Steel Pipe Application: Offshore Pipeline Construction

There are three main methods used to construct an offshore pipeline using steel pipe: tow-in, S-lay, J-lay. While some of these methods overlap, each is best suited for different water depths and water environments.

Tow-In Pipeline Construction

When using the tow-in method, steel pipe is attached to buoyancy modules to keep the pipe afloat, and a tugboat tows the pipe offshore and into place. Once the pipe is in the correct position, the buoyancy modules are removed, and the pipe drops to the seafloor. There are four different types of tow-in pipeline installations: surface tow, mid-depth tow, off-bottom tow and bottom tow.

Surface Tow

Surface tow involves towing the pipeline fully on top of the water. The buoyancy modules used in the surface tow method help keep the steel pipe on the water’s surface as the tugboat pulls the pipe out to location for installation.

Mid-Depth Tow

When employing the mid-depth tow method, less buoyancy modules are used than in the surface tow method. Instead, a mid-depth tow uses the forward speed of the tugboat to keep the pipeline at a submerged level. Once the forward motion has stopped, the steel pipe will naturally fall to the seafloor.

Off-Bottom Tow

Using the off-bottom tow method, a combination of buoyancy modules and chains (for weight) are used to work against each other to keep the pipe near the seafloor. When the pipe has been hauled to location, the buoyancy modules are taken away and the pipe falls the rest of the short distance to the seafloor.

Bottom Tow

Using the bottom tow method, a tugboat drags the steel pipe along the seafloor, and no buoyancy modules are used. This method is only performed in shallow waters where the seafloor is soft and flat.

S-Lay Pipeline Construction

When employing the S-lay pipeline construction method, steel pipe is loaded onto a vessel and driven onto location. As the boat moves forward, the pipe is slowly eased off the back of the vessel, curving down to the seafloor in an S-shape—hence, the method’s name. Sections of steel pipe are welded together on the boat before being dropped into the sea, giving the pipeline its length. Ramps called stingers are used to help guide the welded steel pipe off the back of the vessel and lower them gently into the water.

J-Lay Pipeline Construction

J-lay pipeline installation is similar to S-lay pipeline installation in that steel pipe is loaded onto a vessel and driven onto location, and as the boat moves forward, the pipe is slowly eased off the back of the vessel. The difference is that the pipe is first lifted via a tall tower on the back of the boat and then lowered into the sea. The result is that the pipe makes a J-shape rather than an S-shape. In other words, the pipe has one less curve. This reduces stress on the pipe and allows steel pipe to installed at deeper water depths. Additionally, J-lay pipeline can withstand more motion underwater (such as those caused by currents) than an S-lay pipe can withstand.

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