Subsea Flexible Flowline Installation

Subsea flexible flowline installation issues and soluti solutions ons Kevin Kevin Huang Huang (DMAR Engineering Inc. ，Houston ， Texas 77094 ，USA)

Abstract：Flexible flowlines and risers have been increasingly used for deepwater and ultra-deepwater field applications， partially because of its low submerged weight and better dynamic characteristics comparing to rigid pipelines. The offshore installation of flowline may have advantages as well. However  ，it has special needs for the installation aids ，and it is challenging to install tie-in structures due to its low bending stiffness. This paper is to present some of the challenges during a recent flexible installation project with a total of more than 100 km flexible flowlines，and 24 in-line sleds/pipeline end termination(PLET) in water depth up to 1 300 m. Key words：flexible；installation ；normal lay；in-line sled；PLET

1 Introd Introduct uction ion Flexible Flexib le flo flowlin wlines es hav havee bee been n wid widely ely use used d for  subsea oil and gas field development. Flexible com prises of a multi- layer structure of helically wound meta me tall llic ic wi wire ress an and d ta tape pess an and d ex extr trud uded ed th ther ermo mo-[1，2]  plastics ，and it can be designed to meet the specific req requir uireme ements nts for dif differ ferent ent app applic licatio ations ns bas based ed on conveyed conve yed fluid comp compositio osition n， pressure，temperature， water wat er dep depth th ， servi service ce condi conditions tions ， et etc. c. It al also so ha hass many other advantages when compared to rigid pipelines. However ，flexible is still a relatively new concept， and its ins instal tallat lation ion cou could ld be cha challe llengi nging ng as well， es espe peci cial ally ly wh when en pi pipe pelin linee en end d te termi rmina natio tion n (PLET) and in-line sleds are tied in. Related Relat ed public publications ations on flexib flexible le insta installation llation are still rare. This paper is the first of its kind to focus on the flexible flowline installation，i.e. the common issues associated with flexible installation ，and provide guidelines and recommendations that could lead to solutions. Flexible installation includes the major activitiess su tie such ch as lo load ad ou outt，transpooling，initia initiation tion with PLET，normal laying，in-line sled tie-in， buoyancy module mod ule sel select ection ion，  buoyancy module underwater  transfer ，PLET/sled landing，and curve laying，etc. In this paper ，the flexible initiation and laydo laydown wn with PLET are discussed in Section 2 and 3 respectively.

 Received  20 March

2013

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During PLET initia initiation tion，PLET position control is essent se ntia iall to en ensu sure re th that at PL PLET ET is la land nded ed wi with thin in th thee  planned target box，while during PLET laydown，the layback distance is important to ensure a safe landing of PLE PLET T. It is rec recomm ommend ended ed to ini initia tiate te the 1st end PLET with crane whipline ，and abandon the 2 nd end PLET with A & R winch combined with buoyancy modules. Then typical issues during in-line sled tie-in are presented in Section 4. The issues include sled upending，lowering and landing. Sled maneuvering on the ves vessel sel dec deck k req require uiress a PLE PLET T han handli dling ng sys system tem (PHS). The PHS provides accurate control of the sled  position at the worktable in order to be tied in to the flowline. flowlin e. Exist Existence ence of in-line sled also complicates complicates the lowering and landing procedures. After that ，the  buoyancy module wet parking and subsea transfer are assessed in Section 5 and 6 respectively. Effect of bottom current and rigging snatching loads are the main areas studied. Then this paper continues to discuss the major concerns during flowline trans transpoolin pooling g in Section 7. Flexible transpooling is the area that could be easily eas ily ove overloo rlooked ked by man many y ins instal tallat lation ion eng engine ineers ers， while whi le it is one of the activitie activitiess tha thatt inc incide idents nts occur  occur  most often. Last，flexible curve laying is investigated in Section 8. Curve laying in strong current could be difficult；however ，it cou could ld bec become ome much eas easier ier if  the curve stability is well understood，and vessel is

maneuvered properly based on the current strength and heading. Another option is to pre- install turning  points along the lay corridor ，which will add extra time to install and recover the turning points.

2 Flowline initiation with PLET Flowline is usually initiated with PLET， just like rigid pipe flowline. The PLET could be lowered through the water column with flexible extending to the installation vessel，as long as the flexible ’s tension capacity is sufficient. When the PLET approaches to the sea floor ，it needs to be upended to a level position，and then landed on the target box. Therefore ， the critical stage is this upending process. There are two options to consider. 1）Option 1：use buoyancy modules. The buoyancy modules could be attached to the PLET either on the installation vessel，or through underwater transfer. When the buoyancy module is attached on the installation vessel，clump weight is also needed to ensure the PLET sink under water. This would require some simultaneous operations (SIMOPs) on the vessel. When the buoyancy module is transferred underwater ，a wet parking buoyancy module may be neces-

sary，and remotely operated vehicle (ROV) operations are also required. 2）Option 2：use crane whipline. In this option the crane whipline provides lifting force and PLET. This option assumes the whipline lifting capacity and wire length are sufficient to lift the PLET at the designated water depth. No buoyancy modules are necessary to upend the PLET ，and the PLET positions can  be actively controlled during landing. However ，the crane tied to the PLET and not available for other lifting operations，and it also requires the layback distance to be sufficiently small since the crane can only reach a limited distance from the vessel (maximum lifting radius). Both options are deemed feasible for flowline initiation. Option 1 is more general while Option 2 is limited by layback distance. When layback distance is not a concern ，Option 2 is recommended for better  control on the PLET positioning. This option was also selected during the project (flowline initiation with PLET in more than 1 000 m water depth). Fig.1 illustrates the installation configurations after the whipline is connected to the PLET.

Fig.1 Flowline initiation with PLET Note: DMA—dead man anchor 

3 Flowline laydown with PLET Flowline 2nd end laydown with PLET is different from flowline initiation because the PLET needs to be controlled at upright position throughout lowering， otherwise the flexible torsional moment will rotate the PLET (lift the PLET mudmat on one side，and sink on the other side). Therefore ，it is recommended to attach a buoyancy module to the PLET on the vessel. Yoke could be included in the PLET ；however ，it

is not necessary. Tugger lines are usually needed to control the possible PLET rotation when the PLET is  being lowered into the water (before the buoyancy module is fully submerged and the upright lifting force is in effect). Deployment winch may be used to lower the PLET. Layback distance is to be carefully defined to ensure the bottom tension is within the acceptable range. Fig.2 illustrates the flowline laydown  process.

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Fig.2 Flowline laydown with PLET

4 Flowline laying and sled tie-in In-line sled tie-in usually requires a PHS. It provides 3- degree- of- freedom translational movements and 2- degree- of- freedom rotational movements for  sleds/PLETs positioning during tie- in. In-line sled also requires buoyancy module during lowering. When the sled is lifted up from the PHS and lowered into water ，the buoyancy module is held by the crane and has no effect on the sled. Due to its center of gravity (COG) offset and installation vessel motions，the sled/PLET has a tendency to rotate about the flexible. It was found that if the swivel joints are locked ，then the sled will not rotate. The twisting moment is resisted by the flexible flowline’s torsional stiffness；the residual torque within the flowline is negligible because most of the residual torque is released before the flowline is hung off on the worktable. After the sled is lowered into the water column with buoyancy module attached ，the sled is in nearly vertical position. The static clearance between the flowline and the buoyancy module is quite small ，and  proper analysis is required to ensure the buoyancy

module will not clash with the flowline. When strong current exists，the current drag force may push the  buoyancy around the flowline，and clashing is to be checked under such condition as well. The dynamic installation loads are to be calculated at all installation stages： flange connection， lowering through splashing zone [3]，lowering through water column，and landing. In general the installation vessel may not be able to function as weather vane when the sled is on the work table and being lowered through the splashing zone. When the sled is close to the sea floor ，its installation loads are sensitive to its  position. To capture the maximum loads，the whole  process of sled landing should be simulated continuously. Fig.3 illustrates a typical load time history during sled landing. It shows that the sled landing process takes about 5 min before the sled/PLET is com pletely stabilized on the target box. The flexible installation loads are also related to the flowline conditions， whether they are empty or flooded. Fig.4 shows the snapshots of the flexible installation with in-line sled [4].

Fig.3 Typical installation load time history

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Fig.4 Flexible installation with in-line sled

5 Buoyancy module wet parking Sometimes it is necessary to wet park the buoyancy modules. Usually the buoyancy module has a large cross sectional area that could receive large lateral force if subject to strong current. Excessive lateral force could push the wet-parked buoyancy module away from its intended position，and cause clashing concerns. This effect should be evaluated using typical installation analysis software tools，or estimated through simplified calculation. And buoyancy module installation tolerance should be considered properly to develop the maximum buoyancy module motion envelope.

6 Buoyancy module underwater transfer Buoyancy module underwater transfer is frequently used during flexible installation with structures， especially when the structures have heavy weights. For example，when two buoyancy modules are needed for the heavy sleds installation，one of the

 buoyancy modules is attached to the sled at the vessel. And the 2nd buoyancy module is pre-installed on the sea floor with a 65 m long buoyancy module rigging and an 18Te DMA. Another 25 m long rigging is taped on the buoyancy module and will be connected to the sled when it passes by on its way to target box. As the sled continues to land ， the wet- parked  buoyancy module is passively transferred onto the sled. Numerical simulation indicates that before the wet-parked buoyancy module is fully transferred onto the sled， both 65 m rigging and 25 m rigging could go slack and taut in each cycle the sled moves vertically. To avoid the uncertainty of possible high snatching loads，a 10 m polyester segment is embedded into these two riggings [4]. Dynamic simulations confirm that the addition of these soft slings could effectively suppress the snatching loads ， hence increase the safety margin of the installation. Fig.5 shows the snapshots before and after the buoyancy module underwater transfer.

Fig.5 Example of buoyancy module underwater transfer

7 Flowline transpooling Flowline transpooling could be very challenging  because of the twisting (pig tailing) phenomenon，as shown in Fig.6. Flexible has residual torque during manufacturing. The residual torque could be released  partially or wholly during transportation，storage，or 

load out. It is usually not possible to estimate the residual torque. During installation，the flexible length in suspension is quite long，and installation vessel could also change orientation to release the torque if  necessary. Therefore，this issue occurs more often during flexible transpooling. Theoretically it is not  possible to eliminate this phenomenon，some guide-

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lines to mitigate the risks associated with the flexible  pig tailing are as follows. 1）Know the flexible allowable twisting limits. These limits should be evaluated and provided by the flexible vendors. 2）Use straight line arrangement for transpooling. If straight line arrangement is not possible， sharp turns need to be avoided. 3) Increase the flexible free suspension length， to allow for easy and early identification of the pig tailing，and higher twist angle tolerance. 4) Monitor the flexible closely during transpooling. Once pig tailing occurs，stop and select appro priate contingency plans.

5) Transpooling tensioner could prevent the flexi ble twist from propagating from one side to the other  side，and accumulate the twist at one side. Open the tensioner could balance the torque between the two sides of the tensioner. 6) The transpooling procedures should be reversible. This would allow the flexible to be restored  back to where it comes from，and provide a final safe  plan in case all other contingency plans fail. Overall，flexible transpooling needs to be carefully planned. There has been many incidents resulting in damaged flexible pipe interior layer ，or damaged outer sheath in past projects. Yet this phenomenon has not been fully understood as of now.

Fig.6 Flowline twisting during transpooling

8 Flowline curve laying Curve stability depends on the friction force between the flowline and the seabed soil. Flowline curve laying could be difficult， especially laying small curves with one or more sleds suspended in the water column. The reasons are：a. the layback distance is higher than the normal laying condition (without sleds)，and consequently the flowline static bottom tension is also higher ； b. the existence of the sleds causes higher dynamic tensions within the flowline. When subject to strong bottom current，the current could induce lifting force on the flowline，and reduce the friction force between the flowline and the soil. The flowline bottom tension varies with different sled positions. To determine the curve stability during flowline installation ， several scenarios need to be checked ： a. a curve is being laid while one or more sleds are lowered down through the water column，and in this case the flowline bottom tension at the touch down  point is used for curve stability check ； b. a curve has  been laid not too far away from the sled target box. In this case this curve also requires stability check because the flowline bottom tension is much higher dur-

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ing sled landing. The tension for curve stability check  would be the flowline tension at the touch down point minus the axial friction generated by the flowline straight segment between the curve and the touch down point. For better curve stability it is recommended to position the small curves at a certain distance away from the sled target box. Note that the curve stability during installation also depends on the laying direction. Losing curve stability during installation results in flowline slippage，which could relocate the flowline outside of its laying corridor ，generate unnecessary overlength ，and cause the downstream sled to miss its target box. Usually the flowline needs to be recovered after slippage occurs. Curve stabilizing devices，such as sand bags ，should be considered for small radius curves that have high possibility of losing stability. Fig.7 shows an example of the flexible curve laying with sleds [4]. Installation vessel maneuvering is also critical to ensure the vessel is on the correct position that will not over pull the flexible. One example of the vessel trajectory is shown in Fig.8，where the vessel is laying a 180 degree curve turn with small radius under strong current coming from South [5].

Fig.7 Flowline curve laying

Fig.8 Vessel trajectory during curve laying

9 Conclusions This paper systemically studied many of the to pics covering most areas for flexible installation. Flexible installation could be challenging if its characteristics are not well understood. Flexible flowlines are usually connected to structures such as PLETs and inline sleds for jumper tie- in. The existence of structures complicates the flexible installation，and is the major area of concern. Flexible jumper installation is another area deserves some attention，and many issues have been observed during offshore installation ， such as connector upending ， jumper twist，connector  orientation correction，and connection. These areas are not included in this paper. In summary，typical issues during flexible installation have been discussed ，

and recommendations based on actual project experience are given. References [1] American Petroleum Institute. API 17B Recommended practice for flexible pipe[S]. 3rd edition. 2002. [2] American Petroleum Institute. API Specification 17J Specification for unbonded flexible pipe[S]. 2nd edition ，1999. [3] Det Norske Veritas. DNV 156 Rules for planning and execution of  marine operations Part 2 Chapter 6 special sea transports[S]. 1996. [4]

Huang K ，Ji A，Uribe E. Deepwater in- line sled installation methods and its application to frade project[C]// Proceedings of  the Offshore Technology Conference， OTC 19805. Houston， TX，the United States ，2009.

[5]

Huang K ，Diao W. Flexible laying in strong current[C]//Deep Offshore Technology Conference ，DOT presentation. Houston， TX，the United States ，2010.

Author Kevin Huang，male， born in 1971，graduated from Tsinghua University. He is currently an engineering manager at DMAR Engineering Inc.，in Houston. Dr. Huang has published more than 20 technical papers in the international journals and proceedings. He has more than 18 years experience in the offshore oil and gas industries，and is expertized in floating production system design ，riser system engineering ，flowline and umbilical engineering，and subsea installation. He has employment experience with the major oil and gas service companies and installation contractors ，including ABB Deepwater ，Aker Kvaerner ，Acergy，and Technip. He can be reached by E-mail：[email protected]

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