GB2482370A - Apparatus for controlling a suspended load or a moored vessel - Google Patents

Abstract

An apparatus 1, 220 for controlling a load comprises a body 2, at least one energy storage element 3 attached to the body 2, a first elongate flexible element 14, 215 such as wire rope of chain attachable to a suspended load (such as a tender vessel 110 fig. 1a) or a moored vessel (250 fig. 14) or to an element (103 fig 1a) supporting such a load, in which the first elongate element 14, 215 acts upon the energy storage element 3. The first elongate element 14, 215 may form a drag line to stabilise a load against swinging especially when raised by a davit 101 on a mother ship 100 which is pitching, rolling or yawing or it may stabilise a vessel (250 fig. 14) moored to a structure such as an off shore wind turbine tower (200 fig. 10). The energy storage element 3 may be a compressible element in the form of an hydraulic ram 4 coupled to an hydraulic accumulator 43 and a relief valve 41. A carriage 17, 219 may be slidably mounted on rails 32 on the body 2 with the first elongate element 14, 215 attached to one side of the carriage 17 by a pulley 11, 217 and from there to a fixed point 27, 219. A second elongate element 15 may connect the carriage 17 to a fixed point on the body 2 via pulleys 13 on the body 2 and 12 on the piston 5. The elongate element 14 may be wound on a reel (122 fig. 8) which may include a brake whose actuation is controlled by tension in the elongate element 14 to pay out elongate element 14 and avoid damage to the components of the apparatus.

Description

Apparatus for Controffing a Load

Field of the Invention

The present invention relates to an apparatus for controlling a load, and in particular to an apparatus for controlling the movement of a load suspended on an elongate element.

Background of the Invention

It is often necessary to suspend loads from elongate elements. For example, cranes typically use a wire rope or a chain which is wound around a drum to raise or lower a load. The jib of the crane may be a great distance above the object to be lifted. Once an object is attached to a rope or chain, the object may start to swing under the influence of wind forces, or the force exerted on the rope or chain by movement of the jib, for example when the jib is moved through an arc. Where the distance between the object and the point where the rope or wire joins the jib, relatively modest forces can cause relatively significant movement of the object. Such movement can cause danger.

The suspension of loads on elongate elements is not limited to cranes of the type used on construction sites. A type of crane known as a davit is used on ships to lift tender vessels in and out of the water. A problem associated with lifting a tender vessel out of the water is that during the lifting procedure the ship on to which it is being lifted may experience pitch, roll and yaw motion, which motion is transferred to the tender vessel during the lift. Depending on the frequency and amplitude of such motion and the characteristics of the davit, its lifting gear and the tender vessel, the motion may begin to resonate. The tender vessel being lifted may become extremely unstable, and may crash into the side of the lifting vessel. Depending on the type of tender vessel and mother vessel, the crew may be on board during the lift. If the tender vessel is subjected to violent motion, the crew may be endangered. Further, if the forces driving the motion of the tender vessel begin to resonate, the lifting gear and the tender vessel may be subject to very significant shock loads, which may exceed the loading which the components were designed to withstand.

This problem has been recognised. In an arrangement designed to overcome this problem which is currently in use, when a tender vessel is to be lifted out of the water the free end of a rope attached to a winch drum is attached to a point on the bow of the vessel. During the lift, a winch operator is required to wind in the winch in as the tender vessel is raised in order to keep a tension on the winch rope.

Further, if the winch operator considers that the mother vessel is about to experience a change in attitude that is likely to exert a very large load on the winch rope, he must wind out some rope. Operating a winch in rough seas in this manner is very difficult, and also requires an extra person to be carried on the vessel. Attempts have been made to automate control of the winch. However, such control systems require electronic sensors and feedback systems, which are not well suited to operation at sea.

With this system it has been found that the tender vessel is still not well controlled and other devices have been adopted in an attempt to mitigate against damaging effects. One device comprises pads that are deployed from the mother vessel to engage against the tender vessel while it is being lifted.

However, this device has not proved satisfactory, and may either be damaged or cause damage during the lift.

W00174655 describes a device for attachment to a ship's davit to facilitate attachment of a small craft thereto. The bow of the small craft is attached to a line that is supported on a boom extending to the side of the vessel mounting the davit. A tensioning or attenuating means is indicated as possibly acting upon the line supported on the boom.

With this system it has been found that the tender vessel is still not well controlled and other devices have been adopted in an attempt to mitigate against damaging effects. One device comprises pads that are deployed from the mother vessel to engage against the tender vessel while it is being lifted.

However, this device has not proved satisfactory, and may either be damaged or cause damage during the lift.

WOO1/29366 describes a hoisting system, typically for an offshore drilling mast and includes a pneumatic or hydraulic compensator that acts upon one of the pulleys of the hoisting system.

JP2002167161 describes a an accessory for a floating crane, which comprises two compression springs and is intended to be situated between the end of the lifting cable of the crane and the load that the crane is intended to lift in order to absorb vertical movement of a wave.

JP2005320146 describes a device for suppressing rocking vibration cause by elongation and contraction of a rope and includes winding devices and an elastic body such as a string associated with each winding device.

It would therefore be desirable to provide an improved apparatus for controlling a load that may be used at sea to lift tender vessels on to mother vessels.

In addition to the problem outlined above in relation to the lifting of tender vessels on to mother vessels, there are other sea related applications where the changing height of the sea presents problems.

For example, where access is required to an at sea structure, typically it is necessary to bring a vessel alongside the structure. When there is a sea swell, it is difficult to moor to such a structure without either damaging the structure or the vessel being moored. This problem is particularly acute for the servicing of off-shore wind farms, as it is likely to be necessary to gain access to such structures in sea swells of some metres.

It would therefore be desirable to provide a device that provides for the safe mooring of a vessel to a fixed structure, such as the mast of a wind turbine.

Where access to the structure from the vessel is required it would be desirable to provide a safe means of moving personnel from the vessel to the structure.

Summary of the Invention

According to a first aspect of the invention there is provided an apparatus for controlling a load comprising a body, at least one energy storage element attached to the body, a first elongate flexible element attachable to a load to be controlled or to an element supporting such a load, wherein the apparatus is configured such that the first elongate element acts upon the energy storage element.

Advantageously, the energy storage device comprises a hydraulic actuator which preferably is a hydraulic ram. Preferably, the ram mounts an elongate element support member adapted to support the first elongate element, which member may comprise a pulley wheel. Preferably, the energy storage device includes a hydraulic accumulator, which advantageously is a gas filled accumulator, and preferably of the piston type, although other types of accumulator may be used. The elongate element support member is advantageously mounted on the piston rod of the hydraulic ram. The hydraulic ram and the accumulator may be hydraulically coupled together, for example by means of a conduit, such as a pipe, connected to respective orifices in the ram and accumulator.

The hydraulic ram and/or accumulator may be connected to a pressure relief valve.

The body may include a track adapted to limit the degrees of freedom of movement of the hydraulic ram. Advantageously, the body includes a pair of spaced apart tracks. An element attached to one end of the piston rod may extend into the said track or tracks.

Preferably, the apparatus includes a slider element slidably mounted on the body. The first elongate element may be attached to the one side of the slider element. The first elongate element may be attached to the slider element by means of an elongate element support, such as a pulley wheel.

Advantageously, one end of the first elongate element is attached to a fixed point, for example on the piece of equipment to which the apparatus is mounted, or to the body. The apparatus may further include a second elongate element, and preferably one end thereof is attached to the slider element and the other end is attached to the body.

Advantageously, at least one elongate element is arranged on elongate element supports so as to multiply the distance moved by the end thereof for each unit of distance moved by the energy storage element. Preferably, each elongate element is arranged on elongate element supports to multiply the distance moved by one end thereof with respect to the other end. In this way an object attached to an elongate element may move by a significantly greater distance than the corresponding distance moved by the energy storage element or part thereof In this way, the apparatus can be made relatively small in size.

Not only can thc distance movcd by a part of the energy storage element by multiplied by the arrangement of the support element(s) and elongate element(s), but the energy storage element itself may include means to multiply the distance moved by one part thereof by a multiplication factor, so that one part thereof may move through a shorter or longer distance than another. This may be provided for in a hydraulic system by selecting pistons of different surface areas.

The body may support at least one rail on which the slider element is mounted, and may support two spaced apart rails, and the slider element may be mounted on both of the rails.

The apparatus may form part of a vessel and the elongate element that is arranged to attach to the load to be controlled may form a painter line and extend over a painter boom.

It has been found that the apparatus of the invention is very effective in controlling forces acting on the tender vessel that cause pitch motion of the tender vessel, which is the motion that one would expect the apparatus to control. However, whilst the apparatus is only attached to the forward end of the tender vessel by a single line, it has been found that roll motion of the tender vessel is also attenuated.

The control of roll motion is particularly effective where the length of the elongate element attached to the controlled load is set such that the apparatus exerts a force on the load to be controlled when the load is static.

The apparatus of the invention does not require a winch, although it may include a winch. As well as being operable in conjunction with a winch, the apparatus may include a reel, one elongate element being wound on to the reel. The reel may be provided with a brake which locks the reel against rotation and thereby provides an elongate element of fixed length. The apparatus may further include brake actuation means configured to release the reel brake when the elongate element is subjected to a kad above a threshold load. When so equipped, if the apparatus is subjected to a load in excess of that which may damage part of the apparatus or part of the load being controlled, the brake is actuated and released allowing the reel to rotate, paying out the elongate element. When the load falls below the threshold, the brake returns to its braking configuration. Such a reel may also include a winding means to wind the elongate element hack on to the reel to bring the length of the elongate element hack to a previous setting. The brake actuation means may include an elongate element support, such as a pulley wheel, that is movable between two positions, one releasing the brake and one engaging the brake. The brake actuation means may also include a hydraulic circuit and source of pressurised fluid, and a valve.

Preferably, when the threshold load is encountered, the support moves against a biasing means, such as a spring, and in doing so opens a valve, which allows pressurised hydraulic fluid to flow to a hydraulically actuated brake to release the same. When the load falls below the threshold load the spring returns the support to its previous position and the reel is braked.

Further, the apparatus of the invention is reactive. The more extreme the force that is exerted on the apparatus, the greater the damping force generated by the apparatus. There is no requirement for feedback loops or control systems, whether mechanical or electronic. By virtue of this the apparatus is very simple and therefore well adapted to operation in harsh environments, such as on the seas. Also, the chain link attachment system described allows the length of the elongate element to be set, and a reel may be provided which pays out the elongate element if the controlled load exerts a force on the apparatus greater than a threshold value.

Another aspect of the invention relates to an access device including at least one apparatus for controlling a load according to the first aspect of the invention or the preferred embodiments thereof attached to a structure to which a vessel may moor, and an access means attached to the structure, the access means including at least one moveable element that is moveable with respect to the structure, the at least one moveable element being attached to the apparatus for controlling a load and to a line for attachment to an actuator mounted on a vessel.

Preferably, the at least one moveable element includes a platform.

The access means may be a lift, which may include a lift cage constituting the moveable element, mounted on at least one track attached to the structure. Alternatively, the access means may comprise a kidder including at least two parts, at least onc bcing fixed with respect to the structure and at least one being moveable with respect to the structure.

Another aspect of the invention relates to a mooring system for mooring a vessel to a structure, the system comprising at least one apparatus for controlling a load according to the first aspect of the invention or the preferred embodiments thereof attached to one of a structure or a vessel for mooring to the structure, at least one float mounting means attached to the structure having a float slidably mounted thereon and a line attachment means for attaching a line of the apparatus for controlling a load to the float.

In one embodiment, the at least one apparatus for controlling a load is mounted on the structure and the line extends therefrom to the float, and a further line is provided for attachment to an actuator mounted on a vessel. The actuator is preferably a winch.

In another embodiment, the at least one apparatus for controlling a load is mounted on the vessel and a line attached thereto is attachable to the float. Preferably, the apparatus for controlling a load includes an actuator which is extendible and retractable to provide slack in and to take slack out of the line attachable to the float. Advantageously, the actuator is a hydraulic ram. A pump such as a hand pump may be provided to extend and/or retract the tam. A valve may be provided to release hydraulic pressure in the ram to allow it to retract and/or extend.

Preferably two apparatus for controlling a load are provided, so as to moor both the bow and stern ends of the vessel to the structure.

Another aspect of the invention provides a vessel including components of the mooring system above. Preferably, the vessel includes at least one boarding area.

Brief Description of the Drawings

In the Drawings, which illustrate preferred embodiments of apparatus according to the invention and are by way of example: Figure Ia is a schematic representation of a mother ship equipped with apparatus according to the invention with a tender vessel positioned to be lifted on to the mother ship; Figure lb is a schematic representation of the mother ship and tender vessel illustrated in Figure Ia, with the tender vessel attached to the davit and partially lifted from the water; Figure Ic is a schematic representation of the mother ship and tender vessel illustrated in Figures Ia and Ib, with the tender vessel lifted to its stowed position.

Figure 2 is a schematic representation of an apparatus for controlling a load according to the invention; Figure 2a is an end view of a part of the apparatus illustrated in Figure 2; Figure 2b is a plan view of another part of the apparatus illustrated in Figure 2; Figure 3 is a schematic side view of some components of the apparatus illustrated in Figure 2; Figure 4 is a plan view of the apparatus illustrated in Figure 3; Figure 5 is an end view of the apparatus illustrated in Figure 3; Figure 6 illustrates plan, end and cross-sectional views of the carriage of the apparatus illustrated in Figures 3 to 5; Figure 7 illustrates a retainer plate adapted secure the carriage in place; Figure 8 illustrates another embodiment of apparatus according the invention; Figure 9 illustrates a chain link forming a part of an apparatus according to the invention; Figure 10 is a schematic representation of a lift apparatus of a device for mooring to a structure at sea;

S

Figure II is a plan view of part of the at sea structure; Figure 12 is a front view of a part of the at sea structure; Figure 13 is the plan view of Figure II with components of the mooring device attached thereto; and Figure 14 is a vessel for mooring to the at sea structure.

Detailed Description of the Preferred Embodiments

The apparatus I illustrated in the Drawings provides an apparatus having an element that is attachable to a load that is to be controlled or to an element supporting such a load. The apparatus includes a means for dissipating forces applied to the load and in this way controls the position of the load.

Figures Ia to Ic show the apparatus of the invention in use on a mother ship 100 when lifting a tender vessel 110 from the water (see Figure Ia) to its stowed position (see Figure Ic). The mother ship mounts a davit 101 which includes two arms 102 from each of which depends a rope 102. To lift the tender vessel 110 from the water the ropes 102 are attached to hooking points III on the tender vessel 110. The mother vessel has a painter boom 105 which is telescopic. As is described in greater detail below, a line 14 forming part of the load control apparatus extends around a pulley (not shown) of the painter boom 105 to the forward hooking point III. As can be seen from Figure Ic, once the tender vessel has been lifted to its stowed position the line 14 is detached.

Referring now to Figure 2, the load control apparatus I comprises a body 2, which mounts a compressible element 3 in the form of a hydraulic ram 4 comprising a piston 5 and a cylinder 6. The piston is attached to the body 2 by means of a pin 7 which passes through a bracket 8 attached to the end of the cylinder 6. The pin 7 extends through aligned bearings 9 attached to the body 2.

The apparatus mounts a number of pulley wheels and elongate flexible elements, which are ropes in the present example that extend around the pulley wheels, Of course, ropes may be replaced by wires or chains (in the case of chains the pulley wheels would be cogs). Still further, the elongate member that is attachable to the load to be controlled may include a part that allows the length of the said elongate member to be adjusted, such as a chain connected to lengths of rope by suitable fasteners (see Figure 9 and description relating thereto). To adjust the length the operator simply releases a part of the fastener from the chain, and aligns the fastener with another link in the chain. The apparatus I illustrated includes four pulley wheels 10 to 13 and two wire ropes 14 and 15. The pulley wheels work in pairs, each of a pair co-operating with one of the wire ropes 14, 15. The pulley 10 is mounted on one end of the body 2 by means of a bracket 20 and pin 21 which extends through the bracket and the pulley wheel 10. The second pulley wheel 11 of the pair is mounted on a carriage 17 which is arranged to slide with respect to the body 2. The pulley wheel 11 is mounted on the carriage 17 by means of a bracket 22 and pin 23 which extends through the bracket 22 and pulley wheel 11. Rope 14 extends from a tie off point 27 on the vessel, around the pulley wheel 11, through the body 2 and over the pulley wheel 10, From the pulley wheel 10, the rope 14 may pass over a number of pulleys of a painter boom assembly and in use to a hooking point on the tender vessel.

Of the second pair of pulleys 12, 13, the pulley 12 is attached to the piston 5 of the hydraulic ram 4 by means of a bifurcated clevis 23 and pin 24 which passes through holes in the clevis and the pulley wheel 12. This pin 24 extends beyond the outer sides of the clevis 23 and into channels 27 mounted in the body 2. The pulley wheel 13 is mounted on the body 2 by means of bracket 25 and a pin 26 which passes through aligned holes of the bracket and the pulley wheel 13. The rope 15 extends from the carriage 17 to which it is attached, over the pulley wheel 13, along the underside of the hydraulic ram 4, around the pulley wheel 12 and to a tie off point 16 on the body 2.

The constructional arrangement of the device illustrated in Figure 2 will now be described in greater detail with reference to Figures 2 to 5. In the illustrated example, the body 2 is formed of side walls 30, each wall 30 comprising two elongate members 31 mounted one on top of the other and joined together, by a plurality of fasteners in the form of bolts (it should be noted that any suitable fastening means may be used). On top of the upper member 31 of each side wall 30 there is mounted a rail 32, which has a substantially circular cross-section. The rail 32 is attached to the side wall by spaced apart fasteners 33. The carriage 17 includes spaced apart channels 34 which are so shaped, dimensioned and kcated such that the carriage is retained on and may slide relative to the rails 32. It can be seen in Figure that the cross-sectional shape of the channels 34 allows the carriage 17 to be presented up to the ends of the rails 32 and the rails 32 and channels 34 aligned. It is possible then to slide the carriage 17 on to the rails 32. The channels 34 extend beyond the centre line of the rails 32, thereby preventing the carriage lifting off the rails 32.

Once the carriage 17 is mounted on the rails, a retaining plate 36 as illustrated in Figure 7 is attached to the end of each rail 32.

The manner of operation of the apparatus will now be described with reference to Figures Ia to Ic and 2. The free end of the rope 14 is attached to an object such as a tender vessel. If the mother vessel encounters a large wave, the mother vessel 100 may pitch severely, which could cause the tender vessel 110 to start to swing on the ropes 103. The force generated by pitching motion of the mother vessel causes a force to be exerted on the rope 14, which force is transferred to the carriage 17. The force on the rope 14 causes the carriage 17 to move in the direction of arrow "a" in Figure 2. Movement of the carriage 17 in the direction "a" pulls the rope 15 in the direction "a". The end of the rope 15 is tied off at 16 and hence the hydraulic ram 4 is compressed with the piston moving in the direction indicated by arrow In the particular application illustrated in the Drawings, it has been found to be advantageous to set the length of the rope 14 such that it is slightly too short to allow the ropes 103 to hang vertically under the load of the tender vessel 110. In use, the tender vessel 110 is brought along side the mother vessel 100 such that the rope 14 may be attached to a hooking point on the bow end of the tender vessel.

The ropes 103 are lowered from the lifting davit and then the tender vessel is lifted out of the water by the davit. As the tender vessel is lifted out of the water, because the rope 14 is too short to allow the ropes 103 to hang vertically, a component of the weight of the vessel is taken up by the apparatus I of the invention, the hydraulic ram 4 being retracted to an extent under the force imposed by the tender vessel.

It has been found that this configuration is particularly effective in damping roll motion of the tender vessel 110 about the hooks 111.

When the load on the rope 14 is released it is possible for the carriage 17 to move in the direction The arrangement of ropes and pulleys provides for the distance moved by the piston 5 to be multiplied so that the object attached to the end of the rope 14 may move a much greater distance than the distance moved by the ram. For example, in the arrangement illustrated in Figure 2, for each unit of distance moved by the piston 5, two units of length of rope 15 are released, which allows the carriage 17 to move by a corresponding two units of distance. Again, because of the pulley arrangement 10, 11 the movement of the carriage through two units of distance gives rise to four units of length of rope 14 being released. Since one end of the rope 14 is tied off at tie off point 27, it is the free end of the rope 14 that moves through four units of length. Hence, where the ram 4 provides a piston travel distance of 1.5 metres, an object attached to the free end of the rope 14 may move through 6 metres. By using different arrangements of pulleys and ropes it is possible to provide for the free end of the rope 14 to move by the distance moved by the piston multiplied by a different multiplication factor.

The direction and speed of movement of the piston 5 within the cylinder 6 is controlled by a hydraulic circuit 40 which comprises a pressure relief valve 41, a pressure relief tank 42, and an accumulator 43, which in the example is a nitrogen over oil piston type accumulator. During the commissioning of the apparatus, the cylinder 6, the relief valve 41 and the conduits connecting the cylinder 6 to the accumulator are filled with oil. The accumulator 43 is filled with nitrogen and both the oil and the nitrogen are pressurised to a threshold pressure, which in the example is 50 bar. Pressure gauges 50 are provided to give indications of the hydraulic pressure and the gas pressure. Also, the accumulator 43 is provided with a valve to allow gas to be introduced into or released from the accumulator. The threshold pressure is selected according to the load imposed on the piston 5. In order for the piston 5 to lie in the fully extended configuration before the rope is subjected to any shock load, the threshold pressure must at least be sufficient to resist the load imposed by the ropes and carriage 17 and any anticipated change in load resulting from increasing the potential energy of part of the apparatus.

To explain, the example described here is in relation to vessels. When a so equipped vessel rolls for example, the carriage 17 may be raised, increasing its potential energy and inclining it to slide on the rails 32. The threshold pressure would be set to resist such motion of the carriage. In one example it has been found that a threshold pressure of 50 bar, and a relief pressure of 350 bar at which relief valve 41 allows hydraulic fluid to flow to the pressure relief tank are sufficient.

When a load is exerted on the rope 14 which increases the pressure on the piston head 5' above the pre-set pressure of the hydraulic fluid, but not above the operating pressure of the pressure relief valve 41, the piston head 5' is caused to move in the direction "b", pushing hydraulic fluid through the pressure relief valve 41 into the oil chamber of the accumulator 43. This pushes the piston 44 of the accumulator in the direction "a", compressing the nitrogen in the accumulator 43. The pressure in the hydraulic fluid also increases; the pressures of the hydraulic fluid in the cylinder 6, accumulator 43 and connecting pipe work and nitrogen in the accumulator 43 being substantially equal. Hence, in the present example, after the free end of the rope 14 has been attached to a hook 111 on the forward end of the tender vessel, if the two vessels meet a sufficiently large wave, the force exerted on the tender vessel and reacted through the rope 14 and hence apparatus 1, may well be sufficient to overcome the force exerted on the piston head 5' by the pre-pressurised hydraulic fluid. The energy associated with the load pressurises the hydraulic fluid and gas in the accumulator, storing the said energy.

The resistance provided by the apparatus of the invention depends on the momentum it is trying to resist. In the present example, when the tender vessel is attached to the rope 14 and a large wave hits the bow of the tender vessel, the tender vessel is forced backwards with respect to the mother vessel.

This momentum is reacted through the rope 14 and hence the apparatus. Where hydraulic fluid is forced through an open orifice, the damping force generated in the apparatus depends on the square of the velocity of the driving force. Hence, the more rapidly the rope 14 is pulled, the greater the damping force exerted. In the example, the maximum damping force is set by the pressure relief valve which opens if the pressure exceeds 350 bar.

Conversely, the restoring force that brings the apparatus back into the configuration illustrated in Figure 2, where the hydraulic ram 5 is fully extended, is always the same, irrespective of the load that has been applied to the rope 14.

The nature of an elongate element means that when it is under tension a force is applied to the piston 5, and via the piston head 5' to the hydraulic fluid as described above. However, when the load is released, i.e. when the load that is being controlled swings in an opposite direction the force exerted on the piston 5 by the controlled load is close to zero. This is because the elongate element is flexible and therefore cannot transmit a compressive load. Hence, once the velocity of the controlled load has been brought to zero and the controlled load begins to swing back towards the apparatus, the controlled load exerts no force on the piston 5. Therefore, the pressure in the hydraulic ram falls instantaneously and the piston 5 returns to its extended position under the pressure of the gas in the accumulator which returns to the pre-set threshold pressure. This threshold pressure is sufficient to pull the elongate element and keep it taught.

When the load on the rope 14 is instantaneously released the hydraulic fluid is forced back through the same orifice under the gas pressure in accumulator. The speed at which the ram 5 travels has been found to be constant. The rate which hydraulic fluid flows from the accumulator 43 into the cylinder 6 determines the restoring force on the piston head 5'. It has been found that a constant load is exerted on the piston head 5' and hence the rope 14 until either the ram 4 is fully extended or has reached a position of equilibrium.

As can be seen from the Figures, the piston and accumulator may be specified with different diameters. This means that the distance moved by the piston head 5' does not correspond to the distance moved by the accumulator piston 43'. This is because the hydraulic fluid is incompressible, so the volume displaced by the piston head 5' must correspond to the volume displaced by the accumulator piston 43' and hence because the diameters of the respective pistons are different the pistons must move through different distances to displace the same volume.

When the load exerted on the rope drops to such an extent that the force exerted on the piston 43' of accumulator due to the pressurisation of the nitrogen therein generates a force on one side of the piston 5 that exceeds the load on the other side piston 5, which is due to the load on the rope 14, then the piston 5 moves in the direction "a", back towards its fully extended state to take up a position where the pressures on each side of the accumulator piston 43' are substantially equal.

The pressure relief valve 41 is set at a pressure that will protect the components of the apparatus, i.e. if the load exerted on the rope 14 is greater than a load that would damage the apparatus, the load will be dissipated by hydraulic fluid being released into the pressure relief tank 42.

In the embodiment illustrated the rope 14 is of fixed length, being tied off at tie off point 27.

However, it is possible to use the invention with a rope 14 wound on to a drum, which could be a drum of a winch. With such an arrangement, the rope 14 can be paid out and the end of the rope attached to the desired object. The drum may then be locked, and the apparatus then functions as described above.

Alternatively, where the drum is part of a winch, after attaching the free end of the rope 14 to an object, the winch may be wound in such that the rope 14 is of a desired length and then the winch locked. The apparatus then functions as described above.

A hydraulic load system is particularly suited to the application described in relation to the Drawings. The system allows for a long stroke of the piston 5, without requiring the piston of energy storage means, i.e. the pressurised gas, to move by the same distance, and this can be achieved using parts that are well suited to working in a harsh environment. To achieve the same with springs would require an arrangement of levers and cranks. Also, in a hydraulic system it is possible to protect the apparatus against loads that might damage the apparatus, which whilst achievable using a mechanical system, would be more complex.

In another embodiment of the invention illustrated in Figure 8, the rope 14 rather than being tied off at point 27, is wound on to a reel 122. The rope 14 is supported on pulley wheels 120 which have a fixed position and on a pulley wheel 121 which is mounted so as to be movable in the direction indicated by arrow "y". The pulley wheel 121 is mounted on a spring 126 and is operatively connected to a hydraulic valve 125 such that when the spring is compressed the hydraulic valve is opened to allow pressurised hydraulic fluid stored in accumulator 123 to pass through 124 to release a hydraulic brake associated with reel 122, thereby allowing the reel 122 to rotate and pay out rope 14. The spring 126 is selected such that the force required to compress said spring and open the valve 125 is marginally less than the strength of the weakest component in the system, which may be a part of the apparatus itself, or a part of the load to be controlled to which the apparatus is attached. For example, in the case of the tender vessel illustrated in Figures Ia to Ic the weakest part is the hook to which the rope 14 is attached.

Hence, if the force is so great that a part may fail if the force on the apparatus were to increase further, the pulley wheel 121 rises up against the force of spring 126 and more rope 14 is paid out. When the force reacted through the rope 14 falls below the pressure exerted on the pulley 121, the hydraulic valve is closed and the brake re-applied to the reel 122, The load is then controlled through the damping members of the apparatus.

Figure 9 illustrates an alternative arrangement of the elongate element comprising two pieces of wire rope 14 and a chain 14a comprising a plurality of links 14a'. One of the links 14a' is attached to one of the pieces of wire rope. The other piece of wire rope has a bracket 14b attached to one end thereof.

The bracket 14b is attached to one of the links of the chain by a pin I4c. If it is necessary to change the ength of the elongate element 14, this can be done by attaching the bracket I 4b to a different link I 4a'.

Another application of the invention is in mooring to structures at sea. Figures 10 to 14 illustrate an at sea structure to which access is required. The problem in gaining access to such structures is that when there is a sea swell, it is difficult to moor to the structure without either damaging the structure of the vessel being moored. However, if off-shore wind farms are to be developed it will be necessary to gain access to such structures in sea swells of some metres.

Figure 10 to 14 illustrate a structure 200 which is situated at sea, and to which there is a requirement to moor a vessel. The structure 200 could be a part of a wind turbine. A ladder 201 is attached to the side of the structure 200 extending up to a platform 202. A lift 203 is attached to the structure 200. The lift includes a pair of spaced apart vertically oriented tracks 204 on which the lift cage 205 is slidably mounted by brackets 206. The tracks are provided with apertures along the length thereof through which pins 207 may be passed to mechanically lock the lift cage 205 at a certain elevation.

Another safety device that prevents the lift cage 205 from falling in the event of failure of a part is an inertia reel 208, which is connected to the lift cage 205 by a rope 209. In the illustrated example, the inertia reel is an 800kg inertia reel which retracts the lift cage 205 in the event of failure of another line.

Also connected to the lift cage 205 is a line 210 which attaches to the bottom of the cage 205 at hitch point 211, passes around two pulleys 211 mounted to the underside of a plate 202', and around a third pulley 212 which is mounted to the underside of platform 202. The free end of the line 210 has an eye 213 to which a winch mounted on a vessel may attach. A flotation ring 214 is attached to the line 210 proximate the eye 213. The flotation ring 214 facilitates retrieval of the line 210 from the water from a vessel wishing to moor to the structure 200. When a vessel has moored to the structure 200, the lift cage 205 is operated by a winch situated on the vessel, the winch line of which is connected to the eye 213.

The lifting cage 205 also has another line attached thereto, namely line 215 which attaches to an apparatus 220 for controlling a load as illustrated in Figures 2 to 7. The line 215 passes over pulleys 216 and 217 of the apparatus 220, and is fixed to bracket 219.

The apparatus 220 functions in the same manner as described with reference to Figures 2 to 7 with the device being set by the programmable control equipment 221 to raise the lift cage 205 when carrying a load equivalent to the weight of a man at a rate that is fractionally faster than the speed at which the waves travel. For example, in a swell of up to 4 metres a wave speed of 2 metres per second may be expected, in which case the apparatus 220 could be set to raise the cage 205 at 2.2 metres per second. The winch on the vessel pulls in the line 210 in thereby lowering the cage 205 against the force exerted on the cage 205 by the apparatus 220. The the carriage 219 of the apparatus 220 moves upward.

In the event that the vessel attached to the eye 213 rises quickly, the force exerted on the lift cage 205 would fall below the force exerted on the cage by the apparatus 220 and the cage would be lifted at speed substantially matching the speed of movement of the vessel. If the vessel were to descend rapidly, the mass of the boat would pull the cage 205 downward via the line 210. To allow the lift cage 205 to rise up on the tracks 204, the winch on the vessel is released. The lift cage 205 is then raised by the force exerted thereon by the apparatus 220. If the winch is suitably equipped the speed of ascent of the lift cage 205 can be controlled. For example, if the winch has a brake, the winch drum speed can be controlled by the brake. Other means of controlling the release of line from a winch are well know, and any suitable means could be deployed.

The programmable control equipment 221 allows the speed at which the cage 205 is raised by the apparatus 220 to be controlled, and for the anticipated weight of payload carried in the cage to be compensated for. The manner of adjusting the apparatus 220 is described earlier in this specification.

Figure 11 is a plan view of an at sea structure 200. The structure may be a wind turbine tower comprising a base 230 and mast 231. The apparatus illustrated in Figure lOis situated in a recess 232 in the base and leads up to a set of steps 233 from which access to the base 230 and mast 231 may be gained. An anchor point 234 is located at each side of the recess 232. The anchor points 234 are shown in more detail in Figure 12.

In Figure 12 it can be seen that each anchor point 234 comprises a substantially vertically oriented shaft 235 secured at each end to the base 230 by brackets 236. An anchor in the form of a double ended eye 237 is secured to a float 238 which is slidably mounted on the shaft 235. Attached to one end of the double ended eye 237 is a line 239 which has a float 240 attached thereto. By attaching the float 240 to the line 239, an operator of an approaching vessel can easily retrieve the line 239 with a hook so that the line may be attached to apparatus on the vessel. The construction illustrated in Figure 12 is for use with a vessel of the type illustrated in Figure 14, which includes load control apparatus of the type described above and in the claims. Figure 13 illustrates an alternative example in which the load control apparatus are mounted on the base 230 of the structure. The construction allows a vessel that is not equipped with the load control apparatus to be used.

Figure 14 illustrates a vessel for use with the device as illustrated in Figures 10 to 12. The vessel 250 is equipped with fore and aft load control apparatus 251 and may be moored with either the port or starboard sides to the structure 200. In Figure 14 the vessel is set up to moor port side to the structure, with the line 252 which has an eye 253 at the free end thereof passing through three pairs of rollers 254 and a guide 255. Similar sets of rollers 254 and guides 255 are provided on the starboard side of the vessel. If the vessel is to moor starboard side to the structure 200 the line 252 is passed through the rollers 254 and guides 255 situated on the starboard side. The vessel 250 is also provided with port and starboard side winches 256 each supporting a line 257 which is supported by a guide 257. The vessel 250 is also provided with anti-scuff rollers, which may be damped and which may be retractable. The purpose of the anti-scuff rollers is to prevent or limit damage to the surface of the structure 200.

The load control apparatus 251 each have a hydraulic ram 258 attached to the carriage thereof (corresponding to the carriage 219 in Figure 10). The hydraulic ram 258 is extended to allow the line 252 to be attached to the line 239 in Figure 12. In the apparatus illustrated in Figure 14 a hand pump 259 is provided to release and retract the ram 258. Of course the ram may be released by opening a valve rather than powering the ram out. Whilst the actuator shown is a hydraulic ram 258, other actuators may be used to achieve the same objective which is to provide slack in the line 252 to allow it to be attached to the line 239 and then to take up that slack.

In use, the vessel 250 comes along side a structure 200, the rams 258 are extended. The line 252 is attached to the line 239. The rams 258 are then retracted. The vessel is now secured to the structure with the anti-scuff rollers engaging against the side of the structure. Any rapid change in position of the vessel due to wave motion is dampened by the load control apparatus 251. The line 257 of the winch 256 is attached to the eye 213 of line 210 (see Figure 10). The winch 256 is then wound in until the lift cage 205 descends to the level of the vessel 250. Personnel on board the vessel then walk via the boarding area 260 into the lift cage 205, which includes a fold down door 205' which opens on the deck of the vessel 250 in the manner of a gang plank. The winch operator then releases the winch 256, allowing the lift cage to ascend to the level of the exit and entry steps 233. The lift cage 205 is provided with a door or doors on the opposite side to the door 205' through which the personnel gain access to the structure. Other safety devices such as line attachments to secure personnel in the cage may also be fitted.

Referring now to Figure 13, a load control apparatus 240 is associated with each of the anchor points 234. The line 241 would be attached to the float 238, which would have a line 239 and float 240 as shown in Figure 12. The vessel would be equipped with winches (or another device capable or paying out and bringing in a line) fore and aft. To moor to the structure, the vessel comes along side and hitches the fore and aft winch lines and winds in the winches to bring the vessel alongside the base 230.

Thereafter the manner of use is as described above.

Whilst the structure shown in Figures 10 to 13 is part of a wind turbine, the arrangement illustrated in those figures is not limited to such a structure. The structure could be part of an oil rig, a harbour wall or jetty for example. Further, the lift may be replaced by a two part ladder where one part is slidable with respect to the other. In such an arrangement one of the parts would be secure to the structure, with the other being arranged to slide with respect thereto, and being attached to the lines 209, 210 and 215 in substantially the same way as the cage 205 is attached.