The present invention relates to pipeline laying apparatus and a method of laying pipeline from a vessel, particularly a reel lay vessel for laying continuous steel pipeline, generally referred to as rigid pipe.
When pipeline is being laid from a reel lay vessel, a portion of the pipeline is suspended between the vessel and the seabed (this is known as the catenary). The weight (tension) of this portion must be supported by the vessel's pipe laying equipment. The pipe laying equipment must also be capable of paying out the pipeline under this tension during the reel lay operation. Pipe is diverted (bent) from the reel into the laying axis, before the tension is applied. Straightening is also performed after the diverter, to remove the deformation of the pipe caused by reeling.
It is common in the industry to provide pipe laying vessels with tensioners, such as track type tensioners described in detail in U.S. Pat. No. 5,527,134 in order to meet this requirement. As an alternative to tensioners, some towers may be provided with a travelling clamp mounted on a hoist which allows the pipeline to be lowered during the laying operation. Each of these two alternative systems are, in many applications, relatively satisfactory; however, recently there has been an increasing demand for pipelines to be laid in much deeper water. In such situations, a much greater length of suspended pipeline is needed to span the vertical distance between the vessel and the seabed. This increased length of pipeline exerts a far greater load on the vessel's laying equipment with the result that many vessels are limited to laying pipe in relatively shallow water. Tensioners have limited capacity, and are very heavy items. Furthermore, when compared to a clamp, tensioner are relatively long, and are much heavier, for a given capacity. In situations when the pipe laying equipment is mounted on a raised tower, the use of such large tensioners results in a decrease in the overall vessel stability which can ultimately lead to a need for a larger vessel. Therefore, it is not straightforward to increase the capacity of such vessels, without compromising their stability and cost effectiveness.
According to the present invention, there is provided a method of laying offshore pipeline from a reel lay vessel, the method comprising:—
This allows a single tower to lower the pipeline from the vessel using either the travelling clamp or the tensioners as is appropriate to each particular case. In particular, although the laying process using the travelling clamp is generally slower than using a tensioner, the invention allows a vessel to lay exceptionally heavy or deep pipe on occasion, while having a tensioner of a more modest size, cost and weight, sufficient for efficient performance of the majority of tasks.
The method may include using the tensioner apparatus to support a substantial part of the tension in the suspended pipeline, while simultaneously supporting the remainder in the travelling clamp.
In such a case, one of the tensioner and the travelling clamp may be operated in a constant tension mode while the other is operated in a constant velocity mode. In that way, one of the devices acts to support a portion of the pipe weight, moving effectively as a slave of the other device.
The constant tension control may be applied to the tensioner apparatus, while constant velocity control is applied to the travelling clamp.
In using the travelling clamp to pay out the pipeline, the further clamp is used to hold the pipe stationary while the travelling clamp releases the pipe and returns to the top of its stroke to grip the pipe again and pay out another portion.
The method may then include repeating the above steps in order to provide semi-continuous deployment of the pipeline from the vessel. The travelling clamp may be provided below the tensioner apparatus.
The further clamp may alternatively be provided above the tensioner apparatus. Alternatively the further clamp may be the same as a hang-off clamp located below the tensioner apparatus.
The hoist means may be provided with a stroke length which allows the travelling clamp to stroke below the water line.
The method may further include providing a straightener above the tensioner apparatus for reversing plastic deformation of the pipeline prior to applying tension to support the suspended weight.
Apparatus for laying offshore pipeline from a vessel, the apparatus comprising:—
The hoist means may comprise a winch and cable arrangement which may provide a supportive connection to the pipeline laying tower on each side of the travelling clamp.
The tensioner apparatus may comprise track type tensioners.
The apparatus may be provided with the travelling clamp at a location which is below the tensioner apparatus and, the further clamp means at a location which is above the tensioner apparatus.
Preferably, further clamp means is provided and may comprise a hang-off clamp. Optionally, the further clamp means is provided below the bottom of the travelling clamp stroke length.
Alternatively, the further clamp means is provided above the top of the travelling clamp stroke length. In this embodiment, the hoist means and the travelling clamp means may be adapted to provide a stroke length which extends below the water line.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Referring to
In this arrangement, the tensioners 12 control movement of the pipeline P down the tower 10 by applying a clamping force around the outer surface of the pipeline wall. This allows the weight of the pipeline between the vessel and the seabed to be suspended from the tensioners 12. The straightener 16 removes the bend which rigid steel pipe acquires through being wound onto the reel 20. The length of pipeline P suspended between the vessel and the seabed may be a number of kilometres long and may therefore exert a very large tension force on the tensioners 12. The diameter of the pipe, its wall thickness and gas/liquid contents also determine the weight of the pipe in water, and thus the tension required to support it during laying. During this operation, the hang-off clamp 14 is disengaged so that it has no effect on the pipeline P passing therethrough. The hang-off clamp 14 is normally only activated when it is necessary to temporarily remove the pipeline load from the tensioners 12. One example of when this might be necessary is during installation of an In-Line Tee or a Pipe Line End Termination on the pipeline P.
It will be appreciated by the skilled reader that the above described apparatus relies upon the vast majority of the pipeline tension being supported by the tensioners 12 (the remaining portion of the tension being supported by back tension in the reel mechanism). Hence, in such systems the capacity of the tensioners limits the depth and or type of pipe that can be laid. Tensioners for extreme tensions are extremely large, expensive and heavy, if available at all. Given that the majority of operations will concern lighter pipe and/or shallower operations, the provision of tensioners of very high capacity is rarely going to be a justifiable capital expense.
A novel pipelaying apparatus is shown schematically in
It should be noted that although a single tensioner 112 is shown in
A further clamp means in the form of hang-off clamp 114 is provided in a laying slot at the rear of the vessel V and is capable of clamping around the outer circumference of the pipeline P in order to prevent it from moving therethrough. The hang-off clamp 114 is also releasable in order to allow free movement of the pipeline P therethrough, when desired.
Travelling clamp 124 has a similar clamping capability as the hang-off clamp 114. However, it is mounted on hoist means 126 comprising a pair of hoist and cables on either side of the tensioners 112. The hoist means 126 allow the travelling clamp 124 to be lowered and raised in the stroke direction A which is oriented in line with the direction of downward movement of the pipeline P along the tensioner axis during the laying operation (pipe laying direction). In this regard, the position of the travelling clamp 124 at the top of its stroke is depicted with solid lines and the position of the travelling clamp 124 at the bottom of its stroke with broken lines in
A regulating mechanism may be provided to keep the reel drive (back-tension) in step with the drive for the tensioner 112. In such a mechanism the mounting for the diverter wheel 118 allows it to move slightly to absorb additional tension or slackness caused by differences in the speed of the reel and the tensioner. The drive system responds to this movement to increase or decrease the speed of the reel, or stop it entirely, in order to maintain a pre-set back-tension and avoid undue strain on either the pipe or the apparatus. An example of such a drive system is described in U.S. Pat. No. 6,761,506.
In a second embodiment, depicted in
With particular reference to
The above describes one lowering stoke and one resetting stroke of the travelling clamp 124. This only lowers the pipeline P by a relatively small distance; it will be appreciated that the method can be performed in a repeated cycle such that continued lowering of the pipeline P is possible. This repeated interruption is a major reason why laying all product using the travelling clamp would be unattractive, even though it is useful for its ability to handle the greatest loads.
Furthermore, it should be noted that, in the simultaneous operation mode, at no point in the cycle is the full tension of the pipeline P suspended solely by the tensioner 112. In steps 1 to 5, the tension is optionally distributed between the tensioner 112 and the travelling clamp 124, and in steps 6 to 8, the tension is distributed between the tensioner 112 and the hang-off clamp 114. As the transfer of the tensions occurs, either the hang-off clamp 114 or the travelling clamp 124, or both, support the pipeline P tension. This has a considerable advantage when laying pipeline in deep water since, for a given length of pipeline, the maximum load exerted on the tensioners 112 throughout the laying operation is reduced compared to prior art methods which rely either on tensioners only or a travelling clamp only, to hold the pipeline P tension. The novel arrangement therefore allows smaller, less powerful tensioners, to be used for efficient operation in the majority of pipelaying tasks, while including the capacity to handle the large jobs with lower speed, but with much lower cost equipment than tensioners of such large capacity. The tension capacity of the tensioner 112 may be for example less than 1000 t (tonnes), or less than 500 t. The travelling clamp 124 and hang-off clamp 114 may have a tension capacity in excess of 1500 t, or even 2000 t.
The greater stroke length of the travelling clamp 224 in the second embodiment allows the pipeline P to be lowered by a greater distance in each lowering stroke. This can increase the maximum laying speed of the heaviest products since the time spent transferring loads between the fixed clamp 214 and the travelling clamp 224 and resetting the travelling clamp 224 to the top of its stroke is minimised per unit length of pipeline deployment. In other words the length of pipeline P which may be lowered in a single downward stroke is increased. In contrast with the
Although in the above description, the laying of heavy product has been performed using the tensioner and travelling clamp in tandem, it should be understood that the invention encompasses also laying such items by the travelling clamp and fixed clamp alone, with the tensioner simply disengaged. It should also be appreciated that the methods described are intended for use on exceptional tasks, with the majority of pipe lay operations being performed using the tensioner alone. In that regard, the travelling clamp and any separate fixed clamp may be made demountable to free up space and reduce weight during less demanding operations.
While the apparatus has been shown mounted at the stern of a vessel, it may equally be mounted to lay pipe via a moonpool.
The above and other modifications and improvements may be made to the foregoing without departing from the spirit and scope of the present invention.
Number | Date | Country | Kind |
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0715999.9 | Aug 2007 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB08/50719 | 8/18/2008 | WO | 00 | 2/9/2011 |