Pipeline Installation method in shallow water

S-lay Method by Conventionally Moored Lay Barges

The traditional method for installing offshore pipelines in relatively shallow water is commonly referred to as the S-Lay method because the profile of the pipe as it moves in a horizontal plane from the welding and inspection stations on the lay barge across the stern of the lay barge and onto the ocean floor forms an elongated "S." As the pipeline moves across the stern of the lay barge and before it reaches the ocean floor, the pipe is supported by a truss-like circular structure equipped with rollers and known as a stinger. The purpose of the stinger in the S-lay configuration is to control the deflection of the pipe in the over-bend region above the pipeline inflection point in order to return the angle of the pipeline at the surface to the horizontal. The curvature radius of the stinger corresponds to at least the maximum bending stress. To avoid a bending moment peak at the last roller, the pipe must lift off smoothly from the stinger well ahead of the lower end of the stinger.

In extremely deep water the angle of the pipe becomes so steep that the required stinger length may not be feasible. Deeper water depths will result in a steeper lift-off angle of the suspended pipe span at the stinger tip. This will require the stinger to be longer and/or more curved to accommodate the greater arc of reverse curvature in the overbend region. Accordingly, greater stinger buoyancy and/or structural strength will be necessary to support the increased weight of the suspended pipe span.

The practical water depth limit for a large, conventionally moored lay barge that uses the S-lay method is about 1,000 ft, based on a ratio of anchor line length to water depth of about five to one. Therefore, construction of pipelines by conventionally moored lay barges, if used in conjunction with the development of deepwater oil or gas discoveries in the Gulf of Mexico, will probably be limited to those portions of the pipeline routes located in water depths less than 1,000 ft. The term "conventionally moored" means that the location or position of the installation vessel (lay barge) is maintained through anchors, associated anchor chains, and/or cables.

Smaller lay barges, in the 400 ft long by 100 ft wide size range, typically require eight anchors each weighing 30,000 lbs, and a larger barge operating in 1,000 ft of water typically requires 12 anchors (3 anchors per quarter), each weighing 50,000 lbs or more.

In general, the larger the vessel, that is, the greater the target area presented to wind, wave, and current forces, and the heavier the vessel, the higher the holding requirements will be for the mooring system. The rated holding capacity of an anchor system is a function of the weight and size of the anchor and the tensile strength of the chain or cable that secures the anchor to the vessel. An important factor to be considered when there is a choice to be made between a conventionally moored lay barge and a lay barge that uses other means, such as dynamic positioning, to remain on station is the matter of handling the anchors. To deploy and recover the anchors of a lay barge operating in 1,000 ft of water, two anchor-handling vessels with a horsepower rating of 8,000-10,000 each would be required, and there is a shortage of such vessels. On the other hand, a smaller lay barge operating in shallower water requires only one 3,000-5,000 hp anchor-handling vessel.

The number of anchor relocations per mile of offshore pipeline constructed will be dependent upon the size of the lay barge, the water depth, ocean floor conditions in the vicinity of the pipeline installation, and the amount of anchor line that can be stored, deployed, and retrieved by the lay barge. Assuming a lay barge is operating in 1,000 ft of water and is following the accepted practice of deploying an amount of anchor line equal to five times the water depth, the anchors would have to be relocated after each 2,000 ft of pipeline installed.

Minerals Management Service regulations at 30 CFR 250.1003(a)(1) require, with some exceptions, that pipelines installed in water depths of less than 200 ft be buried to a depth of at least 3 ft. The purpose of this requirement is to protect the pipeline from the external damage that could result from anchors and fishing gear, and to minimize interference with the operations of other users of the OCS. For deepwater pipelines, burial issues are a possible concern only for those pipelines that terminate onshore or at shallow-water host facilities.

The burial of a pipeline is carried out during the construction process and is usually accomplished by either a plow or a jet sled towed along the seafloor by the lay barge. Whether a plow or jet sled is used, the distance of the device from the lay barge is adjusted to position the plow or jet sled just ahead of the point where the pipe contacts the seafloor (the touchdown point). Through the action of high-pressure water jets, a jet sled creates a trench in the seafloor into which the pipeline settles. The jet sled, which generally creates more temporary turbidity in the water column than a plowing device, has an operational advantage over a plow. The area of seafloor disturbed by the pipeline burial process is typically just slightly wider than the outside diameter of the pipeline, for example, a trench approximately 15 inches wide by 3 ft deep for a 12-inch pipeline.

S-Lay Method by Dynamically Positioned Lay Barges

The term "dynamically positioned" means that the location or position of the lay barge is maintained by the vessel's very specialized propulsion and station-keeping system which, instead of or in addition to the conventional propeller-rudder system at the stern, employs a system of hullmounted thrusters near the bow, at midship, and at the stern. When in the station-keeping mode, these thrusters, which have the capability to rotate 360o in a horizontal plane, are controlled by a shipboard computer system that usually interfaces with a satellite-based geographic positioning system.

Dynamically positioned lay barges can be used in water depths as shallow as 100 ft, but generally they are not used in water less than 200 ft deep, depending upon pipe size, the nature of the job, and the location. Dynamically positioned lay barges outfitted with the equipment necessary to install reel pipe are sometimes used in shallow water.

The impact on air quality is one of the most significant differences between using a dynamically positioned lay barge and a conventionally moored lay barge to construct a pipeline. In the case of a conventionally moored vessel, the hydrocarbon-fuel-consuming prime movers that drive the propulsion system are typically shut down or operating at minimum speed, fuel consumption, and pollutant emission levels while the vessel is not under way, that is, while the vessel is engaged in pipeline installation activity. The probable requirement for tug assistance to move from station to station during an installation project and the requirement for the services of anchor-handling vessels to deploy, retrieve, and re-deploy anchors contribute to the pollutant emission levels. Contrast this to a dynamically positioned lay barge which, in order to remain on station during a pipeline installation, must constantly operate its prime movers, which drive the propulsion system.

Some examples of deepwater pipelines installed by the S-lay method from a dynamically positioned vessel (the Allseas ship Lorelay) are the 25-mile long, 14-inch gas and 12-inch oil export pipelines constructed from Shell Offshore Inc.'s Ram Powell tension leg platform at Viosca Knoll (VK) Block 956 to VK 817, and from VK 956 to Main Pass (MP) Block 289, respectively. The water depth along these routes ranges from 3,218 ft at VK 956 to 670 ft at VK 817 and 338 ft at MP 289. The Lorelay also installed three 6-inch gas pipelines in water approximately 5,400 ft deep between three subsea wells in Mississippi Canyon (MC) Block 687 and a subsea manifold in MC 685 (Shell's Mensa project).