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 (remotelyoperated 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 overboards the ship.
Using this method quickens operations on account that an onshore spoolbase is nearby.