Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Reexamination Certificate
2002-08-27
2004-05-04
Lee, Jong-Suk (James) (Department: 3673)
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
C405S154100, C405S166000, C405S173000, C138S093000, C285S024000
Reexamination Certificate
active
06729802
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the laying of pipelines and in particular to a method for abandoning (or abandoning and recovering) a pipeline. The invention also relates to a system for use in such a method. Such a system and such a method are referred to herein as an A/R system and an A/R method.
During laying, it is sometimes necessary to abandon a pipeline and recover it later. For example, a pipeline might need to be abandoned because of severe weather conditions or mechanical problems aboard the laying vessel.
Conventionally, such abandonment and recovery is carried out by a method such as: welding an abandonment/recovery head onto the end of the pipeline being laid; connecting a steel rope to the head; transferring pipeline tension from the pipeline tensioning arrangement on the laying vessel that is used during normal laying to a winch on the vessel; and laying the pipeline and head on the sea bed. In severe weather conditions the vessel may also have to disconnect the rope from the winch and abandon the rope for later recovery; the end of the rope may then be marked by a buoy connected to it directly or via a pennant line.
Use of a conventional A/R system in deep water causes several problems.
The steel rope associated with the winch must have a very high tensile strength to support the high load resulting from the long length of pipeline, which initially extends from the seabed to the vessel. That requires a steel rope having a larger diameter, which, in turn, increases its weight. A higher load capacity is then required of the winch. The problem is increased because a very long steel rope and a very long pennant line are required, which creates further weight. The factors just described mean that the provision of a conventionally designed A/R system on a vessel able to operate in deep water is unduly expensive.
OBJECTS AND SUMMARY
The effect of the factors referred to above is to make the provision of an effective and economical A/R system problematic when pipelines are being laid in deep water.
It is an object of the invention to provide a method and apparatus of laying a pipeline which overcomes or mitigates the above-mentioned problem.
According to a first aspect of the present invention there is provided a method of abandoning a pipeline being laid by a vessel, including the steps of reducing the tension at sea level of the sealine being laid by the vessel, and thereafter lowering the end of the sealine towards the seabed using a line from a winch on the vessel. Preferably, the tension is reduced by at least 15%. Advantageously, the tension is reduced by at least 30%, and more preferably by at least 40%.
By reducing the tension at sea level of the sealine being laid by the vessel before the winch is used to lower the sealine, it becomes possible to employ a winch whose maximum load bearing capacity is substantially less than the maximum tension under which the vessel is designed to lay the pipeline (the tension when the pipeline is being laid at the greatest depth). The maximum load bearing capacity of the winch may therefore be less than the maximum load bearing capacity of the pipe laying and tensioning arrangement that is used for laying the pipeline. Similarly the load bearing capacity of the winch line can be reduced thereby enabling its diameter to be reduced. In an example of the invention the load bearing capacity of the winch is reduced to about 50% of the maximum tension under which the vessel is designed to lay a pipeline. A reduction in the load bearing capacity (SWL) of the winch is critical in enabling an effective and economical A/R system to be provided even on a vessel designed to be capable of laying large diameter pipelines in deep water.
Where reference is made in the specification to a “sealine” it should be understood that the term is referring to the pipeline laid on the seabed together with any pipeline or other elongate members extending upwardly from the pipeline.
In embodiments of the invention described below, the step of reducing the tension at sea level of the sealine being laid by the vessel comprises connecting one or more light elongate members to the end of the pipeline and lowering the elongate members into the sea. Where reference is made to a “light” elongate member, it should be understood that the member is to be light per unit length, when submerged in water, by comparison with the weight in water of the pipeline to which it is connected, so that the effect of connecting the light elongate member in the sealine is to reduce the weight of the sealine. It will be appreciated that any reduction in weight of the sealine extending between the vessel and the seabed will reduce the tension in the sealine at sea level. In the case where the pipeline is flooded, connection of a further length of pipeline sealed at both ends to prevent flooding represents connection of a light elongate member and reduces the tension in the sealine. Provided the water in which the method is carried out is not too deep, the use of such sealed pipeline or similar members may alone reduce the tension to a level that the winch is able to bear. In deeper water, however, even when the pipeline is not flooded, the tension in the pipeline may be too great for the winch to bear; in this case connecting a length of sealed pipeline will not reduce the tension and a lighter member is required to be connected; preferably such a member is sufficiently light that it is buoyant and, more preferably, generates an upward buoyancy force in water of the order of one half its weight in air or more. In especially deep water it may be desirable to connect together a series of buoyant members.
Said at least one buoyant elongate member may be composed principally of a lightweight material, for example, one selected from the group comprising glass fibre reinforced resins, other composite materials, carbon, titanium and aluminium. In an embodiment of the invention described below the material is a glass fibre reinforced vinyl ester resin. Parts of the elongate member, for example, end fittings, may be made of heavier material, for example, steel.
An intermediate elongate member is preferably connected between the end of the pipeline and a buoyant elongate member. The intermediate elongate member is preferably hollow and sealed against the ingress of water. The intermediate elongate member may be of sufficient weight not to be buoyant when empty. Preferably the member is provided with a valve to enable the interior of the intermediate elongate member to be flooded by sea water. Especially in the event that the sealine cannot be recovered by connecting a winch line to its end, the method may further include the step of cutting the intermediate elongate member, connecting the winch line to the end of the elongate member that is connected to the pipeline and recovering the end of the pipeline. The ability to flood the interior of the intermediate elongate member enables pressures to be equalised, thereby facilitating the cutting of the member.
When the one or more elongate members first begin to be lowered into the sea the tension that has to be applied to them is substantially the same as when laying pipe. It is a much preferred feature of the invention that the one or more elongate members are lowered using a pipe laying and tensioning arrangement on the vessel that is used for laying the pipeline; usually there is only one such tensioning arrangement although it may have several component parts. In order to facilitate use of the same tensioning arrangement, it is preferred that the diameter of the light elongate members is substantially the same as the diameter of the ordinary lengths of pipe making up the pipeline. Indeed the light elongate members are preferably of the same general dimensions as an ordinary length of pipe. As will now be understood, the invention enables the load bearing capacity of the winch to be substantially less than the load bearing capacity of the tensioning arrangement.
In some cases there will be no need to disconnect
Giovannini Umberto
Penzo Vincenzo
Lee Jong-Suk (James)
Saipem S.p.A.
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