S-Lay

S-lay is the most applied method for offshore pipeline installations, especially for relatively large diameter pipelines (d > 16”). This lay method is applicable for pipe installation in both shallow and deep water areas. The installation technique is characteristic with the s-curve of the pipeline during laying, and is a result of the stinger and tensioners on the vessel. Maximum depth in which a pipeline can be laid is dependent on the stinger length, -curvature, tensioning capacity, tip slope and longitudinal trim of the vessel.

After passing through a number of welding stations, inspection phases and tensioners, the pipeline will lift off from the stinger located typically at the end of the vessel. The stinger will set the curvature for the upper end of the pipeline, the so-called overbend (figure 2-2). Rollers secure the pipeline support during the offloading into sea, from where the pipeline continues as an unsupported span until interacting with the seabed. Here, at the lower part, the pipeline gets a curvature directed opposite of the overbend. Pipe curvature in the sagbend is a result of the tensioners and weight of the pipeline, and can be controlled by the tension applied to the pipe from the vessels tensioning system.

6.1.1 Steep S-Lay

Steep S-lay is a variant of conventional S-lay, making it more applicable for deep water pipeline installations by modifying the stinger and increasing the structural utilization of the pipe (figure 6-1).

According to Perinet and Frazer (2007) the method includes setting the stinger in such a way that the liftoff point of the pipeline will be as close to vertical as possible, which reduces the tension in the pipe compared to the traditional S-lay method. The steep liftoff angle implies that the curvature has to be increased, in order to keep the stinger to a reasonable size. As a result of the increased stinger curvature the strains in the overbend will increase.

Chapter 6 Offshore Pipelaying

Figure 6-1 Steep S-Lay Configuration [Perinet and Frazer, 2007]

Advantages

• No limitations to pipeline diameter and -length. The vessels can install varying pipeline diameters in different projects, making them feasible for many S-lay installation projects.

• Requires minimal on-shore support ones the installation has started.

• Numerous pipeline tasks can be performed at the same time, including welding, inspections and field joint applications, due to the horizontal transportation across the vessel.

• Several contractors with S-lay experience, which gives advantages due to technical and economical competition.

• Laying speed is quite high, even for large diameter pipelines, and is typically between 2 and 6 km/day (Iorio, Bruschi and Donati, 2000). This is dependent on seabed topography and water depth, among other factors.

Disadvantages

• Limited installation depth. Tension capacities at the vessels are likely to be exceeded at ultra-deep waters for large diameter thick walled pipelines.

• Long stingers are vulnerable to wave and current forces, which is typical for S-lay vessels in deep waters.

• High tension is undesirable as the tensioners can damage the pipeline coating, as well as having to be balanced by the mooring or dynamic positioning system of the vessel.

• High strains in the overbend are common for deep water installations, with a high probability of exceeding the given strain criteria.

Chapter 6 Offshore Pipelaying 6.1.2 S-Lay Main Installation Equipment

The pipeline installation procedure for S-lay vessels is done by the following main equipment:

Tensioners

Tensioners are normally located near the stern of the ramp. Typically rubber pads put a pressure at the top and bottom of the pipe surface. These apply a tension to the pipe, controlling the curvature during installation. Their function is to give sufficient tension in order to secure the integrity of the pipe. The required tension depends on factors such as water depth, length of stinger, stinger radius, pipe diameter and -weight. For deep waters the required tension is higher than for more shallow waters, as the total pipeline segment has a higher weight. The S-lay vessel Solitaire (Allseas Group) has a total tension capacity of 1050t, allowing pipeline installation down to approximately 3000m.

In deep waters the tension capacity of the vessel is usually the limiting factor for how deep a pipeline can be laid. According to Perinet and Frazer (2007) the transfer of tension between tensioner device and pipe is the most critical factor. Tension can be applied to the pipe by:

- Long tensioners and low squeeze - Short tensioners and high squeeze - Shoulders with collars on the pipe

Tension is transferred to the pipe by friction between the pipeline and the tensioning machine. To avoid damage to the pipe coating the area exposed to friction must be large enough. This supports the use of large tensioners with low squeeze.

One way of increasing possible pipe installation depth is by applying tension after the overbend section with use of submerged tensioners. Advantages would be present by not combining the tension force and bending effect, as lower strains would arise in the overbend section, according to Perinet and Frazer (2007).

Stinger

The stinger is an open frame structure with rollers to support the pipeline during installation, and gives the pipe its curvature in the overbend region. It will often be constructed by several hinged sections, giving the possibility to adjust the stinger curvature and shape. Stinger lengths are depending on the lay vessel, but normally the lengths are above 100m for vessels installing pipelines in deep waters. Solitaire has a stinger length of 140m which makes it able to perform deep water installations.

To keep the strain levels in the overbend within the given criteria, long stingers are required for deep water laying. Short stingers will be problematic, as the bending strains would exceed the allowable strain criteria at the end of the stinger, potentially resulting in buckling (figure 6-2).

Chapter 6 Offshore Pipelaying

Figure 6-2 Buckling during S-Lay [Kyriakides and Corona, 2007]

Stingers in themselves have to withstand several forces acting on them during operation:

• Waves and current forces

• Contact forces between the pipe and the stinger

• Forces caused by the weight of the stinger

• Forces acting on the stinger due to vessel movements

In order to increase the applicable water depth of pipeline installation, the liftoff angle at the sVnger Vp should be close to 90 ̊. This can be done by reducing the stinger radius, which will also increase the strains in the overbend region. To make S-lay practical for deep waters this would be preferable, in addition to having a stinger rigidly connected to the lay vessel (Perinet and Frazer, 2007). Stinger configurations applied today normally include:

• Rigid stingers, fixed to the laybarge

• Articulated stingers, flexible or rigid segments joined by hinges

Modern stingers, such as the one on Solitaire, are articulated. This gives possibilities of controlling the curvature of the stinger by setting different angles for the segments, whereas rigid stingers are limited to their given configuration. Installing pipelines in deep waters will require longer stingers to avoid excessive bending at the stinger. There are limitations to how long the stinger length can be due to the increased environmental loads acting on it.