GB2046693A - System for mooring a vessel - Google Patents

Abstract

The present invention relates to a mooring system for mooring a vessel to an offshore structure 6, such as a semisubmersible or a platform resting on the sea bed. The mooring system comprises a winch and a motion compensator associated therewith, the winch being rigidly supported by the platform. The winch comprises a driven drum 2, around which one end of a mooring line 1, from the vessel is wound. In addition the winch comprises one or more separate drums 3, which may be rotated independently of the first drum. One end of a separate line 4, is wound around each drum 3, the other end of which is attached to the platform 6, through a damping means 5. The drums may be made to rotate synchronously. Due to said synchronous rotation the forces appearing in the mooring line may be transferred directly to the motion compensator through the rotation of the drums. <IMAGE>

Description

SPECIFICATION Mooring system for mooring a vessel The present invention relates to a mooring system particularly but not exclusively for mooring a a vessel to an offshore structure such as a semisubmersible or a platform resting on the sea bed. The mooring system comprises a winch and a emotion compensator associated therewith, the winch being rigidly supported by the offshore structure. The winch comprises a driven drum around which one end of a mooring line from the vessel is wound. In particular, but not exclusively, the present invention is intended for mooring a supply ship to an offshore platform.

When mooring a supply ship to an offshore platform it is important to reduce to a minimum the forces induced in the mooring system caused by the motion of the ship due to waves, wind, current etc. Further, it is important to incorporate sufficient safety against breakage of the mooring line in the system. Such safety is commonly achieved by incorporating motion compensating means in the mooring system, the motion compensator producing sufficient slack in the line when required.

In a conventional mooring system the elasticity of the mooring line is often used as a motion compensating means. Alternatively, a winch having a hydraulic cylinder associated therewith may be used as a motion compensator. A major limitation of the conventional systems is the reduced durability of nylon ropes when subjected to mechanical wear, for instance caused by friction and wear when being pulled over a drum, or pulley. Such type of wear may cause breakage of the line even after a short spell of use. The impact caused by such abrupt failure may cause injury to the crew and/or damage to the equipment. The conventional motion compensators are relatively heavy and require a rather large space, limiting the applicability of such equipment due to the limited available free space on the offshore platform.Further, such conventional motion compensators are relatively expensive.

Most commonly a supply ship is moored to an offshore structure, floating with the stern up against the platform. The supply ship is kept in position by means of two stern mooring lines. In addition, the supply ship is maintained in its position relative to the platform by means of its propulsion machinery and its bow thrusters, if any.

Alternatively the supply ship is maintained in position by means of its anchor system.

When mooring a supply ship to an offshore platform the following procedure is most commonly used: The supply ship goes astern to a position where the stern of the vessel is up under the platform superstructure. In this position the stern mooring cable(s) is taken aboard and made fast to the stern. The stern cable(s) has a predetermined, fixed length and is, during its inactive phase temporarily suspended from the platform.

4lternatively, the stern cable consists of a 12" hawser. When the mooring line has been taken on board and made fast to the stern, the supply vessel goes ahead to its working position, producing a predetermined tension in the stern mooring line(s). During this stage forces appearing in the mooring line and/or the motion of the supply vessel are compensated by utilizing the elasticity of the mooring cable(s). It should be appreciated, however, that other means for motion compensation and damping, such as for example hydraulic cylinders, may be used as a motion compensating means.

In general, a nylon rope or a nylon hawser is particularly vulnerable to mechanical wear.

According to the conventional solutions, the mooring line(s) may be exposed to excessive mechanical wear due to the rubbing effect between the mooring lines and the corresponding metal components of the winch.

Further, it should be appreciated that repeating mechanical wear under the influence of tension and shock loading in the line gradually reduces the strength of the mooring line. Thus, such reduction in strength may eventually produce rupture of the hawser or the mooring line even after a short spell of use.

Accordingly the present invention provides means for mooring a surface vessel to an offshore structure, comprising a winch, a mooring line and damping and motion compensating means, the winch comprising two or more drums which may be interconnected so as to rotate synchronously, on one of which drums the mooring line is wound while one or more separate lines are wound on the other drum or drums, said one or more separate lines being connected to the damping and motion compensating means.

With the arrangement of the present invention these and other inherent disadvantages are reduced. A mooring system is obtained having a large inherent flexibility both with regard to required free space and weight.

This is achieved by the winch having one or more separate drums which may be rotated independent of each other and independent of said first winch drum, but which may be coupled with each other and to said driven first drum. Said drums may be arranged so as to allow unidirectional rotation or rotation in opposite direction. One end of each of the separate lines is wound around the separate drums, the other end being attached to the offshore structure through a damping element (motion compensator).

Preferably each of said separate lines is wound around a separate drum in opposite or in the same direction as the direction of the windings on said first, driven drum, the reciprocal winding direction being dependent upon the direction of rotation of the drums. Accordingly the mooring line may be connected directly to the offshore structure through the damping element or the motion compensator, whereby the forces induced by the vessel are transferred directly to the damping element/motion compensator by means of the rotation of the drums.

According to a preferred embodiment of the present invention the mooring system incorporates a winch comprising at least two independent drums, preferably rotating around a common shaft. One of said drums operates as a conventional winch drum, around which the mooring line is wound. The other drum or drums may be connected directly to the first winch drum subsequent to the arrival of the vessel to be moored, in its working position. At this stage the motor driving said first drum is disconnected through a conventional clutch means. The drums are then arranged to rotate synchronously. Due to the synchronous rotation of the drums forces appearing in the mooring line will be transferred directly to the motion compensator through the drums.

The motion compensator of the damping element may comprise hydraulic cylinders, spring devices, elastic rubber elements or hawsers for example made of plastics material fibres. If such a hawser is used, care should be taken to avoid mechanical wear on the hawser.

The mooring system according to the present invention may easily be modified to be used on any type of offshore structures.

It should be appreciated that the present invention is not limited to an arrangement where the drums rotate around a common shaft. The drums may rotate around parallel shafts, using conventional transmission means such as gears, with or without reduction, hydraulic transmission means, chains, driving belts or electrical transmission means. This implies that the drums either may rotate unidirectionally or rotate in opposite direction. Such feature only effects the reciprocal direction of windings on the drums.

If the winch in addition to the first, driven drum comprises two or more separate drums, each of said separate drums may be releasably connected with each other and with said first drum through a conventional clutch means. Alternatively, each of said separate drums may individually be connected with said first drum. Thereby, said separate drums may rotate independent of each other and independent of said first drum, but may rotate synchronously with one or more of said separate drums and/or synchronously with said first drum. According to this embodiment the forces imposed by the vessel may be distributed to the different lines and damping means.

The present invention provides a mooring system enabling a vessel to pick up the mooring line at a distance away from the hazard zone around the platform. Further, a mooring system is provided where the supply vessel may be winched in towards the platform in a controlled manner, thus avoiding the risks of manoeuvring the vessel up under the platform in order to pick up the mooring line.

Further, a mooring system is provided where the hawser may be equipped with permanent means to enage fast with the stern such as a loop eliminating the difficult handling of large and/or stiff hawsers.

In order to promote a fuller understanding of the above, and other aspects of the present invention, some embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a winch embodying the present invention, comprising one driven first drum and one separate drum; and Figure 2 shows schematically a second embodiment of the present invention, showing a winch which comprises one driven drum and three separate drums.

Figure 1 shows schematically a winch comprising one driven drum 2 and one separate drum 3. The reference number 1 denotes a mooring line which is wound around the driven drum 2. The separate drum 3 is rotatably arranged on the same shaft as said first driven drum 2. A line 4, for example of steel wire, is wound around said separate drum 3. The line 4 is connected to a damping means or a motion compensator 5, the remote end of which is rigidiy supported by the platform 6. A clutch 7 (not shown in detail, but indicated by the reference number 7) is arranged on the shaft between the two drums 2, 3.

When the mooring line 1 has been taken on board the vessel to be moored (not shown) and made fast to the stern, the winch 2 is connected to the motor while the separate drum 3 is disconnected. The vessel is then winched into a predetermined, working position. At this stage the motor is disconnected while the separate drum 3 is connected to said first drum 2. The drums 2, 3 will now rotate synchronously. The supply vessel is now directly connected to the damping element 5 utilizing the common rotation of the drums 2, 3 so that the forces appearing in the mooring line 1 will be directly transferred to the motion compensator 5 through the rotation of the drums 2, 3.

Figure 2 shows schematically a second embodiment of the present invention. According to this embodiment the winch comprises one driven, first drum 2 and three separate drums 3, 3', 3". The only major difference between the embodiment shown in Figure 1 and the embodiment shown on Figure 2 is the number of separate drums 3, 3', 3". Each of said separate drums 3, 3', 3" may be releasably connected to each other and with said first drum 2 through a respective clutch means 7, 7', 7". If as an example a length of 3 m mooring line 1 is wound onto the first drum 3, a length of 1 m may be wound on' each of the separate drums 3, totalling the three metres unwound on the first drum 3. This may be achieved by winding the first metre on the separate drum denoted 3, this drum then being disconnected and held, whereafter the second metre being wound on the drum denoted 3', this drum then being disconnected and held, etc. Only drum 3 is connected to drum 1 during the winding of the first metre, only drum 3' to drum 1 during the winding of the second metre,-etc. Thus the vessel may be moored by way of the line 1 and the three damping units, 5, 5', 5".

It should be appreciated that the present invention is not limited to a mooring system in connection with the mooring of a vessel to an offshore platform, but may be used in a variety of ,connections without deviating from the invention idea.

Claims (6)

1. Means for mooring a surface vessel to an offshore structure, comprising a winch, a mooring line and damping and motion compensating means, the winch comprising two or more drums which may be interconnected so as to rotate synchronously, on one of which drums the mooring line is wound while one or more separate lines are wound on the other drum or drums, said one or more separate lines being connected to the damping and motion compensating means.

2. Means for mooring a vessel as claimed in Claim 1, in which the damping means comprises at least one hydraulic cylinder.

3. Means for mooring a vessel as claimed in Claim 1, in which the damping means comprises a spring device.

4. Means for mooring a vessel as claimed in Claim 1, in which the damping means comprises rubber elements.

5. Means for mooring a vessel as claimed in Claim 1, in which the damping means comprises a hawser made of plastics material fibre, etc.

6. Means for mooring a surface vessel to an offshore structure substantially as herein described with reference to the accompanying drawings.