Ultra deepwater pipelay, in water depths of two or more miles, has always been regarded as the exclusive territory of J-lay pipelayers, where pipe sections arc welded in an almost vertical firing line.
J-lay is a very slow method of pipelay. Only one welding point is available. Traditional S-lay techniques utilize a horizontal firing line, allowing multiple welding stations and very fast pipelay, but have always been thought not usable in ultra deepwater due to equipment limitations. However, the hydraulic pipeline tensioner presented in this article, combined with a high departure angle stinger, allow existing pipelay vessels to be readily modified to perform ultra deep S-lay pipelay.
The gas pipelines to be installed between Oman and India will consist of two 1100-km long, 24-in. diameter pipelines running across the Arabian Sea in water depths of up to 3500 meters.
Preliminary engineering shows the pipeline will have a 41-millimeter wall thickness with no concrete coating. Scheduled first gas flow is for 1999. Estimates put the overall cost of the installed pipeline at between US$1.5-2.0 million/km and a total pipeline costs of US$3.3-4.4 billion.
Deepwater pipelay
All pipelay vessels are fitted with tensioners and stingers (sometimes called ramps).
The tensioners hold the suspended weight of pipe between the vessel and sea bed, preventing pipeline buckling. Stingers are tubular steel structures with a number of rollers that support the pipeline and allow it to bend, in a controlled radius, from horizontal to the required departure angle.
Pipelines are laid dry. The inside of the pipeline is not flooded with seawater.
Occasionally though, through material defect or buckling, the pipeline will accidentally flood with water. Under those circumstances, the tensioners must be able to hold the much heavier suspended weight of the flooded pipeline while repair and de-watering are performed.
Large diameter pipelines have been installed in water depths of up to 610 meters. At this depth, a 24-in. diameter pipeline requires a maximum of 300 tons tension to support it and a stinger giving a pipeline departure angle from the vessel of 45.
At a water depth of 3,500 meters, using conventional S-lay pipelay requires a tension capacity of 1,100 tons for a dry pipeline and 2,20 tons if flooded. The stinger must provide a departure angle of 70-75 from the horizontal.
With a high departure angle stinger, the pipelay vessel's forward thrust or anchor force to hold station while laying pipeline is about 275 tons.
Pipeline tensioner
A hydraulic pipeline tensioner is based on a number of hydraulically operated moving clamps. The tensioner can feed out or reel in an Oman-India 24-in. pipeline with the required 1,100 tons tension at a speed of about 24 meters/minute, a comparable speed to existing electric tensioners. If the pipeline is accidentally flooded, then the tensioner can hold the resulting 2,200 ton load.
At 16 meters long, a hydraulic tensioner can be easily accommodated within a pipelay vessel, while allowing room for one or more conventional electric tensioners. The tensioner consists of eight individually operated hydraulic clamps.
Each clamp is controlled by three hydraulic cylinders. Two 1.45-ton load capacity, 500-millimeter stroke cylinders hold the clamp closed and move it 0.5 meters while providing 290 tons tension. The third cylinder, with a 10-ton load capacity and 300 millimeter stroke, opens the clamp by pushing on a release mechanism, returning it to the original position, and closing it onto the pipeline again.
During normal pipelay, four clamps will be closed, providing up to 1,160 tons tension and moving the pipeline. The main hydraulic cylinders of each clamp are at a different stage of retraction. The other four clamps will be open and returning to their original positions. Each 10-ton cylinder clamp opening and returning the main hydraulic cylinders to full extension.
Tensioner features
The top and bottom half of each clamp holds the pipeline due to friction between the corrosion-resistant outer coating of the pipeline and a removable neoprene liner. To achieve 290 tons of friction, a clamping force of 440 tons is required, with a typical friction coefficient between the liner and pipeline of 0.67.
The clamping force is supplied via the two 145-ton hydraulic cylinders through four pinned hinge plates. The distance from the central pin to the hydraulic cylinder connection is 1.5 times the distance to the top clamp shell connection. This gears the cylinder load by 1.5 and hence supplies 440 tons clamping force.
The Oman-India 24-in. diameter steel pipeline can easily withstand both the clamping force and the tension force as its wall thickness and steel grade are governed by the pressure of water at 3,500 meters below sea level. The neoprene liner can be unclipped and removed once worn. Bonding or gluing to the clamp is not necessary as friction between the liner and steel clamp will hold it in place.
Tensions lower than 290 tons per clamp are achievable by adjustment of the fluid pressure in the main hydraulic cylinders. The 145-ton load capacity, 500-millimeter stroke hydraulic cylinder on which the design is based is a commercially available cylinder operating with oil as a hydraulic fluid at a maximum 700 bar pressure. The system for pressurizing and controlling the hydraulic fluid is adjustable to provide a range of lower pressures. Lower tensions are required for installing the 24-in. pipeline in water depths less than the maximum 3,500 meters.
Control of hydraulic fluid pressure is governed by the measured rate of pipeline movement which is continuously monitored and fed into the control system for continuous revising of hydraulic fluid pressure.
The tensioner and control system will allow one clamp to be disconnected from the system with the tensioner still working. This may be required for changing a neoprene liner, routine maintenance, or replacement of cylinders or clamp. With one clamp removed from the system, four will hold tension in the pipeline with three open and moving back to their starting position.
Speed of moving the pipeline through the tensioner is then governed by the return time of the open clamps, and is reduced to 18 meters per minute. Cycle time for a single clamp is envisioned as 10 seconds, based on standard hydraulic cylinders and pumps.
If accidental flooding of the pipeline occurs, all eight clamps will automatically close onto the pipeline to provide up to 2,320 tons of holding force. Once a temporary repair and partial de-watering has been performed, the tensioner is able to pull the pipeline back into the vessel.
As the pipeline is retrieved, pipe sections can be cut off the end until the damaged area has been removed. The hydraulic tensioner can retrieve a pipeline by reversing the steps required to lower it.
The shore approach sections of the Oman-India pipelines are in shallow water, where current and waves can act on them. These sections will require a concrete weight coat for stability. For these sections, all clamps on the hydraulic tensioner will be in the open position with neoprene liners removed. One or more of the pipelay vessel's electric tensioners will be used.
Suitable vessels
Several existing S-lay pipelay vessels can be readily converted to install the Oman-India pipeline. These are both semisubmersible and the latest generation of ship-shape vessels.
The 16-meter long hydraulic tensioner can replace two existing electrical. tensioners, typically leaving one or two of the vessel's electric tensioners in place.
A high departure angle stinger can be fitted to any of the existing semisubmersible pipelay vessels. The ship-shape Allseas vessel Solitaire already possesses one, which could prove suitable without modification.
Stationkeeping in 3,500 meters water depth would be most easily achieved by a dynamically positioned vessel such as the Solitaire. Most semisubmersible vessels can be converted to dynamic positioning. However, the possibility exists for a self-propelled semisubmersible unit with positioning assisted by tugs to be used.
Up to 1,500-2,000 meters water depth, the semisubmersible would use its anchors for positioning. Beyond this depth, the relative crudeness of using built-in propulsion and tugs has little effect on pipeline installation as the large amount of pipeline suspended between the vessel and seabed will translate even large vessel movements into very low pipeline stresses.
Lorne Gifford is an offshore structures and pipelines engineer. His experience covers design and construction of offshore platforms, subsea structures, pipelay stingers, and pipelines. He holds sole patent rights for the tensioner described in this article.
Copyright 1995 Offshore. All Rights Reserved
http://www.offshore-mag.com/articles/print/volume-55/issue-6/news/pipeline/pipeline-technology-s-lay-vessels-can-be-converted-to-install-oman-india-pipeline.html
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