Free spa n inte rvention in the pla tform exclusion zone – risk a ssessment a nd the method for correctionLjiljana D. Oosterkamp,

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Introduction

• Before new subsea structures a nd pipelines a re insta lled a t the sea bed, de ta iled geotechnica l investiga tions a re ca rried out.

• Environmenta l loa ds a re ta ken into a ccount when doing the engineering ca lcula tions to ensure sta bility in opera tion

• Avoid a rea s prone to the sea bed mobility a nd erosion• Presence of la rge structures on the sea bed such a s Pipeline End Ma nifold (PLEM), Subsea

Va lve Sta tions (SSVS) a nd la rge dia meter pipelines ca n significa ntly cha nge the current pa tterns.

• In a rea s with soft soil or sa nd, insta lla tion of these structures often promote sea bed erosion.

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Introduction

• Free spa ns ca n form on pipelines connected to la rge structures a nd/or to the risers, a fter sta rt-up of opera tions.

• Free spa ns ca n be critica l both in te rms of una ccepta ble bending moment due to sa gging a nd environmenta l bending moment due to current a nd wa ve a ctions a nd Vortex Induced Vibra tions (VIV).

• Free spa ns close to offshore insta lla tions a re often difficult to correct a s this opera tion is a ssocia ted with the risk of collision between the vesse ls conventiona lly used for correction a nd the offshore insta lla tions.

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Pipe line system

• La ngeled Pipeline System is (norma lly) opera ted a s two pipelines: • 42” La ngeled Pipeline North (LPN) a nd • 42”/44” La ngeled Pipeline South (LPS). • Ga s is diverted via import 30 ” flow lines to Sle ipner Riser Pla tform (SLR),

it is mixed with ga s from other sources to a chieve the correct composition, further exported via export 30 ” flow line to La ngeled South.

Overa ll dimensions including pup pieces 3820 0 mm by 14463 mm

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Free spa n deve lopment

• Following the onset, the scour sprea ds a long the length of the pipeline genera ting a free spa n.

• When the length of the suspended spa n of the pipe becomes sufficiently la rge , the pipe begins to sa g into the scour hole .

• Conta ct with the sea bed will block the flow, scour process will come to the end a nd the ba ckfilling process will sta rt.

• However, this na tura l process of ba ckfilling is sometimes prevented by a loca l topogra phy often crea ted by e .g. rock interventions

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Free spa ns a t both ends of 30 inch flow lines

As-insta lled free spa ns were engineered a nd new configura tion did not develop a s predicted by design. A deta iled study wa s ca rried out.

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Finite Element Model

• ABAQUS softwa re bea m pipe e lements (PIPE31) were se lected to simula te concentra ted a nd distributed loa ding, tempera ture loa ding a nd interna l a nd externa l pressure loa ding including the end ca p effect.

• Flowlines were first a ligned a nd tied-in to the risers a nd subsequently connected to the SSVS (a s wa s done during insta lla tion).

• A “deflect- to-connect” opera tion wa s performed to a lign a nd recover misa lignment between the 30 ” flowlines a nd the SSVS stick-out pipes.

• Therefore , horizonta l offse ts a nd rota tions ha ve been a pplied to the Finite Element Model to a ccount for "deflect- to-connect" opera tion induced loa ds.

• “Deflect- to-connect” opera tion during tie - in ma inly genera ted bending stresses a nd torsion stresses were of a sma ller order of ma gnitude .

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Results of the FEM a na lyses

• Eigenva lue Ana lysis a nd Deta iled Response Ana lysis were performed for the worst free-spa ns, which were a t SSVS side .

• Fa tigue da ma ge a fter 10 yr exposure wa s found to be well under the limit, considering a pplica ble environmenta l da ta (Weibull pa ra meters, Hs-Tp wa ve sta tistics, directiona lity).

• Residua l fa tigue life wa s found to be a ccepta ble a nd over 50 yr design life .

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Sensitivity a na lysis

• Although reported differently, modelled pipeline configura tion suggested tha t the flowlines a t the riser side were still in conta ct with the sea bed, so free spa ns were less critica l compa red to the spa ns a t SVSS side , where there wa s no conta ct.

• To eva lua te the potentia l critica lities a ssocia ted with a possible increa se of free-spa n length a t riser side , sensitivity a na lyses ha ve been performed simula ting the loss of conta ct with the sea bed to eva lua te the potentia l critica lities in ca se of increa se of free-spa n length.

• Sea bed configura tion re leva nt to survey 20 16 ha s been modified to lea ve the flowlines on the riser side in free spa n condition.

• It wa s found tha t the tota l bending moment for the export flowline exceeds the a llowa ble va lue .

• FE model reproduced the flowline geometry by fitting the survey da ta , a ssocia ted with the uncerta inties, so the decision ha s been ta ken to rectify a ll the free spa ns

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Ca lcula tion results

• Decision wa s ta ken to sta bilize the sea bed a nd perma nently improve the configura tion, by rock dumping of the entire section from the “goose necks” touch down to the loca tion of the nea rest post- la y rock berms.

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Risks a ssocia ted with the correction

• Free spa n correction is conventiona lly ca rried out using dedica ted rock dumping vesse ls.

• A collision study ca rried out using a n in-house developed tool for qua ntifying the risk of collision between a vesse l performing the opera tion a nd nea rby pla tforms ha s been ca rried out.

• The wea kest of a ll the pla tforms in the fie ld is Sle ipner Riser pla tform (SLR) a nd a s pressurized risers connected to the ja cket structures a re exposed, a n a pproa ch of a conventiona l rock dumping vesse ls to SLR wa s una ccepta ble .

• The scope for rectifica tion ha s been split in two.• Rectifica tion of free spa ns a t Subsea Va lve Sta tion (SSVS) ha s been

ca rried out using one of the sma ller subsea rock insta lla tion vesse ls, “Sea horse”.

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Risk a ssocia ted with the correction, SSVS side

• For all drive -off situations , (tha t is a loss of position without a loss of power) • With proximity opera tions 50 -80 m from Sle ipner T, impa ct energy wa s estima ted to be over Design Accidenta l

Loa d of 14 MJ , so ca ta strophic outcome of a collision wa s a nticipa ted. • Time for intervention of a drive-off wa s ca lcula ted to be be tween 60 a nd 90 seconds) depending on dista nce to

SLT/SLR. • 60 seconds (a nd a bove) is sufficient for a successful recovery before collision with a fa ilure proba bility of 0 .1.

• Opera tion ha s been successfully comple ted by rock dumping vesse l “Sea horse”

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Risk a ssocia ted with the correction, SSVS side

• For a ll drift -off scenarios , (a ssocia ted with the bla ckout a nd drifting with wea ther) it is very difficult to a nticipa te the time for recovery

• This mea ns tha t a collision will occur if wea ther conditions a re such tha t drifting is towa rds the pla tforms.

• For this rea son, drift-off conditions (tha t is wea ther a wa y from a pla tform) a re a n a bsolute necessity

• drift-off requirement towa rds Sle ipner T, Sle ipner Fla re a nd Sle ipner riser pla tform with recommended hea ding 0 80 .

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Free spa n correction a t the rise r side

• Further work wa s required to find a n a lte rna tive method for correction, a s it wa s not possible to a llow a ny vesse l a t the side of the riser pla tform where the free spa ns were loca ted, due to exposed pressurized risers a t Sle ipner Riser Pla tform.

• Decision wa s ta ken to do a correction using the dredger, ca pa ble of repositioning of the stones from the designa ted a rea 10 0 m from the riser pla tform to the free spa n loca tion.

Vessel position during deployment of Sca nma chinein gra veldump position 1-2 hours

Vessel position during deployment of Sca nma chineoffse t gra veldump position 1-2 hours

Vessel position during deployment a nd emptying filte r units 24 hours

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Free spa n correction a t the rise r sids, Sca nma chine

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Risk a ssocia ted with the correction Normand Ocean :

• Proximity opera tion; 10 0 -110 m@SLR , >120 m@SLT• Hea ding ~ ma inly side towa rds R• Hs ma x. ~ 3.0 m ; Dura tion = ~ 74 hrs Risk picture• Drive-off results in a n impa ct energy of 31 MJ @SLR, significa ntly la rger tha n Design

Accidenta l Limit (14MJ ) for SLR/SLT/SLF• Risk for ca ta strophic outcome (loss of SLR). Ga s riser sticking out of ja cket structure .• Time a va ila ble for successful recovery/intervention towa rds SLR (~59s)

• Drift-off 17 MJ @SLR)• Drift-off critica l towa rds riser, a nd thus ca ta strophic outcome if hit of riser!

• SLT a nd SLF ha ve simila r risk picture .Risk reducing measures and recommendations:

• Drive-off intervention tra ining required• Drift-off requirement towa rds SLA, SLT, SLR & SL• Hea ding such tha t vesse l ha s side ma inly towa rds SLR• Residua l risk due to Drive-off potentia l is a bove structura l ca pa city of SLR/SLT/SLF, which

implies a Major accident risk to personnel a t vesse l, pla tforms, a sse t a nd reputa tion. Due to a sepa ra tion of minimum 10 0 m a nd estima ted a va ila ble time for drive-off intervention (a bout 1 min. tra nsverse), the risk is a ssessed to be a t a “normally tolerable” level.

Ga s riser

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Releasing and emptying FU in the dump area Emptying FU in dump area

Scanmachine, suction end at gravel ROV a t the blowing end

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As-left survey 30” export flowline As-left survey 30” import flowline

Opera tion ha s been successfully comple ted

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Opera tion ha s been successfully comple ted

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Dr. Ljilja na Dja pic Oosterka mp, Specia list Pipe & Tra ns Pipe Opera tions

Free spa n inte rvention in the pla tform exclusion zone – risk a ssessment a nd the method for correction

Specia l tha nks to:Andrea Ba ldoni a nd Alberto Ba tistini, Sa ipemLa rs Tronsta d (Advisor Sa fe ty) a nd Tone Utha ug (Principle Engineer, Subsea Opera tions), Equinor a nd

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