Offshore Pipeline Installation

Laying pipe on the seafloor can pose a number of challenges, especially if the water is deep. The
most common methods of pipeline lay installation methods are:

  • S-lay (Shallow to Deep) 
  • J-lay (Intermediate to Deep)
  • Reel lay (Intermediate to Deep)

Shallow water depth ranges from shore to 500 feet. Intermediate water depth is assumed to be 500 feet to 1000 feet. Deepwater is water depths greater than 1000 feet. Offshore magazine produces a survey of most of the pipeline lay barges that work in the US every year. This survey does not cover all the lay barges of all the countries that do offshore work, but it does cover the bigger international ones Heerema, Saipem, Stolt, Technip, Allseas, McDermott, Global, and Subsea 7. Other methods that have been used for pipeline installation are tow methods consisting of:
  • Bottom tow
  • Off-bottom tow
  • Mid depth tow
  • Surface tow
Tow methods can be used for installing pipelines from shallow water depths to deepwater depths depending on the design requirements.

In pipeline installation buoyancy affects the pipelay process, both in positive and negative ways. In the water, the pipe weighs less if it is filled with air, which puts less stress on the pipelay barge. But once in place on the sea bed, the pipe requires a downward force to remain in place. This can be provided by the weight of the oil passing through the pipeline, but gas does not weigh enough to keep the pipe from drifting across the seafloor. In shallow-water scenarios, concrete is poured over the pipe to keep it in place, while in deepwater situations, the amount of insulation and the thickness required to ward of hydrostatic pressure is usually enough to keep the line in place.

1. S-lay method

The most common method of pipeline installation in shallow water is the S-lay method. In the S-lay method, the welded pipeline is supported on the rollers of the vessel and the stinger, forming the over-bend. Then it is suspended in the water all the way to the seabed, forming the sag-bend. The
over-bend and sag-bend form the shape of an ‘‘S.’’
S-lay configuration
Source: Offshore pipelines - Dr. Boyun Guo (2005)
In the S-lay method, tensioners on the vessel/barge pull on the pipeline, keeping the whole section to the seabed in tension. The reaction of this pull is taken up by anchors installed ahead of the barge or, in the case of a dynamically positioned (DP) vessel, by thrusters. These barges/vessels are fitted with tension machines, abandonment and recovery (A&R) winches, and pipe handling cranes. The firing line for welding the pipe may be placed in the center of the barge or to one side. The firing line consists of a number of stations for welding, NDE, and field joint application. The field joint station is located after the NDE station and the tension machines.

S-lay vessel

2. J-lay method

To keep up with the discovery of deepwater oil and gas fields, the J-lay system for pipeline
installation was invented. In this system, lengths of pipe are welded in a near vertical or vertical position and lowered to the seabed. In this configuration, the pipeline from the surface to the seabed is one large radius bend resulting in lower stresses than an S-lay system in the same water depth. There is no over-bend, and a large stinger required in S-lay to support the pipe in deepwater is eliminated. The horizontal forces required to maintain this configuration are much smaller than required for an S-lay system. This lends itself for DP shipshape vessels and derrick barges to be equipped with a J-lay tower. 

J-lay configuration
Source: Offshore pipelines - Dr. Boyun Guo (2005)

Normally, the J-lay process is slower than S-lay, but since the large J-lay towers are capable of handling prefabricated quad joints (160 feet long), the speed of pipelaying is increased. The J-lay method is normally used in water depths greater than 500 feet. These water depths are normally too great for moored lay vessels to operate, because the required tensions and pipe bending stresses are too large.

J-lay vessel

3. Reel lay method

Reel pipelay is a method of installing pipelines in the ocean from a giant reel mounted on an offshore vessel. Pipelines are assembled at an onshore spool-base facility and spooled onto a reel which is mounted on the deck of a pipelay barge. The first application of the reeled pipeline was on D-Day when the allies were supplied with fuel across the English Channel using a small diameter pipeline unreeled from a vessel. Commercial application of reeled pipeline technology was not available until the early 1970s when Santa Fe Corporation built the first reel vessel.

Reel lay configuration

The reel method reduces labor costs by permitting much of the welding, x-raying, corrosion coating, and testing to be accomplished onshore, where labor costs are generally lower than comparable labor costs offshore. After the pipeline is reeled onto the drum of the pipelaying vessel, it is taken to the offshore location for installation. The reeled pipeline can be installed in an S-lay method or J-lay method depending on the design of the reel vessel and the depth of water. Reel vessels can have vertical reels or horizontal reels.
The pipe is unreeled, straightened, de-ovalized, and connected to the wire rope from the seabed pre-installed hold back anchor. The sagbend stresses are controlled by the tensioning system on the reel vessel. The vessel moves ahead while it slowly unreels the pipeline from the drum. When the end of the pipeline on the drum is unreeled, a pullhead connected to a wire rope is attached. The end of the pipeline is lowered to the seabed by paying out the A&R wire rope from the reel vessel slowly in a controlled method always maintaining sufficient tension in the pipeline. A buoy is attached at the end of the A&R cable. The reel vessel returns to the spool base to load more welded pipeline on the reel drum. On returning, it pulls the end of the pipeline using the A&R cable, removes the pullhead, and welds it to the pipeline on the drum. It then begins the unreeling process again.

Reel lay vessel

 4. Tow method

Tow-in installation is just what it sounds like, the pipe is suspended in the water via buoyancy modules, and one or two tug boats tow the pipe into place. Once on location, the buoyancy modules are removed or flooded with water, and the pipe floats to the seafloor.  

There are four main forms of tow-in pipeline installation. The first, the surface tow involves towing the pipeline on top of the water. In this method, a tug tows the pipe on top of the water, and buoyancy modules help to keep it on the water's surface. 

Surface tow
 Using less buoyancy modules than the surface tow, the mid-depth tow uses the forward speed of the tug boat to keep the pipeline at a submerged level. Once the forward motion has stopped, the pipeline settles to the seafloor. 

Mid-depth tow
Off-bottom tow uses buoyancy modules and chains for added weight, working against each other to keep the pipe just above the sea bed. When on location, the buoyancy modules are removed, and the pipe settles to the seafloor. 

Off-bottom tow

Lastly, the bottom tow drags the pipe along the sea bed, using no buoyancy modules. Only performed in shallow-water installations, the sea floor must be soft and flat for this type of installation.

Bottom tow


Offshore pipelines - Dr. Boyun Guo (2005) 


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