Moving on with our series; What Jobs Are Done With DP?
Laying Pipe subsea is a much more active form of DP & installations come in different forms. You may see a simple hard lay style pipe barge or an enormous flexible lay vessel, such as Allseas “Solitaire”.
This style of work requires the operator to be constantly making adjustments to the vessels position using the DP console.
Types of Pipelay
As it concerns DP vessels, there are 3 styles of pipelay that you may encounter: S-Lay, J-Lay & Flexible Reel
If it is an S-Lay installation then the vessel will move forward one pipe length (12m or 24m) every few minutes so that a new piece can be welded on to the end of the line. Meanwhile, the last joint welded will move down the assembly line where it gets X-Rayed & Mag Particle tested, or some other type of NDT.
The name is derived from the fact that the pipe makes 2 bends, forming an S shape, on its way to the sea floor.
The pipe will be fed into the water off of a Stinger, which extends far past the stern. This allows the pipe to be controlled as it is deployed, as well as controlling the curvature made by the pipe as it enters the water.
With a J-Lay installation, the pipe is lifter into an installation tower where it is welded in line. The angle of which the pipe enters the water can now be much steeper & with reduced stress on the pipeline. This allows installation in much deeper water.
This style is used to install either small diameter or flexible pipe. The welding of the pieces is done ashore. They are then rolled onto the reel, which is installed onboard the vessel in either a horizontal or vertical fashion. If installed horizontally then the pipe will pay off the reel in an S-Lay fashion & if installed vertically it will pay off in a J-Lay fashion.
This style of installation reduces both cost & time, as the welding is done in the yard & the vessel is able to move continuously forward while paying out pipe. Some vessels are equipped with multiple reels that can be swapped over at sea, thus lengthening their time at sea before resupply is required.
Points to Consider
The tension on the pipe being held by the vessel will act as an external force on the DP system & must be taken into consideration by the operator. This tension is controlled by rollers onboard the vessel as well as by the forward pull of the vessel itself, controlled by the DPO. This is especially important in an S-Lay installation.
The point at which the pipe touches down on the sea floor will need to be carefully monitored for survey purposes. This can be done in some cases by the vessel itself or else by a dedicated ROV chase boat. Keep in mind that the point where the pipe line actually touches the sea bed may be miles away from the ship! Some vessels are installed with Modeling Software that can accurately show the pipes shape as it is in the water.
Depending on the depth that the pipe is installed, it may require concrete mats to be laid over top of it, in order to keep the pipeline in place & to ensure it is not snagged & dragged by an anchoring vessel.
Here is a video showing some flexible pipelay in action onboard the Lorelay
Cable lay works very similar to pipe lay, with the vessel moving forward under operator control as the cable is being paid out.
The submarine cable will in most cases need to be buried, so that it is not fouled by an anchor or fishing nets. This can be done in 2 ways.
- During the cable lay operation with use of a trenching plough
- After the cable is laid on the seabed, using an ROV
With the first method, the cable is fed into a trenching plow which is deployed off the vessels stern on a davit. This will be towed behind the vessel on the sea floor, digging a trench into which the cable is placed. It will close the trench up behind it, thus protecting the cable.
Some of these trenching machines may be fitted with jetting equipment & all of them will be equipped with beacons, in order to monitor its position sub sea.
Burying to cable after it has been laid will generally be accomplished by using an ROV equipped with jetting equipment, although in some cases this job would be done by a diver, for short distances.
As with pipe laying, a cable lay vessel will be equipped with tensioning equipment which will help control the speed at which the cable is paid out, but will also act as an external force on the DP that the operator will need to be aware of.
Both of these operations involve a lot of DPO input & the bridge will be working closely with the deck & survey team in order to complete the sub sea installation in accordance with the job scope.