Reel lay method installs offshore pipeline by sending the pipeline from a reel mounted on a special pipeline installation vessel. Instead of connecting each joint of pipe line at an offshore location like other methods (S-lay and J-lay method), the pipeline is pre-assembled in a spool which is mounted on the deck of the reel barge. This method can lay pipe up to 16” diameter and water depth capacity for 16” pipeline is about 800 m (2,600 ft). This method cannot lay such a big size pipe because the big pipeline is not flexible enough to be rolled into a reel.
The reels can be installed either horizontal or vertical. Horizontal reel barges can do only S-lay installation; however, vertical reel barges can perform both J-lay and S-lay pipeline installation.
The reel lay method is considered to be the fastest laying method because the majority of welding and inspections are performed onshore in order to minimize time for installation. Once all pipe on the reel is laid, the barge either head back to shore of another reel or lift a new reel from a supply boat. This is dependent of each vessel.
Some of the images of reel lay barges are shown below;
Figure 1 – Vertical reel-lay barge in S-lay configuration
J-Lay pipeline installation method is frequently used in deep water pipeline installation. The curvature of pipe line is similar to J shape (Figure 1) while the subsea pipeline is being installed. J-Lay method can handle a full range of pipeline size and it is particularly suitable for deep water pipeline installation up to 2,000 m (6,560 ft). Furthermore, the J-Lay method can withstand higher underwater current and sea state than the S-Lay method.
The J-lay method puts less stress on the pipe line because the pipeline is installed in an almost vertical position. Whereas the S-lay method puts on more stress due to two curvatures at the sag bend and over-bend region. The pipeline is sent into water at a small angle reference to a vertical line and continues at a steep angle until a sag bend is formed.
Figure 1- J-Lay pipeline vessel
This works very well in deep water environment where distance from a vessel to a touchdown point is quite far because sag bend is not too much. If the short distance from a vessel to a touchdown point. Continue reading
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