Laying pipe on the seafloor can pose a number of challenges,
especially if the water is deep. There are three main ways that subsea
pipe is laid -- S-lay, J-lay and tow-in -- and the pipelay vessel is
integral to the success of the installation.
Buoyancy affects the pipelay process, both in positive and negative
ways. In the water, the pipe weighs less if it is filled with air, which
puts less stress on the pipelay barge. But once in place on the sea
bed, the pipe requires a downward force to remain in place. This can be
provided by the weight of the oil passing through the pipeline, but gas
does not weigh enough to keep the pipe from drifting across the
seafloor. In shallow-water scenarios, concrete is poured over the pipe
to keep it in place, while in deepwater situations, the amount of
insulation and the thickness required to ward of hydrostatic pressure is
usually enough to keep the line in place.
Tow-In Pipeline Installation
While jumpers are typically short enough to be installed in sections
by ROVs, flowlines and pipelines are usually long enough to require a
different type of installation, whether that is tow-in, S-lay or J-lay.
Tow-in installation is just what it sounds like;
here, the pipe is suspended in the water via buoyancy modules, and one
or two tug boats tow the pipe into place. Once on location, the buoyancy
modules are removed or flooded with water, and the pipe floats to the
seafloor.
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| Surface Tow Pipeline Installation |
There are four main forms of tow-in pipeline installation. The first, the surface tow
involves towing the pipeline on top of the water. In this method, a tug
tows the pipe on top of the water, and buoyancy modules help to keep it
on the water's surface.
Using less buoyancy modules than the surface tow, the mid-depth tow
uses the forward speed of the tug boat to keep the pipeline at a
submerged level. Once the forward motion has stopped, the pipeline
settles to the seafloor.
Off-bottom tow uses buoyancy modules and chains for
added weight, working against each other to keep the pipe just above the
sea bed. When on location, the buoyancy modules are removed, and the
pipe settles to the seafloor.
Lastly, the bottom tow drags the pipe along the sea
bed, using no buoyancy modules. Only performed in shallow-water
installations, the sea floor must be soft and flat for this type of
installation.
S-Lay Pipeline Installation
When performing S-lay pipeline installation, pipe is
eased off the stern of the vessel as the boat moves forward. The pipe
curves downward from the stern through the water until it reaches the
"touchdown point," or its final destination on the seafloor. As more
pipe is welded in the line and eased off the boat, the pipe forms the
shape of an "S" in the water.
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| S-Lay Pipeline Installation |
Stingers, measuring up to 300 feet (91 meters) long, extend from the
stern to support the pipe as it is moved into the water, as well as
control the curvature of the installation. Some pipelay barges have
adjustable stingers, which can be shortened or lengthened according to
the water depth.
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| Pipe being lowered into the water via a stinger for S-lay installation |
Proper tension is integral during the S-lay process, which is
maintained via tensioning rollers and a controlled forward thrust,
keeping the pipe from buckling. S-lay can be performed in waters up to
6,500 feet (1,981 meters) deep, and as many as 4 miles (6 kilometers) a
day of pipe can be installed in this manner.
J-Lay Pipeline Installation
Overcoming some of the obstacles of S-lay installation, J-lay
pipeline installation puts less stress on the pipeline by inserting the
pipeline in an almost vertical position. Here, pipe is lifted via a
tall tower on the boat, and inserted into the sea. Unlike the double
curvature obtained in S-lay, the pipe only curves once in J-lay
installation, taking on the shape of a "J" under the water.
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| J-Lay Pipeline Installation |
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| J-Lay Pipelay Vessel S7000 |
Types of Pipelay Vessels
There are three main types of pipelay vessels. There are J-lay and S-lay barges
that include a welding station and lifting crane on board. The 40- or
80-foot (12- or 24-meter) pipe sections are welded away from wind and
water, in an enclosed environment. On these types of vessels, the pipe
is laid one section at a time, in an assembly-line method.
On the other hand, reel barges contain a vertical or
horizontal reel that the pipe is wrapped around. Reel barges are able
to install both smaller diameter pipe and flexible pipe. Horizontal reel
barges perform S-lay installation, while vertical reel barges can
perform both S-lay and J-lay pipeline installation.
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| Vertical Reel Barge |
When using reel barges, the welding together of pipe sections is done
onshore, reducing installation costs. Reeled pipe is lifted from the
dock to the vessel, and the pipe is simply rolled out as installation is
performed. Once all of the pipe on the reel has been installed, the
vessel either returns to shore for another, or some reel barges are
outfitted with cranes that can lift a new reel from a transport vessel
and return the spent reel, which saves time and money.
Reference : http://www.rigzone.com/training/insight.asp?insight_id=311&c_id=19
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