Major deepwater pipelay vessel starts work in North Sea

Final touches are applied to Allseas' Solitaire pipelay vessel at the Swan Hunter yard, Tyneside, U.K., where the vessel arrived in May 1996. In February 1998, Solitaire began sea trials in preparation for her maiden pipelay project for Statoil this month.

Edward P. Heerema
Allseas Group S.A.
Châtel-St. Denis, Switzerland

Industry's deepwater pipelaying capability has received a boost this year with the entry into the world's fleet of Solitaire, a dynamically positioned pipelay vessel of about 350 m including stinger.

The converted bulk carrier, formerly the Trentwood, will arrive on station in the North Sea and begin laying pipe this month on Statoil's Europipe II project, a 600-km (372.6-mile), 42-in. OD gas pipeline from Norway to Germany.

Next year, the vessel will install pipe for the Exxon U.S.A.'s Gulf of Mexico South Diana development (East Breaks Block 945) in a water depth of 1,643 m and for Mobil Oil Canada as part of the Sable Island Offshore and Energy Project offshore Nova Scotia.

Using the S-lay mode, Solitaire is particularly well-suited for laying large lines economically, including the deepwater projects anticipated for the U.S. Gulf of Mexico. Table 1 [64,839 bytes] presents Solitaire's technical specifications.

Why now?

The requirement for gas as a clean and efficient fuel is leading to development of large gas-gathering and distribution systems throughout the world.

Until late 1997, one such area experiencing rapid economic development was Southeast Asia. Its economic setbacks have trimmed but not eliminated its potential energy-demand growth which in turn will require extensive gas-transmission pipelines.

Large and efficient pipelay vessels exist and can generally serve the market well. There are situations, however, in which it would be beneficial to lay heavier pipe in deeper water. This offers a particular challenge for a larger and more powerful unit.

Unfortunately for the contractor, those projects arise infrequently. This requires the contractor which introduces a large and sophisticated pipelay vessel to seek other work which also falls within the capability of most other large pipelay units. Such a situation depresses lay prices.

But a large unit such as Solitaire can survive these low prices by working, always on a lump-sum basis, quickly and efficiently so that each job generates a reasonable cash flow and by carrying out a large volume of work every season.

In this way, a massive instrument can be gradually depreciated in competition with units that have been amortized many years earlier.

The downside is that there will not be enough work for all pipelay units. The enormous production capacity of Solitaire is likely to lead to the withdrawal of some pipelay equipment that is now older than 20 years. This is a normal pattern in every industry and has been seen often in the offshore industry in variety of disciplines.

But the pipelay business is too small worldwide to bear replacements on a large scale. This is especially true because the Solitaire has the capacity annually to install nearly all large-diameter pipelines in the world and because much upgrading has recently been done to existing units.

Designing, building, and operating Allseas' Lorelay taught designers several lessons on what the ideal pipelay vessel should look like. The result was a desire to design and build a unit which no other entity in the world would be capable of designing at that moment.

This, an engineer's paradise, must be treated with great caution, for it is tempting to go too far and build a vessel that cannot survive commercially.

Concept, design

Knowing in advance that the vessel was going to be very expensive to build, with all its inevitable sophistication, prompted from the outset a design parameter to make the vessel very fast working, preferably laying a large-diameter pipeline in half the time of existing vessels. This meant a sustained lay rate of some 6 km/day. The direct result is a very long unit to support many welding stations.

Dynamic positioning (DP) was readily chosen because it had proven to be of great benefit to the Lorelay. The main benefits of DP, however, are evident when a vessel is working near platforms. For a vessel designed primarily for long pipelines, this is not a primary consideration.

The requirement to lay pipelines in very deepwater made the decision easy, along with the ability to abandon and recover pipelines quickly (a safety consideration). Not having to depend on anchor-handling increases workability so long as the vessel itself has high workability. There is less mechanical downtime, compared to an anchored vessel, because wear and tear on an anchoring system are intense.

A disadvantage of DP is that the vessel needs to operate in sufficient water depth to accommodate the thrusters protruding under the vessel. As a result, use of Solitaire requires the help of a small flat-bottom barge to lay shore approaches of pipelines down to about 15 m water depth.

If, therefore, a vessel is to be built to peak technological capabilities, the question arises, why not choose a semisubmersible concept?

The answer is that, if a ship-shaped hull is well designed, no significant benefits for use of a semi remain. The key is to give a vessel roll and pitch periods that approach those of a semi.

A pipelayer does not require a large static stability because crane loads are comparatively small. The vessel can therefore be allowed a small metacentric height, which is achieved by making a high build-up and choosing a hull that is not too wide.

A good pitching behavior is achieved by greater vessel length. At the stinger tip, vertical motions may be considerable because of the long arm, but the excitation angles are limited by the vessel's length being sufficiently great relative to prevailing wave lengths (Fig. 1 [52,383 bytes]).

It must be kept in mind that the workability of a spread is limited by its support vessels, such as the anchor handlers and pipe carriers. As a result, a semi will readily experience that its own movements in inclement weather do not determine the workability of the entire spread.

A ship-shape, furthermore, with its comparatively greater length, affords the opportunity for a long firing line. Additionally, a bulk carrier is strong enough to bear uneven loading and offers ample space for pipe storage. This capacity is beneficial when weather permits pipelaying to continue but prohibits cranes from loading pipe from a pipe carrier alongside. A long, deep-draught vessel can shield a pipe carrier, thereby extending the weather limits to pipe offloading.

Finally, a converted ship makes for high transit speed, an attribute significant, for example, when the vessel needs to work in the North Sea in summer and in the Far East in winter.

Construction; welding system

The original vessel, Trentwood, later named Akhdeniz then even later the Comship, was purchased in September 1992. Hull repairs were executed in fall 1992; the firing line was built inside the hull by mid-1993 by Sembawang Shipyards in Singapore.

In early 1993, all large equipment items (diesel engines, generators, high-voltage equipment, thrusters, DP system, tensioners, pipe conveyors, abandonment and recovery winch, pipe-transfer cranes, etc.) were ordered, and they had arrived by late 1994.

Well before the hull preparation was finished, the prefabrication and preoutfitting of sections for the new construction parts were started in the various workshops and construction sites. New construction and outfitting were undertaken in a total of 11 main blocks, and the conversion works in the existing hull were divided over four main zones.

In second half 1994, the accommodation block and the new aftship sections were installed, as well as the diesel generators.

In October 1995, the contract with Sembawang was terminated; in February 1996 the vessel was towed to the Swan Hunter yard, Tyneside, U.K., for the following construction phases. The vessel arrived in May 1996.

In February 1998, Solitaire left Tyneside for her sea trials.

Solitaire's required production rates, along with the impact of weld repairs and related vessel downtime, make ensuring weld quality even more important than usual. Adherence to strictly controlled, prequalified welding parameters maximizes the vessel's sustained production rates.

Allseas, therefore, developed the Phoenix automatic welding system for offshore pipelines 6 in. OD and larger. The system's programmable welding allows repeated use of parameters for each joint.

The operator uses a hand-held remote control unit to select the pass number, position the torch, and start and stop the process. Minor adjustments to some parameters are permitted, within the limits defined by the welding procedure, to allow for small variations in the geometry of the aligned pipe ends (Fig. 2 [13,707 bytes]).

All passes are applied externally, with the root pass using retractable backing shoes positioned on the internal line-up clamp.

The modular system is suitable for gas metal arc or fluxcored arc welding in the uphill or downhill welding direction. Phoenix is also suitable for pulsed MIG (metal inert gas) or TIG (tungsten inert gas) welding applications.

Phoenix has been used aboard the Lorelay since summer 1993 on a variety of pipelines with low repair percentages and high production rates.

S-lay pipelaying

Because of lay speed, S-lay is more economical than J-lay for which all preparation, welding, testing, and coating activities must take place in sequence on one spot, the deck level. Such a procedure is slow, even when long pipe strings are used.

The use of J-lay is determined by the water depth at which for S-lay pipe tensions become too large for the tensioners to hold the pipe and pipe-bending stresses exceed allowable values.

Economics have driven attempts to extend the limits of S-lay. Pipe-bending stresses can be limited to acceptable values by fitting the vessel with a very long stinger having a large bending radius. Added tensioners will bear the large weight of the suspended pipe.

Drawbacks to these measures are, in addition to investment costs, the workability limits imposed by a long stinger, extending far into the water, and the reduced firing-line length as a result of added tensioners. Some pipelay speed is therefore sacrificed.

Nevertheless, such provisions are making it possible to lay pipelines in unprecedented water depths. It is not possible to specify the maximum achievable water depth because every pipe is different in weight and strength.

So far, Allseas has installed small diameter pipelines in up to 1,643-m water depths using Lorelay in S-lay. In general, in S-lay, large-diameter lines can be laid in more than 1,000-m water depth.

Much can be achieved in extending the limits of S-lay if the conventional stress-based requirements are relaxed. Ongoing research is suggesting that the common limiting criteria still adopted by many oil companies are unnecessarily conservative. It seems likely that these installation criteria will be modified in the near future.

The older criteria were established several decades ago, but since that time pipe manufacturing processes have been considerably improved. Strict criteria were easily accepted for a long time, possibly because waters in which pipelay work occurred were relatively shallow and therefore allowable limits were easily maintained.

The acceptability of low limits, of course, changes now that the industry is moving to greater water depths.

Designs of subsea pipelines that use the so-called "strain-based" design method are becoming widely accepted. If the pipeline is subjected to a curvature and if this curvature is permanently fixed and controlled (for example, the pipeline continuously following an irregular seabed contour), the plastic bending stresses need not be combined with circumferential and longitudinal stresses occurring in the pipe wall.

The philosophy is that the bending stresses alone cannot lead to pipeline failure because they are well-controlled and thus constant. The limit-state pipeline design methods therefore no longer require combination of all pipe stresses but allow residual pipe deformation to be assessed separately.

Design codes DnV81 and BS8010-3 already specify less-stringent residual pipe strain limits in case of controlled permanent pipe curvature.

Modern computer programs are able to predict accurately actual stresses and strains in the pipe, taking into account nonlinear stress-strain relationships and effects of roller loads on the pipe in the overbend.

Fixed stingers, as used on the Lorelay, Solitaire, and other large semis, unlike floating stingers, have no dynamic deformation increases in the overbend, which should readily lead to permitting stresses in the overbend much higher than 72% of specified minimum yield strength (SMYS). Fig. 3 [113,614 bytes] shows how Solitaire's length compares to three other major pipelay vessels.

As the curvature of a pipe on a fixed stinger is well controlled, higher stress or strain levels are justified, reducing not only the top tension requirement, but also the bottom tensions and therefore free-spanning.

For example, DnV81 allows for the overbend of a pipe on a fixed stinger, which implies, for steel grade API 5L X-65, an allowable temporary strain of approximately 0.4%.

It is in the client's interest to allow more relaxed installation criteria to avoid possible remedial measures on the seabed to eliminate free spans (such as rock dumping) and to increase the depth of laying in the more economical S-lay mode: generally by a factor of between two and three.

In S-lay, very large departure angles can be achieved because the pipe leaving the stinger is almost a J-lay configuration.

Ovalization limits, possible "corkscrewing," and stress intensification at the field joints are limiting criteria to the allowable strain. For corkscrewing, experience shows that pipes can be designed to remain within acceptable limits.

On-bottom inspection of pipelines laid by the Lorelay with 0.2% residual strain have shown no corkscrewing effects.

Corkscrewing is also counteracted by the residual bottom-pipe tension.

Stress intensification at the field joints is a matter for attention on concrete-coated pipe on which it occurs because of the stiffness of the concrete coating. Solutions to this problem exist, such as the prefabricating of intermediary crevices with soft infill in every pipe joint.

Stresses calculated in the commonly used computer programs have been verified by instrumentation of pipelines laid by the Lorelay in 1993 and 1994, and correlation has been found to be good.

Justification

Building such vessels as the Solitaire, with its large initial investment, benefits from being carried by a company in full private ownership: There are no shareholders to please with good quarterly results.

Profits can be considered with a very long-term view: The participating owner may accept that real success comes only many years after an investment was initiated, something a distant investor does not exactly look for.

This touches upon the question of why Allseas has invested in such an expensive vessel as Solitaire in such an uncertain market.

It is true that there are not enough large-diameter, lengthy, deepwater projects to require yet another large pipelay vessel. But Allseas believes that old equipment will become outmoded as better technologies emerge.

In a privately owned company, a decision to continue spending heavily on research and development can be made even while the market is poor and results are decreasing. This allows the company to be technically more advanced than the competition when new opportunities arise.

In a company that is part of a conglomerate or with mainly third-party shareholders, such R&D expenditures are difficult to obtain approval for in a poor market.

On the other hand, the privately owned company may well have the disadvantage of more limited access to financing than a subsidiary of a conglomerate company. Growth comes with difficulty.

The commercial justification for building Solitaire has been mentioned: The belief that existing technology will be replaced by new and better in the long run. Meanwhile, however, the existing large laybarges have been upgraded and rejuvenated to a fair extent, so that they will be around for quite a while.

Solitaire will have to justify its vast investment through added performance: faster laying, higher tension capacity, higher workability through the absence of anchor handlers, shielding of pipe carriers, and a large pipe-storage capacity; faster mobilization due to the ship's shape, and safer operations in congested areas with the absence of anchors.

When the schedule is not extremely tight, or the pipe not extremely heavy, or the location not too remote, or the start-up or lay-down locations not too congested, these advantages do not provide an overriding edge over the competition.

When the water depth becomes great, deeper than 600 m, the DP system will give the vessel an unusual edge. Working on routine jobs is commercially possible for Solitaire because the daily running costs, depreciation aside, are in the same order of magnitude as for existing, slower lay vessels.

A risk of bringing to life a new concept is that one grossly underestimates the investment at the start. Once the project is well under way, there is no way back when it costs much more. Not only may financing difficulties then arise, but also it is much more questionable whether there will be a proper return on investment.

We have seen companies seeking Allseas' cooperation underestimate costs by factors of two to three. Although the change of shipyard has obviously led to some unexpected cost overruns on Solitaire, the project was much more thoroughly budgeted.

The Author