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).
