5th Offshore South Eost Asia 21-24 February 19S3 Singapore

SSSSICN 11: FLOATING PRODUCTION FACILITIES ]2933

OPERATIONAL PERFORMANCE OF THE BUCHAN FIELD FLOATING PRODUCTION

AND OFFSHORE LOADING SYSTEM

A A MierusBP Petroleum Development Limited

United Kingdom

ABSTRACT

This paper describes the operational history to date of the Euchan Field inthe British Sector of the North Sea. The development is based on subsea com-pletions connected to a floating production facility via a subsea manifoldand a tensioned multiple riser system. Stabilised crude oil is exported fromthe platform to a tanker via a subsea pipeline and a CALM busy. Operationalproblems and successes are discussed. The Field is operated by BP PetroleumDevelopment Limited on behalf of the Buchan Field Joint Venture Partners.

The paper concludes that even in the hostile environment of the North Sea a

floating production system is a viable method of producing marginal fields.It is suggested that a purpose built platform would be a better propositionthan a conversion such as was used for Ruchan. Careful selection of theloading system with respect to weather conditions is essential.

FIGURE 1: BUCJ4AN ALPHA PRODIKHON PIATFORM

11-68

over 0.1 md - 2 dcrcies (Figure 3). In the central horst porosity valuesran~e ??om 6-15% and Nater saturation values are hisn at 65-70%. tiet oay

thickness is about 4001m(maximum) .

FIGURE 3: BUCHAN WELL LOCATIONS

THE 8ucHAN FII!LO 1.3.3

,ru .

LCOCNO

. —“ -m

--- ,mIOOOc-

///// -m.. --

@ ‘-” --

s —0 @b ML,

+ —0 0!, .,.

● — 0!...

S*.

I

I

I

II

This central area is bounded by a flank area af much poorer quality (net pay120m, average permeability 3 md and water saturation 85%) which has beenpenetrated by one well. It is estimated that 370 MISTS are contained inplace in the central horst with only 38 MMSTB in the much larger flank area.The whole reservoir is highly over-pressured with an initial pressure of7,506 psig at datum (2805 mss). Recoverable reserves were estimated at 50MMSTB (without secondary recovery) which represents approximately 14% of thehorst oil-in-place.

At the initial sta~es of develo~ment many factors aF+ecting the reservoirwere unclear - in particular, the extent to which the matrix would contri-bute to overall production had not been defined. It was, however, assumedthat the matrix would contribute, and that the Field would produce at a ?eakrate of 72,000 3PD with an average maximum daily rate of 48,000 BPD (when

11-70 SPE1243?

INTRODUCTION

The 9ucnan Field is located aooroximately 160km ENE af Aberaeen and 55 km

wNW of the Forties Field in water cbout 1 15m deep (Figure 2! . It was dis-

covered in August !$’74 by the Transworld Group and Texac~, and +urther

appraisal wells were drilled between then and 1976 to delineate the Field.

sp ,etroleum ~evelapm,ent +armed into the llcence in early 1977’ and teak over

as Operator 0+ the development. The bulk of the Field lies within Block

21/1 with an extension into Black 20/5.

FIGURE 2: BUCHAN FIELD LOCATION AT BLOCK 21/1

n O@

d’ ?\t~-o

x o \---...-SHETLANU

.

T b

FRIGG

.

~o :

u

% &g 1“

o 9*b BERYL I,$.

2PIPER

I*CLAYMOREO

,(MAUREEN

TARTANy

\

I 58011

‘“cHAN’i-Fi!E- “\

t

* El!E’T’Es’\ABERDEEN

I

SCOTLAND

570r!

1 “f/

.

‘1 I\

. /

v’.

~.o 59

,r\ .-w \ “\

.Kms

I I“P

I f ,

BUCHAN FIELD LOCATION AT BLOCK 21/1

THE RESERVOIR

The main Bucnan reservoir has a complex pre-Cretaceous histary and comprises

a Fluviatile sandstone probably of Devonian age. It is extensively frac-

tured, these fractures having a marked effect on permeability whichranges

11-69 SPE124~3

downtime caused by weather, equipment maintenance and tanker changeover was

taken into account) . This production would be mointcined for obout 1!4 years

before starting to decline, ~with an anticipated field life of about five

years. In order to keep production at a reasonable rate when reservoir

qt-essures had declined, and to increase total recovery by about 8 MMST8, it

was proposed to install gas lift facilities. Becouse of the nature of thereservoir and because o? the constraints imposed by the production platform

c3n?iguration, pressure maintenance by water or gas injectian was not consi-dered to be feasible.

From the foregoing it can be seen that this complex reservoir, with aiffi-

cult drilling conditions and with uncertain recovery, made Buchan a trulymarginal field with considerable commercial risk.

DEVELOPMENT SCHEME

Because of this commercial risk ond the foreseen short life of the Field it

was necessary for the development to proceed as quickly as possible and tominimise capital investment. To this end a fiaatifig -A,.-+< -mprvuutib.wu. Syst!?n! ,

utilising --..--+-,4 a .h~a drilling rig:a ecmver.=u =emi=subR?e~sA- subsea wells and off-

shore loading were chosen for the development (Figure 4). This meant that

the drilling of the wells, the installation of the subsea pipelines and theloading buoy and the conversion of the semi-submersible and the tankerscould all proceed in parallel thus minimising the total time required befarefirst Qil production.

In the event, mainly due to difficulties encountered in the canversian ofthe drilling rig, the development exceeded budget and pragramme. These were

originally set at Q135 million and 30 months but the final cost was same 40%greater and the programrne was 20 months late. Nevertheless, helped by the

increase in oil prices, the development has proved ta be an economic propo-sition.

PRODUCTION AND LOADING SYSTEMS

The Buchan production wells are all sub sea; faur drilled thraugh a template

below the production platform, one just off-template and three satellite.The oil flows from the wells to a subsea manifold an the template and then

via indiviciuai 4“ “risers te a three st=~e ~ep=r=tor trgin on the platfann.Flow from the wells is controlled by chokes on the platform. The three

stage separator train is designed for 72,000 BPD and provision is made for

draw off of formation water. All gas is flared.

The degassed crude is pumped through meters dawn the 12” export riser and

then, via the subsea manifold, the export Line and the CALM buoy, to the

tanker. The 12” subsea export line terminates at a manifold beneath the buoy,to which it is connected by a flexible hose, at about 1.9 km from the plat-form. The CALM buoy (Catenary Anchor Leg Moaring) is 15m in diameter and is

designed to allow the moored tanker to “weathervane” through 360° under the

action of wind and tide.

Two tankers, each of 100,700 DWT, were converted to bow mooeing and loadingfor exclusive use of the ~uchan Field. They carry 30,000 tons of segregated

baliast and deliver crude to parts in the UK and North West Europe.

11-71

.. . .

11-72

..T

BUCliAN GUIDE ARM SPIDERAND RISER ARRAN-MEW

, ..’- .,.-’

?GL.Y

Pa

m-0mU

N

&-

W

w

PRODUCTION, EXPORT AND SERVICE RISER ASSEMBLY 4.2.1

.. ........nlskn

7jJY’/.@@~

_.

~.FIII1WICI1ON

mm walND&—SVAIXUH.WMA1LVb

IiAslltlRlsins -

lWIWII

4“,WAWIPUNVICE IWCRS

II EAw\

tWWI1nlskn --

I* S&nt:llx

(WIDE AflM SPIDER {TYPICAL)

FIGURE 5: BUCHAN RISER ASS~LY

txrolll tllstn /.,m, vtiflSAL

JOIN1

Ill kKVOH1f14SEll- ,CW.lC+iEfl$ 10FlltL LltC.Kb

10 Wm9tlcn 00?4— - R13E13TLN$IONEHS

IIW!I1L OEC$.I

-[ XP(JII1 ftiscti CAMI~tN~I=*

\ PHHW:;l HIStR

\ ,bN,,Lm co, L*H

._Gllll>t AIWSWMM II VPILALI

I.411MtavvWA1 ,

,W,”rl

i

. Si,lJSEAUAN46(11U

PIATFORM OPERATIONS

....

In general the plant and equipment on the platform has operated satisfac-

carily - bearing in mind that not all of it was new equipment installed whenthe conversion from a drilling rig was carried out.

?~e ean--atinn nlmni- hne ~~rforrned very wel~,==W-,=-*”,~ r--,.- ,--- the main problem being one of

foaming during StOrt-1J13 with a c~id plant or when operated at very nigh

production rates. Foaming is controlled by the injection of small quanti-

ties of antifoam typically 2-5 ppm.

Tine effluent treatment plant (ETP) is designed to pracess 25,000 BPD of

produced water to give 40 ppm oil in water quality before discharge ta the

sea. Water fram the production and test separators is routed to G deg=~sifi~vessel then through an inclined plate separatar befare being finally polis-hed in a four cell gas flatation separator unit.

———The ETP is proving rather more troub~esome due principa~~y to the extreiiiel ytight oil water emulsians being farmed. High asphaltene content of the crude

and low PH of produced water contribute significantly to this. In order toachieve oil water separation high dosage rates of emulsion breaker are

required, typically 150 ppm. The process is further hindered by low temper-

ature oil from satellite wells. Difficulty in achieving discharge qualitywater is attributed in some degree to the high residue of emulsion breakerin the water phase which stabilises the oil in water emulsion. Alternativechemicals are being investigated.

The production and effluent treatment processes are periodically treatedwith biocide to prevent build-up of SRBS. In addition corrosion and scaleinhibitors are used to a limited extent as required.

There were fears that the riser system would pase problems due ta itscamplex nature but this has nat proved to be the case (Figure 5). Theproduction and export riser is a complicated system comprising one central~~. -----~ .expw! u f-i~=~

-....-*,.-A-A h.. +-- Afl ~~s=~~,awl 8 Uwlawmu u, bc$mn - e~gh~ of wh~ch Qre far produc-

tion and two are service risers. These 4“ risers are restrained by guidefunnels at 25’ intervals down the main export riser. The export riser issupported by eight 60K tensioners. As ane can see frcm the illustration itis a very complex arrangement (Figure 6). Although na major problems havebeen experienced with the system it is very weather sensitive especially inrelation ta the aperational requirements of the platform itself.

n9ecfiuse of the Cmstmints placed GpGE 2s by tbs Certifyhg Authority it isnecessary to bring the platform up to what is known as “survival draught” of18 metres to permit sufficient clearance under the base of the platfotm deck

during extreme conditions of waves and wind. Further, we are permitted toremain in production only with the platfarm at a draught between 22 and 21metres and can anly leave the riser bundle cannected to a minimum draught of20 metres. Riser retrieval operations are also limited by the amount ofheadroom available above the riser bundle in the moonpool area and in thisconnection the platform heave has to be taken into account. It can thereforebe seen that the state of the weather has an important bearing on riser

operations and in consequence on production. As it can take up to thirtyhours to pull the tatal riser bundle we are very deoendent on weather fore-casting, and in fact during the winter we have a resident weather forcasteron board the platform.

In practice we shut dawn and recaver the production risers on a cieteriara-ting forecast of wind and sea conditions af worse than 35-40 knats and ofseas up to 12 metres maximum. We also remove the export riser gooseneck and

11-74

.1

2

mT1

m-Iv-f=wW

FIGURE 6: BUCHAJ’J ALPHA - MOOWOOL AREA

hose at this time in order to provide additional headroom. Onlyextreme

underweather conditions when the platform heave becomes greater than 5

metres and when there is a requirement to come to survivial draught of 18me~res do we in proctice unlarch and recover the export riser. However,

riser operating envelopes developed from the original stress analysis have

never been requirea to be applied..

Tine prc?cticci~ &anSideFCitimS Ci7 hSGVe

and personnel access have generally become the main determining influences.During the two and a half years of operation we have pulled the productionrisers five times due to adverse weather and the export riser only once. Thehydraulic control umbilicals have not been pulled on account of weather.

It should be nated that while we may not fully disconnect until heave levelsare quite high, reconnection cannot normally start until the heave is lessthan 0.5 metres. Such relatively calm conditions are required in order tostab the risers accurately into the connectors on the manifold and waitingan these conditions can cause additional downtime.

The response of the platform itself to its environment has been more than

satisfactory, being more stable than was predicted. The platform is mooredby ten 70nTn diameter wires each about 3% km long attached to 18 or 22 tonDelta Flipper anchors. As on active mooring system is employed it is there-

fore possibie to maintain the piat+onn on station over the mani+oid by... . --h: --“&t, **, &a,y in Gn the w+.A4*.FI e~r!m..*!!”””, “ -.”-. ~.i~ ..-h.. p=ttern hQ~ ~~rfQ~d“..- .-, Weli ;

keeping the platfazm in position without undue tensions in the wire even invery severe conditions.

The platform is of Pentagone design - the same design as the ill-fatedAlexander Keilland. The Keilland accident happened while 8uchan Alpha wasbeing converted and led to a very extensive structural design review andinspection of the platform to confirm its soundness. This resulted in alarge number of relatively minor modifications such as reshaping of stif-feners, removal of redundant penetrations, etc. A number af minor defectswere also identified and in arder to monitor the condition af these defectsand of the structure in general, continuous NOT and visuai inspection pro-gramnes have been operated since the platform went on station in September1980. It is encouraging to state that none af the lagged defects has shownany sign of propagation since that time.

Becouse of stability considerations careful weight control is exercised.Every item of stores and equipment, including fuel and water, coming on tothe plotform or being off-loaded is weighed and the vertical centre ofgravity is calculated daily to ensure that it remains within the laid down

rL.-iiiiii~~.

--1-..,-., . ~1.- A--a ~s+a” *jcr ~hamna= in ~Fg@t.Illi= LLJA&L4 blb~=ln is UAaW uws,w w, baa w, ,“, ,~we

-------

SUBSEA WERATIONS

Apart from problems with downhole safety valves which are dealt with later,

the total subseo system has performed satisfactorily. The subsea volves andriser connecters are operated by a direct hydraulic system using a waterbased fluid. The fluid is supplied from the platform to the subsea systemthrough four retrievable 65 x %“ care umbilicals providing a reasonablepercentage far redundancy. No problems have been experienced with the WKMactuators or the three valves although we have experienced severe galvaniccorrosion of the initially fitted cadmium plated carbon steel crimp connec-tors on the umbilical cores and individual subsea hydraulic hoses. Theseconnectors are being replaced by ones made of 316-S12 stainless steel (BS970 pt.4). As there are about 1,000 of them, this is a lengthy and expen-sive task.

11-7’6 spE~2433

Problems have been experienced with dawnnole safety valves. The valves used

in tAe Suchan completions are Ccmcc ~~~=~-~~?, t~~ing retrievable a nc! the

failures have been attributed to mud solidslwell debris entering the inter-nal chambers curing completion and resulting in sticking of the moving parts.Two DHSVS failed in the open position necessitating early remedial action asthe UK Department of Energy reauires that wells must have an operable DHSVwnicn will close in case of damage to the Xmas tree. On another well the

sea 1s in the DHSV failed and we were unable to open it. In each of these

cases central of the well was re-established by wireline work involvinglocking open the tubing retrievable OHSV and installing a wireline retriev-able DHSV. One of these wells was a satellite well which necessitated bring-i ng in a semisub to do the wireline work - an expensive business. The two

others were template wells and the wireline work was done from the olat+orm.

The special Bucnan wireline BOP Stack is unfortunately rather large. Itweighs 37 tonnes and is cannected back to the surface via a two core riser.Because of the size of the BOP and the space restrictions in the moonpool

area, there is considerable risk ef ifip~ct with the productim risers, pro-duction has to be shut down and the riser bundle pulled before the BOP Stackcan be run. This causes significant production downtime.

In December 1982 we shut down to wireline a template well to insert a DHVS.Although the wireline work taak only two days we were out of production fromthe 8th December until the 16th February due mainly to weather deloying therunning and pulling of the wireline BOP Stack. Again, we shut down in August1983 to fit a wireline retrievable OHSV. The actual wireline work took 2days. Added to this it took 2~2 days to run and retrieve the BOP Stack, 6days to pull and run the riser bundle and there were 10 days spent waitingon weather. This was a total of nearly 21 days against 12 planned. We arecurrently looking at the development of a subsea lubricator assembly forwireline work which we can run and pull while the riser bundle is in place.

On onother satellite well we last control of the DHSV due to failure of thehydraulic control line below the tubing hanger. We had to bring in a semisubto kill the well, pull the tubing and recomplete the well. Because of theoverpressured nature of the reservoir it is necessary to use high gravitykill fluid of SG 2.09. Because we believe that the use of high solids mudwas the cause of foilure, through sticking, of other DHSV’S we used a solidsfree Calcium Bromide/Zinc Bromide fluid for this workover. This is veryexpensive and has been retained and reconditioned far future use i% required.The new tubing retrievable DHSV installed in this well is a Cameo TRDp-~Awhich we believe will be less subject to failure.

A three chamber saturation diving spread is installed on the platform toenable us to orovide diver access to the subsea manifold and template wellsfar maintenance and inspection.

MOORING AND LOADING OPERATIONS

it Was -A s--- ● L tkfit the l~ading operationreal is=u I I VIII Gsle Start . ..”. wouldweather sensitive and would probably be the most severe constraint

tional continuity and this has proved to be the case.

be veryon opera-

The tanker is moored to the buoy by a 225’ long 21” circumference nylonhawser fitted with flotation callars. A length of 3“ chafing chain is incor-porated at the tanker end of the hawser to take care of wear where the moor-ing passes over the bow of the tanker (Figures 7 and 8). Tension is measured

bv means of a load cell incorporated into the hawser stopper plug retainingassembly. It was initially expected that the tanker would be able to moor

71-77 SPE12433

IvP

wu

BUCltAN FIELD TANKERLOAOING FACILITV

940WOW

If LOAIIUIWI

CALM●lov

,,, , Lmm.–a .2”

PI?ELINE EMOMANIFOLD ,

1

‘“’:’’-7”\ 2,MHUW”““”’%s--4d./ :*,,.D..J-.....9--“

. ....-**’”””’““’‘-,,- ,.

.. Min-liw-%’iik”

WINKEN --,, ,OA,, N,, ,,ubELIGHTS

1*

/011 *LIH 10 I ANt. tll

CALM BUOV 6.1.1

“14

,,0S; EIUIMANOLIWG

UN( 1

?DtUIC.41L11 l* N#f HSUAEILW At4G81SM& EUSt. INIC,,A,,

FIGURE 7: BU(:tlAN TANKER LOADIING SYSTEM

in sea states of up to 3.Om (sig) and remain moored untilthe seas reached

This has not been Found to be the case with the TJanKera

._ -- !.- -- L-., + ““,, UVA!I=

4.5m(sig).

to unmoor when the seas reach about 4m and being unable tore-moor until

this has reduced to 2-2.5m. In practice more than just sea state has to be

considered and we use hawser tensions as the guide. Pr~duction is shut down

at a tensian of 75 tonnes, the hose disconnected at 85 tonnes and the tanker

Unberthed at a tensian af 100 tonnes.

FIGURE 8: BUCHAN TAN~ LOAOING SYSTEM

.— --— .- ..- :---.- ,-.. - _--- -..

a---z+----

.**’ : .- -- ,.. . . .

----- ---- —

11-79SPE1243S

... . . .

The nylon hawsers have given good service although in the first year wehad two failures cc indicated loads of far less than the nominal fcilure

laad of 540 tonnes. One hawser had been in use far five months while thesecond failed a;ter oniy ten cloys light usage. On inspection the manufac-turer ciaimed that the hawser snowed signs OF high cyclic loading whereas

the record of tension measurement in the tanker snowed normai loads. In

practice we changed out hawsers after three months usage in summer and one

month in winter - weather and o~portunity permitting.

3ecause of rapid wear of ‘L- “,. :*+1-ndldPmd Q ~~~~~Qn ofL[1c5 Ck1G7a Chaifi W= .,1.1 ------ stel-

lited chain which improved the life significantly. We now use a covering ofcobalt which has given a further improvement. We have also adaed 75’ doublenylon strop at the buoy end of the hawser which has improved the character-

istics of the string whilst the tcnker is mcred cd f~~i~ita~e~ hawser

changeaut. Further, because of doubts about the accuracy of thetension

existingmeasuring device we have been experimenting with a strain gauged

shackle in the chafe chain outboard of the bow of the tanker. While we have

had same results we have had problems with the signal line from the shacklewhich is subject to damage during maoringlunmooring operations.

As mentioned previously the loading buoy is an IM~DC~ designed CALM buoy and

at the time of its installation was one of the largest in use. Apart from

one major problem the buoy has performed generally os predicted although

access far regular maintenance such as greasing, bolt tightening, inspection,etc. is difficult, requiring sea states of less than 2m (maximum) - not aregular occurrence during the winter months.

In January 1982 we had a significant failure when the pad-eye, to which the

shackle on the buoy end of the hawser is attached, was torn a++ the end 0+

the mooring arm. In order to effect repairs the arm, weighing some 12%tonnes, had to be removed, taken ashare and then replaced. This caused a

production shutdown of some 47 days with bad weather delaying arm remova 1

and re-installation.

~~e ~~=y ,+=~ ~h~~~ ~~rn~, .moorin~, ~~~d~ng and a balance arm set at approxi-

mately 120 intervals. They are supported centrally by a 54” taper rollerbearing and the outer ends each have a pair of two wheeled bogies, which runon a track on the periphery of the buoy. These mooring arm bogies, which arebolted to the arm, incorporate a lug which impinges on the lip of the buoyto prevent the arm from lifting with the vertical angular pull of the hawserExamination subsequent to the incident indicated that the retaining boltsfailed resulting in the loss of both bogies. This in turn permitted the arm~~ e~~e in eent=et with the deck Qf the buoy where it became jammed against

a lifting pad-eye. The tanker, under the effect of wind and current, swunground and eventually took a horizontal angular pull on the end of the arm(for which it was not designed) causing the failure, just inboard of the

mooring pad-eye, of the 15” x 3“ steel arm under a load of some 40 tonnes.

The bogie retaining bolts which were of cadmium plated ASTM 07 steel wererecovered and appeared to have suffered brittle failure caused by hydrogendiffusion resulting from corrosion or possibly the cadmium plating. The

bolts now used are Manel 500 - unplated.

It is Considef-ed that the +Oliure 0+ ~~eseboits was ifiiticted by s,~ctdh:,mjof the hawser which im~osed vertical loads on the ‘oogies higher than antici-

pa ted and an improved, stronger restraint system has now been instailed onthe 8uchan buoy.

The i2° diameter fioat:ng hose used to convey the oil from the OUOY to thetanker is the part which one would consider most vulnerable to wave and boat

11-80

. . damage. However, on Buchan, apart from small leaks at flanges, we have neverhad o hose failure. Hoses are, however, changed out on an approximatelyannual ‘oasis, at a time wnen for other buoy maintenance reasons there is osuitable vessel available. We believe this success is due to the use of

nyion reinforced “kinkable” hoses which have no steel in their construction

and dc net buckle permanently.

We did however have same cancern about the performance of the ufid=FbueY

hose; especially the first two lengths beneath the buoy which bear the brunt0+ the bending maverrtent caused by the buoy mations. Because of this mostNarth Sea Operators change such hoses after two winters in use. The Buchan

buoy and hose system was installed in mid 1979 and because of this weattempted an underbuoy base changeout in the spring of 1981 prior to the

commencement of production. This attempt was frustrated by weather as was a

second try in September 1981 using a programmed shutdown. We then accepted

the risk of going through a third winter after consulting the manufacturerwho advised that the hose stood a gad chcfice d surviving but, without

data, could not be more specific. The hose was eventually changed in June1982 and appeared visually in perfect condition. This was confirmed by the

manufacturer ta whom it was returned for examination.

OPERATIONALEFFICIENCY

Forecasting of operationa~ efficiency ?o~ = systsm like Buchqn is made

extremely difficult by the effects of weather on the Platform and the Moor-ing and Loading system. Prior to production the downtime was categorisedinto three major items:

1. Effects af weather on buoy, tanker and riser operations.

2. Lost time due to tanker change-overs and repairs.

3. Lost time due to platform/subsea problems unrelated to weather.

There was no weather history for the Buchan location and the assessment had

to be made on the limited data available from Forties - about 55 km distant.An overall annual ~pereting tim ~f ~7~ ~~ arrived at with a minimum of 50%

in December, January and February and a maximum of 90% in June, July andAugust. In our first year of operation, kiay 19811MoY 1982, we achieved anoperating time of some 52% and in the second year this was 51%. These per-

centages are to some extent distorted by the shutdawn for buoy works inJanuary/February 1982 and the weather effects on wireline works in December1982 and January 1983. If these are discounted the efficiency improves tothe high fifties for two years when the weather was worse than the expectedaverage. We believe that the most we can expect to achieve from our existingsystem would be about 62-63%.

Because we did not meet our expected time efficiency and because of theproblems we have experienced with our subsea wells we” did not meet our---A..-6*-* +nr-nm+epT UUWG.*”1$ .“. ~---- In our first year we averaged 31,200 bpd and 30,700 bpd

in the second year.

RESERVOIR BEHAVIOUR

The reservoir has behaved generally as predicted. Monitoring has beencarried out on the platform using flowing and clased-in wellhead aressures

along with production test data. During the early part of production the

data collected showed a rapid decline in reservoir pressure indicative

11-81

... ..

of production from the fractures only. However, the rate of decline a~ainszcumulative ?roduction has reduced progressively thus indicating in-$low fromthe matrix. Material balance calculations have indicated that the estimatedfigure o; Sa million barrels for recoverable reserves remains realistic. We

have had water breakthrough fram two we~~= b~t dO flGt CC~S~=~~ ~ha~ this

will affect the ultimate recoverable oil.

FUTURE PLANS

There is to be one further major change in the Ouchan operation and thiswill be the installation of gas lift equipment to increase ultimate recavery.It is considered impractical to install this on location and the platformwill be brought inshore during the fourth quarter of 1984 for these works ta

be carriea out. The facilities comprise a gas turbine driven three stagecompressor rated at 15 mmscf/d with a discharge pressure o; 2500 psig. Thegas will be dried using molecular sieves and start up nitrogen will be pro-vided by a pressure swing absorption unit. Gas lift will of course imposethe requirement far eight additional risers for gas injection to the wells.These will be of 2“ diameter and will be attached “piggy back” fashion tothe existing production risers. Careful design work is required in thisarea to minimise the additional time requirement for pulling and runningrisers.

Wells are completed with two side pocket mandrels at 1355 and 1370 mss inwhich a single orifice type gas lift valve is installed. This simplifiedoppraach is designed to eliminate the need for wireline servicing. It doeshowever require a very high kick-off pressure, resulting in o cafnpressardischarge of 2500 psig.

It is expected that the use of gas lift will increase the recovery from thereservoir by about hn STBO to 58 mm STBO.

confusions

In papers like this it is much easier and perhaps more interesting to des-cribe the problems and their solutions than the successes.

Nevertheless we believe Buchan to be a success and we have learnt much fromour first two and a half years of operation. Despite the development beinglate in commissioning, running well over budget and production targets notbeing met, the operation is viable and profitable. In retrospect it couldwell be that a purpose built platform would have been better than a conver-sion and a loading system less sensitive to weather conditions would haveimproved time efficiency. The cost of accessing subsea wells must also bereduced by some means. It must be added, however, that the capital cost ofthe project was recovered less than two years after commencement of produc-tion.

ACKNOWLEDGEMENTS

The author wishes to thank British Petroleum ?!-C and its Partners in the

Buchan Fieid Joint Venture, Cinarterhall Oil Limited, Clyde petroleum PLC,

Gi3AL Petroieum PLC, Lochiel Exploration Limited, St. Joe Petroieum (UK)Corp. , Sulpetro Limited, Texaco North Sea UK Company, Transworld petroieum

(UK) Limited and Tricentrai Oil Corporation pLC for permission to producethis gaoer.