DEVELOPMENT AND QUALIFICATION OF DEEPEST WATER POWER UMBILICAL

Transcription

1 Clse and Return A th Internatinal Cnference n Insulated Pwer Cables A.7.5 DEVELOPMENT AND QUALIFICATION OF DEEPEST WATER POWER UMBILICAL Arild FIGENSCHOU, Jan Ole DUNSERUD, Aker Slutins, (Nrway), arild.figenschu@akerslutins.cm, jan.le.dunserud@akerslutins.cm Daniel ISUS, Grup General Cable Sistemas S.A., (Spain), disus@generalcable.es Emmanuel BIC, Silec Cable, (France), emmanuel.bic@sileccable.cm ABSTRACT This paper describes the wrld s first umbilical system t deliver substantial pwer at 2700m water depth. The system is develped t deliver pwer t subsea pumps thrugh a dynamic pwer cable hanging frm a mred turret buy in a catenary installatin dwn t the seabed. Attentin is fcused n the develpment, design and qualificatin f the umbilical and its main cmpnents, the pwer cables, perfrmed in clse cperatin between the different cmpanies invlved. KEYWORDS Pwer umbilical, dynamic, submarine, deep water, bsting, pumps INTRODUCTION As the il and gas industry lks fr ways t keep pace with the grwing wrldwide demand fr il, subsea develpments are mving int ever increasing water depths and peratrs are lking twards subsea bsting technlgy as a means f getting the mst ut f their reservirs. The pwer distributin umbilicals, which are needed t supply electrical pwer t the subsea bsting pumps, will be challenged by mre extreme tensin lads due t the deeper waters. This paper presents a technical descriptin f a pwer umbilical t be installed in the area f the central Gulf f Mexic at 2700m water depth; the first f its kind t transmit pwer ver this depth sufficient t pwer subsea bsting pumps. The requirements f the pwer umbilical system required a transmissin f 1.7MVA including tw three-phase supplies and a spare three-phase circuit; therefre, a ttal f nine cables, rated 12/20 kv with a cnductr crss-sectinal area f 150mm 2 each. In additin, due t the lng length f the umbilical cable, including dynamic (that part f the umbilical suspended in water) and static (that part f the umbilical laying n the seabed) sectins, 2 sets f splices were included in static sectin. Attentin is fcused n the develpment and design principles f the umbilical and its main cmpnents, presenting the challenges encuntered, which t date are unique. Als, a thrugh qualificatin prgram is described that has verified the umbilical s ability t survive the rigurs f installatin and peratinal life at the mst extreme cnditins. ULTRA DEEPWATER CHALLENGES The traditinal way t limit the strain in umbilicals is t increase the axial stiffness by adding steel armuring. But fr very deep water applicatins, steel as armuring material des nt wrk efficiently t reduce the strain f an umbilical. If a structural member suspended frm a tp pint is hanging vertically and expsed t its gravity lad nly, the strain clse t the hang-ff pint is prprtinal t the free hanging length and density, but the ppsite applies in relatin t its stiffness. Therefre materials used fr stiffness enhancement lse effectiveness as the water depth grws. At a certain water depth the added weight frm steel armuring increases the strain rather than reducing it. This has made the industry lk fr light but still stiff material fr umbilical stiffness enhancement. As a result f many years f develpment and testing, a carbn fibre rd reinfrced umbilical system has been develped and patented. The carbn fibre rds, which are bundled int the umbilical alngside the ther elements in the umbilical, have abut the same stiffness but nly ne fifth f the density cmpared t steel. This change has eliminated all practical water depth limitatins in terms f strain and stress induced by the gravity lads, and has been used in a number f steel tube umbilicals ver the last several years. The cntrl f strain is particularly imprtant fr high and medium vltage electrical pwer umbilicals. A typical safe lng-term strain limitatin f pwer cables is in the range f 0.15%; this can easily be achieved fr dynamic pwer umbilicals using the carbn fibre rd system. Furthermre, by use f a special hang-ff system described belw (Umbilical mechanical prperties), the cables are free flating in the dynamic bending zne, thus aviding any uncntrlled lads upn the cables in the zne and limiting the strain t that caused by bending nly. Cmparisn f carbn fibre rds vs. steel as armuring material In shallw waters in cmbinatin with a harsh envirnment a high weight t diameter rati is required fr the dynamic umbilicals in rder t perfrm well. This is nt the case fr pwer umbilicals in deep waters. Instead, the primary cncern is t reduce the glbal strain f the umbilical system. The cntrl f glbal strain f an umbilical in deep waters may be achieved by tw methds: reductin f the underwater weight, and/r by increasing the axial stiffness f the umbilical. The fllwing example prvides a cmparisn f steel versus carbn fibre rds as a stiffness enhancement system fr an umbilical: The density f steel is 7850kg/m 3 in air cmpared t 1600kg/m3 fr carbn fibre reinfrced vinylester rds. The stiffness mdulus f steel is MPa cmpared t MPa f a cmpsite carbn fibre reinfrced rd. As the strain is prprtinal t the weight and the inverse applies t the stiffness, reducing strain by the use f

2 Clse and Return A th Internatinal Cnference n Insulated Pwer Cables A.7.5 carbn fibre rds is much mre efficient than using steel. By cmparing the Yung s mdulus t the density fr steel and fr carbn fibre rds, the factr is 8.9, (density f sea water is set t 1025kg/m 3 ): Steel: Carbn fibre rds: Rati f stiffness enhancement efficiency: Wrking example: The fllwing is a cmparisn f a steel reinfrced pwer umbilical t a carbn fibre rd reinfrced pwer umbilical. In this example the calculatin is based n a pwer umbilical with six medium vltage pwer cables, six ½ and ne ¾ steel tube service lines and tw standard signal cables. The stiffness cntributin frm the pwer cables is disregarded due t thermal expansin and creep, thus the pwer cables shuld nt be cnsidered as strength members in peratin. The diagram gives the glbal strain f the umbilical systems as a functin f the amunt f armuring in mm 2, in the crss-sectin f the umbilicals. deepwater umbilicals. POWER UMBILICAL DESIGN The umbilical design is based upn the bundling f the internal elements in a gradual spiral alng the length f the umbilical. The angles f the spiral relative t the centre line f the umbilical range between ne and three degrees nly. This gradual spiral prvides several advantages, such as lw rtatinal frces, and a high capacity t withstand axial cmpressin frces f the umbilical. The internals f the umbilical are held in place and separated by stiff plastic spacers that run alng the length f the umbilical. The spacers are shaped such that when bundled tgether they frm internal lngitudinal vids r cnduits thrugh which the umbilical elements pass. Each cnduit is exactly dimensined t suit the internal elements such as a tube r cable that it will cntain, with an all-rund gap f apprximately 1mm. This prvides the free mvement within the umbilical during handling, and allws the umbilical t be spled and reeled t the respective bending limits f flexibility f the internal elements. The use f plastic spacers als prvides benefits in the distributin f frces thrughut the crss-sectin f the umbilical when it is subjected t high squeeze pressures. Such a benefit is especially advantageus fr deep water installatin, as it helps prevent the caterpillar grip pressures frm causing damage t the umbilical internals. Anther imprtant feature f the umbilical with bundled elements in gradual spiral alng the length f the umbilical is the imprved fatigue life due t lw frictin between the cmpnents. In Figure 2 the crss-sectin f the present umbilical is shwn, including a triad frmatin f three-phase circuits with a ttal f nine cables rated 12/20 kv with a cnductr crss-sectinal area f 150 mm² each. Fig. 1: Umbilical strain versus steel r carbn rd armuring in case f 3,000m water depth The design limit f cpper cnductrs is nrmally in the range f 0.15% strain. As shwn in the graph abve, this can easily be achieved by reinfrcing with carbn fibre rds t achieve glbal strain f 0.15% by including 2750 mm 2 f rds in the example crss-sectin. This crrespnds t abut 83 carbn fibre rds f 6.5mm diameter. The rds can easily be integrated in the crsssectin withut changing the glbal diameter f the umbilical and withut any serius weight impact. T achieve the same glbal strain limit by use f steel reinfrcement it takes in excess f 15000mm 2 f steel t reach a glbal strain f 0.15%. This result is due t the added weight frm the steel. As a further illustratin, if a single steel member is hung free frm the surface 3000 meters vertical int sea water the strain is 0.1% by nly suspending its wn weight. It is therefre clear that steel is nt a suitable material fr reinfrcement f ultra Fig. 2: Pwer umbilical crss-sectin (dynamic sectin sme rds are replaced with plastic fillers in the static sectin)

3 Clse and Return A th Internatinal Cnference n Insulated Pwer Cables A.7.5 cmbinatin f material creep and temperature effects (expansin and cntractin) in the cpper cres makes the evaluatin f the fatigue life very uncertain. Table 1: Table f pwer umbilical cmpnents in Fig. 2 Umbilical electrical prperties There is n transfrmatin f the vltage, s with a mtr vltage f 4.4kV, it is necessary t have an utput vltage frm the variable speed drive f 6.1kV t btain the rated vltage fr the mtr. This crrespnds t a vltage drp f 28% and reduces the air gap trque. This and the vltage increase due t lad reductin were fund t be acceptable fr the peratin. The magnetic field set up by the lad current will induce currents and vltages in nearby cnductive elements. The induced current in the grunded elements such as screens, steel weight elements and carbn fibre rds gives additinal lsses that can be handled by thermal analysis t check if any elements are verheated. The vltage induced int the cres within the same circuit and int the neighburing circuit is mre cmplicated because it affects the peratin f the pump mtr. This negative effect can be cancelled by twisting the circuits within the crss-sectin. That methd wuld increase the required number f splices and the cst dramatically and was rejected. Anther slutin is t have the 3 circuits in different layers with ppsite twisting directin. This wuld increase the uter diameter significantly and a large number f weight elements wuld be required t maintain the weight t diameter rati needed. It was therefre decided t keep the mst efficient design and study mre clsely what impact the induced vltage wuld have n the peratin f the mtr. By use f a Finite Element methd it was fund that the negative sequence vltage was less than 1%. This is acceptable accrding t IEC withut de-rating the mtr. T minimize the fatigue f the cables in the dynamic bending zne, a cncept was develped with the electrical pwer cables free flating in the dynamic bending zne, thus aviding any uncntrlled lads upn the cables in the zne. The carbn fibre rds are anchred at the tp hang-ff, but the electrical cables run thrugh the hang-ff withut any cnstraint and are therefre free t expand and cntract. Instead f a tpside cable terminatin, the cables are hung-ff inside the umbilical with a sft clamp system. This system is basically a vulcanized rubber sheath that is applied t the utside f the cable ver a length f ten metres, increasing the uter diameter and prviding frictin against the inside f the cable cnduit in the umbilical, withut creating a hard pint between the surfaces. This is lcated sme 15 t 20 metres belw the bend stiffener, and becmes the hang-ff pint f the pwer cables. Frm this pint and up thrugh the dynamic bending zne, the cables are free t expand thermally, and are expsed t bending nly, and therefre the frces upn the electrical cables becme highly predictable. Descending belw the hang-ff pint, the pwer cables are secured by frictinal pints in a special pattern ver a length f 2 metres at intervals f 50 metres t cunteract any lng-term creep/defrmatin f the cpper cres. This system prtects the electrical cables frm strain hardening and fatigue failure in the dynamic sectin. Pwer cable design The maximum current required by the pumps was in the rder f 225 A. Due t the length f 22.7 km f ne f the umbilicals and the cnsequent vltage drp, the supply at the hst was at 6 kv apprximately t achieve a vltage required by the mtr terminals f 4.4 kv. These requirements were btained by using a cable with a cnductr crss-sectinal area f 150 mm² and, whereas an insulatin rating f 6/10 kv wuld have been sufficient, 12/20 kv rating was used t achieve a much lwer electrical stress and cnsequently a greater safety factr. A drawing f the pwer cable is shwn in Figure 3. The induced vltage frm the neighburing circuit als causes a variatin in the air gap trque when the pwer frequencies are different. The impact f this was als studied and it was cncluded that the amplitude f the air gap trques was maximum 2.8% and was acceptable because it was within a frequency range that was nt critical fr the peratin. Thermal analysis was als perfrmed t visualize the temperature distributin in the crss-sectin. One f the 3 circuits was spare and it was interesting t knw the temperature difference between the elements as an input t the mechanical analysis. Umbilical mechanical prperties One f the main challenges f designing a safe and reliable high pwer dynamic umbilical fr deep water applicatin is t cntrl the frces and strain in the cpper cres f the cables. The material prperties f cpper make it difficult t predict the frces ver time as the Fig. 3: Pwer cable drawing

4 Clse and Return A th Internatinal Cnference n Insulated Pwer Cables A.7.5 Item Descriptin 1 Cnductr 2 Cnductr screen 3 XLPE Insulatin 4 Insulatin screen (nn metallic) 5 Cpper wires screen 6 Outer sheath Table 2: Table f pwer cable cmpnents in Fig. 3 The cnductr was a multi-stranded Class 2 type design accrding t IEC While fllwing the requirements f the mentined standard, several design parameters can be adapted in rder t achieve the required final cnductr mechanical prperties. Hence, after a prttyping and trial testing prcess, it was f great imprtance t well define and cntrl the material type and the drawing, annealing, stranding and cmpacting prcesses. Electrlytic Tugh Pitch (ETP) was used and every layer f wires was semi-cmpacted; in additin, n single wire weld, due t pssible factry lgistic limitatins r breaks during prductin, was allwed during the whle cnductr lengths manufacturing prcess. The final bjective was t btain prper cnductr behaviur under all expected lads taken during the installatin prcess and peratinal life, while als assuring the cnstant prperties f it in the whle manufactured length. The insulatin system was cnstituted f a semicnductive cnductr screen, Crss-Linked Plyethylene (XLPE) and a semicnductive insulatin screen. The three layers were extruded at the same time by means f a triple extrusin head and the curing and cling prcesses were perfrmed in a dry nitrgen catenary line. Bth semicnductive layer thicknesses were increased cmpared with a standard MV cable design t avid any pssible insulatin damage caused by a cnductr r screen cpper wire embedding int thse layers during the dynamic related mvements. The metallic screen was cnstituted f 60 cpper wires laid up nt the uter semicnductive layer with a ttal crss-sectinal area f 16 mm². This screen design was chsen after several trials, mainly taking int cnsideratin the prper behaviur under bending fatigue and the shear lad capacity between the insulatin system and the uter jacket; this last prperty being crucial in the verall transfer f the cpper cnductr lad thrugh all cable layers t the external umbilical strength members (carbn fibre rds). Other types f screens, including a cpper tape helicidally applied, were tested with unsuccessful results. A High Density Plyethylene (HDPE) uter sheath was applied nt the metallic screen t prtect it mechanically as well as t minimize the water diffusin t the insulatin. strength members were playing an essential rle, limiting mechanical wrking cnditins f pwer cable, the umbilical design was adapted accrding t the pwer cable characteristics established thrugh testing. Subsequently, when pwer cable wrking cnditin limitatins were determined and impsed by the final umbilical strength members design, the secnd imprtant gal was t verify the characteristic cable peratin at thse expected critical wrking cnditins (with an extra safety margin) by means f mechanical testing fllwed by electrical testing. Testing sequences T verify the electrical suitability f the pwer cable during and after the applicatin f an external pressure f 1.5 times the equivalent t the maximum installatin water depth, the fllwing test sequence was made n ne cable sample: Hyperbaric Test at 410 bars fr 24 hurs; applying vltage n the sample while pressurized. Partial Discharge Test accrding t IEC Impulse Vltage Test accrding t IEC Vltage Test accrding t IEC Visual Inspectin (dissectin). The pwer cable stress-strain curve under axial tensile lad was btained fr infrmatin by means f tensile testing. Eventually, multiple samples frm multiple manufacturing batches were tested t find any variatin in batches, using the mst cnservative results t fine-tune the umbilical calculatins. T verify the electrical suitability f the pwer cable after the applicatin f the maximum wrking tensile lad (with an extra safety margin), the fllwing test sequence was made n anther cable sample: Tensile Test up t 0.3% elngatin. Partial Discharge Test accrding t IEC Impulse Vltage Test accrding t IEC Vltage Test accrding t IEC The pwer cable prperties under axial cmpressin lad were als measured. QUALIFICATION TESTING Pwer cable qualificatin testing Due t the specific particularities f the prject (deepness, dynamic behaviur, sea water envirnment ), a specific qualificatin testing plan was designed t simulate the mechanical and electrical cnditins f the applicatin during bth peratin and installatin f the cable. The first imprtant gal f the defined testing sequences was t cllect the mechanical prperties f the cable bth under axial tensile and cmpressin lads. As umbilical Fig. 3: Axial cmpressin testing rig at TECNALIA- Labein

5 Clse and Return A th Internatinal Cnference n Insulated Pwer Cables A.7.5 T verify the electrical suitability f the pwer cable after the applicatin f the maximum wrking cmpressin value (with an extra safety margin) experienced in sme cases due t temperature differences inside the umbilical, the fllwing test sequence was made: Axial cmpressin test up t 0.2% cmpressin strain. Partial Discharge Test accrding t IEC Impulse Vltage Test accrding t IEC Vltage Test accrding t IEC T determine the pwer cable creep behaviur during the peratin f the dynamic umbilical length at bth maximum wrking axial tensile and cmpressin lads and at maximum expected wrking temperature (50ºC), tw creep tests were dne with a ttal duratin f 720 hurs each. See the cnstructed testing rig in Figure 4. Bend testing Lw cycle fatigue testing under tensin t simulate wrst case installatin lads (perfrmed n a similar cable splice fr anther pwer umbilical prject) Electrical testing accrding t IEC Dissectin Umbilical qualificatin testing In additin t standard umbilical testing as defined in ISO , a sample f the cmpleted umbilical was subjected t rigrus mechanical testing. The fllwing tests were perfrmed: Flex fatigue test The purpse f the test was t verify that the umbilical and its bend stiffener wuld prvide sufficient flex fatigue life fr use in a prductin envirnment where the umbilicals are suspended frm a flating platfrm An umbilical sample cmplete with tpside terminatin and bend stiffener and subject t peratinal lads distributed t the varius strength members (see figure 5) survived the flex fatigue test prgram withut mechanical r electrical faults. It fully cmplied with the requirements f Sectin f ISO/FDIS Standard : N tensin lad was applied t the pwer cables, as the hang-ff system described abve under Umbilical mechanical prperties avids any tensin in the cables in the dynamic bending zne. Fig. 4: Axial tensile and cmpressin creep testing rigs at TECNALIA-Labein In additin, the pwer cable was subjected t a cmplete Type Testing accrding t IEC Splice qualificatin testing Ensuring that the cable splices in the static sectin f the umbilical wuld survive the installatin frces at 2700m water depth withut unduly influencing the umbilical design r functin, and withut impsing special installatin requirements was a particular challenge. A prprietary splicing methd was specially develped and qualified fr this applicatin, meeting the gals abve. Qualificatin testing included: Fig. 5: Flex fatigue test setup at TMT Labratries Pull test The purpse f this prprietary test was t determine the frictin capacity f the sft clamp design f the umbilical. Displacement f individual umbilical cmpnents including the pwer cables was measured during pulling t verify that the sft clamp system perfrmed as expected. Tensile testing at the lad crrespnding t the frce induced in the unspliced cable by the glbal strain in the static sectin f the umbilical during installatin, plus a 50% safety factr Hyperbaric testing under nminal vltage

6 Clse and Return A th Internatinal Cnference n Insulated Pwer Cables A.7.5 Fig. 6: Pull test setup at TMT Labratries shwing displacement transducers attached t umbilical cmpnents Additinal testing was perfrmed n a ne meter sample f umbilical, where the sft clamp was applied t all elements and weight was applied t each element, pulling against the plastic prfiles (see Figure 7). This prvided a mre thrugh verificatin f the creep behavir in the sft clamp system, shwing that the extraplated creep ver time was well within the acceptable range fr the system. Figure 7: Creep test rig fr sft clamp system Electrical final acceptance testing (FAT) The cmpleted pwer umbilicals, ne f 22,7km and the ther f 5,6km, each spled t an utdr carusel, were subjected t DC resistance, TDR and HV testing as part f the umbilical final acceptance testing. SUMMARY The unique pwer umbilical design described here cmbines field prven technlgies with new slutins specifically develped in clse cperatin with the invlved cmpanies fr ultra deepwater pwer cable applicatins. This has resulted in the successful qualificatin and manufacturing f the wrld s first umbilical system t deliver substantial pwer at 2700m water depths. The key technlgies enabling this achievement as described here are: Carbn Fibre Rds Sft Clamp system Cable design Splicing Limit glbal strain f the umbilical, including the pwer cables Overcme the limitatins f steel armuring in deep water, allwing fr ptential dynamic applicatins dwn t much greater water depths than therwise practical Eliminate the need fr armuring in the individual pwer cables Prvides a hang-ff pint fr the pwer cables belw the dynamic bending zne, allwing fr free thermal expansin f the cables in the httest area f the umbilical system, and minimizing fatigue in the cables by eliminating the axial lad in the dynamic bending zne Cunteracts the effects f creep n the cables in the full dynamic sectin dwn t the seabed by ensuring sufficient frictinal cntact with the cnduits frmed by the plastic prfiles, effectively transferring frces t the strength members Careful design f the individual pwer cables ensures sufficient shear lad capacity thrughut the layers and int the umbilical Careful design f the cnductr, semicnductive layers and metallic screen ensures prper behaviur under bending fatigue The specially develped static cable splice allws fr safe installatin withut impacting the umbilical design r placing limitatins n the installatin parameters