How do pipelay vessels work?
Updated: Apr 15, 2021
It’s Fun Fact Friday and today we’re going to take a look at pipe laying vessels!
Pipelay vessels are any ships that install pipe on the ocean floor. These ships are purpose built for this process, carrying, welding, and laying pipe thousands of meters under the ocean surface. They are equipped with dynamic positioning systems that keep them on course and provide redundancy in the event of a failure. Some vessels have a spool where pipe can be rolled off into the water, similar to the baskets on a cable lay vessel. These reel barges load up pipe that has been welded onshore and lay the pipe until the reel runs out, at which point they can go to shore for more or load a new reel from a transport vessel. The reel method can be used on pipes 100-400 millimeters in diameter for lengths of up to 90 kilometers. If the pipes aren’t rolled out using the reel method they need to be stovepipe welded on the ship. Barges supply pipes to the ship, a conveyor takes the pipes in to be welded, after which they are ultrasonically tested for defects. They receive an anti corrosion coating and are moved to the aft of the ship, where a special boom, called a stinger, guides the pipe into the water at an optimal angle to prevent deformation.
“Royal IHC Reel lay vessel Seven Oceans”. From Royal IHC.
The pipes are installed three main types of ways: S-lay, J-lay, and tow-in. Jumpers can be installed in sections by ROVs, but long pipelines require different installation. Tow-in is achieved by simply towing the pipe into place with a tugboat. With surface tow-in, the pipe is moved into place and then any buoyancy modules are removed and the pipe is sunk. Mid-depth tow uses speed to keep the pipe off the seafloor, and after the vessel slows, the pipe settles and sinks. Off-bottom tow is the same concept, with the pipe sitting closer to the sea bed. Bottom tow is the last method, where the pipe is dragged across the bottom of the ocean, and can only be used if the floor is flat, soft, and shallow.
“Surface Tow Pipeline Installation
Source: www.pipelife.no”. From Offshore Fleet.
S-lay pipe install is very fast and can be used in many circumstances. The name comes from the shape of the pipe as it is being lowered onto the seabed, forming an “S” from the touchdown point to the pipe laying vessel. A stinger supports the pipe in the water and guides it down, controlling the curve. Some of these stingers are length adjustable for multiple water depths, and must keep proper tension on the pipe as the ship moves forward in the water. This method works in depths of up to 6500 feet, with kilometers of pipe being laid per day.
“S-Lay Pipeline Installation
Source: www.pbjv.com.my”. From Rigzone.
J-lay install is similar to S-lay except that the pipe is at a near vertical angle on the vessel. While it might seem counterintuitive with the pipe being laid horizontally, this method only puts one bend on the pipe at the seafloor. It works well for deep waters and is less affected by underwater currents than S-lay. J-lay does have its disadvantages: it's slower due to the single position of welding and can’t be used in shallow waters because of the vertical angle.
“J-Lay Pipeline Installation
Source: TechnipFMC”. From Rigzone.
Once the pipe is in the water, it can be fixed to the bottom a number of ways. Oil passing through the pipeline can be enough weight to keep it on the seafloor, but for gas it won’t stay in place. According to Offshore Fleet, “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 [off] hydrostatic pressure is usually enough to keep the line in place.”
Next up we’ll take a look at a closer look at the features of an S-lay pipelaying vessel, the Allseas Solitaire.
Allseas Solitaire on the ocean. Image credit: Allseas.
The Solitaire is one of the world’s largest pipe laying vessels, operated by the Dutch construction conglomerate Allseas. It was actually built in the 1970s in a Mitsubishi shipping yard as a cargo carrier with the name Trentwood. It was eventually sold to Allseas and in 1998, was re-christened the Solitaire and started pipe laying activity. The ship has laid many deepwater pipelines, and set a record in 2007 for ultra-deepwater pipeline installation at a depth of 2775 meters, or 9100 feet. It can lay down medium and large diameter pipes at high speed, shortening construction times. Solitaire has laid pipe on the ocean floor at a speed of more than 9 kilometers per day, according to Wermac, which translates to over 1000 kilometers per year. Significant pipelines include Troll Phase 3, Nord Stream2, Polarled, and Wheatstone.
Allseas Solitaire (foreground) with Allseas Pioneering Spirit (background right). Image credit Allseas, from Offshore Energy.
Solitaire is a long vessel with a ship shape and a 22,000 ton capacity, reducing the need for offshore pipe resupply. Pipe lay ships may operate in hostile areas, so the faster they can complete the job the better. The 397 meter long ship has a maximum speed of 13.5 knots. 51,480 kW of total power is supplied to 10 Wärtsilä-Lips azimuth thrusters at 5550 kW each. It’s dynamically positioned with a fully redundant Kongsberg DP system. It has two cranes on deck, one 850 ton crane for special purposes, and two 35 ton pipe transfer cranes. Included is a 150 hp ROV with 4,000 meter depth capability for inspection and interventions. Solitaire can lay pipe from 2 inches in diameter all the way up to 60 inches.
If you’d like to see more videos of the Solitaire, check out Allseas video section.