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How Does Offshore Stainless Steel Pipeline Installation Work?

DataTime:[5/25/2015]
Laying pipe on the seafloor can pose a number of challenges, especially if the water is deep. There are three main ways that subsea stainless steel pipe is laid -- S-lay, J-lay and tow-in -- and the pipelay vessel is integral to the success of the installation.

Buoyancy affects the pipelay process, both in positive and negative ways. In the water, the stainless steel 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 stainless steel pipe requires a downward force to remain in place. This can be provided by the weight of the oil passing through the Stainless Steel Pipeline, but gas does not weigh enough to keep the stainless steel pipe from drifting across the seafloor. In shallow-water scenarios, concrete is poured over the stainless steel 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.

Tow-In Stainless Steel Pipeline Installation
While jumpers are typically short enough to be installed in sections by ROVs, flowlines and Stainless Steel Pipelines are usually long enough to require a different type of installation, whether that is tow-in, S-lay or J-lay.

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

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

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

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

Lastly, the bottom tow drags the stainless steel 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.

S-Lay Stainless Steel Pipeline Installation
When performing S-lay Stainless Steel Pipeline installation, pipe is eased off the stern of the vessel as the boat moves forward. the stainless steel pipe curves downward from the stern through the water until it reaches the "touchdown point," or its final destination on the seafloor. As more pipe is welded in the line and eased off the boat, the stainless steel pipe forms the shape of an "S" in the water.

S-Lay Stainless Steel Pipeline Installation
Stingers, measuring up to 300 feet (91 meters) long, extend from the stern to support the stainless steel pipe as it is moved into the water, as well as control the curvature of the installation. Some pipelay barges have adjustable stingers, which can be shortened or lengthened according to the water depth.

Pipe being lowered into the water via a stinger for S-lay installation
Proper tension is integral during the S-lay process, which is maintained via tensioning rollers and a controlled forward thrust, keeping the stainless steel pipe from buckling. S-lay can be performed in waters up to 6,500 feet (1,981 meters) deep, and as many as 4 miles (6 kilometers) a day of pipe can be installed in this manner.

J-Lay Stainless Steel Pipeline Installation
Overcoming some of the obstacles of S-lay installation, J-lay Stainless Steel Pipeline installation puts less stress on the Stainless Steel Pipeline by inserting the Stainless Steel Pipeline in an almost vertical position. Here, stainless steel pipe is lifted via a tall tower on the boat, and inserted into the sea. Unlike the double curvature obtained in S-lay, the stainless steel pipe only curves once in J-lay installation, taking on the shape of a "J" under the water.

J-Lay Stainless Steel Pipeline Installation
The reduced stress on the stainless steel pipe allows J-lay to work in deeper water depths. Additionally, the J-lay Stainless Steel Pipeline can withstand more motion and underwater currents than stainless steel pipe being installed in the S-lay fashion.

J-Lay Pipelay Vessel S7000
Types Of Pipelay Vessels
There are three main types of pipelay vessels. There are J-lay and S-lay barges that include a welding station and lifting crane on board. The 40- or 80-foot (12- or 24-meter) pipe sections are welded away from wind and water, in an enclosed environment. On these types of vessels, the stainless steel pipe is laid one section at a time, in an assembly-line method.

On the other hand, reel barges contain a vertical or horizontal reel that the stainless steel pipe is wrapped around. Reel barges are able to install both smaller diameter pipe and flexible pipe. Horizontal reel barges perform S-lay installation, while vertical reel barges can perform both S-lay and J-lay Stainless Steel Pipeline installation.

Vertical Reel Barge
When using reel barges, the welding together of stainless steel pipe sections is done onshore, reducing installation costs. Reeled pipe is lifted from the dock to the vessel, and the stainless steel pipe is simply rolled out as installation is performed. Once all of the stainless steel pipe on the reel has been installed, the vessel either returns to shore for another, or some reel barges are outfitted with cranes that can lift a new reel from a transport vessel and return the spent reel, which saves time and money.
Source: Yaang Pipe Industry Co., Limited (www.yaang.com)

Tel No:+86-18267732328 / Fax No :86-0577-56809281 / Email:sales@yaang.com
Address:Longwan District, Wenzhou, Zhejiang Province, China.

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