OTC 17627

Recent Achievements and Present Trends in Deepwater Pipe-lay Systems E.P. Heerema, Allseas Group S.A.

Copyright 2005, Offshore Technology Conference This paper was prepared for presentation at the 2005 Offshore Technology Conference held in Houston, TX, U.S.A., 2–5 May 2005. This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Over the past ten years, deepwater pipelaying has gone through an impressive development. Production from oil and gas fields is foreseen in up to 11,000 feet water depth, and this may not yet be the limit.

The various existing installation methods have their own merits in the deepwater market.

In-line structures and steel catenary risers can be installed safely and efficiently, and rotation can be well controlled.

Repair procedures are proven technology. Potential limits to deepwater pipelay are discussed.

Introduction Deepwater pipelaying has gone through a spectacular development. Before 1995, a water depth of 1,000 feet was considered “deep”; since then, pipelaying in water depths of 5,000 feet and over has become normal practice. Future installations in over 8,000 feet are a reality and installations in 11,000 feet are being studied.

See figure 1. Methods of deepwater pipeline installation Three basic deepwater pipelay methods are commonly in use: reeling, J-lay and S-lay. The author assumes these methods to be generally known to the reader. Reeling is often the most economical method of installing small-diameter lines (up to 16”) of limited length. Specific market conditions, and project conditions such as the use of exotic materials, however, have justified reeling of larger lengths.

J-lay, dependent on the size of the vessel selected, can be technically very suitable for deepwater pipelines. As J-lay is inherently not a fast system (welding, non-destructive testing and field joint coating all need to take place in one or two stations in the J-lay tower), it is generally a competitive

method when deepwater pipelines are heavy or short; especially when combined with heavy lifting work, saving mobilization of separate vessels. For this reason, J-lay has found a place in the market primarily in installing deep pipeline ends, notably steel catenary risers. On some occasions, J-lay vessels have installed long pipelines.

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Figure 1 - Pipelines in water depths over 3,000 feet

S-lay quantitatively dominates the deepwater pipelay work in the Gulf of Mexico – over three-quarters of all pipelines, measured by length, in over 3,000 feet water depth have been installed in this manner. S-lay requires heavy tension equipment and a long stinger. To effectively utilize the tension equipment, the departure angle of the pipeline leaving the stinger should be near-vertical, so a long stinger is needed to provide sufficient guidance for the pipe from the horizontal to the near-vertical. To limit overbend strains in the pipeline, for larger pipe diameters a relatively large stinger radius is required and therefore a longer stinger to meet the near-vertical departure angle.

Integrating pipeline end manifolds and large in-line structures has proven to be well feasible and safe, as is demonstrated by the vessels’ track record.

Another reason for wishing to achieve a near-vertical departure angle in S-lay is to limit the tension force in the pipeline on the seabed in areas with an uneven seabed, thus avoiding free spans.

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Upgrades in capacity have moved previously existing boundaries. As a result even the heaviest pipelines, such as pipe-in-pipe systems and water-filled pipelines, can be installed well in S-lay. For example, in 2002 “Solitaire” successfully installed an 18” pipeline in a flooded condition in 6,300 feet water depth. Limitation of overbend strain in deepwater S-lay Once a deepwater pipe has left the pipelay vessel’s stinger, essentially the pipe configuration is the same for every deepwater installation method: the allowable sagbend strains determine the required angle of departure from the stinger. This departure angle, in combination with the stinger length, determines the required stinger radius. This radius yields a certain overbend strain, and this overbend strain has to be checked against client specifications or allowable strain levels defined in international codes.

A widely accepted code such as DnV OS-F101 requires that every installation case is evaluated individually by analyzing a number of relevant limit states. Pipeline limit states during pipelay are for example local buckling and ovalization on the stinger. This approach usually leads to significant overbend strains being allowed. In current practice, values allowed by clients are generally lower, in the order of magnitude of up to 0.45%, which represents a residual strain in the pipe of 0.25%.

One of the concerns arising from high overbend strains is potential rotation of the pipeline during installation and consequent twisting (“cork-screwing”) on the seabed. Although pipeline rotation is being observed, widespread experience from seabed surveys shows that pipelines laid at such residual strains on the seabed are straight. Prevention of in-line structure rotation Preventing rotation of in-line structures such as “wye”, “tee” and valve assemblies is obviously very important in order to achieve the required orientation relative to the seabed. Requirements are strict: tolerances on orientation are generally plus and minus 5 degrees.

Figure 2 - Site Integration Testing of WYE piece with mudmats

This is achieved by the use of yokes and buoyancy provisions. With all installation methods mentioned above, this works well. In S-lay, vessels such as "Lorelay" and “Solitaire” have successfully installed some 12 deepwater in-line structures in the past ten years within the required verticality requirements without corrective measures. Figures 2 and 3 show examples of such in-line structures applied in the Gulf of Mexico.

Figure 3 - WYE piece ready for inserting in firing line

Steel Catenary Risers For connection of pipeline systems to production facilities, many steel catenary risers (SCRs) have been installed by all pipelay methods. This can be done in ”first end mode”, during start-up, or ”second end mode”, at the lay down end.

In S-lay, second-end SCRs are well feasible, for example by laying down the pipe from the stinger and retrieving it in J-mode in a frame on the side of the vessel. Alternatively this can be achieved by means of a direct lateral hand-over from the pipelay vessel’s stinger to the production facility. A winch on the platform or on the pipelay vessel can pull in the SCR.

See figure 4.

Figure 4 - Hand-over of 2nd end SCR to platform

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A J-lay crane vessel may in some cases, which is when the platform topside does not have a large overhang, have an operational advantage here, as the crane can hand the SCR directly to the production facility.

First-end SCRs are naturally straightforward for S-lay vessels, whereby the SCR is pulled directly from the vessel’s stinger into the hang-off seat located on the side of the platform. This method has an advantage when SCRs have to be installed in between an anchor pattern or risers already in place. Repairs Pipelaying in deep water may appear straightforward using a dynamically positioned vessel, but when problems arise, the consequences are normally costly and very time-consuming. Deepwater pipeline repairs have been executed by a number of contractors successfully and as such have led to industry- accepted contingency methods.

As early as 1996, a 12” pipeline was lost in the Gulf of Mexico in 5,300 feet water depth. The pipeline was cut and successfully retrieved by remotely operated tools commonly used in the drilling industry. The entire procedure, including de-watering from the other end, however required many weeks and led to considerable financial impairment of the contractor’s project. Necessary upgrades of present equipment to cope with upcoming projects Large-diameter pipelines such as 24” O.D. in water depths up to 8,000 feet are planned for installation as early as 2006. These developments require both tension and abandonment and recovery (A&R) capacities on an S-lay vessel of some 1,450 kips (650 metric tons). This has made upgrades necessary. For example, in the course of 2005 “Solitaire” is being upgraded to a total tensioner capacity of 1,930 kips (875 metric tons), and more in later years; and an A&R capacity of 2,220 kips (1,000 metric tons).

Figure 5 - Construction of new 140m stinger

In addition to this upgrade, in March 2005 the vessel was fitted out with a new, massively powerful stinger of 460 feet (140 m) length, capable of holding pipes with a weight of 2,200 kips (1000 metric tons) in anticipation of the heaviest upcoming projects (see figures 5 and 6).

Figure 6 - New 140m stinger with new hang-off system

In order to limit strain concentration (“point loads”) on rollers, the new stinger has been equipped with more roller supports than before, and also equalizers, resulting in a smoothly distributed strain level. Market conditions Naturally, the majority of pipelay projects is much lighter and therefore most can be installed by lighter pipelay equipment. Many contractors have invested in pipelay equipment of varying capacities over the past years and as a result the market generally, other than in 2005 and 2006, suffers from an over-capacity of deepwater pipelay equipment. Most pipelay contractors jumped on the bandwagon and in such an environment, making a profit is not easy or common. Moreover, mistakes are dearly paid for. Dropping a pipe from a pipelay vessel in “shallow” water may require a week or so in pipe cutting, de-watering and retrieval; recovery of a lost pipe in deep water can cost many weeks. Lost time is no longer covered by the standard Construction All Risk policy as a result of the tightness of the insurance market, and getting

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paid for the lost time by the client if the client is self-insured is very difficult, particularly in the contracting environment of the recent past. Price levels in the past years did not allow for the necessary margins to cope with the consequences of accidents, and this has left many pipelay contractors financially in dire straits. At present, the market is momentarily better, and contractors have learnt their lessons in what risks they can accept, so that onerous contracting conditions are less prevalent. Limits to deepwater pipelines? Where are the limits to pipe size and weight? Hydrocarbons have been identified in water depths of over 11,000 feet. But preventing hydrostatic collapse of gas lines clearly limits pipeline size. For example, in 8,000 feet water depth a 24” O.D. line requires a wall thickness of in excess of 30 millimeters. The present wall thickness limitation for SAW pipe is 40 to 45 millimeters. A 24” line with such wall thickness would have a water depth limitation of roughly 12,000 to 13,000 feet. Pipe mills have limitations in the achievable diameter to wall thickness ratio. In deeper water, the pipe diameter would therefore have to be reduced.

A pipe of 24” in these water depths would generate a static tension and A&R requirement of roughly 2,200 to 3,300 kips (1,000 to 1,500 metric tons) on the pipelay vessel. Oil lines can be laid water-filled to avoid having to meet the hydrostatic collapse criterion. As a result, however, these are very heavy during laying. Pipe-in-pipe systems can also be very heavy; when diameters become large, such as 12” in 18”, high tensions are necessary. Further upgrades are possible; on ”Solitaire”, for example, an arrangement named “S-Lift” can be installed, enabling the installation of pipelines with a static holding capacity requirement of 3,300 kips (1,500 metric tons) and a dynamic capacity of 4,400 kips (2,000 metric tons).

The basic idea of S-Lift is to reduce the load on the stinger, the tensioners and the A&R system through a system of two submerged clamps which take the load of the pipe below the stinger. The pipe clamps will be suspended from a cantilever structure installed on top of the cross-over extending 203 feet (62 m) aft of the stern of the vessel.

The system can be used either to provide contingency holding in case of pipe flooding, or for continuous pipelay.

For contingency holding, only one clamp will be deployed, which will passively slip over the pipe as normal pipelay progresses. Only in case of a calamity such as accidental flooding of the pipe, leading to excessive tension, the contingency clamp is activated. The pipe can then be safely lowered by the contingency clamp to the seabed for de-watering.

For continuous pipelay, two clamps will be deployed, working according to the linear winch principle which is known from other industry applications.

When using the S-Lift method, the axial pipe strain will be reduced to a small value. Therefore most of the strain capacity of the pipe can be used for global bending.

The cantilever structure has been fabricated and a number of long lead components such as winches have been purchased, but the timing of final investments and installation of the S-Lift system on board will be determined by upcoming project requirements. Conclusion Deepwater pipelay has in a few years’ time become safe and economical. Although the various installation methods have proven their suitability and each have their specific advantages, the choice of equipment is determined primarily by pricing policy. On small diameter lines of limited length, reeling is very competitive. J-lay has clear advantages when combining heavy-lift work with SCR installation and can be attractive for heavy, short lines. S-lay is fast and economical, and dominates the market for deepwater pipeline installation. It can deal with SCRs and in-line structures, and can avoid their rotation equally well as the other pipelay methods. By carrying out the required equipment upgrades, the current trends in deeper water pipeline systems can be accommodated by the S-lay installation method.

The contracting industry has responded well to the operators’ requirements. Acknowledgements The author wishes to thank Sil Draaisma, Johan Drost, Arjen Korver, Johan Vermeer and Onno Weustink, Allseas, for their valuable contributions to this paper.