Vuyk Engineering Rotterdam (VER), a specialist in basic design and engineering for the offshore market, has delivered basic design packages for a number of pipelay vessels including the Sapura 3000 and the GLOBAL 1200.

VER Managing Director Marc Oele explains the basic design process. “Pipelay vessel designs should always be client specific. We start by analysing the function of the vessel and the pipe that needs to be laid, including the diameter, weight, depth, allowable bending radius, required tension, and the process of constructing the pipe. From that base, the required method and equipment are developed.

“The vessel design must provide the space and carrying capacity for the equipment and supporting services. The final details of the equipment determine the detailed layouts of the work areas and integrated support structures of the vessel, thus requiring a great amount of additional engineering and design work,” Mr Oele says.

Choosing the right pipelay system

During the design process, a pipelay system is chosen based on the needs of the client. Dutch pipelay system engineers and manufacturer specialist Huisman provides an overview of the different pipelay systems.

A Flex-lay system is used on vessels which are required to install flexible pipelines, risers and in-line structures. A Flex-lay system comprises a vertical ramp equipped with one or more tensioners and a chute or wheel on top. Two fixed struts between the ramp and the deck keep the ramp upright; however it is possible to install an adjuster system to change the working angle of the ramp. The flexible pipe is spooled from a reel, carousel, or basket, bent over the chute and guided into the tensioners and onto the seabed.

“The advantages gained by the less complex installation, abandonment and recovery procedure for pipelines and in-line structures, and the fact that the pipelines are less sensitive to fatigue due to their flexibility, make Flex-lay an efficient method for their installation. As a Flex-lay system is not suitable for the installation of rigid pipe, it is often combined with other pipelay methods onboard,” Huisman says.

The J-lay installation system is used for installing subsea pipelines in deepwater. Pipe stalks with a length up to six joints are upended and welded to the seagoing pipe in a near vertical ramp.

The ramp angle is chosen in such a way that it is in line with the curve of the pipe to the seabed. The J-lay method is very suitable for deepwater as the pipe leaves the lay system in an almost vertical position. The pipeline is only bent once during installation (at the seabed) which is advantageous for installing pipelines that are sensitive to fatigue.

Compared to other lay methods, Huisman says that J-lay has a relatively low production rate due to the limited number of workstations. The J-lay method is less suitable for shallow waters as this requires a steep departure angle.

Reel-lay is a method for installing both flexible and rigid subsea pipelines. Long pipe segments are welded, tested and coated onshore and then spooled onto a large vertical pipe reel in one continuous length. Once the Reel-lay vessel is in position, the pipe is unspooled, straightened, and lowered overboard as the vessel moves forward.

Huisman says that the advantages gained by the high production rate as well as the controlled welding and inspection conditions onshore make Reel-lay an extremely efficient method for the installation of pipelines up to 20 inches in diameter in all waters.

“A Reel-lay vessel requires a spoolbase nearby to reduce transit time to and from the installation site. Reel-lay is therefore often combined with other pipelay methods onboard,” the company says.

The S-lay installation method is a system where onboard welded pipe joints leave the vessel nearly horizontally and are guided by a stinger – a structure on the back of the ship that supports the seagoing pipe string to control its bend radius.

“Due to its high production rate and the possibility to install concrete-coated pipe, S-lay is extremely suitable for pipe installation in shallow and intermediate waters. Larger water depths are equally possible but require a very long stinger, turning S-lay into a less practical and efficient solution.”

Sourcing the right equipment

Once the pipelay system has been selected, the right equipment must be purchased or fabricated according to the design requirements.

Common system components include:

• Tensioners
• Winches
• Bevel machines
• Welding and non-destructive testing stations
• Line-up tables
• Pipe rollers and conveyors
• Overhead gantry cranes
• Gantry/pipe handling cranes
• Pipe elevator
• Pipe repair davits.

Putting the pieces together

Ship manufacturers are responsible for bringing all of the pieces together. Sembawang Shipyard says that in completing some of its larger projects, works are carried out in different parts of the world.

The completion of the Sapura 3000 involved major works in three countries. Completion involved the construction, outfitting, and commissioning of an 8,000 t new-built hull into a self-propelled, DP 2 heavy-lift and pipelay vessel with a Huisman 3,000 t heavy-lift crane and S-Lay pipelay system and accommodation for 330 crew and employees. Fabrication of the heavy lift-crane and pipelay equipment was completed in the Netherlands and China, steel hull construction in China, and systems outfitting and final commissioning in the Sembawang Shipyard, Singapore.

Once commissioned, the pipelay vessel is ready for its first job on the open sea.

The challenges ahead

The development of offshore oil and gas fields is increasingly occurring in deeper waters. This is because of the amount of resources to exploit in these areas, and the increased viability of these fields due to a trend in higher oil and natural gas prices.

This poses challenges for the design of pipelay vessels. Mr Oele explains “As projects are pushed deeper offshore, pipelay vessels need to have sufficient tension on the pipe, adequate abandoning and pipe-recovery procedures, as well as ample bending radius control. The challenges in designing a pipelay vessel are to keep up with increasing laying depth, diameters and laying speed requirements.”