Pipe-laying on the seafloor requires you to consider a
number of things to ensure that the installed pipeline will be able to last
many years, deliver its purpose and function efficiently. The deeper the depths
of the pipes are lain on, the more challenges you’ll have to consider.
Buoyancy plays a role in the pipe laying process. Both
negatively and positively.
If pipes are filled with air, they weigh lighter in water.
When the pipeline is slowly discharged off the pipelay barge, this lessens the
stress they put on the barge.
Although the same air that makes the pipeline buoyant, makes
it harder to keep in place on the seafloor. A downward force is required to
keep the pipeline in place.
In shallow water situations, concrete is used to fix the
pipeline in its place. Concrete is poured on it so that it won’t float from
place to place.
In deepwater situations, however, the application of cement
may not be required. Instead, the amount of insulation and the thickness required
to ward off hydrostatic pressure is usually enough to keep the pipeline from
drifting up.
The weight of the oil flowing within the pipes can also
provide enough to have the pipeline remain in place. But gas is not enough to
keep the pipes from drifting on the seafloor.
There are three common ways pipe is installed on the
seafloor. That is the Tow-in method, S-lay and J-lay.
Installation in deep sea usually uses ROVs (remotely operated
vehicles). But these equipment are only used for relatively lightweight and
short items such as jumpers.
Pipelines are a different story. Pipelines are
individual pipes welded together and form a longer system for fluid
transportation. Pipelay barges or lay vessels are used in pipeline
installations.
1. Tow-in Installation Method
It’s just as the name suggests. A long segment of welded
pipes are towed in over the location for the pipeline system by barges. They
are suspended by floating modules in the water. Once the barges arrive in the
pipeline’s location, the floating modules are removed or filled with water to
let the pipes sink down to the seafloor.
Surface
Tow is a variant of the tow in pipe-laying
method where the pipeline segments are pulled by tug on the surface of the
water. Floating modules keep the pipes afloat until they arrive over the
location and before they are removed or filled with water.
Mid Tow is
another variant of the tow in method, but unlike the surface tow, this uses a
lesser number of floating modules. This tow method uses the forward speed of
the tug boat to keep the pipes submerged. Upon arriving to the location and the
absence of this forward speed, the pipes are left to sink to the seafloor.
Off Bottom Tow is
another variant of the tow in method. In this method, floating modules work
hand in hand with chains for added weight to keep the pipeline segment drifting
just above the seafloor. The floating modules are then removed to let the pipes
settle on the seafloor.
Bottom Tow is
the final tow in method, where pipeline segments are dragged on the seafloor
towards the location. No buoyancy modules are used. This method is restricted
to shallow situations where the seafloor is soft and flat for the pipeline
segments to be dragged on.
2. S-Lay Installation Method
This pipe-laying method uses a special kind of
barge with a tail called stinger. The stinger is the ramp-like platform that
provides transition for the pipeline from a horizontal configuration to a
vertical configuration as it settles to the seafloor.
Individual pipes are welded, coated and controlled in a
continuous production system based in different workstations in the ship before
they get fed down the discharge line, and down the stinger.
Stingers, measuring up to 300 feet long extend from the
stern of the ship to support the pipe as it is discharged into the water, as
well as to control the curvature of the installation. Some pipelay barges have
adjustable stingers, which can be shortened or lengthened according to the
water depth.
The pipeline curves downward from the stern
through the water until it reaches the “touchdown point,” or its final
destination on the seafloor. As more pipes are welded in the line and eased off
the boat, the pipe forms the shape of an “S” in the water.
Because of the pipe departure angle at the stinger tip,
S-lay intrinsically requires much higher tensions (than J-lay) to control the
pipe curvatures. Furthermore, the long lay back distance of pipe touchdown
point in deepwater with S-lay is also problematic and restricts its ability to
lie around a curve.
S-lay is usually used for shallow operations.
3. Steep S-Lay Installation Method
To reduce high tensions on the pipes and the barge using the
conventional S-lay method, the stinger departure angle can be increased
to 60°- 80° range (near vertical angle) in a configuration.
This, however, means a longer and deeper stinger leading to
more awkward handling problems and greater weather sensitivity.
Furthermore, the high pipe weight tension combined with the
steep radius of curvature will introduce an elasto-plastic bending and a
residual pipe strain. Slightly deforming the pipes as they settle on the
seafloor.
4. J-Lay Installation Method
Pipes depart from the lay vessel at a near vertical angle,
lessening the tension on the pipe-laying vessel and pipes and overcoming some
obstacles posed in using the S-lay method.
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.
Since the pipe is departing from the vessel in the near
vertical position, this implies that the welding should be done in the near
vertical as well.
Since it is not practical to have a vertical ‘firing line’
as in the S-lay, the pipe is assembled (welded) by stalk of 2, 4 or 6 joints. A
6 joint stalk is considered to be the current practical limit for up-righting.
The pipe needs to be held (in the vertical position) while welding of the stalk
joint is taking placed.
J-lay method can operate in deeper installation depths.
5. Reel-Lay installation Method
This installation technique can be considered
similar to the J-lay approach. The main difference is that the pipe is
continually fed down an adjustable lay ramp from a large storage reel mounted
on the lay vessel.
Barges used in reel- lay are able to install both
smaller diameter pipe and flexible pipe. Prefabricated pipe lengths are welded
together and spooled onto the ship reel at an onshore fabrication yard
(spoolbase). This minimizes the offshore time or field duration required for
pipe laying operations.
During pipelay, the pipe is unspooled from the reel. Then
guided via an aligner to a straightener on the lay ramp. The pipe then passes
through the straightener, down the ramp, through the tensioners and overboard the ship.
Using this method quickens operations on account that an
onshore spoolbase is nearby.
Originally posted in aly.palawanblogger.com
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