itself is part of the rotating system.
3. Yes 3. soft formation 11. No. We will look at a shale shaker in
4. No. The rotary table is part of the Section 3. It is a component in the circulating rotating system which we will look at in system. Section 2.
12. Yes 5. Yes
6. No. This is also part of the rotating system.
7. Yes
8. Yes
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~ Check Yourself 3
To obtain 50 OOOlbs weight on bit with fluid buoyancy factor of 0.833 will Therefore the total weight required is the weight of the drill collar + the require: excess weight
= 60 000 + 15 000 50000 = 60 024 Ibs actual collar weight 0.833 = 75 000 Ibs
Let's say 60 000 Ibs If each joint weighs 4 410 Ibs the number of joints required
If 25% excess weight is required, this will be: = 75 000 4410
60000 x 25 = 15000 Ibs 100 = 17 joints
255
~
~ Check Yourself 4
1. False A bit sub is used to connect the bit to the collars but it has box connections, not pin connections at each end.
2. True
3. False Drill collars allow just part of the weight of the collars themselves to be applied to the bit. This means that the drill pipe is held in tension.
4. False Crossover subs are used to connect different sizes of drill pipe or collars.
Check Yourself 5
swivel bail +
tri-cone roller +
drill collar + + drill bit
synthetic diamond insert +--
kelly bushing • • drill string
gooseneck +
stabilizer • :+ rotating mechanism
master bushing •
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Check Yourself 6
The words missing from the sentences are shown in bold type below.
A rotating system has three main subsystems, the drill bit, the drill string and the rotating mechanism.
There are a number of different drill bit designs available such as drag bits, tri-cone roller bits, diamond bits and polycrystalline diamond bits.
Under reamers and hole openers are sometimes placed above a bit to enlarge or maintain a hole size.
In a drill string, drill collars are used to hold the string in tension and maintain weight on bit.
In a conventional rotating mechanism the rotary table turns the kelly bushing, which transmits the rotary motion to the kelly and from there to the drill string and bit.
Drilling fluid enters the drill string via the gooseneck in the swivel.
Of the remaining words, a top drive is one of the alternative rotating systems, a bit sub connects the bit to the collars, and stabilizers help to maintain a straight hole of full diameter. A bail is simply a handle by which the swivel is suspended from the drilling hook.
~
~ Check Yourself 7
If gas cut mud is recirculated a number of problems may arise. These will include a reduction in mud weight (or density), giving rise to pressure control problems, which you will see in Unit 7.
Also, the mud pumps will have difficulty in dealing with mud which is gas-cut.
Check Yourself 8
The correct sequence is:
Mud pits IT] Stand pipe [I] Mud pumps ~ Kelly ~ Swivel @] Drill pipe o Drill bit ~ Annulus ~ Shale shaker ~ Drill collars [!] Rotary hose [TI Mud conditioning equipment ~
Check Yourself 9
I'm sure that you wrote the same components as me, i.e.
* the drawworks
* the rotary table
* the mud pumps
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~ Check Yourself 10
The 3 main functions of a BOP system are as follows:
* Close the top of the hole.
* Release any fluid under controlled conditions.
* Permit the pumping of new mud into the hole.
Check Yourself 11
A pair of pipe rams can only make a seal around one particular size of pipe, ie. the diameter of the cut out in the face of the ram must match the diameter of the pipe. If different sizes of pipe are used, the rams must De changed for ones with the correct size of cut outs. Often more than one set of ram type BOPs is used with different sized rams to accommodate two different sizes of pipe.
Check Yourself 12
Your answer should have included the following.
Two of the BOPs will be ram type preventers. One would be fitted with pipe rams to close round drill pipe in the hole, the other would be fitted with blind / shear rams. This preventer could close the well with nothing in the hole or in an emergency could cut the drill pipe and make a seal. The uppermost preventer would be an annular type. This unit could close around any size of drill pipe, a kelly or even empty hole.
Between the two ram preventers would be a drilling spool. The choke and kill lines would be connected to the drilling spool. The choke and kill lines allow controlled release of well pressure and permit pumping into the annulus when the preventers are closed.
Finally on top of the stack is the bell nipple. The mud return flowline is connected to this nipple which also has a connection for a fill up line. The top of the bell nipple is flared to guide the drilling tools into the hole.
If you have missed any of these components, go through Section 5 again and satisfy yourself that you are now familiar with them.
2.59
~
-------------------------_.....--------------------------------------------_.
~ Check Yourself 13
Your table should look like the one opposite.
hoisting system
rotating system
circulating system
power system
blowout prevention system
drawworks, links, elevator, deadline anchor, brake
swivel, kelly bushing, bit sub, master bushing, saver sub
shale shaker, stand pipe, desilter, centrifuge
generators, SCR
choke, accumulator, drilling spool, blind ram
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