The present invention relates to a system for attaching a vessel, in particular to attaching a water borne vessel to a structure. The present invention also relates to a method for attaching a vessel, in particular a water borne vessel to a structure.

The transfer of personnel and/or items, such as cargo, between a water borne vessel and a structure is commonly required in a wide range of operations. The structure may be a fixed structure, for example a structure secured to the bed of the body of water such as a wind turbine tower, or a floating structure, such as another vessel or tethered floating structure, such as a floating offshore wind platform (FOWP). Methods for the transfer of personnel and/or items are known in the art and generally fall into one of two types.

First, it is known to position the vessel adjacent or close to the structure using a dynamic positioning (DP) system. The DP system comprises sensors to determine the position of the vessel. Data from the sensors is used to maintain the vessel in position relative to the structure. In particular, the position and attitude of the vessel in the water is controlled using the propellers and/or thrusters of the vessel. Currently, many such DP systems are able to maintain the vessel in position at an accuracy of less than 1 metre and with an attitude variation, in particular yaw, of just a few degrees.

The advantage of using a DP system to control the position and attitude of the vessel relative to the structure is its flexibility and accuracy. The position of the vessel can be varied easily, using the control system. In addition, contact between the vessel and the structure is not required and in many cases can be avoided. However, the DP system is complex and expensive to install and operate. As a result, it is generally only larger vessels where the cost of the DP system and its operation can be justified.

Second, it is known to manoeuver the vessel into contact with the structure. To maintain the vessel in position, the engines of the vessel are used to maintain propulsion and thrust the vessel against the structure. This technique is referred to as ‘push-on’. The objective of the push-on technique is that sufficient force is generated between the vessel and the structure that the vessel remains in position and that there is little to no movement of the vessel relative to the structure during the operation. The push-on method has the advantage of being relatively inexpensive, as it requires little additional equipment, and can be employed with smaller vessels. However, the technique requires that the structure and the vessel are able to withstand the considerable loads that are needed to be applied by the vessel to the structure. In addition, there is a risk of the vessel slipping and moving relative to the structure. There is also a limit to the prevailing conditions that allow this method to be used. In particular, the push-on technique cannot be used safely in rough conditions, which in turn results in the technique and vessels employing this technique, to have a relatively narrow range of safe operating conditions.

There is therefore a need for an improved system and method for allowing the transfer of personnel and/or items between a water borne vessel and a structure. It would be advantageous if the system and method could be relatively simple and inexpensive to install and use. It would also be advantageous if the system and method could be safely employed in a wide range of weather conditions.

US 5,275, 119 discloses a boat mooring device employing a pair of cables, each of a predetermined length.

WO 95/018038 discloses a method of manipulating a connecting element in shipping. The method finds use in connecting a tug with a vessel or structure to be towed.

More recently, WO 2012/067519 discloses an arrangement and method for connecting a vessel to an installation employing a telescopic bridge mounted on the vessel. The arrangement and method are particularly intended for the transfer of personnel between the vessel and the installation.

Despite these developments, there remains a need for an improved system and method for allowing the transfer of personnel and/or equipment between the vessel and the structure.

In a first aspect, there is provided a system for positioning a water-borne vessel close to a structure, the system comprising: a tension link assembly comprising a tension link for extending between the vessel and the structure; and an actuating system for releasably attaching the tension link to one of the vessel and the structure; in use, the tension link being attached to both the vessel and the structure and under tension.

The system of the present invention is operable to position a water-borne vessel close to a structure. In this respect, references to the vessel being ‘close to a structure’ are references to the vessel being adjacent the structure or at a distance from the structure such that personnel and/or items may be readily transferred between the vessel and the structure.

During use of the system, the tension link is connected to both the structure and the vessel, with the vessel positioned a distance from the structure corresponding to the length of the tension link. The tension link is kept under tension, in particular by means of the engines of the vessel being operated to move the vessel away from the structure. In this way, the position of the vessel relative to the structure may be maintained in a wide range of weather conditions, allowing for the safe and quick transfer of personnel and/or items, such as cargo or equipment, between the vessel and the structure.

The system is used together with a water-borne vessel. The vessel may be any water-borne craft that is required to be positioned close to a structure. The vessel may be of any size and configuration. The present invention is particularly advantageous as it can employ smaller vessels, such as vessels of the size used for the known push-on technique. This allows the invention to be employed at relatively low cost. In addition, the invention increases the range of weather conditions in which the smaller vessels can safely operate.

The present invention positions the water-borne vessel close to a structure. The structure may be a fixed structure, for example a structure extending from the bed of the body of water. Examples of fixed structures include wind turbine towers and oil and gas platforms. Alternatively, the structure may be floating. The floating structure may be anchored to the bed of the body of water, for example a floating offshore wind platform (FOWP), a floating production system (FPSO), a single point mooring (SPM), or a floating gas or oil production system. The floating structure may alternatively be a floating vessel. It is preferred that the structure has no significant, more preferably limited movement under the action of the water, such as waves and/or currents in the water. If the structure is a floating structure, it is preferred that the structure is not unmoored or able to weather-vane, that is rotate about a single mooring under the action of wind and/or currents in the water, and/or other forces, such as wind. The system comprises a tension link assembly. The tension link assembly includes a tension link. As described hereinbefore, during use, the tension link is attached to both the structure and the vessel and is placed under tension. In this way, the position of the vessel relative to the structure is maintained.

The tension link is releasably attached to one or both of the structure and the vessel. In one embodiment, the tension link is attached or mounted on the structure and is releasably attached to the vessel during use, the tension link being detached from the vessel after use. In this embodiment, the tension link may remain on the structure during periods of non-use and releasably attached to a vessel when the transfer of personnel and/or items between the structure and the vessel is required. In this embodiment, different vessels may use the tension link at different times for different operations. The tension link may be releasably attached or mounted to the structure, such that it is temporary and m ay be removed or repositioned.

In an alternative embodiment, the tension link is attached or mounted on the vessel and is releasably attached to the structure during use, the tension link being detached from the structure after use. In this embodiment, the tension link may remain on the vessel during periods of non-use and releasably attached to a structure when the transfer of personnel and/or items between the structure and the vessel is required. In this embodiment, the vessel may use the tension link at different times for operations on different structures. The tension link may be releasably attached or mounted to the vessel, such that it is temporary and may be removed or repositioned, for example mounted to a different vessel.

The tension link may be flexible. For example the tension link may comprise one or more cables or ropes. Alternatively, the tension link may be rigid. For example the tension link may comprise one or more bars, tubes or rods. In one preferred embodiment, the tension link comprises an articulated arm assembly comprising a plurality of rigid arm portions, with each rigid arm portion being hingedly attached to one or two adjacent rigid arm portions. A preferred embodiment in which the tension link comprises an articulated arm assembly is described in more detail hereinafter.

In one embodiment, the tension link is of a fixed length. More preferably, the tension link is adjustable, such that the length of the tension link may be varied. In this respect, references to the ‘length’ of the tension link are to the distance between the vessel and the structure that is required to allow the tension link to be connected at one end to the vessel and at the other end to the structure. By having the tension link of a fixed length, during operation of the system it is necessary for the vessel to be positioned at a fixed distance from the structure corresponding to the length of the tension link. By having the length of the tension link variable allows greater freedom in positioning the vessel relative to the structure, while still allowing the tension link to be engaged with both the vessel and the structure and placed under tension.

Having the length of the tension link variable is particularly preferred in cases where the structure is not fixed and may be subject to movement in the water.

Any suitable arrangement may be employed to allow the length of the tension link to be varied. In one embodiment, the tension link is flexible, as noted hereinbefore. Alternatively, the tension link is rigid and has a length that may be varied. In particular, the rigid tension link may comprise two or more portions that are moveable with respect to each other. In particular, the tension link may comprise a first portion and second portion, the first portion being moveable with respect to the second portion. For example, the rigid tension link may comprise one or more rigid sections arranged telescopically. Alternatively, the rigid tension link may comprise a plurality of rigid sections interconnected by a pivot or hinge.

In embodiments where the length of the tension link is variable, it is preferred that the tension link further comprises a locking assembly, to lock the tension link at a given length. For example, the tension link may comprise one or more hydraulic rams, which may be locked. Alternate locking means include a brake assembly employing friction to prevent movement of the components of the tension link relative to one another.

As described hereinbefore, the tension link is releasably attachable to at least one of the vessel and the structure. Therefore, the tension link assembly preferably further comprises a releasable engagement assembly for releasably attaching the tension link to one of the structure and the vessel. The engagement assembly is preferably disposed at an end of the tension link and is configured to releasably engage the end of the tension link with the structure or the vessel, as appropriate. Any suitable assembly for releasably engaging the end of the tension link with the structure or the vessel may be employed. Examples of suitable assemblies include mechanical engagement assemblies, in which the end of the tension link is provided with a latch for releasably connecting to a complementary form on the structure or the vessel. Alternative engagement assemblies include, for example, electromagnetic assemblies.

In one embodiment, the releasable engagement assembly employs a feature on the structure or the vessel to form the releasable connection. Examples of a suitable feature include a rail, a bollard, a pad-eye, or another feature sufficiently strong to withstand the forces applied to it by the tension link during use.

In one preferred embodiment, one of the tension link and the structure or the vessel is provided with a male latch member and the other of the end of the tension link and the structure or the vessel is provided with a female latch member, wherein the female latch member is releasably engageable with the male latch member. The male latch member may have any suitable form. In one preferred embodiment, the male latch member comprises a rounded latch portion, preferably a spherical cap or spherical dome. The female latch member may have any suitable form complementary to the male latch member. In one preferred embodiment, the female latch member comprises a receiver for receiving the male latch member, the receiver provided with one or more moveable locking fingers for engaging with a portion of the male latch member, for example a recess or groove formed in the male latch member. The locking fingers may be moved into and/or out of engagement with the male latch member by any suitable means, for example hydraulically or electrically, such as by one or more solenoids. The female latch member preferably comprises a guide for guiding the male latch member into engagement with the locking fingers. The guide may have any suitable form, for example conical.

As described above, in use at least one end of the tension link is releasably attachable to one of the structure or the vessel. If a single releasable engagement assembly is employed for connection with one of the structure and the vessel, the second end of the tension link is mounted to the other of the structure and the vessel. Any suitable mount may be employed. Preferably, the mount is a pivotable mount, allowing the tension link to pivot in one plane about the mount, or a rotatably mount, allowing the tension link to move in more than one plane about the mount. Suitable mounts are known in the art.

Alternatively, the tension link may be provided with a releasable engagement assembly at each end for releasably attaching the tension link to both the structure and the vessel. As described hereinbefore, during use of the system, the tension link is deployed to be releasably connected to one of the structure and the vessel. The tension link is either permanently mounted or releasably connected to the other of the structure and the vessel. In this respect, the tension link can be considered to be moveable between a stowed position, in which the tension link is stowed on the vessel or the structure, and a deployed position, in which the tension link extends between the structure and the vessel, holding the vessel in position relative to the structure and allowing for the transfer of personnel and/or items.

The system further comprises an actuating system for moving the tension link between the stowed position and the deployed position. The actuating system also functions to releasably attach and detach the tension link to one of the vessel and the structure. In use, before the transfer of personnel and/or items between the vessel and the structure, the actuating system operates to deploy the tension link from the stowed position on one of the structure and the vessel and releasably engage the tension link with the other of the structure and the vessel.

The actuating system may be mounted on the structure. In such cases, the actuating system may be operated from the structure and/or the vessel, for example the actuating system may be operated remotely from the vessel to deploy the tension link from a stowed position on either the structure or the vessel. More preferably, the actuating system is mounted on the vessel. In such cases, the actuating system may be operated from the structure and/or the vessel, for example the actuating system may be operated remotely from the structure. Preferably, the actuating system is mounted on the vessel and operated from the vessel.

In one preferred embodiment, the tension link assembly and the actuating system are both mounted on the vessel, with the tension link moveable between a stowed position on the vessel and the deployed position, in which the tension link is releasably connected to the structure. Preferably the actuating system is operated from the vessel.

In one embodiment, the actuating system comprises an actuator assembly. In use, the actuator assembly moves the tension link between the stowed and the deployed positions. The actuator assembly preferably comprises a base. The base is mounted on the vessel or the structure. The base may be any component or assembly of components that can secure the assembly to the structure or to the vessel. In one preferred embodiment, the base comprises a fixed base assembly for securing to the structure or the vessel and a moveable base assembly mounted to the fixed base assembly for movement relative to the fixed base assembly. Preferably, the moveable base assembly is rotatably mounted to the fixed base assembly. With the base at rest, the moveable base assembly preferably moves in a substantially horizontal plane. In an alternative embodiment, the moveable base assembly is mounted to the fixed base assembly by a gimbal and rotatable with respect to the fixed base assembly. This is particularly preferred when the actuator assembly is mounted on a vessel, so as to isolate the moveable base assembly from movement of the vessel. The gimbal preferably has three axes of rotation, allowing the moveable base assembly to move independently of the fixed base assembly and for motion of the fixed base assembly and the vessel on which it is mounted, in particular pitch, roll and yaw, not to affect movement of the moveable base assembly.

The actuator assembly has a proximal end and a distal end. The proximal end of the actuator assembly is preferably mounted to the base. In embodiments in which the base comprises a fixed base assembly and a moveable base assembly the proximal end of the actuator assembly is mounted to the moveable base assembly. The distal end of the actuator assembly is moveable relative to the base to which the proximal end of the actuator assembly is mounted. The distal end of the actuator assembly is moveable by an actuation control system. The distal end of the actuator assembly is moved by the actuation control system in response to signals received from a movement control system, as described in more detail hereinbelow.

The actuator assembly preferably comprises one or more actuator members.

In one embodiment, the actuator assembly has a single actuator member. The single actuator member is preferably mounted at its proximal end to the base. In embodiments, in which the base comprises a fixed base assembly and a rotatable base assembly, the actuator member is preferably mounted to the rotatable base assembly. The actuator member may have any suitable form. In one embodiment, the actuator member is an elongate member or arm. Alternatively, the actuator member may comprise a frame member, for example a rectangular or triangular frame member. For example, the actuator member may comprise an A-frame having a wide end and a narrow end, preferably with the A-frame mounted with its wide end proximal to the structure or vessel on which it is mounted and the narrow end distal of the structure or vessel.

In a preferred embodiment, the actuator assembly comprises a plurality of actuator members, preferably wherein the actuator members are connected in an articulated arrangement to form an articulated arm. By providing the actuator assembly with a plurality of members, the actuator assembly is able to be manoeuvred to allow its distal end to access any position within a volume of space defined by the limits of the movement of the distal end.

In one preferred embodiment, the actuator assembly comprises an articulated arm assembly having a first actuator member mounted to the base, preferably pivotally mounted to the base, and extending from the proximal end of the actuator assembly and a second actuator member pivotally mounted to the distal end of the first actuator member and extending to the distal end of the actuator assembly.

Alternatively and more preferably, the actuator assembly comprises an articulated arm assembly having more than two actuator members, for example an articulated assembly having a first actuator member extending from the proximal end of the actuator assembly, a second actuator member pivotally mounted to and extending from the distal end of the first actuator member and a third actuator member pivotally mounted to and extending from the distal end of the second actuator member to the distal end of the actuator assembly. In one preferred embodiment, the actuator assembly comprises an articulated arm assembly comprising a first actuator member pivotally mounted to the base and extending in the distal direction, a second actuator member pivotally connected to the distal end of the first actuator member and extending distally, a third actuator member pivotally connected to the distal end of the second actuator member and extending distally, and a fourth actuator member pivotally connected to the end of the third actuator member and extending to the distal end of the actuator assembly. In embodiments where the actuator assembly is pivotally mounted to the base at its proximal end, the pivot mounting preferably allows the actuator assembly to move relative to the base about a substantially horizontal axis, when the base is at rest.

Each actuator member may have any suitable form. In one preferred embodiment, each actuator member comprises an elongate arm.

The actuator assembly comprising one or more actuator members is mounted at its proximal end to the base. The mounting allows the one or more actuator members to move in at least one plane or direction relative to the base. In particular, the actuator assembly may be pivotally mounted to the base at its proximal end to allow the distal end of the actuator assembly to be raised and lowered in one plane. With the base at rest, the actuator assembly is preferably mounted to the base so as to allow the distal end to be raised and lowered in a substantially vertical plane.

Alternatively, the actuator assembly comprising one or more actuator members may be mounted to the base and the arrangement configured so as to allow the distal end to be raised and lowered in a first plane, as just described, and the proximal end rotated in a second plane, perpendicular to the first plane. With the base at rest, the first plane is preferably substantially vertical and the second plane is preferably substantially horizontal. In embodiments in which the base has a moveable base member rotatably mounted on a fixed base member, this may be achieved by mounting the proximal end of the actuator member to the moveable base member.

As a further alternative, the actuator assembly comprising one or more actuator members may be mounted to the base and the arrangement configured so as to allow the distal end to be raised and lowered in a first plane, as hereinbefore described, the proximal end to be rotated in a second plane perpendicular to the first plane, as just described, and the actuator member rotated about the mounting in a third plane, wherein the third plane is perpendicular to the first plane and the second plane. In embodiments in which the base has a moveable base member mounted on a fixed base member by a gimbal and able to rotate relative to the fixed base member, this may be achieved by mounting the proximal end of the support assembly to the moveable base member. Suitable articulated actuator assemblies having a plurality of actuator members are known in the art. An example of an articulated arm assembly is the Neptune 20M+ system available from Submarine Technology Limited, United Kingdom.

An arm assembly suitable for use in the system of the present invention is described and shown in GB 2,336,828 A. The arm assembly comprises a base with a fixed part to be mounted in use on a host structure or vessel and an articulated assembly having two arm members for controlled movement in three mutually perpendicular planes or directions relative to the fixed part of the base. An arm assembly is mounted at one end on the articulated assembly for luffing and reach movement.

WO 2021/239728 discloses an arm assembly suitable for use in the system of the present invention. The arm assembly is similar to that of GB 2,336,828 A and comprises a base having a stationary base part and a moveable base part rotatable relative to the stationary base part about a substantially vertical axis. The arm assembly further comprises an articulated arm comprising two arm members mounted to the moveable base part and moveable about a substantially horizontal axis.

It is especially preferred that the arrangement, preferably the base and the actuator assembly, is configured to allow the distal end of the actuator assembly to have a freedom of movement that allows the movement of the distal end to compensate for movement in six degrees, namely heave, pitch, roll, yaw, surge and sway.

In one embodiment, the actuator assembly is employed together with a tension link. In use, the actuator assembly is employed to move the tension link from the stowed position to the deployed position and to complete the releasable connection between the tension link and the structure or the vessel. For example, with the actuator assembly and the tension link both mounted on the vessel, the actuator assembly moves the tension link from its stowed position on the vessel to its deployed position, in which the releasable connection between the end of the tension link distal of the vessel and the structure can be formed. In such cases, it is preferred to use a releasable latch assembly having a first latch member on the tension link and a second latch member on the structure, the actuator assembly engaging the first latch member on the tension link with the second latch member on the structure. The actuator assembly is also used to return the tension link from the deployed position to the stowed position on board the vessel once the operation has been completed and the tension link released from engagement with the structure. In an alternative example, the actuator assembly and the tension link are both mounted on the structure. The actuator assembly is used to move and engage the tension link with the vessel in an analogous manner to that immediately hereinbefore described.

In an alternative embodiment, the actuator assembly itself provides the tension link. In this embodiment, the actuator assembly is moved from its stowed position on one of the structure and the vessel to its deployed position and releasably engaged with the other of the structure and the vessel. Once engaged, the actuator assembly is put under tension and itself functions as the tension link to hold the vessel in position relative to the structure.

The actuating system preferably comprises one or more actuators for moving the actuator assembly and its components to move the tension link between the stowed position and the deployed position. For example, in embodiments in which the actuator assembly comprises articulated actuator members, the one or more actuators operate to move the actuator members. In addition, in embodiments in which the actuator assembly comprises a base having a fixed base assembly and a moveable base assembly, which is moveable, for example rotatable, relative to the fixed base assembly, the one or more actuators also operate to move the moveable base assembly relative to the fixed base assembly.

Suitable actuators are known in the art and include electric actuators and hydraulic actuators. Hydraulic actuators, such as hydraulic rams, are particularly suitable for use in the present invention.

The system of the present invention preferably further comprises a movement control system. The movement control system may be located on one or both of the vessel and the structure. In one preferred embodiment, the movement control system is located on the vessel.

In use, the movement control system provides signals to control the operation of the actuating system and control the movement of the actuator assembly. The movement control system comprises a processor for controlling the movement of the components of the actuating system. The movement control system is preferably configured to receive data regarding the movement of the vessel. In use of the system, the movement control system functions to control the actuating system to move the tension link assembly in response to data received regarding movement of the vessel. In this way, the movement control system instructs movement of the components of the actuating system to allow the tension link to be engaged or disengaged, that is to make or break the link between the vessel and the structure. Depending upon the operation being carried out, the movement control system is configured to operate in a number of modes, as described in more detail below.

As noted above, the movement control system is preferably configured to receive data relating to the movement of the vessel. A vessel floating in the water has six possible degrees of freedom of movement, three rotational movements and three linear movements, namely heave, pitch, roll, surge sway and yaw. The movement control system is preferably configured to receive data relating to one or more of the six degrees of freedom of movement, that is the heave, pitch, roll, surge, sway and yaw of the vessel. The movement control system is more preferably configured to receive data relating to two or more, more preferably three or more, still more preferably four or more, more preferably still five or more, especially all of heave, pitch, roll, surge, sway and yaw of the vessel.

In the case of the vessel being equipped with a system for detecting motion in the water, the movement control system may be configured to receive data from the motion detection system of the vessel. For example, many vessels are equipped with a system for sensing the movement of the vessel on the water as a component of the control systems of the vessel. In such cases, data regarding movement of the vessel can be obtained by the movement control system directly from the onboard system of the vessel. Such data can be transmitted to and received by the movement control system in any suitable manner, for example by a wired connection or by a wireless connection. A wireless connection is preferred in embodiments in which the movement control system is mounted on the structure and is to receive data regarding movement of the vessel from the vessel systems themselves. In cases where the vessel does not have its own system for detecting motion of the vessel, the system of the present invention may comprise a vessel motion sensing system for detecting motion of the vessel and providing data regarding the movement of the vessel to the movement control system. The vessel motion sensing system may be disposed on the vessel.

One preferred system for sensing motion of the vessel from the vessel itself comprises an Inertial Measurement Unit (IMU) and a global navigation satellite system (GNSS). An alternative preferred system is Real Time Kinematic (RTK) GPS. Suitable motion sensing systems are known in the art and are commercially available. Examples of commercially available systems include the CPT7 of Novatel, USA. The use of such systems is preferred, even in cases where the vessel has an onboard motion detection system, as the onboard systems may not provide sufficient accuracy to allow the movement of the actuator assembly and the tension link to fully compensate for the six degrees of freedom of movement of the vessel in the water.

Alternatively, the vessel motion sensing system may be disposed on the structure and capture data relating to the movement of the vessel remotely, for example using an optical system. In one embodiment, the vessel motion sensing system located on the structure is based on optical sensing of the position of the vessel. In particular, one preferred system employs one or more, preferably two or more, cameras mounted on the structure for detecting movement of the vessel. The vessel may be provided with suitable markers on its outer surface, such as colour markers, which may be tracked by the one or more cameras. In one preferred embodiment, the motion sensing system comprises one or more pairs of cameras, the cameras in each pair operating in stereo.

In cases where the structure is fixed, for example where the structure is anchored to the bed of the body of water or is firmly anchored or tethered so as to not to move under the action of waves or currents in the water, it may be necessary only to sense the motion of the vessel for sufficient control of the actuating system to safely and effectively engage and disengage the tension link, so as to couple or decouple the vessel from the structure. However, in cases where the structure is subject movement, for example under the action of waves or currents in the water and/or the wind, it is preferred for the movement control system also to receive data relating to movement of the structure.

In one embodiment, the movement control system receives data relating to movement of the structure from an external system. For example, the structure may have its own system for detecting its own motion and transmitting data relating to such movement. An example of this is where the structure is a large vessel provided with an onboard system for detecting movement of the vessel as described above.

In an alternative embodiment, the system of the present invention further comprises a structure motion sensing system for detecting movement of the object, including allowing the motion of the object to be tracked. The structure motion sensing system provides the movement control system with data relating to the movement of the structure.

The structure motion sensing system may be located on the structure. Examples of suitable motion sensing systems for this function preferably comprise an Inertial Measurement Unit (IMU) and a global navigation satellite system (GNSS). Commercially available systems of this kind are indicated hereinbefore.

Alternatively, the structure motion sensing system may operate remotely of the structure, in particular from the vessel. Any suitable system for detecting movement of the structure may be employed. In one preferred embodiment, the structure motion sensing system is located on the vessel and is based on optical sensing of the position of the structure. In particular, one preferred system employs one or more, preferably two or more, cameras mounted on the vessel for detecting movement of the structure. The structure may be provided with suitable markers on its outer surface, such as colour markers, which may be tracked by the one or more cameras. In one preferred embodiment, the motion sensing system comprises one or more pairs of cameras, the cameras in each pair operating in stereo.

The movement control system is configured to receive data relating to the movement of the structure. In particular, the movement control system is preferably configured to receive data relating to one or more of the heave, pitch, roll, surge, sway and yaw of the structure. The movement control system is preferably configured to receive data relating to two or more, more preferably three or more, still more preferably four or more, more preferably still five or more, especially all of heave, pitch, roll, surge, sway and yaw of the structure.

Data regarding movement of the vessel received from an onboard sensing system of the vessel will be data regarding the actual movement of the vessel in the water. In contrast, data regarding movement of the vessel received from a detection system located on the structure may be data relating to movement of the vessel relative to the structure, which may differ from the actual movement of the vessel. The movement control system may be configured to receive and process both data relating to the actual movement of the vessel and data relating to movement of the vessel relative to the structure. Similarly, data regarding movement of the structure received from an onboard sensing system of the structure itself will be data regarding the actual movement of the structure in the water. In contrast, data regarding movement of the structure received from a detection system located on the vessel may be data relating to movement of the structure relative to the vessel, which may differ from the actual movement of the structure. The movement control system may be configured to receive and process both data relating to the actual movement of the structure and data relating to movement of the structure relative to the vessel.

The movement control system is configured to determine the relative movement between the vessel and the structure. This determination is based on the data received regarding movement of the vessel and data received regarding the movement of the structure. In cases where the structure is stationary, there is no movement of the structure to be considered and the actual movement of the vessel will also be the movement of the vessel relative to the structure.

As noted hereinbefore, the movement control system of the system of the present invention is operable in a number of different modes.

In a first mode, the movement control system receives data relating to the movement of the vessel. The movement control system controls movement of the actuating assembly to move the tension link of the tension link assembly to be engaged or disengaged with the vessel or the structure. In particular, with the tension link being deployed from one of the structure and the vessel, the movement of the releasable engagement assembly of the tension link is controlled to match the movement of the other of the structure and the vessel. This mode of operation is herein referred to as the ‘stabilised mode’. This mode of operation is particularly advantageous when the structure is fixed.

In a second mode, the movement control system determines the movement of the vessel relative to the structure. In embodiments where the movement control system receives data regarding actual movement of the vessel, such as from a motion sensing system on board the vessel, and data regarding the actual movement of the structure, such as from a motion sensing system on board the structure, the movement control system makes a determination of the relative movement of the vessel and the structure. In embodiments where data relating to the relative movement between the vessel and the structure provided to the movement control system, for example when the system comprises and employs a sensing system operating to determine movement of the structure from the vessel or vice versa, the data received by the movement control system relate to the relative movement between the vessel and the structure. Using the received or determined data relating to the movement of the vessel relative to the structure, the movement control system controls movement of the actuating system to move the releasable engagement assembly of the tension link of the tension link assembly along a path to match the relative movement of the vessel and the structure. In particular, with the tension link being moved from one of the structure and the vessel to releasably engage with the other of the structure and the vessel, the movement of the releasable engagement assembly is controlled to match the movement of the other of the structure and the vessel, allowing the releasable connection to be made. This mode of operation is herein referred to as the ‘synchronised mode’. This mode of operation is particularly advantageous when the structure is not fixed and may move under the action of the body or water and/or such factors as the wind.

In a third mode, the movement control system is used merely to control the movement of the actuating assembly without processing data relating to the movement of the vessel or the structure. In this mode, the movement of the actuating assembly and the tension link is determined by the actions of the movement control system and the movement of the vessel or the structure. This mode is herein referred to as the ‘unstabilised mode’. This mode is used, for example, to move the tension link assembly on board either the structure or the vessel, for example to move the tension link to a starting position prior to commencing the operation to releasably connect the vessel and the structure or to move the tension link to a stowed position.

In a further aspect, the present invention provides a vessel or a structure comprising a system as hereinbefore described.

As described hereinbefore, the system of the present invention may be employed to releasably connect a vessel with a structure by a tension link, allowing the tension link to be placed under tension, in turn allowing the safe and effective transfer of personnel and/or items, such as cargo or equipment, between the structure and the vessel.

As described hereinbefore, the tension link may be flexible. For example, the tension link may comprise one or more cables or ropes. However, it has been found that, when the tension link is flexible, for example made from a flexible material such as rope or cable, and tension is applied by the attached vessel being moored by applying thrust, there is a risk that the vessel will acquire sufficient kinetic energy to cause a snatch load that will be sufficient to break the tension link. Therefore, when employing a flexible tension link, it is preferred that the arrangement prevents the tension link being subjected to loads that exceed the breaking load of the tension link.

In one preferred embodiment, the flexible tension link has a sufficient length to be able to extend under load to such a degree that its strain energy matches the vessel kinetic energy without exceeding the breaking load of the tension link. Preferably, the flexible tension link is resilient, that is formed at least in part along its length from a resilient material. In one preferred embodiment, the tension link is formed entirely from a resilient material. Suitable materials for forming the resilient tension link are known in the art and include polyamides, such as Nylon. A resilient material that stretches easily is particularly preferred, for example a rope formed from Nylon, rather than less resilient materials such as polyester.

However, when employing a resilient tension link, it may be difficult to control the speed of the vessel applying tension to the tension link sufficiently to ensure that the snatch load is less than the breaking load. This is particularly the case when a short tension link is employed between the vessel and the structure, as the strain energy of the tension link is directly proportional to the length of the tension link. In such cases, it is preferred to employ a tension link that is significantly longer than the distance between the vessel and the structure. Such a long tension link may be employed in a number of different arrangements. For example, the tension link can be extended through a guide or fairlead mounted on the vessel, for example at the bow of the vessel, and securing the tension link on the vessel at a distance from such guide or fairlead. In one embodiment, the guide or fairlead is mounted at the bow of the vessel and the securing point of the tension link is towards or at the stem of the vessel. An analogous arrangement may be employed when the tension link is mounted to and extended from the structure to the vessel.

Having a flexible tension-link that is attached at a sufficient distance from the point where it is it is brought aboard the vessel or structure, such as through a fairlead, can result in the tension link being a safety hazard if it breaks under tension. The tension link may also occupy valuable space, depending upon how it is arranged. To overcome these disadvantages, it is proposed that sufficient length can be incorporated into the tension link by using a multipoint purchase around which the tension link extends. In this arrangement, the tension link extends around or through a plurality of spaced apart purchase members, which are arranged in an array. The array may have a compact arrangement, thereby keeping the space occupied by the multipoint purchase to a minimum. Any suitable form of purchase member may be used, with pulleys being a preferred example. The multipoint purchase preferably comprises a plurality of purchase members. The number of purchase members will depend upon such factors as the space to be occupied by the multipoint purchase and the length of tension link required. The multipoint purchase may have from 2, preferably from 3, more preferably from 4 purchase members. The multipoint purchase may have up to 8, preferably up to 6 purchase members. Preferably, the array of purchase members forming the multipoint purchase is contained in a housing, for example with the purchase members being mounted to the walls of the housing. In this way, the housing acts to protect personnel in the event the tension link breaks under load.

The multipoint purchase may be located at any suitable position on the vessel or structure. For example, the multipoint purchase may be located inboard or outboard of the guide or fairlead through which the tension link is brought aboard the vessel or structure.

In one embodiment, the multipoint purchase is moveable, for example pivotable, between a stowed position and a deployed position. In this way, the multipoint purchase may be stowed in a position to free space on the vessel or structure when not in use.

A single multipoint purchase may be employed for the or each flexible tension link. Alternatively, two or more multipoint purchases may be employed for the or each flexible tension link.

In one embodiment, if the vessel has a wide bow, such as is the case with catamaran vessels, separate multipoint purchases may be used on each side of the vessel. In this arrangement, two separate tension links may be employed.

Alternatively, a single tension link may be used and extend in a bridle arrangement to provide the extra length in the tension-link.

When deploying the tension link, the actuating assembly may be operated to move the tension link from a stowed position to a starting position. In the starting position, the tension link is connected to one of the structure or the vessel at one end. The other end of the tension link comprises the releasable engagement assembly for releasably engaging with the other of the structure and the vessel. This part of the operation may be conducted with the movement control system in the unstabilised mode. In this way, the actuating assembly and the tension link move with the movement of the structure or the vessel.

To have the tension link form a releasable connection between the structure and the vessel, the vessel is manoeuvred into a position where the tension link can be deployed and the releasable connection made.

In the case of a structure that is fixed, the movement control system can be switched to the stabilised mode. In this mode, the tension link may be moved along a deployment path to releasably link the vessel and the structure. The tension link is moved from the starting position on one of the structure and the vessel and matched to the movement of the other of the structure and the vessel to releasably engage with the other of the structure and the vessel.

In the case of a structure that is moving, the movement control system can be switched to the synchronised mode. In this mode, the tension link may be moved along a deployment path to releasably link the vessel and the structure. The tension link is moved from the starting position on one of the structure and the vessel to match the movement of the other of the structure and the vessel and releasably engage with the other of the structure and the vessel.

With the tension link connected to both the vessel and the structure, the tension link is placed under tension. For example, the engines of the vessel may be used to drive the vessel away from the structure, applying a tensioning force to the tension link. Generally, the tensioning force applied by the vessel to the tension link will be along a line coincident with or parallel to the longitudinal axis of the tension link. Preferably, the vessel is operable also to counter any lateral forces applied to the vessel, for example by waves and/or currents in the water or such factors as the wind. In the case of vessels with two or more propellers, these may be operated independently to counteract any lateral forces and keep the vessel and the tension link from pivoting laterally about the connection with the structure. In vessels provided with thrusters, these may also be used to counteract any lateral forces. To release the vessel from the structure, the tensioning force applied to the tension link is reduced or removed. The releasable engagement assembly of the tension link is released, using the mode described above and the procedure reversed.

The length of the tension link determines the distance between the vessel and the structure during the operation and while the vessel is connected to the structure. In embodiments where the length of the tension link is variable, once the tension link is engaged with both the vessel and the structure, the length of the tension link is fixed, as described hereinbefore. This determines the distance between the vessel and the structure for subsequent operations. The length of the tension link may be selected according to a number of factors, such as the nature of the items to be transferred between the vessel and the structure, the manner in which the transfer is carried out, for example the equipment such as cranes or derrick or the like, and the prevailing weather conditions. For example, in severe conditions, such as high winds and/or a high swell, the length of the tension link may be increased. For many operations, a length of about 5m for the tension link is suitable. Longer lengths, such as up to 10m, or shorter lengths, for example 1 or 2m, may also be used, when suitable.

In many cases, the tension link will extend from the bow of the vessel or from the region of the bow of the vessel to the structure. However, the tension link may be positioned at other locations on the vessel, as appropriate for the operation to be carried out.

In a further aspect of the present invention, there is provided a system for releasably attaching a water-borne vessel to a structure, the system configured to be mounted on the vessel or the structure, the system comprising: a water-borne vessel; a tension link assembly comprising a tension link for extending between the vessel and the structure, the tension link being releasably engageable with at least one of the vessel and the structure; an actuating system comprising an actuating assembly for releasably attaching the tension link to one of the structure and the vessel; in use, the tension link being connected to and extending between the waterborne vessel and the structure and placed under tension.

In a further aspect, the present invention provides a method for releasably connecting a vessel to a structure, the method comprising: providing a tension link connected to one of the structure and the vessel; moving the vessel to a position relative to the structure, whereby the tension link may be releasably connected to the other of the structure and the vessel; releasably connecting the tension link to the other of the structure and the vessel; and moving the vessel to apply tension to the tension link.

Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a system according to one embodiment of the present invention;

Figure 2 is a diagrammatic representation of a system according to an alternative embodiment of the present invention;

Figure 3a is an isometric view of an actuating assembly according to one embodiment of the present invention in a first position; Figure 3b is an isometric view of the actuating assembly of Figure 3a in a second position;

Figure 3c is an isometric view of the assembly of Figure 3a mounted on the deck of a vessel;

Figure 4 is a perspective view of one embodiment of a releasable engagement assembly for use in the system of the present invention;

Figures 5a to 5f show a diagrammatic representation of the stages of an operation to engage and disengage a vessel with a structure using the embodiment of the system shown in Figure 1;

Figures 6a to 6e show a diagrammatic representation of the stages of an operation to engage and disengage a vessel with a structure using the embodiment of the system shown in Figure 2;

Figure 7 is a diagrammatic representation of one embodiment of the present invention employing a flexible tension link;

Figure 8 is a diagrammatic representation of an alternative embodiment of the present invention employing a flexible tension link; and

Figure 9 is an enlarged diagrammatic representation of the multipoint purchase for a flexible tension link as used in the embodiment of Figure 8.

Turning to Figure 1 , there is shown a diagrammatical representation of a system according to one embodiment of the present invention. The system, generally indicated as 2, comprises a tension link assembly 4 having a tension link 6 and an actuating system 8 comprising an actuator assembly 8a. The tension link assembly 4 and the actuating system 8 are represented in Figure 1 , in a preferred embodiment, as being mounted on a floating vessel 10. In use, the tension link 6 is deployed to releasably connect the vessel 10 to a structure 12. It is to be understood that the tension link assembly 4 and/or the actuating system 8 may be mounted on the structure 12. The system 2 further comprises a movement control system 20. The movement control system 20 controls movement of the actuator assembly 8a by way of control signals sent to the actuating system 8 via a signal line 22. The movement control system 20 is configured to receive data relating to motion of the vessel 10, indicated in Figure 1 as data signals being received from the onboard system of the vessel 10 via a signal line 24.

In embodiments where the structure 12 is a moving structure, such as a moored or tethered floating structure, the movement control system 20 is further configured to receive data relating to the motion of the structure 12. In the arrangement shown in Figure 1 , the movement control system 20 receives data directly from the structure 12 by way of a signal line 28. Alternatively, or in addition, the movement control system 20 is configured to receive via a signal line 30 data from a motion sensing system 32 mounted on the vessel 10 and configured to sense the motion of the structure 12.

In use, the actuator assembly 8a is used to move the tension link 6 under the control of the movement control system 20. The tension link 6 has a proximal end 6a and a distal end 6b. The length of the tension link 6 is adjustable. To achieve this, the tension link 6 comprises tension link members 7a and 7b arranged telescopically. A locking assembly 7c is provided on the tension link and is operable to lock the tension link members 7a and 7b and prevent relative movement between them.

As shown in Figure 1 , the tension link 6 is connected at its proximal end 6a to the vessel 10. The distal end 6b is provided with a releasable engagement assembly 40 for releasably connecting to the structure 12. During use, the releasable connection between the engagement assembly 40 and the structure 12 is made by the actuator assembly 8a moving the tension link 6 under the control of the movement control system 20.

Turning to Figure 2, there is shown a diagrammatical representation of a system according to an alternative embodiment of the present invention. The system, generally indicated as 2a, is configured in generally the same manner as the system 2 of Figure 1 . Components common to the systems of Figure 1 and Figure 2 are indicated using the same reference numerals and are as described above.

The system of Figure 2 differs from that of Figure 1 in the actuator assembly 8a itself forms the tension link and no separate tension link assembly is provided. The releasable engagement assembly 40 is provided on the end of the actuator assembly 8a, as shown in Figure 2.

In use, the movement control system 20 controls the movement of the actuator assembly 8a to engage the engagement assembly 40 and the actuator assembly 8a with the structure 12. Once connected in this manner, the actuator assembly 8a itself forms the tension link between the vessel 10 and the structure 12.

T urning to Figures 3a and 3b, there are shown two isometric views of an actuating system according to one embodiment of the present invention, generally indicated as 102. The actuating system 102 is shown in a stowed position in Figure 3a and in a deployed position in Figure 3b.

The actuating system 102 comprises a base 110 and an actuator assembly 130. The base assembly 110 has a fixed base assembly 112 comprising three legs 114 extending radially outwards from a central fixed base member 116. A moveable base assembly 118 is rotatably mounted on the fixed base member 116 and comprises a rotatable base member 120. Having the moveable base assembly 118 rotatable about the fixed base member 116 allows for compensation of movement of the vessel, including yaw movements.

The actuator assembly 130 has a proximal end 132 and a distal end 134. The actuator assembly 130 comprises four elongate actuator members, as follows:

The actuator assembly 130 comprises a first actuator member 140 extending from the proximal end 132 of the actuator assembly and pivotally mounted at a first end to the rotatable base assembly 118. The pivot connection between the first actuator member 140 and the rotatable base assembly 118 allows for compensation of the pitch and roll of the vessel 10. A second member 142 is pivotally mounted at a first end to a second end of the first actuator member 140.

A third actuator member 144 is pivotally mounted at a first end to a second end of the second actuator member 142.

A fourth actuator member 146 is pivotally mounted at a first end to the second end of the third actuator member 144 and has a second end forming the distal end 134 of the actuator assembly 130.

The system 2 further comprises a motion sensing system 150, shown in Figure 3c comprising a pair of cameras 152 mounted on a lateral spar 154, in turn mounted on a leg 114 of the fixed base assembly 112. Alternatively, the cameras 152 may be mounted on the fourth actuator member 146. Image data from the cameras are processed by a remotely mounted computer (not shown for clarity), which also forms part of the motion sensing system 150. In use, the motion sensing system 150 is used to detect and track movement of the vessel 10 relative to the structure 12. Data from the motion sensing system 150 is provided to the movement control system 20, as described hereinbefore.

The actuation system 8 further comprises an actuating system 160, which operates to move the components of the actuator assembly 130 and the base 110. The actuator system 160 shown in Figures 3a to 3c is a hydraulic system, by way of example. The actuator system comprises a first hydraulic ram 162 extending from a leg 114 of the fixed base assembly 112 to the rotatable base member 120. A pair of second hydraulic rams 164 extend from the rotatable base member 120 on either side of the first actuator member 140 to a position on the first actuator member 140. A third hydraulic ram 166a extends between the first actuator member 140 and the second actuator member 142. A fourth hydraulic ram 166b extends from the second actuator member 142 to the third actuator member 144. A fifth hydraulic ram 166c extends from the third actuator member 144 to the first end of the fourth actuator member 146 adjacent its pivot connection with the third actuator member 144.

The movement control system 20 is shown in Figures 3a and 3b mounted to a leg 114 of the fixed base assembly 112. In embodiments in which the actuator assembly 130 itself acts as the tension link between the vessel and the structure, the distal end of the actuator assembly 130 is provided with a releasable engagement assembly 40, as shown in Figure 2 and described above. One embodiment of a releasable engagement assembly 170 is shown mounted on the distal end of the actuator assembly 130 in Figures 3a to 3c and described in more detail below with reference to Figure 4.

In embodiments in which the system 2 comprises a tension link 6 and an actuator assembly 8a, the distal end of the actuator assembly is provided with an assembly for engaging with the tension link, such that the tension link can be moved by the actuator assembly.

Turning to Figure 4, there is shown a perspective view of the embodiment of a releasable engagement assembly 170 provided at the distal end 134 of the actuator assembly 130 of Figures 3a to 3c. The engagement assembly 170 is the female part of a male-female engagement system and comprises a generally cylindrical body 172 having a mounting flange 174 disposed at one end for mounting to the end of the fourth actuator member 146. The body 172 has a receiving cavity 176 at its other end, from which extends a frusto-conical guide member 178. A plurality of locking fingers 180 are spaced around the opening of the receiving cavity 176 and extend radially inwards. The locking fingers 180 are each retractable under the action of a respective solenoid 182 acting through a biasing spring 184. The spring 184 biases the respective locking finger 180 into a locking position.

A male engagement assembly 190 comprises a generally circular flange 192, from which extends an elongate engaging member 194 having a spherical cap 196 at its free end. A circumferential groove 198 extends around the engagement member 194 adjacent the flat surface of the spherical cap 196.

One of the female engagement assembly 170 and the male engagement assembly 190 is mounted on the tension link assembly and the other mounted on either the structure 12 or the vessel. In one embodiment, the male engagement assembly 190 is preferably mounted on the structure 12, to be engaged by the female engagement assembly 170 mounted on the tension link assembly extending from the vessel 10. In embodiments in which the tension link assembly is mounted on the structure and to be releasably engaged with the vessel, the male engagement assembly 190 may be mounted on the vessel.

In use, the engagement assembly 170 is manoeuvred close to the male engagement assembly 190 and moved into contact with the male engagement assembly until the spherical cap 196 is received in the receiving cavity 176. The locking fingers 180 engage with the groove 198 in the engaging member 194. To disengage the assembly, the solenoids 182 are activated to withdraw the locking fingers 180 from engagement with the groove 198, allowing the engagement assembly 170 to be withdrawn from the male engagement assembly 190.

Turning to Figures 5a to 5f, there is shown a sequence of stages in an operation to couple and uncouple a vessel 10 with a fixed structure 12 using the system shown in Figure 1 .

Referring to Figure 5a, the vessel 10 is approaching the structure 12. The actuation system 8 and the actuator assembly 8a, in the form of an assembly of the embodiment shown in Figures 3a to 3c, are in a stowed position on the deck of the vessel 10. In addition, a tension link assembly 4 comprising a tension link 6 is also in a stowed position on the deck of the vessel 10.

In the embodiment shown in Figures 5a to 5f, the tension link 6 is in the form of a flexible tether. However, it is to be understood that a rigid tension link may be employed in an analogous manner. The tension link 6 is connected to the vessel at its proximal end 6a. A releasable engagement assembly 40 is mounted to the distal end 6b of the tension link 6.

As shown in Figure 5b, as the vessel 10 approaches the structure 12, the actuator assembly 8a is moved from its stowed position and its distal end engaged with the tension link 6. The actuator assembly 8a can then be used to move the tension link 6 from its stowed position and extended towards the structure 12. With the actuator assembly 8a engaged with the tension link, as the structure 12 is fixed, the movement control system 20 is operated in stabilised mode, such that movement of the distal end 6b and the engagement assembly 40 of the tension link 6 is independent of movement of the vessel.

In cases where the structure 12 is not fixed and may move under the action of the water and/or wind conditions, the movement control system 20 is operated in synchronised mode, such that movement of the distal end 6b of the tension link 6 and the engagement assembly 40 is synchronised with the movement of the structure 12.

With the vessel 10 at the appropriate distance from the structure 12, as shown in Figure 5c, the actuator assembly 8a engages the engagement assembly 40 on the distal end 6b of the tension link 6 with the structure 12. The engines of the vessel 10 are used to move the vessel 10 away from the structure 12, placing the tension link 6 under tension. Any operation, such as the transfer of personnel and/or items, such as tools and equipment, between the vessel 10 and the structure 12 can then be carried out in safety.

During the operation, the actuator assembly 8a may be returned to its stowed position, as shown in Figure 5d.

To uncouple the vessel 10 from the structure 12, with the motion control system in the stabilised mode, the actuator assembly 8a is moved from its stowed position and engaged with the distal end 6b of the tension link 6. The engagement assembly 40 is then released and the vessel 10 is moved away from the structure 12. This is shown in Figure 5e.

The actuator assembly 8a is then used to move the tension link 6 to its stowed position aboard the vessel 10, as shown in Figure 5f. This may be carried out with the movement control system in unstabilised mode. The actuator assembly 8a may then be returned to its stowed position.

Turning to Figures 6a to 6e, there is shown a sequence of stages in an operation to couple and uncouple a vessel 10 with a fixed structure 12 using the system shown in Figure 2, in which the actuator assembly 8a itself acts as the tension link between the vessel and the structure.

Referring to Figure 6a, the vessel 10 is approaching the structure 12. The actuation system 8 and the actuator assembly 8a, in the form of an assembly of the embodiment shown in Figures 3a to 3c, are in a stowed position on the deck of the vessel 10. A releasable engagement assembly 40 is mounted to the distal end of the actuator assembly 8a.

As shown in Figure 6b, as the vessel 10 approaches the structure 12, the actuator assembly 8a is moved from its stowed position and extended forward of the vessel towards the structure 12. As the structure 12 is fixed, the movement control system 20 is operated in stabilised mode, such that movement of the distal end of the actuator assembly 8a and the engagement assembly 40 is independent of movement of the vessel 10.

In cases where the structure 12 is not fixed and may move under the action of the water and/or wind conditions, the movement control system 20 is operated in synchronised mode, such that movement of the distal end of the actuator assembly 8a and the engagement assembly 40 is synchronised with the movement of the structure 12.

With the vessel 10 at the appropriate distance from the structure 12, as shown in Figure 6c, the actuator assembly 8a engages the engagement assembly 40 with the structure 12. The engines of the vessel 10 are used to move the vessel 10 away from the structure 12, placing the actuator assembly 8a under tension to form a tension link between the vessel 10 and the structure 12. Any operation, such as the transfer of personnel and/or items, such as tools and equipment, between the vessel 10 and the structure 12 can then be carried out in safety.

To uncouple the vessel 10 from the structure 12, with the motion control system in the stabilised mode, the engagement assembly 40 is released and the vessel 10 is moved away from the structure 12. This is shown in Figure 6d. The actuator assembly 8a is then moved to its stowed position aboard the vessel 10, as shown in Figure 6e. This may be carried out with the movement control system in unstabilised mode.

Referring to Figure 7, there is shown one embodiment of the present invention, in which a flexible tension link is employed. In the embodiment shown in Figure 7, a vessel 302, having a bow 302a and a stem 302b, is attached to a structure 304 by a tension link, generally indicated as 306. In the position shown in Figure 7, the vessel 302 is applying reverse thrust, thereby applying tension to the tension link in the direction of the arrow A.

The tension link 306 comprises a Nylon rope secured at a first end 308 to a releasable coupling 310 extending from the structure 304. The tension link 306 extends from the structure 304 to the bow 302a of the vessel 302, where it extends through a fairlead 312 mounted at the bow of the vessel 302 and on board the vessel. The second end 314 of the tension link is secured to a bitt 320 mounted adjacent the stem 302b of the vessel 302. As can be seen in Figure 7, the total length of the tension link 306 is significantly longer than the distance between the bow 302a of the vessel 302 and the structure 304, thereby providing a significantly increased capacity for the tension link to accommodate strain energy, and significantly reduce the possibility that the force applied to the tension link by the vessel exceeds the breaking load of the tension link.

It is to be understood that, in an alternative embodiment, the tension link is connected to the structure 304 in a manner analogous to that described above and shown in Figure 7.

Figure 8 shows an alternative arrangement to that of Figure 7. In the embodiment shown in Figure 8, the vessel 302 is again attached to a structure 304 by a flexible tension link 306, connected at a first end 308 to a releasable coupling 310 extending from the structure 304. The embodiment shown in Figure 8 employs a multipoint purchase, generally indicated as 340, mounted to the bow 302a of the vessel 302. The tension link 306 extends through the multipoint purchase 340, through the fairlead 312 at the bow 302a of the vessel 302 and to a bitt 342 mounted in close proximity to the bow 302.

Figure 9 shows a detailed view of the multipoint purchase 340 of the embodiment of Figure 8. The multipoint purchase 340 comprises a housing 350. In the embodiment shown, the housing 350 is generally rectangular in form. The housing 350 has a front end 350a and a rear end 350b. The housing 350 is pivotally mounted to the bow 302a of the vessel 302 by a pivot mount 352. In use, the pivot mount 352 allows the multipoint purchase 340 to be moved between a deployed position, shown in Figure 9, and a stowed position, in which the multipoint purchase extends substantially vertically upwards or downwards from the pivot mount 352.

The housing 350 has a cover, which is shown removed in Figure 9 to reveal the interior of the housing. The housing is provided with a first opening 354 in its front end 350a, through which the tension link 306 extends into the housing. The housing is further provided with a second opening 356 in its rear end 350b, through the tension link extends out of the housing 350.

The multipoint purchase 340 further comprises a plurality of purchase members, in this embodiment pulleys 360a and 360b, arranged in an array within the housing 350. Each pulley 360a, 360b is mounted to the wall of the housing. As shown in Figure 9, the tension link extends around the pulleys 360a, 360b in a convoluted pattern, thereby allowing a significant portion of the length of the tension link 306 to be accommodated within the housing 350. The number and/or arrangement of the pulleys within the housing may be varied to accommodate different lengths of tension link within the housing, as required.

It will be understood that the multipoint purchase 340 may be mounted to the structure 304 and operated in an analogous manner to that described hereinbefore.