A number of DP pipelay vessels are currently in operation worldwide. Dispensing completely with anchors and moorings, these vessels are able to conduct pipelay more quickly and efficiently than the pipelay barges.
Three methods of pipelay are in use;
• S-lay,
• Reel-lay and
• J-lay.
• Seabed Tractors and Trenchers
A seabed tractor or trencher may be configured to lay and bury a cable. These vehicles are tracked crawlers, built to be controlled from the vessel, with operators ‘driving’ the unit as if they were on board. These units usually move slowly,
depending on soil conditions. In some cases an ROV is deployed independently, to record progress and performance.
Trenchers for pipeline burial are much larger and heavier. The trencher is lowered onto the seabed over the pipeline and the DP control system can set the centre-of-rotation of the trencher.
Associated with the business of laying pipelines is the need to protect them from damage. DP-capable vessels are used here also. A pipeline may be trench buried by use of a specialist seabed crawler vehicle. This vehicle will be deployed by an A-frame over the stern of the trenching vessel and will follow the pipeline, excavating a trench of the required depth. This will be done using ploughshare and water jetting. Once a trench has been established, the vehicle will be recovered and re-configured for a back-fill or cover operation. The DP vessel will use a specialist track-follow or vehicle-follow function to maintain station on the trencher. The vessel may also use a specialist position-reference system such as Trimcube or Smartwire, allowing position relative to the vehicle to be monitored.
• Pipelay Operations
Many pipelay operations are conducted by DP lay barges.
Figure 70. - Pipelay methods
• S-Lay Operations
In a typical S-lay barge, the pipe is constructed in a linear pipe fabrication facility called the "Firing Line" in which a number of stages of welding take place. Each operation is conducted at a "station". Further stations conduct X-ray and NDT testing on the welded joints, anti-corrosion coating, and weight-coating if necessary. At intervals, the DPO initiates a move ahead a distance equivalent to the joint-length. Once the move ahead has been completed, the firing-line operations continue.
It is essential that tension is maintained on the pipeline. At the back end of the firing line, the pipe is held by a number of pipe tensioners, or caterpillar tracks clamping the pipe. The tensioners control the movement of the pipe, maintaining a set tension on the pipe string. The pipe is supported aft of the firing line by the "stinger", which is an open lattice gantry extending beyond the stern of the vessel, sloping downwards. Tension on the pipe is needed to prevent pipe damage from
buckling. The set tension is to ensure a smooth catenary to the touchdown point on the seabed. If tension is lost, then damage will occur at the touchdown area.
Pipe tension values are communicated to the DP system which is continually providing thrust commands to maintain tension, position and heading.
Pipelay operations are particularly dependent upon environmental conditions. The vessel must be able to cope effectively with the tides, sea state and wind conditions from most directions, because it is not possible to allow the vessel to weathervane.
• J-Lay Operations
In J-lay operations the pipe is constructed in a long narrow factory called the "firing-line" at deck level. Pipe is fabricated, welded, coated and inspected at a number of stations spaced at 12m intervals along the firing-line. The pipe is controlled by caterpillar-track pipe tensioners that feed it down the "stinger". The stinger is a hefty ramp at the stern supporting the pipe in the overbend area. The pipe is supported by its own tension only in the span between the end of the stinger and the sea bed touchdown point, or the "sagbend" zone.
The DP system must allow the vessel precision positioning on a fixed heading, maintaining pipe tension and moving the vessel ahead an exact 12m on demand. These moves may occur every four minutes. Faster working may be achieved if double-joints are worked, with the vessel moving 24m each time. Pipe tension is fed back into the DP from sensors on the tensioners and must be maintained within specification tonnages.
In deeper water, S-lay is not feasible and J-lay is common. In J-lay operations, the stinger is configured as a tower, angled between the vertical, and up to 20 degrees from the vertical. Pipe lengths are pre-jointed into triple or quadruple joints before being raised to the vertical for welding onto the pipestring.
• Reel-Lay Operations
This type of operation varies from those described in that the pipestring is prefabricated in one length at a shore-based factory. The vessel loads the pipeline straight from the factory, spooling it onto a reel or into a carousel. The vessel can transit to site with the pipe to lay it by feeding it off the reel/carousel via straighteners and tensioners, either singly or as a bundle.
The pipe is laid by passing it from the carousel onto the lay-ramp, thence down the stinger. In very deep water the only suitable method is J-lay. Here the stinger is mounted close to vertical. The pipe is fabricated into triple-joint lengths, which are turned to the vertical at the stinger. Large forces are induced at the stinger due to the heavy weights of pipe involved and these forces must be countered by the vessel's DP capability.
Pipelay vessels routinely conduct complicated evolutions using DP. The operations to commence and complete pipelay, conduct an in-water tie-in or to lay down the end of the pipe if necessary, all involve precision positioning.