OTe 6255

Don Field Subsea Development: UmbilicalStrategy and Practical ExperiencesJ.H. Hall, J.J. Campbell, and P.G.H. Shaw, BP Exploration

Copyright 1990, Offshore Technology Conference

This paper was presented at the 22nd Annual OTC in Houston, Texas, May 7-10, 1990.

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper,as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflectany position of the Offshore Technology Conference or its officers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. Theabstract should contain conspicuous acknowledgment of where and by whom the paper is presented.

The wells are arranged in a cluster around amanifold. This enables the wells to be drilledfrom one mooring position and allows simultaneousproduction and drilling and workover. Each treestructure incorporates a valve package containingan additional wing valve designed to take the flowcutting action on tree shut down and a choke. Acontrol module to control and monitor each well isalso located on the tree adjacent to the valve

The plan finally adopted was to develop the fieldusing subsea production techniques tied back tothe Thistle Platform. Because of the nature ofthe reservoir particularly its multiple faults anduncertain reservoir characteristics, thedevelopment was to be phased.

Two production wells and a water injection wellcomprise the first phase. All three wells arelocated at the Don NE area. The second phasepotentially requires up to a further seven wellsat Don NE area and three wells at the Don SW area.However, the detailed requirements and theirlocation will be better determined after furtherreservoir information has been gained from theearly phase 1 production experience. Both theproduction wells incorporate downhole pressure andtemperatures gauges to assist in maximising thisinformation.

485

References and figures at end of paper

INTRODUCTION

ABSTRACT

The paper also addresses, in particular, problemsencountered with the design, manufacture andinstallation of the electro-hydraulic controlumbilical and their successful resolutionfollowing an extensive test programme.

The Don field is situated in Block 2ll/18a about15 km north of BP's Thistle Platform and is in 170meters water depth. The field is extensivelyfaulted and consists of two main accumulations.These are denoted as Don NE and Don SW with theformer having potentially the more prolificreserves. The reserves for both areas wereestimated as 56 mmstb. Figure 1 shows thelocation of the field.

The Don field was developed as a subsea satelliteof the Thistle platform and is currently thefurthest from the host platform (17.5 km) in theUKCS. The paper presents the umbilical strategyadopted for the first phase of the fielddevelopment.

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DON FIELD SUBSEA DEVELOPMENT - UMBILICAL STRATEGY AND PRACTICAL EXPERIENCES OTC 6255

In addition, there is often a requirement todesign the umbilical to minimise thecross-sectional area. This may be because oflimitations in the J tube diameter or handlingcapacity limitations of transportation or layvessel reels.

The resulting umbilical will then be nearer to anelectric cable with copper being the predominantelement. The longer the umbilical, the more thecopper content per unit length will increase tocompensate for voltage drop. The configuration ofsuch an umbilical will be such that the copperconductor elements will not form a central corearound which the hoses are laid as would be thecase for a dynamic umbilical used for example on aFloating Production system.

2 screened signal pairs;6 electrical power cores (3 phase);2 hydraulic hoses.

DESIGN ASPECTS OF ELECTRO-HYDRAULIC UMBILICALS

Power conductors, signal pairs and hydraulic hosesof different diameters and with differentelasticity properties are then bundled togetherusing either a planetary machine resulting in ahelical lay-up or an alternateclockwise-anticlockwise method known as S-Z. Thislatter method has a distinct advantage in that itcan lay-up longer lengths without joints becauseof the size limitations of cable drums which canbe accommodated on planetary machines.

In addition to the components having differingelasticity, the back tensions of each component asthey are transferred from the reels or drums tothe lay-up machine may be unequal.It is not usual to design an electrohydraulicumbilical which is to be used for subsea wellcontrol to withstand the dynamic stressesassociated with floating systems. Such dynamicumbilicals would have a very high degree of radialsymmetry and the lay-up angles for all conductorscables and hoses would be high to give thenecessary flexibility. They would therefore havea larger diameter and would occupy much moremachine time in manufacture than the equivalentstatic product. As a result, they would betypically 50% more expensive than an umbilical

There is a tendency to request duplication of theelectrical components, both power and signal, inthe umbilical. In the extreme, this can mean anumbilical containing:

injection into the wells and manifold had not beenused previously in thermo-plastic hoses. Althoughchemical compatibility testing was carried out toverify the suitability of the hose liner fortransporting the specific chemicals (andalternatives) there was always the risk that anincompatible chemical could be introduced over thefield life leading to failure. In addition, theaccelerated ageing tests in which the hose linermaterial was tested with the chemicals at elevatedtemperatures, although satisfactory, cannot beconsidered a fully guaranteed guide to long termdeterioration. They are useful in that they willidentify most incompatibility problems in areasonable time.

486

package. The valve package and the control moduleare independently retrievable without having torecover the tree.

Control functions were incorporated in asingle electrohydraulic umbilical. Thisumbilical was designed to provide hydraulicand electric power and signal transmissionfor up to 10 wells and control modules atthe NE area and for extension to 21.5 km toaccommodate 3 wells and control modules atDon SlY area.

The control umbilical and its subseatermination were designed so that theumbilical could be retrieved, quicklyreterminated and re-laid.

The chemical umbilical was designed toaccommodate phase 1 only. It would beretrieved, cut, reterminated and re-laid toservice the Don SlY area. A new chemicalumbilical would be installed for Don NE.

Separate control and chemical umbilicals werechosen because of the possible incompatibilitybetween one of the injected chemicals and the hoseliner. Such an incompatibility could then resultin a chemical attack of the critical elements ie,hydraulic hoses and electric cable insulation withthe subsequent loss of control and a total fieldshutdown. At the time this umbilical strategy wasbeing determined, there were reports in theindustry of umbilical hose failure due to chemicalattack. Three of the chemicals designated for

The umbilical strategy adopted for the first phaseof Don may be summarised as follows:

The cluster arrangement proposed for both phasesis shown in figure 2. The development is alsodescribed in more detail in ref 1.

The umbilical was designed with a minimumnumber of joints in cables and hoses and hadno externally apparent joints.

In order to derive maximum economic benefit from aphased development, Phase 1 equipment wasgenerally limited to that required for the initialstage of the development only and was not extendedto accommodate flexibility initially for futuredevelopment. However, there were areas where theexpenditure of very little capital in an earlyphase was seen to result in significant savings inlater phases and it was recognised that theumbilicals satisfied this criteria.

UMBILICAL STRATEGY

There are a variety of ways to approach a phasedexpansion in two separate areas and a number ofumbilical solutions. The installation of ajunction box in the umbilical to accommodate thetie-in to a second area and the provision ofseparate hose umbilicals and electric cables toeach area are just two examples. A balancebetween minimum initial cost, capability forre-use in phase 2 and long term reliability mustbe made and the weighting against each of thesethree criteria must always be to some extentsubj ective.

2

OTe 6255 HALL, CAMPBELL AND SHAW 3

designed for static use. For long umbilicals suchas those required for Don, a fully dynamic designwould prove very expensive and is in any caseunnecessary provided the design allows for normalinstallation stresses.

The project specification required duplication ofany system which would shut down the whole fieldand so the umbilical functions were duplicated:

After about 2 km had been installed,operational difficulties with the trencherwere experienced. Weather conditionsprevented immediate recovery. The umbilicalwas subjected to flexing in the length ofumbilical in catenary during this period.

After 11 hours' continuity in one of thesignal pairs was lost.

MANUFACTURE

INSTALLATION

Torque balance test under loads to 10tonnes.

The constraints placed on the contractor resultedin the cross section shown in Figure 3.

The 2 km of abandoned umbilical wassubsequently retrieved from the J tube andseabed and returned to the factory along withthe unlaid umbilical.

UMBILICAL EXAMINATION

After about 3 days, continuity was lost inthe 35 mm2 power cores. Wind speeds wereabove 60 knots with significant wave heightsof 12 metres. The umbilical was cut andabandoned to save the vessel.

The signal pairs exhibited occasional kinking inthe conductors. Sometimes this kinking wasclearly visible through the conductor insulation.The breaks in the conductor strands were always atone of these kinks. Figure 6 shows an X-ray of asignal pair, the break and other kinks are clearlyVisible. Broken conductors are difficult todiscern electrically because of the rubberinsulation keeping the two broken ends in contact.

The examination of samples of umbilical from anunlaid section and a spare section which had notbeen loaded onto the vessel showed kinks in thesignal conductors. There were no breaks.

On return to the factory, the location of thesignal core failure was determined using pulsereflection techniques and then selectively cutting10 m samples from the umbilical removing thearmour and examining the cores, initially usingX-ray techniques. The examination of theumbilical was undertaken over several weeks. Theresults are summarised below.

Weather conditions improved to allow thetrencher to be recovered and repaired.However, before it could be re-deployed, theweather worsened considerably, exposing theumbilical in the catenary to even greaterdynamic forces.

There were no kinks in samples of unarmouredumbilical.

The kinks in the signal conductors occurred withvarying severity and varying intensities; theywere generally to be found where the lay directionreverses and the umbilical elements are co-axialwith the umbilical axis.

Evidence of kinking of the 35 mm2 power cores wasfound only in the section of the umbilical whichhad been subjected to extended time in thecatenary. No kinks were found in unlaid ornormally laid umbilical. Examples of kinked andbroken power cores are shown in figure 7. Therewas some evidence to suggest that the kinking inthe power cores had occurred predominantly in thelower region of the stationary catenary.

areaarea

35 mm2 c.s.6 mm2 c.s.9 mm i.d.

12 mm i.d.

4 power conductors2 screened signal cables2 HP (7,500 psi) hoses2 LP (3,000 psi) hoses

During the installation, hose pressures andelectrical continuity were continually monitored.The umbilical was pulled onto the platform and thelaying/trenching process started satisfactorily.The ensuing events are summarised as follows andthe weather during the period is described infigure 5.

Determination of cable characteristics R, L,C &G for the signal and power conductors.

Testing comprised the usual industry standards forboth qualification and routine tests on the hosesand the cables. Tests on the final umbilicalincluded:

Cold bend tests in which a sample was cooledto -150 C and bent 4 times through theminimum bend radius and then tested andpartially stripped down for examination.

Flow rate and pressure response tests on thehoses (fig 4).

The control umbilical was laid up and armoured asone continuous length of 18 km. All internaljoints were at specific locations. This wasachieved by making all the hoses in 9 km lengthsand the electric cables in 4.5 km lengths. Alljoints and splices were factory made so theumbilical lay up was a continuous process for thetotal 18 km length.

The umbilicals were reeled on to the lay vesselDeepwater 1 and were to be simultaneously laid andtrenched with lay being initiated by pulling theumbilical up the riser and laying away fromThistle to Don.

Quality control inspection was extensive duringall stages of the umbilical manufacture, from theextrusion of hose liners and electrical insulationof the individual conductors to final electricaltests after the umbilicals were loaded onto thevessel.

487

4 DON FIELD SUBSEA DEVELOPMENT - UNBILICAL STRATEGY AND PRACTICAL EXPERIENCES OTC 6255

The conductor fractures were clueto stress fatigue catenary tension decreased from about 1900 kgf atfractures and not tensile fractures. the vessel to about 160 kg at the seabed. The

pressure near the seabed will induce both radialSIGNAL CABLE FAILURE AND UMEILICAL DESIGN and axial compressive forces on the umbilical

core. It is possible that in this region the coreIt can be seen that the central hose is not becomes detached from the armour and that theconcentric with the umbilical. This will cause no internal components which will be in compressionproblems unless during the lay up it remains support the umbilical with the compressive forcesstraight with the other elements laid round it. the same as the residual catenary tension. These

effects, coupled with the dynamics imposed byIn this case the form of the laid up bundle would significant veaael motion, could account forhave slightly cork-screw-like or convoluted form severe cyclic compressive forces being applied towith different lengths of each laid up component the umbilical internals in the lower regiona offor unit length of umbilical (Fig8). When such a the umbilical catenary which must have occurred to“wavyttconstruction iS subsequently forced into a cause the observed damage of the power cores.cylindrical form as it will when armoured, therelative lengths of each component will change TESTINGresulting in significant compressive forces. Inthe case of Don umbilical which was laid up using It was not clear at the time whether a completelyalternating directions or S-Z lay, the armouring new control tunbilicalwould be required or whetherprocess could have introduced compressive forces the 15 km which had not been laid could be used.in the signal pairs. If the central hose were An immediate test programme was initiated in whichperfectly straight then with a lay length of 700 samples of the armoured umbilical were flexed withmm the signal pairs would be 0.87% longer than the no tension. The S-Z change over points of thepower cores. If the core is then forced from a signal pairs were positioned on the outside orconvoluted form into a cylindrical form about the inside of the bend so that they would see maximumcentre of the umbilical, the signal pairs would be strain.shorter by 0.3%. This represents a strain of1.18%. These forces were possibly of a sufficient The object was to see how many cycles of strain amagnitude to make the signal pair conductors kink. kinked signal conductor could accept beforeThe kinks would be expected to occur in the region failure in an attempt to determine limits onwhere the S-Z lay up changed direction and the laying conductors. In essence, it was establishedconductors were co-linear with the umbilical. that’cyclic strains of more than 0.3%, equivalent

toa7- 8 meter bend radius, over 40,000 cyclesAlthough such a mechanism could explain the would cause the conductors to fail. However, withproduction of kinks, there may well be others. strain levels of around 1%, the failures occurredThere were no reports from QC inspectors of any after a few hundred cycles. The cycling frequencyconvoluted central core. However, the effect was selected to reflect the anticipated wavewould be small and was not specifically looked frequency.for. Helical lay up would not necessarily preventthe formation of kinks resulting from compressive In parallel with the testing of the unlaid

forces; the kinks would occur randomly since there umbilical, a new 1000 metre length was

would be no preferential locations. manufactured using a variety of laying-up andarmouring parameters. Each new combination was

~OWER CORE D= tested by bending sample sections through bendradii up to 7 metres. There was no significant

The kinking of the power cores did not appear to improvement in the cycles-to-failure figures for

be the result of manufacturing or design. None of any of the samples when compared with the original

the samples from unlaid umbilical showed any sign umbilical.

of irregularity. The kinking and subsequentfracturing occurred only in the section subjected REPLACEMENT UM81LICAL

to catenary laying forces when the vessel wasstationary and subjected to rough seas. As can be A replacement length using the more conventional

seen in figure 7, severe and repetitive planetary lay-up method was manufactured. A four

compressive forces must have occurred during this kilometre length was made and samples taken which

period to cause 35 mm2 conductors to kink and were then flex tested and stripped down. None of

ultimately fracture in this way. The kinks and the samples failed after 50,000 cycles of flexing

fractures all occurred at the S-Z change over through a bend radius of 7 metres. On stripping

positions. the samples, kinks were found in the signalconductors. However, the kinks appeared to be

The exact cause of these compressions has not been fewer in number than in samples taken from thefully established, For flexing to have been the original umbilical. They were also randomlycause, the catenary must have been moving around distributed, this is to be expected because of thequite markedly or the umbilical must have been homogeneous nature of the planetary lay-up.bent at the plough entry point. Observation from Samples from this umbilical for flex testing couldROV during installation did not reveal any severe not of course be oriented so that any kinksbending or flexing either in the catenary or at present would be in the position to receivethe plough entry. Indeed, the bend radius at the msximum strain. It was therefore not possible,seabed with the 50 metre stand off distance was without testing many samples, to conclude withabout 12 meters resulting in strains of only0.14%.

certainty that the planetary lay-up was betterOne possible cause is the effect of than the S-Z form. However, because there are no

hydrostatic pressure on the umbilical core. The areas more susceptible to strain than others, it

OTC 6255 HALL , CAMPBELL AND SH.AW 5

is likely that any kinking should be less severe. tensions during the lay-up process are also veryIt did not appear to be possible to make important in ensuring a cylindrical form free fromdefect-free umbilical of this particular design convolutions. If project timescale permits, ausing alternating or S-Z lay-up. sample of the designed umbilical should be made,

RESOLUTIOarmoured, flexed and stripped down to prove the

N OF THE PROBLEM design and manufacturing process.

The original umbilical was re-completed using two The effects of dynamic forces due to vessel motionnew sections. One section, some 2 km in length, which occur where there is little residual tensionwas joined to the existing umbilical to form the in the umbilical and where compressive forces duesubsea end which would be laid at the manifold.This is the end which may be recovered for

to hydrostatic pressure acting on the umbilicalcore become significant are not fully understood.

regermination at a later stage and would therefore There is clearly scope for modelling the processesbe subject to further stress. A 600 m length was involved so that all forces on components can bejoined to the other end of the umbilical and so predicted,would be the length to be pulled up the J tube.These lengths had the esme geometry as the The distance between the touch-down point of theoriginal design but were made using the planetarymethod. All hydraulic joints and electrical

umbilical or plough and the vessel should be suchas to give a residual catenary tension which

splices were made in the factory and tested. prevent% the core from experiencing compression.

During installation,A stand~off distance of about 110 metres would

the distance between the have given a tension of around 500 kg at thetouchdowm point and the vessel was increased from plough.50 metres to about 100 metres to increase thetension in the catenary at the seabed. Previous experience of umbilical installation in

the North Sea would indicate that the poorThe principle of simultaneous lay and trenchingwas discontinued.

reliability of trenching equipment and predictedThis minimised the time the

umbilical was exposed to catenary laying forces, weather windows combine to make simultaneous layand trench operations over long distances risky.

The umbilical was successfully installed and Umbilicals longer than say 3 to 4 kilometres

trenched using this modified procedure and to dst’ should be laid and subsequently trenched.

is working to specification.

CONCLUSIONS MKNWLEDGMENTSo

It will always be difficult to design a The authors express their thanks to BP exploration

symmetrical electrohydraulic control umbilicaland Don Partners for permission to present this

because of the high copper conductor content. paper.

Ideally, copper conductors should form the centralcore with hoses surrounding. In general, this

~EFERENCE$

cannot be achieved particularly with long1.

umbilicals where for voltage drop considerations,Stoddard B and Campbell J J: “Don - A Cost

the conductor size is substantial. Hose and cableEffective Approach to Subsea Design,” paperpresented at Offshore Europe 1989

...40Y

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Figure 1 Don Field Location Plan

~ Thistk

1

Fiowlinos And Umbilicals

On. x 0“ Productla L!n.

/\Onc x 8- Water Injection Lino

On. x Control Umbilkal

L. Y“’”mbiika’‘\

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Don N.E

Msnifold And Three Wolk ‘)

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Phasa 1

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T- kd!160m

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Manifold Rolocatod

From Don N.E

Ton WON* Maximum\

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Figuro 2 Don Sy8tom Configuration

2-112” HOSOS.(Low Pros*uro) \ ~

2-4mm2 8cro

Signal P

2-S/8” Ho

(High Pros

4-36Powor Coros

Umbilioal Diamotor 88mrw

Umbllioal Woi@M (Filled)

In Air : 15.8KQIM

In Water: 10.2K9/M

250 [

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200 :-\,,, 4

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s 150.\..,,

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+:,k Actusi

Rosporr$o50 ~ \ i

o ~Jo 200 400 600 – 800 1000

limo (8)

Figuro 4 Prossuro Rospon80 Of 1/2= Hoso 17.8Km

In Lwtgth With Water Baaed Hydraulic Fluid

Figuro 3 Eloctro Hydraulio

Control Umbilkai

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l--~—Op.ratiorrm 8.sp.nd.d

hrstaliation 8tartod

Figuro 5 Woathor And Events Chmrt During Installation Of Controi Umbilloal

491

Figure 6 Radiographs Of Umbilioal Elements

Showing Kinks And Breaks In Signal PairsFiguro 7 Radiographs Of Umbilical Elomonts

8howing Kinks And Brcsk8 In S5mm Powor Coroo

Armour

/

Hydraulic Lines 1,

\Signal Pairs \ )1

Power Cores /

Figure 8 Convoluted Umbilical Lay-Up Resulting From Non Central Component

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