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Offshore Pipeline Installation
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.
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.
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.
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.
J-Lay
Pipeline Installation
The reduced stress on the pipe allows J-lay to work in deeper
water depths. Additionally, the J-lay pipeline can withstand more motion and
underwater currents than pipe being installed in the S-lay fashion.
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.
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.
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