conductor design and installation maula for offshore platform

8/11/2019 Conductor Design and Installation Maula for Offshore Platform

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PETRONAS TECHNICAL STANDARDS

DESIGN AND ENGINEERING PRACTICE

MANUAL (SM)

CONDUCTOR DESIGN AND

INSTALLATION MANUAL FOR


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PREFACE

PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication,of PETRONAS OPUs/Divisions.

They are based on the experience acquired during the involvement with the design, construction,operation and maintenance of processing units and facilities. Where appropriate they are basedon, or reference is made to, national and international standards and codes of practice.

The objective is to set the recommended standard for good technical practice to be applied byPETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemical

plants, marketing facilities or any other such facility, and thereby to achieve maximum technicaland economic benefit from standardisation.

The information set forth in these publications is provided to users for their consideration anddecision to implement. This is of particular importance where PTS may not cover everyrequirement or diversity of condition at each locality. The system of PTS is expected to besufficiently flexible to allow individual operating units to adapt the information set forth in PTS totheir own environment and requirements.

When Contractors or Manufacturers/Suppliers use PTS they shall be solely responsible for thequality of work and the attainment of the required design and engineering standards. Inparticular, for those requirements not specifically covered, the Principal will expect them to followthose design and engineering practices which will achieve the same level of integrity as reflectedin the PTS. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting from hisown responsibility, consult the Principal or its technical advisor.

The right to use PTS rests with three categories of users :


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EP/23.4 TEL: (070)

Dear Sirs,

CONDUCTOR DESIGN AND INSTALLATION MANUAL (EP 52510 - DECEMBER 1980)

In response to requests from operating companies for advice on offshore conductor installation we aresending a copies of a Shell Group "Conductor Design and Installation Manual", which we believe willbe of assistance on this subject.

The manual is of loose-leaf construction to permit easy insertion of updates as they becomenecessary.

As with all operationally oriented subjects, feedback from your local experience offshore is essentialfor further development of techniques.

Yours faithfully,

Shell Internationale Petroleum Maatschappij B.V


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References

Figure 3.1

Figure 3.2

4. SPUDDING-IN PROCEDURES

Cleaning-out of Conductor

Pilot Hole

Drilling Techniques

Setting the First Casing

5. INSTALLATION METHODS

Driving

Drill-Drive

Drill and Drive

Drill and Cement


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12. DO's and DON'T's

Do's

Dont's

APPENDICES

APPENDIX I : Records of Meetings with Personnel of various Shell Group Companies.

APPENDIX II-1 : Soil Parameters Required for Conductor Design and Installation Planning.

APPENDIX II-2 : Calculation of the Coefficient of Earth Pressure at Rest (Ko).

APPENDIX III-1 : Use of the Wave Equation in Drivability Analyses.

APPENDIX III-2 : Example of Wave Equation Analyses for Conductor Drill-Drive Sequence.

APPENDIX III-3 : Extract from "Construction Specification for Installation of Steel Platforms",Prepared by SSB (September 1978).

APPENDIX III-4 : Extract from "Outline Installation Procedure for SFDP A", Prepared by SSB.

APPENDIX III-5 : Cormorant Alpha Conductor Installation Procedure, Prepared by Shell Expro(October 1978).

APPENDIX III-6 : Extract from "Drill and Cement Conductor Installation Specification", Prepared byQPPA


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INTRODUCTION

The purpose of this Manual is to serve as a guide for Company staff involved in the design andinstallation of conductors for fixed steel platforms. It is not a complete and infallible rule book for usein all circumstances. It cannot, therefore, replace common sense, sound judgement based on theknowledge of engineering principles, and field experience.

The design and installation of conductors involves engineers from a number of different disciplines.Some may have difficulty in fully appreciating the requirements of others. Problems with conductorsmost frequently occur because of lack of communication between the different disciplines during the

initial stages of the project. It is hoped that the contents of this Manual will enable the dialoguebetween departments to be improved. For this reason some subjects which are usually dealt with byDrilling Department are discussed, e.g. spudding-in procedures. Such discussions are for backgroundpurposes only, and Drilling Department should be consulted for advice on current good practice. Asimilar comment applies to design procedures which are always the responsibility of a platform designgroup, e.g. the computation of stress in a conductor due to environmental loading. If advice is requiredon such matters, Structural Engineering should be consulted.

Any document concerning conductors is complicated by the fact that each discipline uses its ownterminology and system of units. Therefore, a list of definitions has been provided together withcommonly used alternatives. With regard to units it has proved impractical to standardise in thisManual. Sizes are quoted in the vernacular of the relevant discipline, e.g. the majority of the drillersstill use inches and pounds, platform designers generally use SI units. The Group's efforts to achievefull metrication are, however, fully recognized.

During the preparation of the Manual, discussions were held with Engineers in a number of GroupOperating Companies. Brief records of these discussion are presented inAppendix I. From theserecords it is apparent that there, is considerable diversity of practice around the world. This is not


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DEFINITIONS

MARINE CONDUCTOR(see NOTE on next page)

The first pipe to be installed when drilling an offshore well typically 30" in diameter although othersizes are used. The marine conductor usually extends from the seabed up to the cellar deck of theplatform and may have a penetration of a few hundred feet. The purpose is to provide lateral supportto the well, to case off very soft formations below the seabottom, to facilitate circulation of drilling fluidand to guide the drill string until the next casing string has been set. The term "marine conductor"relates specifically to offshore drilling from a bottom- supported fixed or mobile platform (as opposedto a floating platform) with above water well control equipment.

CONDUCTORS(see NOTE on next page)

A pipe which is installed inside the marine conductor typically 20" in diameter although other sizes arealso used. The conductor is run into a predrilled hole and its function is to case off unconsolidatedformations and water sands and to provide protection against shallow gas. This string is normallycemented to seabed and is the first string on which blowout preventers are installed. The conductor isconsidered to be the first casing string of the well.


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1. GENERAL DESIGN AND INSTALLATION CONSIDERATIONS

Functions of Conductors

1.1 The primary functions of a conductor are to facilitate the spudding-in of the well and to provideprotection and lateral support to the well casings. The conductor assists spudding-in byproviding guidance for the drilling tools, allowing the circulation of drilling fluid and supportingweak formations above the conductor tip level. Also, in conjunction with a diverter system, theconductor enables diverting a possible flow, originating from shallow formations. In the longterm the conductor protects the well from external corrosion and provides lateral supportagainst environmental loads by spanning between guides.

1.2 On some platforms the conductor bracing is designed so that the conductors act as laterallyloaded piles and assist the foundations in taking-out shear at mudline. This may have thebeneficial effect of reducing the number of foundation piles required. However, the conductormust be designed for this mudline shear and this can involve increasing the wall thickness ofthe conductor pipe. When there are no advantages in using the conductors as laterally loadedpiles, guides in the bottom horizontal bracing are either omitted or designed with sufficientclearance to prevent conductors acting as piles.

1.3 If curved, slanted or otherwise "deviated" conductors are used they may provide initial

alignment in the preferred azimuth and increase the separation of conductors at tip level. Thelatter reduces the risk of the drill string colliding with a previously installed casing string.

SETTING DEPTH

The depth from some datum to the conductor shoe. This datum is the derrick floor during the drillingphase In the production stage it may be the bottom flange of the wellhead or seabed


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FIGURE 1.1 DESIGN AND INSTALLATION SEQUENCE


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1.4 Under some circumstances the conductor may be required to serve as a temporary supportfor casing strings during cementation.

Sequence of Design and Installation

1.5 The considerations of paragraphs 1.1 to 1.4 together with information concerning:

site and soil conditions

platform and foundation design

environmental conditions

installation equipment

determine the diameter, wall thickness and setting depth of the conductor and the method ofinstalling it.A schematic representation of the design and planning procedure is provided onFigure 1.1. The numbers in each box on Figure 1.1 refer to the relevant paragraphs of thisManual.

1.6 A decision on the number of conductors, their diameter, the setting depth and whether theyare to be vertical, slanted or curved has to be taken before the commencement of finalplatform design. This may be up to four or five years before the first well is installed. It is mostimportant that the detailed planning of the installation of the conductor and the well are startedat this early stage and are not left, as is the practice of some Oil Companies outside theShellGroup, until after the platform has been launched. The reason why conductor designand installation planning must start so far in advance of well installation is that the conductors


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It is common practice to carry out the first three items simultaneously from one vessel equipped forgeophysical work. The soil borings would be made from a different vessel, usually under a separate

contract and at a different time. The TV inspection of the seabed can be made in conjunction witheither the soil or geophysical survey. Alternatively it can be performed as part of a "submarine"survey.

Bathymetric, Side Scan Sonar, and TV Surveys

2.3 These surveys provide data on water depth, seabed topography and obstructions on theseabed such as wrecks, pipelines and boulders. The requirements for platform design will beadequate for the conductors.

2.4 If there are any obstructions on the seabed, such as boulders, it will be necessary to removethem prior to platform or conductor installation, whichever is first. This is usually done by a"trawling" technique.

2.5 Before installing the conductors of a platform, it may be necessary to carry out an additionalsurvey for "junk" (material dropped from the platform during installation). A number oftechniques are available for carrying out this survey, for example TV can be used if there is

no guide at bottom bracing level. In other cases each guide might have to be "fished".

Shallow Seismic Survey

2.6 The shallow seismic survey is made for a number of reasons? for the conductors the mostimportant being:


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Is there any possibility of the drillstring or next casing string standing-up on, or being caughtunderneath an internal shoulder?

Remember that the outside diameter of the conductor usually determines the guide size.Once the design is frozen and the guide size fixed it is difficult to alter the conductor O.D.

Have the structural designers satisfied themselves that the conductor section and grade ofsteel are sufficient to protect the well from environmental forces by acting as:

(a) a beam spanning between guides?

(b) a laterally loaded pile?

If the conductor is to carry axial load has it been checked for combined stresses?

If the conductor is to be driven, have drivability and driving stresses been checked?

(see Section 5)

Finally in case of high steel stresses, are there any possibilities of increasing the section orupgrading the steel to bring stresses within allowable limits?

(iii) Special Considerations

Will corrosion be a particular problem? Could this involve increasing the steel section in thesplash zone?

Should thermal effects be considered?. This might lead to increased stresses in the conductorif the inner casing strings are cemented up to the top of the well


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(ii) Hammers

What hammers are available? The use of steam hammers is only really practical when aderrick barge is alongside the platform (to lift the hammers and to generate enough steam). Ifthe drilling mast or derrick is to be used to handle the hammer then, normally only diesel andcompressed air hammers can be considered (note - "steam" hammers can operate oncompressed air). The use of hydraulic hammers to install conductors is feasible but up to nowthis has not been tried by any Group Company. Will the hammer overstress theconductors ? Could this problem be solved by increasing the conductor section or by using ahigher grade of steel?

(iii) Drilling Rig

Has the rig been designed to handle hammers and what is the maximum weight of hammerthat may be used?

What is the maximum length of add-on that can be handled within V-door, derrick and piperack constraints? Add-ons should preferably be in one of the standard lengths that aredelivered by the mills, i.e. 6, 8, 12 and 16m.

(iv) Contingencies

What plant is available if contingency procedures have to be employed? (see Section 11)


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3. CALCULATING THE SETTING DEPTH

Factors which determine the Setting Depth

3.1 The setting depths of marine conductors are generally determined on the basis of thefollowing considerations:

(i) the conductor shoe level should be such that drilling for the first casing does not havean adverse effect on the foundations of the platform, eg. reduce the bearing capacityof a pile.

(ii) when drilling below the conductor tip for the first casing string, formation breakdown

should not occur. Formation breakdown takes place when the drilling fluid pressure issufficient to crack the soil forming the borehole wall. This increases the permeabilityof the soil to such an extent that fluid flows freely into the formation. It is thenimpossible to obtain returns to drill floor. This phenomenon is termed "lostcirculation", cuttings cannot be lifted from the bottom of the borehole and the holecannot be advanced.

(iii) the penetration of the conductor below mudline should be sufficient to generate therequired "bearing capacity" in outside skin friction, seeSection 6.

(iv) the tip of the conductor should be below any unstable formations.

(v) whenever feasible the conductor tip should be in an impermeable strata such as clayrather than in porous soil such as sand.

The relative importance of each of the criteria listed above will vary from site to site. In fact, in someoperating areas some of the above may not be considered at all. For example some Group Companiesignore item (ii) and are prepared to drill "blind" for the first casing "Blind" drilling is drilling without


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3.3 In most circumstances the soil formation strength at any depth can be obtained from thelesser of:

(i) the effective overburden pressure of the soil

(ii) the effective horizontal earth pressure

The effective overburden pressure is the difference between the density of the soil and the density ofseawater multiplied by the depth below mudline. The effective horizontal earth pressure is equal to theeffective overburden pressure multiplied by the "at rest earth pressure coefficient" for the soil,"Ko,".The value of Kois a function of the soil properties and is generally 0.5 to 3.0. Thus to avoid hydraulicfracture


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3.5 It is recommended that the results of the calculations are presented graphically as on

Figure 3.1. A method for computing Ko is described in Appendix II-2. A more rigorousapproach to calculating the formation strength may be found in Ref. 3.1.

Leak-Off Tests

3.6 The formation strength may be measured in the field by performing a leak-off test. In outlinethe test is carried out a follows:-

(a) drill-out to a depth of 3m to 6m (10 ft to 20 ft) ahead of the conductor shoe: cap conductor and

connect to a pump.

(b) fill the entire conductor with fluid and commence pumping.

(c ) raise the fluid pressure in small increments of approximately 10% of the effective soil overburden pressure. At each increment plot the fluid pressure versus the measured flow rate.The points should fall on a straight line until the "formation intake pressure" is reached. Atpressures greater than this very much greater flow rates will occur for a given pressureincrement, seefigure 3.2, and the points will deviate from the straight line to form a curve ofreducing gradient. Attempts to increase the pressure further will lead to formation breakdown.There are a number of different techniques for performing these tests. The method used willdepend on the circumstances and the equipment available. A description of the standardmethod used by the drillers may be found in Appendix 15 of EP- 40806, "Well Control andBlowout Prevention Manual".

3.7 An alternative form of this test is the "limit test". This is exactly the same as the leak-off testexcept that the fluid pressure is not increased beyond the "maximum required mudholdingcapacity". It will confirm that the formation strength is sufficient for the proposed drilling


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Setting Depth and Spacing between Conductors and Piles

3.11 Having calculated the setting depth to avoid formation breakdown it is necessary to check

whether or not such a depth will be detrimental to the piles. if the conductors were driven to asignificant depth below the tip of the piles, there would be little likelihood of the piles beingaffected by the drilling procedure for the first casing. This is why some oil companiesoperating in the Gulf of Mexico, but not SOC, insist that all conductors be taken below the tipof the piles, (see minutes of meetings with SOC).This rule, although safe, may beunnecessarily conservative in some cases. However, if there is a chance of encounteringshallow gas during drilling, the conductor of the first well to be drilled should be set at least 15m below the tip of the piles.

3.12 If the conductors are sufficiently distant from the piles and provided formation breakdowndoes not occur and shallow gas is not encountered, drilling for the first casing, or theconductor itself, will have no effect on pile capacity. In these circumstances the conductorsetting depth need not be related to the pile tip level. At this "sufficient distance" there mustbe enough clear space such that there is no danger of the conductor being driven into theside of a pile and that washout does not affect the soil in which the pile develops its capacity.To allow for the use of contingency procedures, such as drilling ahead of conductors whichrefuse prematurely, it is recommended that all conductors have this safe clearance from thepiles. This distance will depend upon:

(a) the diameters of the pile and conductor

(b) the soil conditions

(c) whether the pile capacity is developed mainly by friction or mainly by end bearing


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3.14 From the observations inparagraph 3.13the following conclusions may be drawn:

(a) in sand, drilling may result in a hole several times the diameter of the bit. There areindications that, in extreme cases, the washout zone may extend to a distance of fourtimes the nominal hole diameter.

(b) over relatively shallow depths driven conductors may deviate from proposedalignments by 3ft and deviations of one degree per 100ft are common.

3.15 To ensure that there is no reduction in the capacity of a pile, the zone of soil within 2 pile

diameters of the centre of the pile should not be disturbed by drilling. Thus the minimumcentre to centre spacings between piles and conductors for the clearances to be safe are

- for piles embedded in CLAY:

S = (2 x D) + 3 + d 3.5

where: S = the minimum centre to centre spacing between a pile and conductor

D = pile diameter

d = conductor diameter

and all dimensions are in feet. The 3ft term is for deviation and the formula allows for drillingan oversize hole.

- for piles obtaining much of their capacity from end bearing in SAND:


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3.19 The centre to centre spacings between conductors is a separate matter and should bedecided by the drilling department.

Conductor Installation by Drilling or Drill-Drive Technique

3.20 Drilling ahead at relatively shallow depths will be required for conductors installed by the drill-drive technique. The same comment obviously applies to drilled and cemented conductors. Itis essential that the drilling for the conductors does not result in formation breakdown. Mostoperating companies overcome this problem by using seawater as the drilling fluid anddischarging the returns at some convenient level below the drill floor. For example,

Shell Expro when using the drill-drive technique employ a circulating gravity connectorwhich allows discharge below sea level. QPPA, who use drilled and cemented conductors,use seawater for drilling and clean out the hole with slugs of viscous mud. Because of thesmall air gap and shallow water depth at most platform sites offshore Qatar, hydraulic fractureduring conductor drilling is not a problem for QPPA, seeparagraph 3.4

3.21 Drilling close to mudline may result in surface washout or collapse of seabed formations. Thiswill cause a cone of depression around the conductors and if it extends towards a pile, mayreduce the lateral capacity of the pile. This phenomenon has been observed by QPPA, seeminutes of meetings. Washout can also have a serious effect on drilling templates, causingthem to tilt. To overcome these problems it is recommended that conductors penetrate atleast 20ft (6m) before drilling commences. With soft soils the self weight of the conductor willoften be sufficient to cause this penetration. With harder soils, driving, vibrating or some othermeans will have to be employed.


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References

3.1 BJERRUM, L. et al. "Hydraulic Fracturing in Field Permeability Testing" Geotechnique Vol. 221972 pp319-3 32.


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FIGURE 3.1 PLOT OF HYDROSTATIC MUD AND SEAWATER PRESSURE AND LIMITINGSTRESS CONDITION AGAINST DEPTH


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FIGURE 3.2 TYPICAL LEAK-OFF AND LIMIT TEST RESULTS


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4. SPUDDING-IN PROCEDURES

4.1 In this Manual the term "spudding-in" is used to describe the drilling procedures for the firstcasing. It is necessary that all those involved in conductor design and installation have anappreciation of spudding-in procedures since they are the next stage in drilling the well afterthe conductor has been set. Because of this, conductor designand installation and spudding-in procedures are interdependent. For example in Section 3 it is explained how the drillingprocedure for the first casing string determines the conductor setting depth if formationbreakdown is to be avoided. Therefore, this Section of the Manual is included to provideengineers who are unfamiliar with drilling techniques with some background data onspudding-in procedures. It is not intended to be a definitive guide to spudding-in practice.

Cleaning-out of Conductor

4.2 For driven conductors the soil plug in the conductor is removed by drilling. For conductorsinstalled by the drill and cement technique the residual cement plug above the float shoe mustbe drilled out. After the conductor has been cleaned out it is common practice to drill a pilothole which is subsequently opened to the size required for the first casing. However, prior todrilling the pilot hole it may be advisable to drill a full size hole approximately 3m ahead of the

conductor shoe. The reasons for drilling this short hole are:-

(a) to check that the shoe is not damaged

(b) to centralise subsequent holes

(c) to confirm that the axial external skin friction capacity exceeds the weight of the conductor.


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Drilling Techniques

4.5 When drilling for the first casing it is recommended by all Shell Group companies that,if possible, returns should be obtained at deck level. One advantage is that the driller has anearlier warning of a shallow gas influx. To achieve returns, drilling with a controlled

penetration rate may be required to reduce annular pressures and the risk of formationbreakdown.

4.6 SomeShellGroup companies use seawater as a drilling fluid; others use mud. If seawater isused slugs of viscous mud are periodically spotted to prevent sticking of the drillstring. Aftercompleting drilling, the hole must be filled with lightweight viscous mud to keep it open prior tosetting the first casing. These considerations do not arise if mud is used as the drilling fluid.

Setting the First Casing

4.7 The casing string is lowered into the hole from the derrick floor. It is fitted with a float shoe tofacilitate cementation. Normal practice is to cement the annulus between the casing and thehole, and extend the cementation for some distance up the conductor, typically to 30ft or 40ftbelow mudline.

4.8 It is common practice to specify that the cement should not be taken above mudline. Thereasons for this are:

a) the abandonment is easier and the conductor can be recovered and re-used(particularly important for exploration wells)

b) the stress in the conductor at mudline can be determined using simple analyticaltechniques (i.e. there are no thermal or secondary axial effects)


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5. INSTALLATION METHODS

5.1 Although in areas with a long operating experience established practice is usually followed,for new and larger platforms or in new areas the method of installing the conductors shouldbe determined on the basis of a time and cost analysis of various techniques. Importantparameters which influence the viability of different methods are:

the setting depth

the soil conditions

the conductor section

the equipment available

5.2 The most commonly used installation techniques are:

driving (continuous driving)

drill-drive (repeatedly alternating drilling and driving)

drill and drive (single drilling operation followed by continuous driving)

drill and cement (single drilling operation followed by cementation) jetting

Jetting is not recommended as an installation technique for conductors on platforms. This isbecause jetting may adversely affect the integrity of the platform foundations. The other four


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5.6 There are a number of different approaches to driving to be considered:

(i) drive the conductors as soon as pile driving is complete using the derrick barge and

steam hammers. This method of installing the conductors is extensively used byNAM, SSB, BSP and SOC. It was used initially by SBPT for Maui A. However, it isweather dependent and derrick barges are expensive. Therefore, even for conductorswhich can be driven easily, it may not be the best solution in all operating areas.

(ii) drive the conductors from the drilling floor using the drilling derrick or a "pile" handlingrig. This approach is much less weather dependent than item (i) above but mayrestrict the choice of hammer to diesel or steam hammers driven by air. Thesehammers are usually less powerful than those available from a derrick barge and

therefore drivability may not be so good and installation can take longer.

(iii) stab the lead sections with the derrick barge and drive the conductors from the drillfloor as in (ii) above. This enables the boom height of the derrick barge to beemployed together with the "weather independence" of the jacket. Under nocircumstances should the conductors be driven to refusal with the derrick barge in theexpectation that it will be possible to advance them further with the drilling derrick. Infact, apart from the stabbing operation, it is not recommended to install conductorspartly with the derrick barge and partly with the drilling derrick.

5.7 For conductors which are to be installed by driving it is necessary to connect add-ons bywelding. It may be possible to use a mechanical joint provided its fatigue life can be shown tobe satisfactory. However, there is little Shell Group experience on the use of mechanicalconnectors with conductors on permanent platforms.


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Shell Expro generally use 30in diameter conductors. For cleaning-out the conductorand drilling the pilot hole they use a 17 in. bit with a 26 in. hole opener. The hammer isusually a heavy diesel hammer such as a Delmag D55 or D62A. Seawater is used as drilling

fluid with pills of drilling mud being used to clean the hole. Welded connections are used foradd-ons to the conductor.

5.11 An assessment of whether or not the drill-drive technique will be successful at a particular sitecan be made using a modified version of the drivability analyses. The calculation of the SRDmust take into account the effect of cleaning-out the conductor (no inside friction) and pilothole (reduced outside friction and end bearing). The wave equation analysis is the same as

for a driven pile. An example is provided inAppendix III-2.

Drill and Drive

5.12 In the "drill and drive" method a pilot hole is drilled from mudline to the setting depth beforeany attempt is made to drive the conductors below seabed. Open-hole drilling using seawateris employed, i.e. cuttings discharged at seabed level. At Auk, Shell Expro used the

following technique for 30 in. conductors with welded add-on connections:

(i) run conductor to a few feet above seabed

(ii) drill a 26" hole and fill with mud to prevent borehole collapse.

(iii) drive the conductor to the bottom of the hole and beyond until sufficient capacity isobtained. This distance must be calculated, seeSection 6.


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6. AXIAL BEARING CAPACITY OF CONDUCTORS

General

6.1 As a general rule, well casings should not be hung from conductors. In principle, conductorsshould only be required to take lateral forces, not axial forces. The reasons for this are:

(i) depending on the span between supports, the magnitude of environmental forces andman other factors, the strength of the conductor section may be committed to takingbending.

(ii) the axial bearing capacity may be relatively small and difficult to calculate with anydegree of reliability.

(iii) the stress distribution in a series of cemented casing strings is difficult to analyse.There will always be uncertainties as to the magnitude of the load on the conductor,particularly when considering temperature effects.

However, every conductor must be able to support its own weight and that of any equipmentplaced on it, e.g. a diverter, and because of this an estimate must be made of its bearingcapacity.

6.2 If despite the reservations of paragraph 6.1, it is decided to support well casings and otherequipment on the conductor, then the system must be very carefully engineered. It can neverbe assumed that the conductor can take a load simply because it has a deep penetrationbelow seabed.


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6.7 An indication of the capacity of a driven pile may be obtained from its driving records.Frequently, piles are driven to refusal in a sand layer to develop as much end bearing aspossible and "refusal" with a specified hammer is used as an acceptance criterion. However,

the situation with driven conductors is completely different. Because the end bearing isremoved during spudding-in, seeparagraph 6.5, refusal cannot be used as an indication ofthe capacity of a conductor.

6.8 The fact that "refusal" is not an acceptance criterion for conductors does not mean that drivingbehaviour may be ignored. In fact for conductors installed by the drill-drive or drill-and-drivetechniques the driving record may provide the only reliable data by which to estimate

capacity. This is because the pilot hole is of a similar diameter to that of the conductor,typically a 26in. nominal hole diameter for a 30in. O.D. conductor. Washout and other factorsmay result in a hole which is larger than its nominal value. Locally the size of the hole mayexceed that of the conductor. Under these circumstances, the normal techniques of predictingoutside skin friction from the results of soil testing are inapplicable. However, with there beinglittle or no end bearing and no inside frictional resistance to driving, the SRD is generated onlyby outside friction. Thus by backfiguring the SRD versus depth relationship from theblowcount records, an estimate may be made of the bearing capacity of the conductor. Thisinvolves equating that part of the SRD which is clearly attributable to the outside soil

resistance to the bearing capacity.

6.9 The estimation of the bearing capacity of conductors may be further complicated by therelatively small centre to centre spacings between conductors, typically 7ft 6in. andsometimes 5 ft. The capacity of a conductor could be reduced by installation of subsequentadjacent conductors, seeparagraphs 3.11to 3.19, particularly if drilling is involved.


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Conductors Installed the "Drill-Drive" technique

6.13 The capacity of conductors installed by the "drill-drive" technique is difficult to predict because

of the uncertainties regarding the skin friction which can be developed in soil through which apilot hole has been drilled. The friction will be a function of hole size, conductor size, drillingpractice and soil type but there is insufficient data to provide guide-lines. Shell Exprohave used their experience to draw-up a detailed installation specification which they strictlyenforce by providing tight supervision. The capacity can be assessed in two ways. First bybackfiguring an SRD from the blowcount record and equating this to the capacity. Secondlyby assuming that outside friction is developed only over a length near mudline and a lengthnear the tip where pilot holes were not used. An example calculation is provided inAppendix IV-1.

Conductors Installed by the "Drill and Drive" technique

6.14 The same considerations apply to the capacity of a conductor installed by the "drill and drive"technique as for the "drill-drive" conductor of paragraph 6.13.

Conductors Installed by the "Drill and Cement" technique

6.15 The "drill and cement" technique is described in paragraphs 5.14 to 5.16. There are twoconstraints on the capacity of such conductors; the limiting bond stress between grout andsteel and the skin friction at the grout/soil interface. The first is referred to the outside surfaceof the conductor, the second to the nominal diameter of the hole. To avoid confusion and thepossibility of error it is recommended that the limiting bond stress be converted to anequivalent limiting friction at the grout/soil interface thus


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6.17 To determine the ultimate axial capacity of a "drilled and cemented" conductor, the ultimateunit skin friction of the soil (fs) in each layer is computed. In strata in which fsexceeds fg, the

value of fgapplies. The remainder of the computations follow the standard procedures and anexample calculation is provided inAppendix IV-2.

Computing the Axial Load on the Conductor

6.18 The axial load depends upon a number of factors some of which are difficult to determine andmust be simplified (or ignored) in order to make the analysis possible. They are:

(a) the weight of the casings

(b) the precise details of the installation sequence of well casings

(c) the densities of the drilling fluid and the cement for the first casing string

(d) the degree of deviation of the well

(e) temperature gradient

(f) the level of cementation in annulus between first casing and the conductor.

(g) rig tensioning and overpull

(h) the mechanical properties of the grout

The complexity of the situation is one reason why some Group companies avoidcarrying axial forces on the conductors and do not cement above seabed, eg. ShellExpro. Other Group Companies use arbitrary rules which, experience has shown,provide reasonable guidance in a particular operating area eg SSB require the ultimate


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References


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Figure 6.1


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7. SELECTION OF INSTALLATION METHOD

7.1 The selection of the most appropriate installation procedure for the conductors on a particularplatform is not always a simple matter. There is a temptation to adopt the method which wasused on previous platforms in the operating area concerned. On many occasions this is the

correct choice. However, changes in platform type, setting depth, soil conditions, casingprogramme, conductor size, equipment availability and technology will affect the economicand technical viability of a given procedure vis-a-vis the others available. It is recommendedthat for each new project the conductor installation procedure is selected on the basis of anengineering judgement and not necessarily on precedent.

7.2 Descriptions of the various methods currently used within Shell Group companies to

install conductors are provided inSection 5. The methods are:

driving drill-drive

drill-and-drive drill-and-cement

jetting (not recommended)

The purpose of this Section of the Manual is to provide guidance in assessing the suitability ofeach of the above against the others.

7.3 The overriding consideration must be the safety of the platform. The first step in the selectionprocedure is to rule out any conductor installation method which might lower the safety factorsof the foundations or reduce the integrity of the well to below acceptable levels. The next-


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Assessing Driving for Safety

7.5 Provided that there is no danger of the conductors running into the piles, seeparagraphs 3.12

to 3.20, conductors installed by driving are unlikely to have an adverse effect on the safety ofthe platform foundations. With respect to damage to the conductor itself it must be checkedthat driving stresses do not exceed permissible values using wave equation techniques, seeAppendix III-1. If it is possible that a ledge of rock or coral will be encountered specialprecautions may have to be taken to prevent damage to the conductor tip, see paragraph 9.5.it is recommended that add-ons be connected using welded joints. There is little groupexperience of mechanical connections on driven conductors. The add-on length may belimited by the maximum permissible stick-up height, which is governed by allowable steelstress criteria. The method for calculating maximum stick-up height is identical to that for piles

and is described inAppendix V-2. Stick-up height is the distance from the last point ofrestraint to the top of the conductor.

Assessing Driving for Feasibility

7.6 Determine whether or not the conductors can be driven to the specified setting depth using:-

(i) the hammers available on the derrick barge, usually steam hammers.

(ii) the hammers which can be handled from the drilling derrick, usually diesel or smallsteam hammers operated by compressed air.

The method of computation used to assess drivability is described in Appendix III-1.Should itappear that the conductors would refuse before the setting depth is reached, assess whethersoil plug removal would enable them to be driven to grade.


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Assessing Drill-and-Drive

7.10 The safety aspects of the drill-and-drive method are the same as in paragraphs 7.5 and 7.8.However, there is an additional aspect in that erosion and washout of surface soils ispossible. This may have an adverse affect on some types of foundation. The analyticalprocedures for assessing feasibility are the same as in paragraph 7.9. In general the drill-drive technique is to be preferred if the technology is available.

Assessing Drill-and-Cement

7.11 With the drill-and-cement technique there is little danger of damaging the conductorsthemselves and mechanical connections may be used. The principle safety considerationsinvolve the effect the technique may have on the surrounding soil and the platform

foundations. The spacing between the piles and conductors must be checked against therecommendations of paragraphs 3.12 to 3.20. Surface washout may be a problem.Installation should be feasible provided hydraulic fracture of formations with either drilling fluidor cement can be avoided. The method of computation to check this is described inSection 3.

7.12 For a number of recent field development projects wells have been drilled through a seabedtemplate before a jacket was placed. For such cases a 30" conductor is also set, although it

does not extend above the template. In these circumstances the conductors are usuallyinstalled by the drill-and-cement technique from a jack-up or semi submersible rig. Thecementation procedures must be designed so that grout losses into the soil strata areminimised. This means that formation fracture by the grout pressures must be avoided. Thisis because the presence of grout in the soil, particularly for sand layers, may makesubsequent pile driving much more difficult than anticipated. Also formation fracture mayaffect the bearing capacity of the piles supporting the template.


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8. RECORDING THE INSTALLATION AND INSTRUMENTATION

Why Installation Records are Essential

8.1 Over a period of many years, a large number of wells may be installed in any one part icularfield or concession. It is important that data obtained from previous conductors are consideredwhen selecting the techniques to be used in the future. Due to personnel movementsbetween OPCOS it is essential to initiate recording procedures to permit continuity. Withoutsuch records it is possible that the same difficulties with conductor installation will arise time

after time.

8.2 There are a number of reasons, other than continuity, for making conductor installationrecords. They are:

(a) to ensure that the conductors fulfil all aspects of the specification.

(b) the taking and checking of records is a Quality Assurance procedure.

(c) if conductor installation does not proceed as planned, the records can be used todetermine what has gone wrong and to design appropriate remedial measures.

(d) records will provide a data base for improving methods of conductor installation.

Normal practice is to oblige the contractor to take the records. The Shellrepresentative is responsible for checking and approving them.


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Instrumentation

8.6 Hammer performance can be monitored by instrumentation, and instrumentation provides theonly method of assessing the efficiency of a diesel hammer. Both Shell Expro andSOC have suspected that conductors have refused prematurely because of poor dieselhammer performance, but without instrumentation have not had data to substantiate theirviews. For steam hammers, a qualitative assessment of efficiency may be obtained byobserving the free fall. However, if a reliable estimate of efficiency is required instrumentationmust be employed.

8.7 SBPT used strain gauges and accelerometers on a conductor on Maui A to monitor theperformance of a diesel hammer. The theory and practice of such measurements for bothpiles and conductors is described inAppendix V-4.

General Quality Control

8.8 Consideration should be given to setting up a Quality Assurance Procedure for both welding(NDT) and cementation. These are outside the scope of this Manual but the ConstructionDepartment or EP 23.4 can provide advice.

FIGURE 8.1


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FIGURE 8.2


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FIGURE 8.3


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9. CONDUCTOR SHOES

9.1 It is common practice for there to be an internal driving shoe at the toe of a conductor.Generally, such shoes increase the wall thickness at the toe to 1 .5 times the nominal wallthickness, eg. the shoe of a 26"dia. x 0.5" wt conductor would increase the wall thickness atthe toe to 0.75". It is essential that the dimensions of the shoe allow the drilling bits to passwith adequate clearance. Also the transition from the nominal conductor wall thickness to theshoe should be gradual both to allow easy passage of tools and casings, and to reduce stressconcentrations during driving. Although conductor shoes are widely used, they are notobligatory and in some circumstances serve no useful purpose. External conductor shoes, i.e.those which increase the outside diameter of the conductor, should be avoided. This isbecause they may reduce the bearing capacity of a conductor and, if a weak path is formedup the outside of the conductor, they may reduce well security.

9.2 The reasons for using conductor shoes are:

(a) to make driving easier by reducing the friction between the conductor and the soilplug inside it (see paragraph 9.3)

(b) to reduce the probability of damage at the end of the conductor when driving throughhard strata (see paragraph 9.5)

(c) to cause the conductor to deviate in a preferred direction during driving, a specialconfiguration being required to do this (see paragraph 9.6).

From the above it is apparent that the use of a shoe only benefits driven conductors. Becausethere are some disadvantages associated with driving shoes (such as an increased risk of


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damage. If it is expected that the conductors will have to be driven through a hard layer, suchas a ledge of rock or coral, a detailed study must be made of the implications using waveequation techniques. The results of the study should include; predictions of driving stresses;

limits on the blowcount; recommendations regarding wall thickness and drilling out. Acommon practice is to drill ahead once the limiting blowcount has been reached.

Directional Control

9.6 Shell Expro have developed a conductor shoe which, in conjunction with their drill-drive technique, is expected to cause a conductor to deviate from the vertical during driving.The intention is to increase the spacing between conductors at tip level and to have the

conductors aligned in preferred directions. A schematic drawing of the Shell Exproshoe is provided onFig. 9.1A proprietary systems is also available(Ref.9.1)but as far as isknown there is no group experience of its use.

9.7 The Shell Expro shoe has been used on a number of projects in the North Sea. Theresults of directional surveys (Sperry Sun) to confirm that the technique works were awaitedat the time of writing this manual.

9.8 Other methods of increasing the spacing of conductor tips are described inSection 10.

Clearances

FIGURE 9.1


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10. CURVED CONDUCTORS

10.1 If it is proposed to use curved conductors it is normal practice to install the lead sectionsthrough the guides prior to float-out. The uppermost guides are usually vertical, the remainderfollowing the specified conductor curvature. The lead section is fabricated to the samealignment as the guides. Straight add-ons may be used for the remainder of the conductorlength. This means that straight sections of conductor may eventually be driven throughcurved guides resulting in the guides being subjected to static and dynamic lateral forces. Ifcurved add-ons are used a straight follower should be used for driving. A brief note on the useof curved conductors in the Gulf of Mexico, prepared by Shell Oil, is presented inAppendix V-5.

Reasons for using Curved Conductors

10.2 The principal advantages of using curved rather than straight conductors are:

(a) they allow increased coverage of the reservoir from one platform.

(b) the centre to centre spacings at the conductor tips are greater than at deck level. This

reduces the possibility of one casing running into another

Disadvantages of Curved Conductors

10.3 There are several disadvantages associated with the use of curved conductors. These are:


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10.6 Conductors may wander and the results of Sperry Sun surveys indicate that the curvature inthe soil may be 2 less than expected, (see minutes of meetings with SOC, SSB and BSP).

Drivability

10.7 A wave equation program suitable for analysing the driving of curved conductors has beendeveloped by SOC and is available through SIPM . In addition to providing data ondrivability it predicts the forces induced in the conductor guides by driving. A description of theprogram and an input guide is provided in EP 42513 "Driving Analysis for Initially CurvedMarine Conductors". However, this may be difficult to follow for a new user and for this reasona Fugro Report to Shell Expro which comments on its use forms Appendix V-6 of thismanual.

10.8 It is the practice of SOC, SSB and BSP to use the maximum possible wall thickness forcurved conductors, ie. they use 26in. dia x 0.75in. wt for curved conductors and 26in. dia x0.5in. wt for straight conductors. This means there is no shoe on their curved conductors.With this increased wall thickness SOC report that they are just as easy to drive as straightconductors.

10.9 Shell Expro are proposing to use curved conductors on their North Cormorant jacket.The conductors will be installed using the drill-drive technique. A theoretical analysis indicatesthat pilot holes may have to be shorter than for straight conductors, but that overall drivingshould be no more difficult than for straight conductors.


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11. CONTINGENCY PROCEDURES FOR CONDUCTORS WHICH DO NOT REACH SETTINGDEPTH

11.1 If a conductor fails to reach its setting depth the methods proposed for spudding-in andinstalling the first casing string may have to be revised. Generally the drillers have a numberof contingency procedures available to them, some of which may affect the structural integrityof the conductor. It is therefore essential that the drillers and designers jointly agree anycontingency procedures.

Conductors not Satisfying Axial Capacity Requirements11.2 On some projects SOC design conductors to carry the weight of the casing string. If the

specified setting depth is not reached and the conductor has inadequate "bearing capacity"SOC would either:

(a) vary the well design so that internal casings can be carried on the first casing string or

(b) cement the annulus between the conductor and first casing string up to the well headand use the conductor plus casing string as a composite pile.

As described inparagraph 6.1 (iii)the analyses required to justify the latter are complex andsecondary effects become important. For this and other reasons SIPM recommendthat conductors should not be required to carry axial loads.


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Other Contingency Measures

11.6 SSB have reported that if a conductor has refused prematurely during driving from a derrickbarge, they subsequently try to pull the conductor out with the drilling derrick and reinstall itusing a drill-drive or drill-and- drive technique. However, on no account should this procedurebe relied upon as a "cure-all" for conductor installation. On many occasions the drilling derrickmay have insufficient capacity to pull the conductor. For further comments on this matter seeminutes of meeting with NAM andparagraph 5.6.

12. DO'S AND DON'T'S

12.1 This Section is a summary of the discussions which were held with the Group Companies,and the contents of the Manual itself. Its purpose is to briefly highlight accepted good practiceand assist the reader in avoiding the mistakes of others.

Do's

12.2 The majority of the points listed below may appear obvious. However, pressures on time andcost have caused them to be omitted on some projects, occasionally with seriousconsequences. Do:

1. have a soil investigation made at the location of the platform.

2. ensure that there is sufficient clearance inside the conductor for the drilling bit to passfreely ,up and down. This is particularly important if there are any local reductions in


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13. put all of the above in an installation specification, take records (at least sufficient tocheck the contractor's records) and incorporate them in an "as- built" report.

Don't's

12.3 A list of things not to do obviously includes the opposites of many of the items in paragraph.In addition to these,

1. do not economise on materials, e.g. wall thicknesses, to such an extent that theconductors are difficult to install.

2. unless there are overriding reasons to the contrary do not plan to

(a) support casing strings on the conductor

(b) use blind drilling techniques

c) cement the annulus between the conductor and first casing string abovemudline

3 . do not use square shoulders on the inside of the conductor. These may snag thedrillstring or casing string and may cause stress concentrations.

4. do not use mechanical connectors for driven conductors unless there is satisfactoryevidence of their field performance.

5. do not abdicate all control responsibility to the installation contractor.

6. do not assume that the installation will go exactly as planned.

APPENDIX 1


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RECORDS OF MEETINGS WITH PERSONNEL OF VARIOUS SHELL GROUP COMPANIES


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Discussions on the content notes of this Manual were held with a number of Shell GroupCompanies. The range of topics covered at each meeting varied from company to company.Frequently the agenda was influenced by the outcome of previous meetings and the particularinstallation problems in the operating area concerned. In this Appendix the meeting records are

presented in chronological order;

Company Date

SIPM 12 October 1978

SBPT 15, 16 and 22 November 1978

Shell Expro 16 November 1978

NAM 29 November 1978

QPPA 5 and 6 December 1978

BSP 11 to 14 December 1978

SSB 14 and 15 December 1978

SOC (Houston) 22 and 24 January 1979

SOC (New Orleans) 23 January 1979

It should be noted that, except for the meetings with BSP, the records have been compiled by thewriter without reference of the other attendees at the meetings They are not intended to be official


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SESSION 1

E. Toolan Fugro Ltd.

M. Brinded EPE/4

M. Langereis OPE/1

C. Shepherd OPE/15

G. Bayer ECC/22

J. Caminada EDA/23

J. Dawson EDA/22 (Reporter)

This was a generalised discussion to cover the purpose, style, scope and format of the Manuals.

A. GENERAL POINTS

1) The questionnaire, mentioned in the S.I.P.M. notes on the project, had not in fact been

sent to operating companies, as S.I.P.M. and Fugro agreed that mare useful informationwould be obtained from visits to the operating companies.

2 B.S.P. would like to receive the manuals in their Final Draft form, to permit scrutiny andcomment prior to publication by S.I.P.M. and Fugro. A time limit would be imposed forsubmission of comments. (say 1 month)

M T l ld i thi i t ith S I P M d B S P h ld d i d d tl

C. STYLE

f f


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7. The style proposed for the Manuals was generally agreed, with the proviso that all formulasused should be derived or referenced, with constants and parameters specified.

In each section, basic principles would be highlighted, with references where subjects were

not covered in detail.

D. SCOPE

8. There was a request from B.S.P. for greater emphasis, in the pile Manual, on design methodsand parameters.

9. Mr. Toolan stated that to expand the manuals to include all aspects of design would makethem very large and somewhat clumsy.

NOTE: In subsequent discussions, certain topics were pinpointed for coverage of designfactors in greater depth in the manuals.

10. A section will be included on Freestanding and guyed conductors, mudline suspension wells,

and tie-ins to jackets.

11. The main points of interest from Operational Viewpoints were emphasised to be formationstrength, conductor setting depths, and casing cementing setting levels.

Mr. Toolan was able immediately to provide information on calculation methods to determinethe correct conductor setting depth for avoidance of formation fracture.


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SESSION 3

M. Langereis OPE/1

A. Schouten OPE/13

C. Shepherd OPE/15

M. Brinded EPE/4

J. Caminada EDA/23

J. Dawson EDA/22 (Reporter)

This session concentrated on the Drilling/operational criteria for conductor Specifications.

Drilling Department have found from experiences over the past few years that the old conductorinstallation criteria dating from about 1972 have not been completely satisfactory and have suggestedthat those procedures should be revised.

The following points summarise the main areas of concern among Drilling, Operational andEngineering Departments of B.S.P., for inclusion into the Conductor Installation Manual beingcompiled by Fugro.

1. A general point was made that criteria governing freestanding conductors should beincluded - with references.

SECTION 3 - Conductor Setting Depths


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SECTION 7 S l ti f I t ll ti M th d


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SECTION 7 - Selection of Installation Method

5. This section or an appendix should include the results of a wave equation analysis for theconductor sizes and hammers commonly used by B.S.P. For this Mr. Toolan has been

provided with information on all hammers used in B.S.P. offshore areas, also information oncommon conductor sizes and material strengths.

SECTION 9 - Conductor Shoes

6. It was recommended that internal conductor shoes should be used generally 1.5 x conductor

wall-thickness.

SECTION 10 - Curved Conductors

7. The section of the manual dealing with curved conductors should be expanded to discussmitred and slanted conductors and any other techniques used in the Group to maximiseseparation of conductor shoes.

8. A note should be included that it has been B.S.P.'s experience that diameter 20" casing

cannot be run through a diameter 26"x 3/4 conductor at a dogleg severity or more than 5

per 100'.

Furthermore, diameter 18 5/8" casing has become an important size in B.S.P. and has

d) Should difficulties be encountered in this first well, either from loss of circulation or fromshallow gas pockets, it is agreed that for the remaining conductors on the jacket the 20" casing would be set at a shallow depth of typically 75' - 150' below pile tips.


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For this situation, it is recommended that, as in the past, sea-water would be used for drilling,with the casing being set and cemented as in normal practice.

APPENDIX II-1

SOIL PARAMETERS REQUIRED FOR CONDUCTORDESIGN AND INSTALLATION PLANNING


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SOIL PARAMETERS REQUIRED FOR CONDUCTORDESIGN AND INSTALLATION PLANNING


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The soil parameters required for the design and installation planning of conductors are given in theTable below. Also included in this Table are the types of laboratory tests which are used to obtainthese parameters.

In addition to the soil parameters, it is also necessary to know the specific gravity of any drilling fluidsto be used and the discharge height of drilling returns above sea level.

SOIL PARAMETER TEST

_ Visual description

Bulk density density test

Submerged density

Moisture content Moisture content test

Undraine Shear Strength Unconsolidated undrained triaxial test; pocket penetrometer;torvane; fall cone; consolidated undrained triaxial test (if

insitu stresses can be estimated)

Remoulded Shear Strength plasticity index; moisture content; remoulded triaxial test

Strain required to mobilise 50% ofmaximum soil shear strength(cohesive soils only)

unconsolidated undrained triaxial test

unconsolidated undrained triaxial test

APPENDIX II-2

CALCULATION OF THE COEFFICIENT OF

EARTH PRESSURE AT REST (K )


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EARTH PRESSURE AT REST (Ko)

CALCULATION OF THE COEFFICIENT OF


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EARTH PRESSURE AT REST (Ko)

The method of calculation of Ko is dependent upon the soil type under consideration. Basically themethods differentiate between cohesionless and cohesive soils.

COHESIONLESS SOILS

For a cohesionless soil which has not experienced an overburden pressure greater than the existingvalue, i.e. the precompression ratio (PCR) is unity, the value of Kois obtained from the expression:-

Ko= 1 - sin '

where is the angle of internal friction of the soil.

If the PCR of the soil is greater than unity, Schmertmann (Ref. 1)has developed the formula:-

Ko= (PCR)0.42

x (1 - sin ')

COHESIVE SOILS

For a normally consolidated cohesive soil the theoretical value of Kois the same as for a cohesionlessmaterial:-

K 1 i '

References

1) SCHMERTMANN, J., "The measurement of in-situ shear strength". Proceedings ASCEspecialty conference on insitu measurements of soil properties Vol. 2 1975.


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2) SKEMPTON, A.W., Discussion on "The planning and design of New Hong Kong Airport".Proc. Institution of Civil Engineers 7. 1957.

3) LADD C. and FOOTT, R. (1 974), "New design procedure for stability of soft clays", A.S.C.E.,J-GED, July, p 769.

4) BROOKER, E.W., and IRELAND, H.O., (1965) "Earth Pressures at rest related to stresshistory" Canadian Geotechnical Journal Vol. II No. 1 (Feb.).

5) VIJAYVERGIYA, V.N. "Procedure for computing axial pile capacity" Fugro Internal Paper.July 1977.

APPENDIX III-1

THE USE OF THE WAVE EQUATION IN DRIVABILITY ANALYSIS


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THE USE OF THE WAVE EQUATION IN DRIVABILITY ANALYSIS


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General

For most conductor installations, use is made of driving methods for at least part of the installation.With computer programs based on the wave equation, the ability of a particular hammer to drive aconductor to its required penetration may be assessed. Such programs can also determine whether ornot the conductor is overstressed during driving.

To use the wave equation correctly it is necessary to have a basic understanding of the phenomenainvolved in pile driving. The impact of the hammer generates a compressive stress wave in the

conductor. This stress wave travels down the conductor at the velocity of sound in steel, 5000 m/s. Ifthe wave meets a resistance or discontinuity a proportion is reflected back up the conductor, reducingthe magnitude of the downward travelling wave by an equal amount. Once the wave has reached theportion of the conductor shaft within the soil, the downward movement of the conductor behind thewave front generates a frictional resistance at the soil/conductor interface. This causes a reflectionwhich reduces the intensity of the wave. Eventually the wave reaches the conductor tip. If there issufficient power left in the wave to cause permanent deformation in the soil below the conductor tip, orif only elastic movements occur in the soil below the tip, the conductor will be in the same positionafter the blow as before it. In this situation the conductor is said to have refused.

Most conductor driving analyses use computer programs based on one dimensional wavetransmission. Those available within the Shell Group employ finite difference techniques for thenumerical analyses. The hammer is modelled as a falling weight striking a cushion and/or anvil, seeFig. 1. The conductor is modelled as a series of lumped masses of the conductor. The soil springs aredefined by; an ultimate static resistance (Ru); a displacement over which the soil behaves elastically,the quake (Q); and a damping factor which increases the static soil resistance as a function of the

by API method 1 or 2 (whichever is higher, seeAppendix IV-1)for the calculated shaft friction duringdriving The rest of the computation is as before In some hard clay soils the SRD computed after set


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driving. The rest of the computation is as before. In some hard clay soils the SRD computed after set-up may be less than that calculated for continuous driving. This indicates that set-up effects may beignored.

Calculation of SRD versus depth relationship

The SRD at any depth is determined from the following considerations. If the soil inside the conductor(soil plug) remains stationary during driving the SRD must be made up of inside and outside frictionand wall end bearing. In this situation the magnitude of the inside friction which can be mobilised maybe limited by the end bearing capacity of the soil plug. Alternatively the soil plug may move downduring driving in which case the inside friction must be equal to or greater than the plug end bearing.

Generally the conductor will behave in the manner which produces least resistance to penetration.

Thus at any depth the SRD will be the least of:-

where : fs' = unit shaft friction during driving (outside)

fi' = unit shaft friction during driving (inside)

q ' unit point resistance during driving

Calculation of shaft friction in cohesive soils


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The contribution of skin friction to total SRD is calculated on the basis of laboratory test results. Alongthe length of the conductor, the soil at the interface with the conductor wall is strained to failure by

every hammer blow. In a cohesive material the soil is. compressed to accommodate the volume of theconductor as it penetrates. The displacement, shearing and compression remould the soil and causeexcess pore water pressures to be developed. Thus during continuous driving in a clay:

In many cases it may prove impossible to perform sufficient meaningful remoulded triaxial tests in thetime available. This is particularly the case with heavily overconsolidated clays which may have to beground down, reconstituted and reconsolidated prior to shearing.

In order to overcome this problem, considerable reliance is placed on empirical relationships betweenremoulded shear strength and other properties of a soil such as those developed by Skemption andNorthey (Ref. 8) and Houston and Mitchell (Ref. 9). The remoulded shear strength according to

Skempton and Northey depends on a relationship between r and plasticity and liquidity indicesderived from laboratory measurements. This relationship is shown onFig.6.

C l l ti f h ft f i ti d i d i i i l il

Two relationships have now been developed, SRD versus depth and SRD versus blowcount. Theseare used to determine the predicted blowcount versus depth relationship see Appendix III-2


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are used to determine the predicted blowcount versus depth relationship, see Appendix III 2.However, the situation is complicated by the fact that the SRD versus blowcount relationship varieswith

hammer efficiency (which may change during a drive)

conductor penetration (distribution of SRD between tip and shaft, soil damping increasing withdepth, different soils at tip of conductor)

Prior to describing drivability analyses in detail it is essential to return to first principles and considerthe effect of various parameters on drivability.

Parametric Considerations

The discussion of wave transmission earlier in this Appendix-enables some simple conclusions to bedrawn on the effect on drivability of various parameters. A given hammer will transmit approximatelythe same amount of energy, E, to any conductor irrespective of the conductor's dimensions. Theenergy in the conductor may be expressed as:


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And fromequation (3)

Hammer Energy

It would appear from equation (13) that for a given conductor the maximum soil resistance which canbe overcome is proportional to the square root of the hammer's delivered energy.

The improvement in drivability due to increased energy is less favourable than the ratio of squareroots since higher energies involve higher velocities, stresses and elastic compression of theconductor, see equation (9). These lead to increased losses from side friction and damping. Thustrying to improve drivability by increasing hammer size is subject to the "law of diminishing returns".Eventually a stage is reached at which the hammer is so powerful (or heavy) that permissible driving(or static) stress levels in the conductor are exceeded. It is concluded that more powerful hammers

Penetration in Uniform Soils


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As a conductor is driven deeper, the length of shaft in contact with the soil increases. Hence a greaterproportion of energy is lost in soil damping. Thus for a given conductor hammer, the greater the pilepenetration the lower will be the soil resistance which causes refusal. However for penetrations whichare within approximately 30% of that analysed significant errors do not occur, i.e. an analysis madefor 50m would be valid for penetrations within the range 35m to 65m.

The length of conductor above mudline has little if any effect on drivability.

Penetration in Stratified Soils

The drivability of a conductor is affected by the distribution of the soil resistance. Generally the greaterthe proportion of soil resistance at the tip the harder driving becomes. There are a number of reasonsfor this:

(a) high tip resistance is usually associated with sand and much higher point damping is appliedin sand than in clay', seeFig. 2.

(b) if the soil resistance on the shaft is low, velocities are high and energy losses due to soildamping are proportionately higher.

(c) the permanent movement of the conductor is governed by the situation for driving may arisewhen the conductors reach maximum penetration, or at some shallower depth when the tip isin sand.

Yi ld St th

6) DURNING, P.J., and RENNIE, I.A., "Determining Pile Capacity and Pile Drivability in Hard,Overconsolidated North Sea Clay". EUR 47, European Offshore Petroleum Conference andExhibition, London 1978.

7) NAUGHTON, H.R., and MILLER, T.W., "The Prediction and Subsequent Measurement of PileDriving Behaviour at the Hondo Platform in Santa Barbara" EUR 11 European Offshore


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Driving Behaviour at the Hondo Platform in Santa Barbara . EUR 11, European OffshorePetroleum Conference and Exhibition, London, 1978.

8) SKEMPTON, A.W. and NORTHEY, R.D., "The sensitivity of clays". Geotechnique, III, 1,1952.

9) HOUSTON, W.N. and MITCHELL, J.K., "Property interrelationships in sensitive clays".Journal of the Soil Mechanics and Foundation Division, ASCE 95, SM4, July 1969.

FIGURE 1 IDEALISATION OF HAMMER PILE AND GROUND FOR WAVE EQUATION ANALYSIS


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FIGURE 3 BASIC CONCEPTS FOR PREDICTING SOIL RESISTANCE DURING DRIVING FOR ANUNPLUGGED CONDUCTOR


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FIGURE 4 BASIC CONCEPTS FOR PREDICTING SOIL RESISTANCE DURING DRIVING FOR APLUGGED CONDUCTOR


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FIGURE 5 COMPUTATION OF THE ULTIMATE UNIT END BEARING OF A PILE FROM A CONEPENETRATION TEST


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FIGURE 6 LIQUIDITY INDEX .V. REMOULDED SHEAR STRENGTH


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FIGURE 7 SOIL RESISTANCE AT TIME OF DRIVING VS BLOW COUNT


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APPENDIX III-2

EXAMPLE OF WAVE EQUATION ANALYSIS FOR CONDUCTOR DRILL-DRIVE SEQUENCE


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EXAMPLE OF WAVE EQUATION ANALYSIS FOR CONDUCTOR DRILL-DRIVE SEQUENCE

1) The first stage of the analysis is to determine the soil resistance to driving (SRD) of thed t i th t d d d d ib d i A di III 1 A l f h


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conductor, using the standard procedures described inAppendix III-1.An example of suchanSRD calculation for a 28in. diameter conductor, at a North Sea site, is presented on Fig. 1.

2) For a drill-drive sequence, the computed SRD will need modification to allow for the effects ofdrilling-out. The amount of modification is difficult to predict on a purely theoretical basis.Wherever possible, use should be made of back-analyses of conductors installed by similarmethods at nearby sites. An example of this approach is shown on Figs. 2 to 4 of thisAppendix.

Figs. 2 and 3 show the observed soil resistance at the time of driving conductors atShell 's Dunlin Field in the North Sea. From the relationships shown on Figs. 2 and 3the generalised relationship shown onFig. 4has been obtained. It can be seen that the drill-drive procedure adopted at Dunlin generally inhibited the development of skin friction alongthe length of the conductors. Beyond the initial drive into virgin soil (necessary to form a sealto avoid washout at the mud-line) no significant build up in soil resistance occurs with depth.Where resistance has built up, it is due to tip resistance resulting from the conductor reachingthe bottom of the predrilled pilot hole. This resistance is destroyed on drilling out the nextlength of the pilot hole.

3) The next stage is to plot out the calculated SRD versus depth for a purely driven conductor,seeFig. 5. Combining this relationship with the results of a wave equation analysis performed

for the proposed installation plant, it is possible to calculate what percentage of total SRD canbe overcome by the conductor/hammer combination. An example of the wave equationanalysis is given onFig. 6.In this example only approximately 25% of the calculated SRD atsetting depth could be overcome by a Delmag D-55 hammer.

4) The drill-drive sequence is then developed on the basis of the above information. After driving

FIGURE 1 : 28" CONDUCTOR - SRD 1 " W.T


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FIGURE 2 SOIL RESISTANCE DURING DRIVING AGAINST DEPTH DUNLIN A


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FIGURE 3 SOIL RESISTANCE DURING DRIVING AGAINST DEPTH DUNLIN A


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FIGURE 4 SOIL RESISTANCE DURING GENERALISED DRILL-DRIVE SEQUENCE DUNLIN A


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FIGURE 5 SOIL RESISTANCE AT THE TIME OF DRIVING DUE TO EXTERNAL SKIN FRICTIONONLY


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FIGURE 6 SOIL RESISTANCE AT THE TIME OF DRIVING VS BLOW COUNT


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APPENDIX III-3

EXTRACT FROM

"CONSTRUCTION SPECIFICATION FOR INSTALLATION OF STEEL PLATFORMS"


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CONSTRUCTION SPECIFICATION FOR INSTALLATION OF STEEL PLATFORMS

PREPARED BY SSB (SEPTEMBER 1978)

a) The CONTRACTOR shall continue to drive the pile with a higher number of blows per foot,

or


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b) The CONTRACTOR shall jet out the soil plug inside the pile (in accordance with sub-section5.13) and then continue to drive the pile.

or

c) The CONTRACTOR shall drill out the soil plug inside the pile and drill a pilot hole (inaccordance with sub-section 5.14) and then continue to drive the pile.

The above procedures shall be approved by the COMPANY.

5.3.13 The CONTRACTOR shall carry out buckling analyses to ensure that add-on lengths/piledriving hammer to be utilized are compatible.

5.4 Shims

5.4.1 Every attempt shall be made to centralize all piles by using the largest shim plates possible.They should be driven in if necessary. Before commencement of welding shim plate fit-upshall be subject to inspection and approval by the COMPANY.

5.4.2 The number and size of shim plates issued for each installation is as shown on the ContractDrawings.

5.4.3 The shim plates shall be blast cleaned before fitting Once the shim plates are fitted thepile/leg area where the fillet, welds are to be made shall also be blast cleaned to a white

5.5.4 No driving of a conductor is permitted while other conductors are still hung off from the jackettop brace elevation i.e. all conductors must be self-supporting; or when welding out shims

5.5.5 Curved conductor sections shall be accurately lined up to ensure the correct curvature whenmaking field joints in curved sections


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5.5.6 All field welds in conductors shall be made and inspected in accordance with Section 4 of this

Specification. Cut offs shall be held to a minimum

5.5.7 The conductors shall be driven to the target penetration, at which penetration a ' good' drivingresistance should be achieved. 'Good driving' resistance is defined as approximately 40 blowsper foot with a hammer having a rated striking energy of about 50,000 ft. Ibs.

In the event that this 'good' driving resistance is not met at target penetration, driving mustbe continued until this requirement is achieved, or until all available conductor length isutilized.

5.5.8 In general the blowcount should always be limited to a maximum of 200 blows per foot for thetype of hammer recommended above. In the event that such blowcounts are encounteredbefore the target penetration is reached, driving must be discontinued when such hard drivingresistance is obviously caused by a hard layer at the tip of the conductor and not due set upin clay (e.g while making an add-on connection).

5.5.9 Conductors shall be cut off at the correct level, extended up to deck level and cover platesinstalled, as indicated on the Contract Drawings or in the Outline Installation Procedures.

APPENDIX III-4

EXTRACT FROM

"OUTLINE INSTALLATION PROCEDURE FOR SFDP-A"


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PREPARED BY SSB

Before stabbing the initials of skirt piles SA-2 and SB-2 the skirt pile sleeves must be flooded. Theprocedure for the installation/driving of the four skirt piles shall be similar to that for the centre piles.Skirt pile add-ons shall be reworked, and the skirt piles cut off underwater, in accordance with theC t t D i


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Contract Drawings.

The piles shall be driven with hammers having the rated striking energy recommended in ConstructionSpecification N

conductor design and installation maula for offshore platform

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