Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Reexamination Certificate
2001-01-22
2003-02-25
Lagman, Frederick L. (Department: 3673)
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
C405S168400, C405S170000
Reexamination Certificate
active
06524030
ABSTRACT:
The invention relates to the laying of pipelines under water, especially at sea, from a vessel floating on the water.
The technique mainly used up to now to lay pipelines in the sea is that called “S laying”. This technique can be described briefly as follows. The pipe joints, generally 12 meters long, are transported from a port to a lay-barge by means of supply barges or supply vessels, and are loaded onto the deck of the lay-barge. Those pipe joints are added one after the other along a construction ramp, which is usually horizontal or in some cases has a small inclination of 5 or 10 degrees (this ramp being called the “firing line”). On the firing line the operations necessary to complete the connection of the pipe joints are performed in several working stations in order to build a continuous pipeline. When a new joint is added, the barge moves forward and the pipeline, supported at the stern of the lay-barge by an inclined ramp (or floating stinger) curves over the stem of the barge down toward the seabed. The profile of the pipeline, from the lay-vessel to the seabed, is in the form of a long “S” (from which comes the term “S lay”). The upper part of the profile is called the “over-bend”, and the lower part is called the “sag-bend”. In order to reduce the stresses on the suspended part of the sealine on its way from the lay-vessel to the sea bottom, a constant tension is maintained on the pipeline by means of machines called tensioners. There is a maximum to the depth of water in which that method can be used. As the water depth increases, the tensioner pull necessary to maintain the pipe stress within acceptable values increases dramatically, and the horizontal bollard pull on the lay vessel increases correspondingly. A method to reduce the above-mentioned pulls consists of increasing the angle of descent of the sealine in to the sea. If the angle is close to vertical (called “J lay”) the necessary tension on the pipe is very close to the weight of a length of the sealine string equal to the water depth, and the horizontal component is close to zero. This method has the contrary limitation that there is a minimum to the depth of water in which it can be used. because the pipeline must have room to curve through about 90° to lie on the seabed, and if the pipeline is too tightly curved it will be over-stressed.
It is an object of the present invention. considering all the above mentioned problems, to provide an apparatus able to lay pipeline in very deep waters as well as in relatively shallow waters, in conditions favourable for the integrity of the pipe and with high productivity.
The invention provides a pipe-laving vessel comprising: means for propelling the vessel during pipe-laving; means for assembling sections of pipe generally horizontally on the vessel to form longer lengths; a tower at the bow of the vessel, with respect to an intended direction of movement, pivotally mounted so that it can be angled forwards towards the top; means for raising a length of pipe from the deck to a position aligned with the tower; means for joining such a length of pipe to a pipeline being laid; tensioners arranged to grip such a pipeline and to lower it into the water while maintaining a desired tension in the pipeline; one or more clamps arranged to grip the pipeline below the tensioners; a lower ramp provided with rollers and arranged to guide the pipeline as it leaves the vessel; and means provided at a rear portion of the vessel for monitoring the pipe where it touches down on the water bottom.
The invention also provides a method of laying pipes from a vessel, comprising: providing a tower at the bow of the vessel, with respect to an intended direction of movement, pivotally mounted so that it can be angled forwards towards the top; repeatedly assembling sections of pipe in a generally horizontal position on the vessel to form a longer length, raising that length of pipe from the deck to a position aligned with the tower, joining that length of pipe to a pipeline being laid, and gripping the pipeline with tensioners and lowering it into the water while maintaining a desired tension in the pipeline; guiding the pipeline as it leaves the vessel using a lower ramp provided with rollers; monitoring the pipe from a rear portion of the vessel where it touches down on the water bottom; and gripping the pipeline with clamps below the tensioners during any interruptions in laying.
The mounting of the tower (firing line ramp) at the extreme bow of the vessel, and the accompanying inclination of the pipeline axis toward the front of the vessel, greatly facilitate the operation of passing the pipeline to a platform or other similar structure when constructing, for example, catenary risers. The end of the pipeline may be lowered free of the vessel using the abandonment and recovery winch, and then drawn up to the platform using one or more winches on the platform, without the need to pass the pipeline under the vessel or to manoeuvre the vessel out of a position between the end of the pipeline and the platform. Also, the pipeline, on its way to the bottom, runs under the vessel and the touchdown point is relatively close to a point directly below the stern. That makes it easy to monitor the touchdown and control the laying operation by means of a remotely operated vehicle (r.o.v.) based on the pipelaying vessel without the need for an extra survey vessel. The monitoring means may comprise means for operating an r.o.v. from the vessel, and the vessel may include such a remotely operated vehicle.
The means for propelling the vessel may comprise propellers driven by motors on the vessel. Instead, especially in relatively shallow water, the vessel man be propelled by capstans working on ropes attached to fixed anchors or the like.
Having the tower positioned in the bow, rather than amidships as has previously been proposed, allows a very large clear deck area for storing pipe containers and for assembling lengths of pipe. The assembling means advantageously comprises means for welding four sections of pipe to form a single length, preferably by welding them together in pairs and then joining the pairs. Thus, the vessel may be supplied with standard 12.2 meter joints, and raise 48.8 meter lengths to the tower.
The tower is advantageously pivotable between a vertical position and an angle of about 30° forwards. The tower may also be capable of being lowered to a shallow angle for use as a launch ramp or stinger in S laying, but is preferably arranged to be dismounted to allow S laying from that end of the vessel. The end of the vessel that is the bow during J laying may then become the stem during S laying.
The raising means may comprise a cradle for supporting a length of pipe, pivotally mounted at or near the foot of the tower. Such a pivoted cradle can be raised and lowered by a rope from the tower. Preferably, the cradle is only approximately as long as the maximum length of pipe to be raised, in which case the pipe may be raised further up the tower by a vertical conveyor.
The joining means may comprise a first working station, equipped for actually welding the pipeline, and a second working station, one pipe length below the first, at which non-destructive testing and finishing of the join are carried out. One join may then be welded while the previous one is being tested, with a consequent increase in the rate of pipe laying. The second working station may be positioned between the tensioners and the clamps.
The tension machines may be a standard type previously used for S laying. The use of tensioners simplifies and speeds up the pipeline lowering operation, and allows the passage of anodes, buckle arrestors, etc. without problems.
The said clamps may comprise a fixed friction clamp and a movable friction clamp. Each of the clamps is preferably strong enough to support the maximum weight of pipeline that the vessel is capable of laying, even if the pipeline becomes flooded with water.
The lower ramp preferably comprises means for monitoring the load on the pipeline as it passes over
Bianchi Stefano
Giovannini Umberto
Signaroldi Teresio
Burns Doane Swecker & Mathis L.L.P.
Lagman Frederick L.
Saipem S.p.A.
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