OTC 11025

The Siri Production Jack-up PlatformMorten Brerheim, Statoil; Dirk Manschot, MSC; Torgeir Olsen, KOGAS; Henning Eide, Enterprise Oil

drilled in August 1996 on Siri Central. The two wellsgave the basis for establishing the reserves for the fielddevelopment studies. A third appraisal well was drilledon neighbouring Siri East proving non-commercialreserves in that area.

Copynght 1999, Offshore Technology Conference

This paper wes prepared for presentation at the 1999 Offshore Technology Confarence held inHouston, Texas, ~ May 1999

This paper was selectad for presentation by the OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s) Contents of the paper, aspresented, have not been reviewed by the Offshore Technology Conference and are subject tocorrection by the author(s) The material, as presented, does not necessarily reflect anyposition of the Offshore Technology Conference or its officers Electronic reproduction,distribution, or storage of any part of this papar for commercial purposes without the writtenconsent of the Offshore Technology Conference is prohibited Permission to reproduce in printis restricted to an abstrect of not more than 300 words; illustrations may not be copied Theabstract must contain conspicuous acknowledgment of where and by whom the paper waspresented

Abstract

The Siri field is located in 60 metres water depth in theDanish sector of the North Sea. It was developed by apurpose built three legged jack-up standing on the top ofa steel storage tank. The tank was installed in May 1998and the jack-up was installed six months later.

A "fast track" field development strategy was establishedin February 1996. Screening studies of alternatives weredone, and the first feasibility study of what later becamethe selected field concept was initiated in July 1996.Conceptual studies linked with competitive bidding withthree contractors for a full EPCI bid for the completeplatform including an oil offloading system wasconcluded at the end of December 1996. After somefurther development work, the EPCI contract was signedwith Kvaerner Oil and Gas (KOGAS) at the end of March1997. The storage tank was installed medio May 1998,after which drilling started with the Noble GeorgeSauvageau jack-up drilling rig operating in cantilevermode. The 8700 t jack-up production deck was installedmedio November 1998 after 7 weeks waiting onweather. This gives a time span of 18 months fromsignature of EPCI contract to deck ready for towage,which is considered to be very quick for a project of thissize.

The reservoir development requires a maximum of 12well slots, and these will be placed inside a wellheadtower connected to the side of the storage tank andhinged to the deck

This paper will give a brief technical description of thefield development and highlight the challenges in designand installation with an emphasis on the steel structures

The Siri field

Introduction

The licence for exploration and production of the SiriCentral! North structures was given medio May 1995 tothe Statoil Group. The group consist of the following

companies:

The Siri field is located in block 5604/20 in the north-western part of the Danish sector of the North Sea,some 220 km from the coast line. The field is marginal,and various development options were investigated inthe middle of 1996 to find an economically attractivesolution. These included:

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Statoil E & P NS (operator)Enterprise Oil Ltd.Dansk Olie 09 Gasproduktion NS

Phillips Petroleum CompanyDenerco K/S

Jacket full process platform with storage tankerConcrete full process platform with storageCombinations of steel and concrete solutionsCombinations of wellhead and process platformsFPSO ship with sub sea wellheadsConverted jack-up platform with storage tankerNew built jack-ups with storage tanker

The first exploration well in Siri Central was spudded atthe end of November 1995, with a second appraisal well

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The infrastruc:ture around the Siri field is limited, and themost attractivle solution for oil export was via a shuttletanker. In august 1996, the concept of a new built jack-up standing on a steel or a concrete storage tank with awellhead towE~r extending from the tank to the deck waschosen for further study. This is shown in figure 1. Thewells are drillE~d by a jack-up rig operating in cantilevermode. Well maintenance is to be performed with drillingequipment (coiled tubing or snubbing) lifted onboard byplatform cranE~. A 400 m2 deck at the end of the platformtowards the wellhead tower is provided for this purpose.

with fire and explosion walls separating the three mainareas. A flare tower of 96 metres height is installed onone corner of the platform. The height is dictated by thecloseness to the drilling rig. The main hull of the deck is50 m wide and 60 m long. The height is 6.7 m. A "fork"structure at one end of the deck connects the wellheadtower to the hull. The fork arms extend 12 m from theside of the hull and support the well manifolds. The livingquarters are cantilevered out some 7 metres on theopposite side of the platform.

This concept 'Nas further evaluated through the winter1996 / 1997, and a contract for engineering,procurement, construction and installation (EPCI) wassigned on the 21st of March 1997 with Kvaerner Oil andGas (KOGAS:~.

Design data

The main data for the field development is given in table1 below.

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Figure 1. The Siri platform

The deck is supported by three tubular legs of 3.5meters diameter. The legs stand in 13 metres deepsleeves in the tank structure. The gap between the legsand the sleeves are grouted to provide moment restraintbetween the legs and the tank. The grout is not designedto carry vertical loads from the legs.

Platform desc:ription

The Siri jack-up is a fully integrated platform containingwellheads, process equipment and living quarters. Thedeck is shown in figure 1. The deck layout is traditional

The jacking system is a hydraulic ram pin-in-holesystem. Three jacking units are placed around each leg,

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OTC 11025 THE SIRI PRODUCTION JACK-UP PLATFORM 3

as shown in figur'e 2. Each jacking unit contains twohydraulic rams operating on a beam which houses onejacking pin. The three beams are welded together suchthat they form a ring. There is an upper stationaryholding ring that are connected to the top of the jackhouse and a mo\l'able lower working ring around each

leg.

systems around each leg fail. After the deck was jackedup to its correct height on the Siri location, the height ofthe upper ring was adjusted vertically to coincide withthe jacking holes on the leg. The hydraulic cylindersholding the lower ring was shimmed off with steel clampssuch that the hydraulic pressure could be bled off. Eachleg is hence connected to the deck by six pins; three inthe upper ring and three in the lower.

The moment between the leg and the deck aretransferred by circular steel guides to the upper part ofthe jack house and the lower part of the hull. The jacksare supported on rubber bearings to prevent the legmoment being transferred to the jacking system.

The tank is 50 by 66 by 17.5 meter and has an effectivestorage volume of 50000 m3. The bottom of the tank hasskirts of 1.6 and 2 meter depth. The skirts divide theunderside of the tank into compartments and theseassist in transferring the horizontal forces to the bottom.Suction between the seabed and the bottom of the tankwas used to penetrate the skirts, after which theremaining space between the seabed and the tank wasfilled with grout.

The seabed consisted of 5.2 meter of sand above hardclay. The sand was not strong enough to carry thehorizontal forces from the waves and wind, and wastherefore dredged away to simplify the foundations of thetank. The effect of dredging was twofold, it reduced theskirt length by 5 metres and it reduced the effect of thetank on the wave height. When a wave passes a largeobject on the seabed in shallow water, there is a build-upeffect on the wave height. With the top of the tank 12.5metres above the seabed, the 100 years extreme waveheight is calculated to increase by 5 % as the wavepasses over the tank. This value would have beenhigher without dredging.

Benefits of the Siri jack-up solution

During the evaluation of the various field developmentscenarios, it became apparent that the jack-up concepthad several benefits that would assist in reducing thefield development costs:

2.

I=igure 2. The jacking system for one leg

3.The jacking sys,tem has a built in redundancy such thatthe deck still can be jacked if one of the three jacking

The jack-up solution allows substantialcompleteness of the deck structure in the yardthereby reducing the offshore hook-up.The platform installs itself. The cost of an expensivelift vessel is avoided.The combination of a jack-up standing on a storagetank opens up the possibilities of using circulartubular legs since the legs are shorter than without

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cost impact was reasonably small, and it showed thatthis jack-up concept has good tolerance for weightincreases when identified at an early stage of the design.

4.

5. The wellhead manifolds and the "fork" structure

The wellhead manifold structures are placed on two"fork" structures at the bow of the hull, one at each sideof the wellhead tower, as shown in figure 3. The forksalso provide the connection between the wellheadcaisson and the hull. The forks extend some 12 metresfrom the side of the hull. The forks are extensions of thedouble bottom, having a height of 1.4 metres.

6.

7.

the tanl< and the tank gives a fixed bottomconnection for the legs. The circular legs aresubstarltially cheaper than similar steel truss legs.The cir(~ular legs can use a simple and cheap

jacking system.The cir(~ular wellhead tower protects the wells fromwave forces and allows standard dry wellheads tobe use(~.The tanlk with wellhead tower can be installedbefore 1:he deck to allow pre-drilling of wells.All oil piping (to and from the tank and export), seawater lilnes, risers and J-tubes are located insideeither the wellhead tower or the legs. This gives asubstarltial reduction of wave forces on thestructure.The hull is large in area and volume, but cheap tofabricate.

'"opsides d.~sign considerations

The topside~; equipment is placed in small modules of upto 500 t on top of the jack-up hull. The hull is in generalrelatively lar!~e allowing ample space for the topsidesequipment. lrhe hull size is given by a combination ofstrength and buoyancy requirements. The deck spaceitself is hence provided at a relatively low cost.

The initial philosophy was to limit the equipment insidethe hull. With a hull size of SOm by 60m and a hull heightof 6.7 m, it soon became apparent that the hull volumewas too attralctive to be left empty. The hull now housesdiesel and water storage, electrical rooms, generalstorage, venltilation and communication rooms. Thisresulted in a high number of penetrations through thehull, and this gave two major challenges:

Figure 3. The forks. The wellhead decks are inthe middle and the two forks structures with wellmanifolds are on either side of the wellheaddecks.

.The penetrations were a hazard during tow out, andextensive effort was spent in ensuring a watertight hull..Gas ingress into the hull during operation is a veryimportant de:sign condition, and this has affected bothdesign and operational procedures.

The wellhead manifolds are designed with a minimumheight to allow the cantilever of the drilling rig to passover it during drilling. The manifold structures at theiroperational position are therefore marginally higher thanthe hull. This posed problems both for the fabricationand installation phase. For the major part of thefabrication and the tow out phase the draft of the deckwas some 3.7 meters, which means that parts of themanifolds would be below water during both thesephases. In addition, they would in the worst case besubject to wave forces from a 10 year seasonal stormwave or, at best, to forces from transport conditionwaves and the bow run-up effect for towage. This wasclearly not acceptable. Various alternatives weretherefore investigated for protection. The chosen

The weight of the hull was initially estimated to be 7300t. During the early phases of the project it becameapparent that this weight would increase. Based onexpected changes, the new weight limit for jacking wasset as 9300 t, and an active weight control system wasintroduced. The deck, jacking system and legs weredesigned ba~;ed on this weight and a maximumoperational ~(eight of 11 OOOt. Even though the predictedweight at times during the design phase exceeded thedesign weighlt, at the"time of installation the deckweighed 8700 t. The increased weight had small effectson the amount of steel in the hull and the legs. It did,however, result in an upgrade of the jacking system. The

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THE SIRI PRODUCTION JACK-UP PLATFORM 5OTC 11025

2. Static strength due to global axial loads andbending moments due to deck weight and environmentalforces.

3. Fatigue in the circumferential girth welds or in theparent metal at the jacking holes.

solution was to hold the 450 t manifold structures 8metres above the fork and sea fasten them on theadjacent fire and explosion wall. After installation of thedeck, the manifolljs were lowered down to their finalposition by continuos strand jacks. The jacks weresupported by columns running through the main verticalmembers of the manifold structures down to the fork.The columns wer,e free standing and the sea fasteningwas connected directly between the manifolds and thewall. The advantage of this solution was little additionalweight and cost. The disadvantage was that all thepiping between the manifolds and the deck had to bewelded offshore, thereby increasing the offshore hook-up and completion scope considerably.

The minimum yield strength specified for the legs were690 MPa, and the steel was delivered with an actualyield strength of nearly 800 MPa. This extra strength isbeneficial for the yielding in the holes and the staticstrength conditions. For the fatigue conditions, however,this is negative. CTOD tests of the heat affected zoneshowed that there were areas of low fracture toughnessin the heat affected zones of the welds, and it wasshown that very little fatigue crack growth waspermissible before a brittle fracture would occur.

Extensive fracture mechanics and crack growth analyseshave been performed complimented by laboratorytesting of the parent and "as welded" Siri materials. Theresults of this show that the fatigue life is satisfactory,but not as long as aimed for. This means that the legswill have to be inspected at an earlier stage of theplatform life than originally planned.

The connection between the hull and the wellhead towershould give a pinned horizontal translatory connectionand at the same I:ime give vertical and rotational freedomfor the deck. The rotational freedom was necessarysince the wellhead tower is a lot stiffer in rotation thanthe legs are in translation, such that the tower would beoverloaded in torsion if the rotational connection wasfixed. The initial s,olution to this was a steel to steelsliding connection. Soon after installation it wasdiscovered that this quickly wore. The steel to steelconnection could also give sparks when moving. Sparkshas actually beerl observed when dark, and sparks asan ignition source are obviously not acceptable in awellhead area. An alternative solution with two pinnedstruts, one in the longitudinal and one in the transversedirection, was chosen. This provided a cheap solution toan otherwise complex problem

It is the circumferential welds that are critical, especiallywhere these are linked with thickness transitions andhigh bending moments. These locations are just abovethe tank top and just below the lower guide in the hull.

The platform le£ls

The inspection of the legs is a challenge in itself. Eachcircumferential weld is 11 metres long, and the outersurface is ground smooth, so there is no weld cap toidentify the presence of the weld. The grinding of thewelds were done both from a fatigue point of view andfrom the need for a smooth outer surface of the legswhen passing through the guide rings of the jackingsystem. The requirement for smoothness also resulted ina requirement for constant outer diameter. All thicknesstransitions are hence made on the inner surface of thelegs. Due to the local bending moments arising from theeccentricities at thickness transitions, the highestdynamic stresses in the legs are therefore occurring atthe inner surface of the legs.

The legs are 104 meter long, have an outer diameter of3.5 meter and WE!igh some 800 t each. The wallthickness of the legs varies from 65 to 110 mm. Thelower 27 metres of the legs are without holes for thejacking system and are made out of 390 MPa steel. Theremaining parts of the legs have 460 mm diameterjacking holes spaced at 1750 mm and are made of veryhigh strength. The high strength steel was mainlychosen due to the high contact loads from the pins in thejacking system. Each leg was installed as a complete legwith some piping inside while the deck was afloat at theRosenberg yard in Stavanger. Two floating shear-legcranes were uselj simultaneously for this spectacular

operation.

The dimensions lot the legs are governed by threedesign conditiom):

The outer surface of the leg is coated with hot sprayedaluminium, and the inner surface is painted. Normal MPIat the outer surface will therefore be hindered by thealuminium cover, and access to the inside of the legs isrestricted due to equipment placed inside the legs. Evenif it in principle is possible to inspect the legs withtraditional MPI, the practical problems of access, weldlocation and surface protection makes it virtuallyimpossible to inspect the legs by traditionally acceptedmethods to a high standard of quality.1. Yield in thl3 jacking holes due to concentrated

loads from the jacking pins

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weather conditions, and rubber dampers were thereforeinstalled at the bottom of the legs to dampen the impactforces. This raised the permissible sea state duringinstallation to approximately 1.0 m Hs for wave periodsless than 7 seconds and somewhat higher sea states forlower periods. The motions of the deck in wave swellconditions were seen from model test to be critical, andthis was also experienced during installation.

The actual length of the inspection intervals is not yetdefined. The method of inspection is also not yetdefined, but the aim is to establish a mechanisedmethod that can inspect both the inner and outer surfacefrom the outside of the leg from a control centre abovethe water surface. Various methods are beinginvestigated at the time of writing this paper.

Installation The deck was twice towed by four tugs to the Siri field; inthe beginning and at the end of October, but on bothoccasions the weather conditions were not acceptablefor mating to the tank. On the third attempt in the middleof November, the deck was successfully installed inextremely good conditions for the time of the year. Thedrilling rig remained on location with retracted cantileverduring the installation.

The tank was built by Daewoo in South Korea. It wastransported to Stavanger on the heavy lift vessel "MSSwan". After offloading from the vessel and a shortperiod of testing it was towed out to the Siri field by threetug boats. The lowering of the tank down to the seabedwas successf:ully done in extremely good weatherconditions in the middle of May 1998. The tank wasempty during the tow out.

Operation and removalThe tank has three separate buoyancy compartmentseach designed for nearly 60 metres water pressure. Themain tank is only designed for the effect of oil buoyancyand wave pressure at the top of the tank. The buoyancytanks are located around the leg sleeves, and they areused to provide tilt and depth control during lowering ofthe tank to the seabed. 5000 t of concrete was alsoinstalled in Korea in the bottom of the tank to providestability. The concrete ballast was limited due to thecapacity of the heavy lift vessel. Good weather wasnecessary to observe the effects of the trimming of thetank that had a GM value of only 0.6 metre.

The Siri platform will, during normal operations bemanned by 21 persons. The living quarters have acapacity of 60 persons in single cabins, mainly to caterfor maintenance and well work-over operations.

The operational life of the Siri field is estimated to be 10years, and this is the design basis for the topsidesequipment. The life requirement for the structure has,however, been set to 20 years. Being a jack-up, theplatform can with relative small cost be reused on otherfields of similar or shallower water depth. The removaloperation is basically a reversal of the installation, butthe deck and tank is to be removed as one piece.The tank installation was done through the following

steps:The most critical item in terms of feasibility for reuse isthe tank foundations. In some cases it may actually becheaper to scrap the tank and wellhead tower, and builda new structure as foundation support. If storage is notrequired on the new location, the foundation supportcould be a jacket-like structure. This could be built higherthan the Siri tank such that the platform could be used indeeper waters.

a)b)

The main tank was filled completelyThe buoyancy tanks were filled in increments untilthe to~) of the tank was just below the watersurface. The tilt of the tank was carefullycontrolled during this operation.The tank was lowered to the seabed by fillingwater into the wellhead tower.

c)

The deck was ready for tow on the 1 st October. Thestandard requirement for offshore barge transport ofsurvival in a 10 years storm was used as criteria for thetransport. This had a marked effect on the design of theliving quarter module. Two criteria were limiting for thedeck mating to the top of the tank:

Project organisation

The project management philosophy of the Siri projectwas to keep Statoil management to a minimum and togive the whole platform development responsibility toonly one Contractor. Statoil had only three full timepersons in Contractors office. but Statoil disciplineengineers were on a part time basis following up theirown disciplines from the home office.

a)

b)

Horizontal dynamic forces in the legs as they werehitting the guides on top of the tankVertical dynamic forces as the legs were hittingthe bottom of the tank

Kvaerner Oil and Gas (KOGAS) was chosen as theengineering, procurement, construction and installationThe vertical forces were the most critical with respect to

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7THE SIRI PRODUCTION JACK-UP PLATFORMOTC 11025

The project requirements were specified in a 43 pagesproject design basis. Standard specifications(NORSOK), clear definition of responsibilities and thelimited number of interfaces are all contributing reasonsfor KOGAS being able to run the project on schedule inthe short 18 months time span.

The EPGI project cost for the Siri platform and offioadingsystem (but not including drilling) is just above 2 billionNorwegian kroner.

(EPGI) contractor for the complete Siri jack-up platformwith oil offioading system. KOGAS had the responsibilityfor the quality assurance of the project, while Statoilverified this amj ran independent 3. party verificationanalyses of the! main structural steelwork. LloydsRegister was responsible for the 3. party verificationanalyses. In addition, Lloyds Register was selected asWarranty SurvE~yor and assisted Statoil in qualityassurance acti1/ities and adherence to Danish rules and

regulations.

The EPCI contract had two major interface areas:Conclusions

The drillin!~ operationsStatoil Siri operations. The Siri field development has shown that purpose built

jack-ups are cost effective concepts that can bedesigned and built in a relatively short time.Both interfaces were focused on at an early stage, and

the operations department of Statoil had onerepresentative (or at times several) on the project teamfor the duration! of the project.

The jack-up deck can also be placed on top of a jacketthat is terminated some 10 metres below the watersurface. In this way, the possible water depth of the costeffective jack-up concept can be extended.

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