OFFSHORE USE OF FLOATING SHEERLEGS

Proceedings of the ASME 211 3th Interntionl Conference on Ocen, Offshore nd Arctic Engineering OMAE211 June 19-24, 211, Rotterdm, The Netherlnds OMAE211-49394 OFFSHORE USE OF FLOATING SHEERLEGS Vincent Koster SMIT Engineering Kees Jn Vermeulen SMIT Engineering Peter Korteks SMIT Engineering SMIT Nederlnd BV Wlhven O.Z.85 387 BM Rotterdm ABSTRACT The floting sheerlegs of SMIT re used for lrge vriety of tsks in sheltered, ner shore s well s offshore environments. Generlly the tsks vry from recovering opertions (slvge) to inshore instlltion work. Originlly, the usge of floting sheerlegs ws driven by their robustness, needed for recovering work nd shllow drft, needed for working ner shore. In the pst yers the sheerlegs of SMIT becme incresingly utilized for instlltion work offshore. As result, the need for more detiled nlysis showing the fesibility, relted sfety nd relibility of opertions emerged. In the engineering phse of such offshore instlltion projects these spects re nlyzed for ech opertionl phse from lod-out, towge to offshore lifting nd instlltion. This pper ddresses the crucil elements tht need to be ddressed when floting sheerlegs is utilized for offshore instlltion work. The engineering topics tht will be discussed in this pper re stbility, strength nd multi body dynmics. To demonstrte the relevnce nd vlidity of the discussed topics, n offshore instlltion project is nlyzed in terms of mesured motions nd wve conditions. NOMENCLATURE over Overturning rm [m] right Righting rm [m] B Bredth [m] Cb Block coefficient [-] DAF Dynmic Amplifiction Fctor [-] F led Predicted led force [ton] F led_crit Off/side led criterion [ton] Fn Froude number [-] F wind_pp Overturning wind force on the [N] lifting pplince F wind_lift Overturning wind force on the [N] lifting lod g Grvittionl ccelertion [m/s 2 ] H s Significnt wve height [m] k Form fctor [-] L Length [m] RAO Response Amplitude Opertor [m/m;deg/m] T n Nturl Period [sec] T p Wve spectrum pek period [sec] WLL Working Lod Limit [ton] z h Height of centre of ttchment [m] lifting hook z p Height of centre of pressure [m] lifting pplince γ over Prtil lod fctor [-] γ res Prtil resistnce fctor [-] ρ Specific grvity of wter [kg/m 3 ] φ Inclining ngle [deg] Ships displcement [m 3 ] 1 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

1 INTRODUCTION When using sheerlegs for offshore instlltion work number of crucil elements hve to be ddressed to ssure sfe opertion. Amongst others, those elements re stbility of the sheerlegs, strength of the lifting pplince nd dynmic behvior in wves. A sheerlegs is floting pontoon equipped with n A-frme which is positioned t the bow of the pontoon. Optionl the A- frme cn be equipped with jib which cn be used for multi hook lifts of complex structures s top sides, jckets, civil structures, grounded vessels etc. In the figure below, the 22 t sheerlegs Tklift 4 with n FPSO module in the tckles is presented. In this pper, the typicl issues with respect to engineering nd opertion of the sheerlegs in n offshore environment re ddressed. The topics tht will be ddressed re: Strength Stbility Hydromechnics Full scle monitoring 2 STRENGTH The cpcity of lifting pplince is bounded by its lifting curves. These lifting curves do represent the Working Lod Limit (WLL) s function of A-frme nd Fly-jib position. In the figure below, lifting curve typicl for sheerlegs is presented. In order to work offshore with sheerlegs it is required to do n engineered lift in which strength, stbility nd llowble motions need to be ssessed. Furthermore feedbck from the sheerlegs in opertion is of utmost importnce. Therefore, during the opertionl phse the lod nd motions re monitored nd logged for post opertionl nlysis nd vlidtion purposes. The mximum loding presented in the lifting curves is bsed on the strength of the lifting pplince in verticl, trnsverse nd longitudinl direction. As result, the lods in the lifting curves re relted to mximum off-led nd side-led ngles s well. For lifting opertion the mximum lod from the lifting curves my not be exceeded. For ech specific lift the lifting cpcity is ssessed by mens of the lifting curves. 2 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

For offshore lifts the wve induced motions nd derivtives will decrese the lifting cpcity of the lifting pplince. Therefore, the motions nd the effect on hooklod nd lifting pplince hve to be included in the lifting ssessment. By mens of motion nlysis the dynmic lods in the tckles of sheerlegs re determined. Furthermore, the mximum side nd off led re determined. The following formultion presents how the dynmics in the lifting system hve their effect on the lifting cpcity: In which: WLL F sttic F led_crit F led DAF WLL > Fsttic * DAF F F led _ crit led Working Lod Limit from lifting curves Sttic hook lod (inclusive rigging) Off/side led criterion Predicted led Dynmic Amplifiction Fctor The DAF includes the effect of ccelertions on both the lifting pplince nd the hook lod. The predicted DAF nd led ngles together with the sfe working criteri of the lifting pplinces do result in set of criteri which bounds the offshore lifting opertion. While n inshore lifting opertion is normlly only bounded by wind speed nd the lifting curve, the offshore lift is bounded by the dynmics in the lifting system s well. 3 STABILITY Stbility for lifting opertions is not prescribed in detil by Clss societies or IMO. Nevertheless the stbility during lifting opertion shll be quntified nd ssessed ginst criteri. Due to the fct tht for lifting, no generic stbility criteri re prescribed, specific criteri for lifting opertions re developed by SMIT. The bsis for the development of the specific lifting criteri ws the opertion itself. In generl (offshore) lifting opertion cn be defined s: Well plnned Environmentl restricted Short durtion formul below where the res of both over heeling rm curve nd righting rm curves re compred. lod i over res i right In which: lod Prtil lod fctor over Overturning rm res Prtil resistnce fctor right Righting rm Inclining ngle In this formul the lod fctor nd the resistnce fctor represent the required sfety mrgin with regrds to stbility of the opertion. Within SMIT, those sfety mrgins re bsed on both opertionl experience nd stbility nlysis of prticulr sheerlegs. The re below the righting rm curves needs to be lrger thn the re below the overturning rm. The following figure shows the reltion. φ 1 Righting rm φ 3 Overturning rm The ssessment of the res under both the righting rm curve nd overturning rm curve is repeted for three pre-defined heeling ngles φ 1.. φ 3 to cover chnges in geometry under influence of heel. For sheerlegs the overturning moment is minly influenced by environmentl forces. The overturning rm for the stbility ssessment is determined by mens of the following formultion: φ The specific stbility criteri for lifting opertions with sheerlegs re driven by the stbility rnge insted of initil stbility. The bsis for this is pproch is given by the fct tht energy induced by n overturning moment on the sheerlegs should be smller thn the restoring energy tht cn be generted by the sheerlegs. This pproch is formulted in the over = F wind _ pp z p + F ρg wind _ lift z h 3 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

In which: F wind _ pp Overturning wind force on the lifting pplince F _ Overturning wind force on the lifting lod z p z h wind lift Height of centre of pressure lifting pplince Height of centre of ttchment lifting hook Ship s displcement It should be noted tht the overturning force of wind hs its centre of pressure t the lifted lod. Nevertheless the centre of ppliction of this overturning force shll be defined t the loction where the lifting tckles re suspended s cn be seen in the following figure. The following figure presents method sttement of verifying the lifting stbility of sheerlegs. Lod curves START Lower lifting lod NO Criteri meet llowbles YES LCG lifting lods Weight lifting lods LCG lifting configurtion Weight lifting lods Trimming moment lifting lods Trimming moment lifting configurtion Fill tnks 9 & 1 to reduce trim to minimum CRITERIA CHECK VCG lifting lods Weight lifting lods VCG lifting configurtion Weight lifting configurtion VCG tnk fillings nd lightweight Weight tnk fillings nd lightweight END Centre of ppliction Centre of Pressure Also the lifted lod hs its centre of ppliction t the loction where the lifting hooks re supported. If lifting curve hs insufficient reserve stbility, its llowble cpcity shll be lowered in order to meet criteri. For lifting curves which re normlly only bounded by strength criteri, n re should be defined which is specified s the criticl re. If lifting opertion is plnned in this criticl re, it is importnt to quntify clerly the lod cse nd ll other sfety fctors s pplied for strength nd opertions. The totl set of opertionl, strength nd stbility relted sfety fctors shll be combined in order to sses clerly the risks relted to the reserve stbility. The stbility for lifting opertion in n offshore environment is s criticl s n inshore opertion. Looking t sheerlegs lifting curve it cn be concluded tht highest lifting lods re pplicble t minimum outrech if the centre of ppliction is highest. Therefore it my be concluded tht the lifting stbility is governing for minimum outreches. If sheerlegs is equipped with jib it hs lrge number of lifting combintions. All these lifting combintions shll be verified ginst stbility criteri. Nevertheless with incresing outreches, the llowble lifting lod decreses nd sheerlegs stbility becomes less sensitive. If the loding conditions, presented by lifting curve of sheerlegs, hve sufficient reserve stbility for minimum outreches, it cn be concluded tht the totl lifting curve hs sufficient reserve stbility [1]. Becuse sheerlegs is lifting over the pontoon s longitudinl xis, it is importnt to note tht the trim chnge due to picking up lod cn be kept within cceptble limits without the need to bllst during lifting opertion. Bllst tnks t the stern, with mximum lever, cn be loded before the lifting opertion tkes plce to compenste the trim of the sheerlegs. 4 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

4 HYDROMECHANICS The floting sheerlegs utilized by SMIT consists of Lifting pplince nd pontoon. The typicl dimensions of the pontoon determine the behvior of the pontoon in clm wter s well s in wves. In this section, the consequence of some typicl min dimensions of the pontoon will be discussed with respect to clm wter resistnce, roll dmping nd dded resistnce in wves. Furthermore the coupled motion between sheerlegs nd hook lod will be discussed. Clm wter resistnce In clm wter the performnce of the pontoon is determined by the resistnce during trnsit. The resistnce of the pontoon is determined by the shpe of the pontoon nd the reltive speed through the wter. It should be noted tht, lthough sheerlegs re utilized worldwide, the shpe of the pontoons re not primrily optimized for trnsit speed but rther for providing stble pltform during hevy lift opertions. As result, the speed of the sheerlegs in trnsit is reltively low t Froude numbers up to.18. At such low Froude numbers min component of the clm wter resistnce is due to friction between the wetted surfce of the pontoon nd the surrounding wter. Due to the lrge block coefficient (>.8) in the ft body of the pontoon, significnt prt of the frictionl resistnce cn be ttributed to form effects; the form fctors re in the rnge of 1.2-1.4 [2]. Roll dmping In this section the effect of the lrge bredth with respect to the drft of the pontoon on the roll dmping will be discussed. Typiclly, the bredth over drft reltion (B/T) of the trnsverse sections of the pontoon is in the vicinity of 9. In the figure below the section view of sheerlegs is presented. In the figure below, the reltive roll nd pitch dmping re presented s function of oscilltion period. The dmping vlues re bsed on 3-dimensionl diffrction theory hence no viscous effects re included. rletive dmping [-].25.2.15.1.5. Reltive dmping 5 1 15 2 Ocilltion period [s] From the figure it cn be observed tht the reltive roll dmping is in the sme order of mgnitude s the pitch dmping; between 2% nd 25% of the criticl dmping. Compred to the reltive roll dmping for conventionl hull forms (below 5% t zero forwrd speed [6]) these vlues clculted for typicl sheerlegs pontoon re significnt higher. It should be noted tht the bsolute vlue of the pitch dmping is n order of mgnitude lrger compred to the roll dmping. Due to the lrge (dded) inerti in pitch, the effect in terms of reltive pitch dmping is smll. To demonstrte this, in the figure below the dded (hydrodynmic) inerti reltive to the structurl inerti is presented s function of oscilltion period for both roll nd pitch. 2.5 Added mss Roll Pitch 2. Added mss [-] 1.5 1. Roll Pitch This lrge bredth over drft reltion mkes the pontoon n effective shpe for the genertion of roll dmping. The totl roll dmping tht will be generted by the hull is given by two contributors: potentil (wve mking) prt nd viscous prt. The lrge B/T reltion hs the lrgest effect on the wve mking prt of the roll dmping..5. 5 1 15 2 Ocilltion period [s] From the figure, it cn be observed tht the dded mss is fctor 3-4 lrger for pitch with respect to roll. The sheerlegs pontoons re not fitted with bilge keels for the genertion of dditionl (viscous) roll dmping. However, s demonstrted by the tests within the Noble Denton brge reserch project [3] there will be considerble mount of 5 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

viscous roll dmping due to vortex shedding from the shrp edges of the bilge. nturl pendulum period s function of tckle length is presented in the figure below. Due to the reltively lrge roll dmping t zero speed (instlltion condition), the sheerlegs re less sensitive for excittion of wves with periods round the nturl roll period reltive to conventionl ship hull forms. 25 2 Nturl pendulum period Added resistnce in wves Although the wve mking prt of the totl resistnce of the pontoon (the energy dissipted by the genertion of wves due to forwrd speed) is smll in clm wter, the dded resistnce in wves (the energy dissipted by the genertion of wves due to verticl motions t the bow) is lrge [7]. The reson for the lrge dded resistnce in wves origintes from the fct tht the full bow section of the sheerlegs genertes lrge dmping. The dmping is mesure for the mount of energy dissipted by the wves due to the verticl motions t the bow [4]. Due to the reltively lrge dded resistnce in wves nd the reltively low clm wter speed, the involuntry speed loss of the sheerlegs when siling in wves is considerble. Therefore, to be ble to mintin sfe speed in wves tug will ssist the sheerlegs on trnsits where significnt hed ses conditions re expected. In the cse of cceptble se conditions, the sheerlegs will sil on its own propulsion. Coupled motions The suspended hook lod results in coupled motion behvior of both hook lod nd sheerlegs. This behvior is demonstrted in the figure below. RAO [m/m].6.5.4.3.2.1 Hooklod motions 5 1 15 2 Wve period [s] -m otion In the picture bove, the RAO of the longitudinl motion of suspended hook lod is presented s function of wve period for wves coming in from the bow. The longitudinl hook lod motions re chrcterized by two distinct peks. The first pek round 8 seconds is relted to the pitch of the pontoon; the position of the top of the A-frme results in longitudinl motion t the top which exerts the suspended hook lod. The second pek round 13 seconds is relted to the nturl pendulum period of the hook lod-tckle combintion. The nturl pendulum period depends on the tckle length. The Tn [s] 15 1 5 2 4 6 8 1 12 14 16 Pendulum length [m] Due to the fct tht the dmping generting cpbilities of the suspended hook lod re low, the motions of the suspended hook lod cn be mplified to extreme vlues when excittion period (motions of top) nd nturl period pproch ech other. Therefore, it is of utmost importnce to void tckle length tht results in nturl pendulum period of the motions t the top of the A-frme. 5 FULL SCALE MEASUREMENTS Although the use of sheerlegs in n offshore environment is robust solution, the behvior shows some sensitivity with respect to dynmic behvior of both sheerlegs nd suspended lod. To ssist the opertors in plnning nd decision mking, the dynmic behvior of sheerlegs, suspended hook lod nd incident wves is monitored during offshore instlltion projects. Furthermore, the results cn be post processed nd used for vlidtion purposes. In this section some typicl results of mesurements during n offshore instlltion project will be presented. The dt presented in this section is mesured during the offshore instlltion of 11t substtion for n offshore windmill frm. The substtion ws lifted offshore from brge nd instlled onto monopile. The following stges cn be distinguished during this project: Pre-bllsting of the sheerlegs Lifting the substtion from the brge Repositioning the sheerlegs towrds instlltion position Lowering the substtion on instlltion position De-bllsting of the sheerlegs 6 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

In the picture below, the mesured time trces during ech stge of the instlltion re presented. Mesured Mx Roll, Pitch Roll [deg].5 August 11 11:-12: UTC -. 5 5 1 15 2 25 3 35 3. Rol, Pitch [deg] 2. 1.. -1. -2. -3. 11-8-21 :.5 Pre- bllsting 11-8-21 4:48 11-8-21 9:36 Repositioning Lowering Lifting 11-8-21 14:24 De-bllsting 11-8-21 19:12 12-8-21 4:48 12-8-21 : signl roll signl pitch Time UTC [dd-mm-yyyy hh:mm] Tckle Lod [t] Pitch [deg] -. 5-1 -1. 5-2 -2. 5 5 1 15 2 25 3 35 6 4 Pre lift Lift Lift Pre lift 2 5 1 15 2 25 3 35 Hs [m] 3. 2.5 2. 1.5 1..5. 11-8-21 : 11-8-21 4:48 11-8-21 9:36 Mesured Hs, Wve period 11-8-21 14:24 The following quntities re indicted: Roll Pitch Significnt wve height Spectrl pek period 11-8-21 19:12 12-8-21 : 1 8 6 4 2 Time UTC [dd-mm-yyyy hh:mm] Wve pek period [s] 12-8-21 4:48 The bove quntities re presented with 15 minute time steps, for roll nd pitch the mesured extremes during tht period re presented. The mesured roll nd pitch ngles include the sttic heel nd trim hence the pre-bllsting to stern trim of 2.5 deg, lifting, lowering nd de-bllsting to even keel cn be clerly identified. In the following picture the sitution fter the substtion ws lowered onto the monopile before de-bllsting is presented. From the picture the stern trim of 2 due to bllst in the ft tnks cn be clerly recognized. Hs Tp DAF tip [%] 5-5 5 1 15 2 25 3 35 Time [s] In the tble below, the DAF vlues re presented in numericl form for the crne tip nd the tckle lod. The DAF of the tckle lod is defined s the dynmic force vrition from the sttic lod divided by the sttic lod. The DAF vlues re presented s men vlues during pre-lift nd lift stges. The pre-lift stges represent the stge in which the tckles re tensioned up to 2t ech with the substtion still positioned on the brge. Lifting stge DAF tckle lod DAF crne tip Pre lift 12% 3% Lift 3% 3% From the figure nd tble bove, it cn be observed tht the DAF vlues during pre-lift re lrgest. The reson for this origintes from the fct tht when the substtion ws hooked on nd tensioned up to 2t per tckle, the brge with substtion ws still under influence of wve induced motions. The reson tht this mount of DAF cnnot be recognised in the top of the fly-jib cn be explined by the fct tht significnt prt of the dynmic forces is dissipted by the flexibility of the lifting tckles nd fly-jib. In ddition to the quntities described bove, the tckle lods nd verticl ccelertions t the lifting hooks re mesured. The ccelertions experienced by lifting pplince nd hook lod with respect to the grvittionl ccelertion determine the dynmic mplifiction of the sttic lod cse. In the following figure, the time trces of roll, pitch, tckle lods nd DAF crne tip re presented for the totl length of the opertion. The tckle lods re mesured with lod cell in ech block in the fly-jib. The DAF t the top of the fly-jib is determined by dividing the mesured verticl ccelertions t the fly-jib tip loction by the grvittionl ccelertion. 7 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

Spectrum Roll l.16.14.12 Roll [deg 2 s/rd].1.8.6.4.2 5 1 15 2 25 Period [s] To get more detiled insight in the behvior of the sheerlegs, energy spectr re derived from the mesured roll nd pitch signls nd compred to the ctul mesured wve spectr. In the figure below, the mesured wve spectr derived during three periods of the instlltion re presented. Mesured wve spectr Pitch [deg 2 s/rd] x 1-3 2 1.5 1 Spectrum Pitch l 1.2.5 1. S [m^2/hz].8.6.4.2. 5 1 15 2 25 Wve period [s] 7:42 15:12 19:42 From the figure it cn be observed tht the spectrl pek period shifted from 6s t 7:42 towrds 5s t 15:12; lter t 19:42 the spectrum shows multiple peks. The relting significnt wve height rnges between.5 nd.75m. In the picture below the roll nd pitch spectrum re presented for the sitution when the substtion ws lifted from the brge nd suspended from the Fly-jib. 5 1 15 2 25 Period [s] For both roll nd pitch clerly two peks cn be recognized; pek round 7s nd pek round 17s. The first pek cn be relted to the nturl period for roll nd pitch of the sheerlegs while the second pek cn be relted to the free swinging pendulum period of the lod. It should be noted tht lthough the double peks in the pitch spectrum re more pronounced, the ctul spectrl density for pitch is n order of mgnitude smller with respect to roll. Given the mesured wve spectr during the instlltion work, it cn be observed tht the roll nd pitch energy is not solely concentrted round the peks of the wve energy spectrum. In the response spectr peks cn be observed on both roll nd pitch nturl period nd lso t the pendulum period of the hook lod, this illustrtes the effect of coupled behvior between brge nd hook lod. From the nlysis presented in the pictures bove, it cn be concluded tht, lthough this instlltion project took plce in benign wve conditions, the coupled behvior cn be recognized for both roll nd pitch of the brge. 8 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms

6 CONCLUDING REMARKS The use of sheerlegs for lifting opertion provides robust solution for hevy lifts. When using the sheerlegs in n offshore environment, there re number of sensitivities tht hve to be delt with. This pper gives insight in these sensitivities from both technicl nd opertionl point of view. From the nlysis presented in this pper it cn be concluded tht: The dynmic prt of the lods in the lifting pplince should be nlyzed by mens of motion nlysis prior to n offshore instlltion project. The resulting dynmic lod will decrese the Working Lod Limit of the lifting pplince. Bsed on the in-house development criteri with respect to stbility of the sheerlegs, the criticl situtions with steep A-frme ngles should be ssessed crefully. Governing with respect to stbility is the stbility rnge rther thn the initil stbility. REFERENCES [1] Koster, V., Lifting stbility ssessment Tklift 4, report No.9-11-4-R-3 rev. C, July 12 21. [2] Holtrop, J., A sttisticl Anlysis of performnce results, Int. Shipbuilding Progress, Vol. 24, 1977. [3] Noble Denton brge motion reserch project, Report No.L1214/NDA/JBW July 1984. [4] Gerritsm, J. nd Beukelmn, W. Anlysis of the resistnce increse of fst crgo ship in wves. Interntionl Shipbuilding Progress, Vol 18, 1972. [5] Dlling, R.P. Model tests for 91m Sheerleg, mrin report No.2.12769-1-OE/ZT, June 1995. [6] Lewis, Edwrd V (Editor). Principls of Nvl Architecture 2 nd revision, Volume II Motions in wves nd controllbility, Published by the Society of nvl rchitects nd Mrine Engineers, pge 79, 1989. [7] Holtrop, J., De weerstnd vn bkvormige vrtuigen, Schip & Werf de Zee, volume 3, 1991. The reltive lrge roll dmping of the pontoon mkes the roll behvior less sensitive for wve excittion with periods round the nturl roll period. Due to the lrge initil longitudinl stbility of the pontoon the sheerlegs cn be pre-bllsted such tht the lifting opertion cn tke plce without ny bllst trnsfer during the lifting opertion. From the full scle mesurements during n instlltion project, interction between lod nd sheerlegs cn be observed, even in benign wve conditions. 9 Copyright 211 by ASME Downloded From: http://proceedings.smedigitlcollection.sme.org/ on 11/25/214 Terms of Use: http://sme.org/terms