Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Patent
1996-11-26
1998-11-17
Graysay, Tamara L.
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
405158, F16L 100
Patent
active
058367195
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the laying of submarine pipelines.
Known methods of submarine pipelaying fall into two broad classes.
First, there are methods in which pipe lengths are shipped on a vessel such as a lay barge and are welded end to end as laying proceeds. These can be divided into "S lay" where the pipe is welded up in a horizontal path and laid over a stinger, and "J lay" where the pipe is launched from the vessel in a substantially vertical direction and each succeeding pipe length must be erected into a substantially vertical position for welding to the last section.
Each of these has problems and limitations. S lay requires a cumbersome and expensive stinger and can operate only in relatively shallow water. J lay can cope with deeper water, but handling pipe lengths into the vertical and welding them in that position is difficult and time consuming.
The second broad category of known method is reel pipelaying. This has the great benefit of allowing the bulk of the welding to be carried out in factory conditions ashore. However, a complex, specialised lay vessel is required for larger diameter pipelines. Reel pipelaying using existing arrangements is generally preferable on economic grounds for pipelines having an o.d. of 16" or less. However, this is not an absolute limit and may vary depending upon detailed operational requirements.
It should be noted that in the first of the foregoing categories, persons in the art have always considered it important to avoid plastic deformation of the pipe, seeking to control the double bends of S lay and the single bend of J lay such that the elastic limit of the pipe material is not exceeded. In reel pipelaying, the act of reeling solid pipe requires plastic deformation which is subsequently reversed by a straightener assembly.
An object of the present invention is to provide methods of, and vessels for, laying underwater pipelines which do not fall into the foregoing categories. The invention is particularly, but not exclusively, applicable to the laying of large diameter pipe which it is not practicable to coil on a reel and which must therefore be welded on the lay vessel.
Accordingly, from one aspect the present invention provides a method of laying an underwater pipeline, comprising welding pipe sections together along a substantially horizontal axis on the deck of a vessel to form a pipeline and laying the pipeline onto the sea bed as welding progresses, the pipeline being bent as it is laid from the vessel.
Preferably, the bending of the pipeline is followed by straightening thereof.
Preferably also, the pipeline is launched at a substantial angle to the horizontal, said angle being selected to allow the pipeline to form a catenary curve between the launch point and the sea bed.
Preferably, tension is applied to the pipeline and is controlled to ensure that the bending of the pipeline at the point where it meets the sea bed is within the elastic yield limit of the pipe material.
In certain embodiments, the pipeline is diverted about a horizontal axis through substantially 180.degree. prior to being launched via an adjustable-angle ramp, either from the stern of the vessel or through a moon-pool.
In a further embodiment, the pipeline is diverted about a horizontal axis through substantially 270.degree. prior to being launched substantially vertically from the side of the vessel.
From another aspect the invention provides a vessel for use in laying an underwater pipeline, the vessel including a deck area, means on the deck area for aligning pipe sections along a substantially horizontal axis and welding them together to form a pipeline, and means for bending the pipeline to cause it to pass from said horizontal axis to a launch axis having a substantial inclination to the horizontal.
Preferably, straightening means are provided downstream of said bending means for removing the bend in the pipeline.
In one form of the invention, the bending means comprises a fixed curved ramp secured to the hull of the vessel, and the straightening means (if any) is a
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Carmichael Donald
Martin Robert George
Roberts Stephen John
Coflexip Stena Offshore Limited
Graysay Tamara L.
Lagman Frederick L.
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