The present invention relates to a vessel for underwater towing of heavy loads, comprising a moon pool connected to a suspension for the load that hangs down in the water, where a lower sheave block is suspended in a lifting wire and is arranged to be fastened to the suspension down in the water. The invention also relates to a method for assembly and underwater towing of heavy loads on a such vessel.

In connection with underwater towing of heavy loads it has been the norm to use cranes, winches and different types of suspensions. In the cases where a lifting wire for the load has run through the moon pool of a vessel and down to the load, some of the problems that have arisen have been that the lifting wire easily comes into contact with the hull opening of the moon pool. As the load is influenced by the resistance to movement by the body of water during towing this can occur, in particular, in the rear hull opening of the moon pool, but could possibly also happen in the forward hull opening. The contact of the lifting wire with the hull opening of the moon pool leads to a number of disadvantages. For one thing, it can be detrimental to the towing itself as the wandering of the lifting wire in the moon pool must be taken into consideration, and there is a notable risk that the hull opening or the lifting wire can be damaged. The latter can clearly be disastrous for the load in the case of the lifting wire breaking.

The invention relates, in particular, to underwater towing of heavy subsea modules for the offshore oil and gas industry, where the modules can be, for example, foundations or templates that are placed on the ocean bed, or frames that are placed on top of existing equipment on the ocean bed. The latter is often used to protect the existing equipment on the ocean bed against other equipment that is lowered in the water in connection with offshore activities, or against damage from, for example, trawl fishing or similar activities, and can be formed with a lattice structure.

A vessel for towing of heavy loads is known from WO 2009/070034 A, comprising a tower that is placed over a moon pool of the vessel. A lifting wire runs from a winch and via the tower down to a snatch block, which is suspended in the tower and which is further connected to a load underneath the vessel.

Consequently, it is an object of the invention to provide a frame for placing in the moon pool of a vessel and which is constructed for towing and heavy lifts so that the above mentioned problems can be reduced or avoided.

A frame according to the invention can be constructed to lift and tow loads up to, or even above, for that matter, 500 tonnes.

The above mentioned objects are achieved with a vessel for underwater towing of heavy loads comprising a moon pool connected with a suspension for the load, where a lower sheave block is suspended in a lifting wire and is arranged to be fastened to the suspension down in the water, where said moon pool is equipped with a frame, where the frame comprises several sheave blocks arranged to regulate and guide the lifting wire for the load, and a loop to the lifting wire runs down and around the lower sheave block where the loop extends from a sheave block at one side of the frame and to a point in the other side of the frame.

Alternative embodiments of the vessel are given in the dependent product claims.

The frame can be arranged to be removed from said moon pool.

The frame can comprise an upper sheave block that the lifting wire runs over, arranged to regulate and guide the lifting wire down through the moon pool.

Furthermore, the frame can comprise a lower and rear sheave block that the lifting wire runs towards, arranged to regulate and guide the lifting wire away from the rear hull opening of the moon pool and a lower and forward point in the form of a sheave block that the lifting wire runs at least partially around, arranged to regulate and guide the lifting wire away from the forward hull opening of the moon pool.

A first part of the lifting wire can run around a sheave block fastened to the deck of the vessel, where said sheave block is mounted aft of the moon pool, and where the lifting wire runs forward to a winch of the vessel. A second part of the lifting wire can run from the lower and forward sheave block and up to a heave compensator.

The heave compensator can be mounted in the frame.

Furthermore, the heave compensator can be made up of equipment mounted on the deck and which is arranged for active or passive heave compensation.

During towing of the load, one or more guide wires can run from an area in or adjoining the bow section of the vessel and down to the load. Said guide wire can be equipped with weights or guide devices.

The frame can be stored in a cradle on the deck of the vessel when not in used. The point in the other side of the frame can be a fastening point for the lifting wire in the frame.

Furthermore, a heave compensator can be placed between the lower sheave block and the suspension for the load.

Said objects are also achieved with a method for assembling and underwater towing of heavy loads that hang underneath a vessel, where the vessel is equipped with a moon pool connected to a suspension for the load, in which the method comprises the following steps: placing of a frame on the deck of the vessel, threading in of a lifting wire from a winch on the vessel and through the frame, where the frame comprises several sheave blocks arranged to regulate and guide the lifting wire for the load, placing of the frame in the moon pool of the vessel, spooling out of the lifting wire with the help of the winch for the formation of a loop that extends from a sheave block in one side of the frame to a point in the other side of the frame, and where the loop runs down and around a lower sheave block, and to fasten the suspension to the load in said lower sheave block.

Alternative embodiments of the method are given in the respective dependent method claims.

The frame can be placed in a cradle on the deck of the vessel during the threading in of the lifting wire and when it is not in use. The lifting wire can be winched out until the loop is sufficiently far down in the water for the load to be fastened to the lower sheave block, and thereafter the lifting wire can be spooled in for the hoisting of the load. One or more guide wires can be mounted from an area in or adjoining the bow section of the vessel and down to the load to avoid rotation of the load during the towing.

An ROV can be used to fasten the suspension to the load and/or to fasten the suspension to the lower sheave block. After the tow has arrived at the place of destination, the load can be lowered with the help of a winch and thereafter the load is disconnected with the help of an ROV.

The invention shall now be explained in more detail with reference to the enclosed figures, in which:

Figure 1 shows a vessel for the towing of a load, seen from the side.

Figure 2 shows the vessel shown in figure 1 seen from the front, seen partly in section.

Figure 3 shows a section of the vessel in figure 2 along the line A-A and shows a first embodiment of the present invention.

Figure 4 shows a principle diagram of a second embodiment of the present invention.

Figure 5 shows a variant of the second embodiment. Figure 1 shows a vessel 10 for underwater towing of a heavy load 12. The vessel can be, for example, a known anchor handling vessel equipped with a moon pool 42. Through said moon pool 42 is a load 12 in the form of, for example, an underwater template. The vessel 10 arrives at the place where the underwater template is made, normally at a workshop, and where it, with the help of cranes or the like, is placed on the ocean bed outside the workshop. An

ROV is normally used to connect a suspension 14 to the load and to connect the suspension to a lifting wire 26 on the vessel 10. When this has been done the load is lifted and the towing can be carried out. On arriving at the

destination the load is lowered onto the ocean bed and released

correspondingly with the help of an ROV.

During the towing, guide wires or cables 16 can be used to prevent rotation of the load 12. The guide wires can be equipped with weights or guide devices 18 to better control the load. For example, the guide devices can function as "trawl doors" if several guide wires are used. To steer the wires out from the side of the ship the guide wires run from an area in or adjoining the bow section 10a of the vessel and down to the load 12.

As mentioned, the lifting wire 26 can damage the hull opening of the moon pool 42, or the lifting wire itself can be damaged, and the present vessel 10 is therefore equipped with a frame 28 that is placed in the moon pool 42. This frame 28 is a mobile moon pool frame equipped with several lifting blocks or sheave blocks which the lifting wire is threaded through. The frame 28 is formed with a shape somewhat smaller than the moon pool opening, and during use can be welded in place, or fastened in a different way. However, the frame 28 is preferably arranged in said moon pool 42 so that it can be removed, even if it is welded in place or fastened in a different way. The frame 28 can be delivered and lifted over onto the vessel 10 lying in a cradle 24. The threading in of the lifting wire 26 is preferably carried out while the frame lies in the cradle and the frame is thereafter hoisted up and placed in the moon pool opening with the help of, for example, the vessel's own crane 20. When the frame 28 is not in use it can be loosened from the moon pool opening and be placed in the cradle 24 while waiting to be used later. Similarly, the cradle with the frame is hoisted on to land or over to another anchor handling vessel. When the frame 28 lies in the cradle 24 it can be securely locked with the help of locking bolts.

In the following a first embodiment of the invention shall be described. The frame 28 is preferably made in a stiff construction and is equipped with several sheave blocks 32,34,36,38. During assembling and threading in of the lifting wire 26, the wire is lifted from the winch 22 of the vessel and optionally back to a sheave block 30 mounted, for example, on the deck 44 aft of the moon pool 42. The sheave block 30 can optionally be welded on and be placed at a correct distance from the winch 22 so that a flow angle is avoided. Alternatively, the lifting wire 26 can be led directly to an upper sheave block 32 in the frame. Thereafter, the lifting wire 26 is led over the upper sheave block 32 mounted in the frame 28 to lead the lifting wire down in the moon pool. A rear and lower sheave block 34 is mounted in a lower area of the frame 28. This rear and lower sheave block has as a task to prevent that the lifting wire 26 comes into contact with the rear hull opening of the moon pool 42. Furthermore, a fastening point or guiding point in the form of a forward and lower sheave block 38 is mounted on the opposite side of the frame 28, which has as a task to prevent that the lifting wire 26 comes into contact with the forward hull opening of the moon pool. In addition to said tasks, the sheave blocks are also arranged to guide and steer the lifting wire 26. From the lower and forward sheave block 38 the lifting wire 26 runs further up to a heave compensator 40 which is mounted in the frame. The heave compensator 40 can be mounted or anchored in lifting- eyes in the frame and is fastened to the lifting wire. Alternatively, the heave compensator can be mounted on the deck (not shown) and be made from equipment arranged for active and passive heave compensation, such as a heave compensating crane equipped for this purpose.

The suspension 14 of the load 12 is fastened to a lower sheave block 36 arranged down in the water, where this sheave block 36 is placed between the two sheave blocks 34,38, i.e. the lower and forward sheave block 38 and the lower and rear sheave block 34. When the lifting wire 26 is winched out, a loop 26a is formed in which the lower sheave block 36 is placed, and when the winch 22 is winched in, the lower sheave block 36 and of course the

suspension with the load 12, are lifted. Thus, the lower sheave block 36 hangs underneath the frame 28 and takes up the weight of the load 12. Furthermore, the lower sheave block 36 is mounted preferably corresponding to what is described when the frame 28 is placed in the cradle 24.

Alternatively, or in addition, a heave compensator, such as the heave compensator 40, can be placed between the lower sheave block 36 and the suspension 14 for the load 12. The advantage with this is, among other things, that the stroke length of the heave compensator can be used in full. A such heave compensator is regarded to be known by a person skilled in the arts and is therefore not described in more detail. This solution can be used in all embodiments of the invention. With the expressions "forward" and "rear" is meant with regard to how the frame is placed in the moon pool and the direction of the vessel. Similarly, with the expressions "wire" and "lifting wire" must be understood any type of wires or cables that can be used for lifting and towing, and with "sheave block" any type of lifting blocks, pulleys, casters, etc., that are used for heavy lifts and towing. For the fastening of the wires and suspension a standard, known shackle can be used. Figures 4 and 5 show examples of further variants of the present invention and which are used corresponding to the use described above.

A corresponding frame 28 that is placed in the moon pool 42 of the vessel is used. The lifting wire 26 can run from the winch 22, back to the sheave block 30 (as shown in figure 5) or alternatively directly (as shown in figure 4), and around the upper sheave block 32 in the frame 28. Thereafter, the lifting wire 26 runs down in the frame 28 and optionally behind a guide disc 35 and thereafter, at least partially, about the lower and rear sheave block 34. Similarly to what has been previously described, a loop 26a is formed down in the water, between a rear and a forward part of the frame 28, and which in this case comprises the rear sheave block 34 and a forward fastening point 46. One can imagine that this forward fastening point can be made up of the described lower and forward sheave block 38, i.e. that the lifting wire runs round the sheave block 38 before it is fastened in the frame 28. As shown in figures 4 and 5 the heave

compensator 40 is placed between the lower sheave block 38 and the suspension 14 for the load 12.