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.

Figure 1 - Pipelines in water depths over 3,000 feet (Available in full paper)

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 vessel's 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.

Upgrades in capacity have moved previously existing boundaries.

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