Patent Application
20060151768
The invention relates to methods and apparatuses for deploying articles to great depth beneath the sea surface, for example to the seabed in deep waters.
Cranes and winches employing wire rope have been used to deploy loads to the seabed in modest water depth for many years. Some of these cranes and winch systems are fitted with, or used in conjunction with, heave compensators, which take-up and pay out the rope dynamically, to compensate vertical motion (heave) of the ship, barge or other platform from which the rope is supported.
As water depth increases, the weight of wire needed to lower equipment to the seabed increases until it becomes such a significant part of the total load that the method becomes impractical. Man made fibre rope can be almost neutrally buoyant and have strength and elastic characteristics similar to wire rope and is therefore potentially a suitable replacement for wire. Man made fibre rope, however, has a poor tolerance to the fatigue induced by bend cycling under load, and is thus unsuitable for use with current winch designs, particularly but not only those having heave compensation.
The present invention aims to provide novel methods and apparatus for using fibre rope, when deploying loads from a vessel at sea. A particular object for at least some embodiments of the invention is to provide methods that reduce bend cycling of the rope under load. A further aim is to allow operations to depths exceeding 300 m or 1000 m.
In broad terms, in one aspect of the invention a tensioning device mounted substantially vertically is used to grip the fibre rope, supporting the load and facilitating the payout of the rope.
The tensioning device may in particular be a continuous track system (linear winch) and can be made up from multiple units mounted around the fibre rope.
The tensioning device may alternatively comprise at least one clamp comprising a plurality of pads arranged longitudinally and circumferentially around the fibre rope, and mounted on a movable carriage
Substantially the entire load in the fibre rope is taken by the tensioning system; the rope entering the tensioning system is not under substantial load. This allows the fibre rope to be stored on a storage reel or carousel without bending under load. Of course some back-tension may be maintained on the reel for control of the rope.
The tensioning device may have a general form and features in common with clamps and/or track-type tensioners used conventionally for pipe laying operations. In preferred embodiments, however, at least the shoes of the tensioner are specially adapted to the different characteristics of the fibre rope, and would not be suitable for smooth conduit.
Several specific adaptations of tensioner and clamp are described below, by way of example only. These may be used individually or in any combination, and the invention does not exclude other adaptations, nor the use of an existing pipelay tensioner.
The tensioner may be mounted so as to suspend the rope from beside the vessel, or via a moonpool. A tower arrangement for vertical deployment of flexible conduit through a moonpool is known for example from WO 91/15699 A (Coflexip). As is also known in the pipe laying art, vertical or steeply inclined towers of other constructions can be applied. “Vertical” in the present context is intended to encompass a range of deviation from the vertical, particularly (i) the load for whatever reason acts in a direction inclined from the vertical (in which case the tensioner may be tilted to aligned with the load direction) and (ii) where fatigue under bend cycling is serious only beyond a certain bend angle. An offset tower permitting pipelay with an inclined tensioner is known for example from WO 02/57675 A.
Tower arrangements providing a pair of moving clamps are disclosed in WO 99/35429 A (Coflexip) and in our co-pending application GB 0302279.5, not published at the present priority date. Suitable clamps are described in our co-pending application GB 2 364 758 A (63566 GB). The contents of these documents are hereby incorporated herein by reference.
Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:
Tracks or the like arrayed around the axis of the rope 14 are pressed radially inward by suitable rams, levers and the like to grip the rope, and to release it again when required. Each track comprises a series of individual shoes linked together. While two tracks are shown for the sake of illustration, three or four tracks will be more usually provided, spaced at 120° or 90° intervals around the rope axis respectively.
The detailed construction and operation of the structures for supporting these tensioners in vertical and/or inclined positions above the sea surface can be readily envisaged by the skilled person, for example by reference to prior art in the field of pipe and cable laying, including those documents mentioned already above.
Ideally, to use fibre rope in combination with a multi-track tensioner, equipped with pads on the tracks, the pad design should be adapted to the rope. A rope behaves different than, say, an umbilical or pipeline (flexible or rigid), when it is fed through a tensioner, compressed by the pads and brought under tension. Unlike pipes and umbilicals, the diameter of the rope can change significantly with increasing load onto the pads as well as with increasing tension to the rope. Furthermore the danger of pinching the rope between the pads is significant. Therefore a proper fit of the rope between the pads should be always ensured, regardless of the load to the pads or the tension to the rope.
This will happen even under tension of the rope. According to this, the cross sectional area described by the closed pads needs to be less than the nominal cross sectional area of the rope.
During deployment and/or recovery both clamps 210, 220 move relative to each other, in a sequential manner to and from the middle of the tower, to hand over the grip on the rope from one clamp to the other. This action results in the paying in or out of the rope, and can be controlled to provide continuous movement. (With a single movable clamp and a fixed clamp, only intermittent movement could be achieved.)
Three other possibilities have been considered for adapting the tensioner specifically for gripping of the fibre rope.
The above adaptations are provided by way of example only, and the skilled reader will appreciate that other arrangements are possible within the spirit and scope of the invention. In particular, it will be noted that the adaptations of
The method can be applied beneficially in oil & gas field development (sub-sea construction) in depths beyond 300 m. General lifting and lowering operations can also be envisaged in depths down to full oceanic depth, for example for Salvage, Oceanography, and Military purposes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0205252.0 | Mar 2002 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB03/00932 | 3/6/2003 | WO | 6/14/2005 |
