S-lay method is the oldest and the most commonly used method for offshore pipeline installation. This is named as “S-lay” because the shape of the pipe line while being installed looks like S-shape (Figure 1).
Figure 1 – S-Lay Method (Courtesy of Allseas)
While installing pipe line using S-lay method, the pipe line is eased off the stern of a pipeline installation boat as the vessel moves forward. The pipe line is transferred into the sea until it reaches the sea bed which is called the “Touch down poin.t. Each joint of pipe line is welded on the installation vessel and eased off the boat. A stringer located at the stern, whose length can be up to 300 ft., helps supports the pipeline when it is transferred into the sea. Some installation barges can be equipped with an adjustable stringer which is used to control the length of a stringer. This stinger is vulnerable to damage in bad weather.
Figure 2 – S-Lay Diagram
The S-lay method can handle the full range of pipe sizes and it is considered most suited to water depth around 2,000 ft (600 m) of water depth. The pipe line is welded horizontally in a fire line (Figure 3). Depending on the vessel capacity, speed of laying the pipe can be up 6 – 8 km per day and the pipe size can be up to 60” in diameter.
Figure 3 – Pipeline Welding on the S-Lay Barge
This method requires high longitudinal tension in order to avoid excessive bending at the touchdown point (sag-bend) and the departure point (over bend). Two ways that restrict ability to quickly change a route direction are high tension and long lay-back distance.
These are some examples of installation barges.
S-Lay Barge – Allseas vessel
S-Lay Barge – Allseas vessel
S-Lay Barge – SEMAC-1
S-Lay Barge – SEMAC-1
One of the most critical pieces of equipment is the pipe tension, which will keep the pipe in the required tension while laying pipe with the S-lay method (Figure 4).
Figure 4 – Pipe Tension System
Departure angle at a stinger is typically about 30- 40 degree reference to the horizontal plan. However, the angle can be increased to 60- 70 degree in deep water operation.
Over-bend radius is proportional to the departure angle and inversely proportional to lay tension. The over-bend is controlled by a stinger. Sag-bend radius is inversely proportional to lay tension and proportional to suspended weight of pipeline. Proper tension must be maintained while laying the pipeline, otherwise risk of buckling will drastically increase.
When tension is not sufficient, the sag bend will increase. Hence, the chance of buckling is higher in the sag bending area. Even though longitudinal tension is minimum, hydrostatic pressure at this point reaches almost maximum.
References
OMBugge, (2009), Barges [ONLINE]. Available at: http://i566.photobucket.com/albums/ss102/OMBugge/Offshore%20Vessels/Barges/P1010044.jpg [Accessed 16 July 2016].
James G. Speight, 2014. Handbook of Offshore Oil and Gas Operations. 1 Edition. Gulf Professional Publishing.
Trond Bendiksen, 2015. Commissioning of Offshore Oil and Gas Projects: The manager’s handbook. Edition. AuthorHouse.
Joseph A. Pratt, 1997. Offshore Pioneers: Brown & Root and the History of Offshore Oil and Gas. Edition. Gulf Professional Publishing.
Offshoreenergytoday.com, (2014), S-Lay Barge – SEMAC-1 [ONLINE]. Available at: http://www.offshoreenergytoday.com/wp-content/uploads/2014/06/Pipeline-installation-begins-at-Ichthys-project.jpg [Accessed 16 July 2016].
OMBugge, (2014), S-Lay Barge – Allseas vessel [ONLINE]. Available at: http://i566.photobucket.com/albums/ss102/OMBugge/Offshore%20Vessels/Misc%20Offshore%20vessels/Audacia/Audacia_01.jpg [Accessed 16 July 2016].
Allseas, (2014), Allseas vessel [ONLINE]. Available at: http://allseas.com/wp-content/uploads/2015/07/01-Pipeline-installation_00-S-lay-1024×648.jpg [Accessed 16 July 2016].