The present invention relates to a vessel for installing a platform on an offshore support structure,

comprising a floating body, a lifting device which is tiltable about a tilting axis with respect to the floating body and a tilting force member for applying a tilting force to the lifting device _f which lifting device is provided with a static part that is tiltable about said tilting axis and a movable part which is movable in vertical direction with respect to the static part.

Such a vessel is known, for example the vessel as described in GB 2 165 188, The known vessel is used for

installing and removing a module on or from, an offshore support structure, also known as float.-over operations. Usually, the offshore support structure is fixed to the seabed. The lifting device of the known vessel is suitable to lift a sea-going platform and set it down, on the support, structure. After the plattorra is seated, upon the support structure the lifting device is lowered further and the vessel leaves the location of the support structure. In practise the floating body will be subject to wave motions at open sea.. As a consequence, the floating body will be moved in vertical and horizontal directions. During transferring the platform to the support structure the platform should be kept in a stable position, with respect to the support structure. This is performed inter alia by compensating the horizontal movements of the floating body through displacing the platform relatively to the floating body by means of a tilting force member comprising a hydraulic cylinder, A disadvantage of the known vessel is that the hydraulic cylinder for horizontal compensation limits a large displacement of the platform in vertical direction.

It is an object of the present invention to provide a vessel which comprises a flexible lifting device in terms of freedom of movement.

This is achieved by the vessel according to the invention of which the tilting force member engages a drive location, at the static part. Under operating conditions the tilting force member can exert a. tilting force on the drive location of the static part of the lifting device so as to tilt the lifting device with respect to the floating device or to limit tilting of the lifting device with respect to the floating device.

The advantage of the vessel according to the invention is that the tilting force member and the displacement of the movable part with respect to the static part are functionally decoupled. As a consequence, the lift height is independent from the tilting force member, which makes the lifting device

flexible in respect of freedom of movement. It is noted that the static part is a static or non-movable portion of the lifting device when considering the static part and the movable part only, bat as described, hereinbefore the static part is tiltable with respect to the floating body.

The movable part is raoved in horizontal direction upon tilting the static part by means of the tilting force member . This provides the opportunity to compensate horizontal movements of the floating body during installing a platform when the platform is supported by the movable part of the lifting device.

In. a preferred embodiment the drive location, is located below the tilting axis. This provides the opportunity to locate the tilting force member at a low position with respect to the lifting device _f for example close to the floating body. This may create a relatively large free space at an upper portion of the lifting device.

In. a practical embodiment a support member for supporting a platform may be pivotaliy mounted to the movable part. The advantage of this configuration is that during a tilting motion of the lifting device the support member may stay in the same position with respect to the platform that is supported .

The static part may be tiltable through a ball joint such that the movable part can be displaced in all horizontal directions. In this case an infinite number of tilting axes extending through the centre of the ball joint are present. The displacement may be supported by a tilting force member which, is adapted such that it can displace the drive location in

different horizontal directions. The tilting force member may comprise a drive unit so as to actively control the force of the tilting force member exerted on the static part. In this case the tilting force member may be called an active member. In practice the drive unit may comprise at least two hydr.au.lic cylinders which are operable in mutually different directions, for example

perpendicular to each other.

Alternatively, the tilting force member comprises a resilient element. In this case the tilting force member may be called a passive member. The resilient element may be a springy a rubber element or the like. The tilting force that is exerted by the tilting force member on the static part is then dependent on inter alia the stiffness of the resilient element.

The static part and the movable part may be formed by a hydraulic cylinder. Preferably; the hydraulic cylinder can be controlled such that the movable part is retracted quickly. This minimizes the risk of a collision between the lifting device and the platform during retracting after the platform has been transferred to the support structure and the vessel is free from. the platform.

The invention will hereafter be elucidated with

reference to the schematic drawings showing an embodiment of the invention by way of example.

Fig. 1 is a perspective view of an embodiment of a vessel according to the invention.

Fig. 2 is a schematic side view of a lifting device of the embodiment as shown in Fig. I.

Fig. 3 is a perspective view of the embodiment as shown in Fig. 2.

Fig. 1 snows an embodiment of a vessel 1 according to the invention. The vessel 1 is used for installing a sea-going platform 2 on an offshore support structure 3. The platform may be a drilling unit, a production unit, a high voltage station or the like. The support structure 3 rests on the seabed, and the vessel 1 including the platform 2 is brought into a position directly over the support structure 3.

The vessel 1 comprises a null or a floating body 4 on which a plurality of lifting devices 5 are .mounted . The lifting devices 5 carry the platform 2. Figs. 2 and 3 show an embodiment of one lifting device 5 in more detail. The lifting device 5 is provided with a static part 6 and a movable part 7, The static part 6 is til table with respect to the floating body 4 about a tilting axis. The static part 6 is tiltable through a ball joint 8. This means that the tilting axis extends in. different

horizontal directions and in fact the ball joint has an infinite number of tilting axes. In this embodiment the bail joint 8 is formed by a spherically shaped member which is supported by spherical portions of supporting elements 9 which, are attached to the floating body 4,

The static part 6 and the movable part 7 are formed, by a hydraulic cylinder. Upon operating the hydraulic cylinder the movable part 7 is moved in vertical direction with respect to the static part 6.

Under operating conditions the lifting device 5 can be tilted with respect to the floating body 4 by means of a tilting force member. In the embodiment as shown the tilting force member comprises two pairs of opposite hydraulic cylinders 10 as a drive unit. Both pairs of cylinders extend perpendicularly with respect to each other. As a. consequence, the movable part 7 of the lifting device can be displaced in all horizontal

directions. Since the movable part 7 is also displaceable in vertical direction with respect to the static part 6 the movable part 7 is displaceable within a three-dimensional coordinate system.

Driven sides of the hydraulic cylinders 10 of the tilting force member pivotally engage a drive location 11. at the static part 6 and stationary sides thereof are pivotally mounted to the floating body 4. In the embodiment as shown in the drawings the drive location 11 is located below the tilting axes of the ball joint 8. The static part 6 functions as a. lever;

depending on the dimensions of the parts of the lifting device 5 the relative displacements of the movable part 7 and the drive location 11 can be selected.

The lifting device 5 is further provided with a support member 12 for supporting the platform 2. The support member is pivotally mounted, to the movable part 7 by means of a bail joint 13, as well, in the embodiment as shown the support member 12 has a substantially flat upper surface which can be kept more or less horizontally when the vessel 1 including the platform 2 is on open sea and the lifting device 5 is operated to compensate for movements of the floating body 4 due to waves.

In an alternative embodiment the tilting force member may function passively. For example, the drive unit or hydraulic cylinders 10 of the embodiment as described hereinbefore may be replaced by one or more resilient elements . A spring or rubber material as a resilient element are conceivable.

The invention is not limited to the embodiment shown in the drawings and described hereinbefore, which may be varied in different manners within the scope of the claims and their technical equivalents. It is conceivable that the drive location is present above the ball joint, for example. Furthermore _f the vessel may comprise a pontoon on which a crane, for example a land crane, is supported through the lifting device such that the position of the crane can be stabilized with respect to an offshore structure that has a fixed position on the seabed-