Technical Field

The invention relates to method for connecting or disconnecting a turret mooring on a floating structure. The invention relates, in particular, to the method of connecting or disconnecting a floating power generator, adapted to be moored to the seabed through a disconnectable turret mooring.

The invention also relates to a mooring system for connecting or disconnecting the floating structure to or from the mooring device.

Background Art

In the oil and gas offshore industry disconnectable turret mooring have been in operation for more than 30 years. One of the prior art systems is based on US5456622. This system has been in service around the world since the early 1990’s with very good results. It is based on a conical buoyancy buoy with a turret shaft complete with bearings and mooring line attachments points. The buoy is preloaded into a cavity reverse conical recess in the bottom of the hull with a locking mechanism consisting of two or more locking arms. Dedicated vessels with full DP and buoy pull-in systems operate on these loading buoys and the cost is very high.

This patent application is primarily for the floating structures having a detachable turret mooring similar to the principle disclosed in the US5456622. The invention is particularly advantageous for the floating power generators where there is a demand for much more cost-effective solutions and as little permanent equipment located on the floating power generator (FPG).

The floating power generators are preferable designed with only the necessary equipment installed and have as little extra space as possible.

The main purpose of the invention is to simplify disconnection and reconnection of the floating structure having a detachable turret mooring for towing the floating structure near the shore for service and repair.

This method and mooring system according to the invention will not only save space on the floating structure, but also decrease the cost and time required for the mooring of these structures. It makes it also possible to use the same service vessel and mooring system on a repeated number of floating structures in an efficient way since all necessary equipment are arranged on the service vessel. Summary of invention

The invention relates to a method for connecting a floating structure having a disconnectable turret mooring system, said turret mooring system comprising a turret arranged on the floating structure and a mooring device adapted to be releasably connected to the turret, said method comprising the following sequential steps of

- moving a service vessel comprising a handling frame towards the floating structure,

- connecting the handling frame to the floating structure so that the floating structure and the service vessel are held at a fixed distance along the longitudinal axis B of the handling frame,

- extending a first wire from a winch arranged on the service vessel via a guide pulley towards the mooring device, said guide pulley being attached to the handling frame and/or attached to the floating structure in a fixed position above the turret, -performing the steps of coupling the first wire to the mooring device, lifting the mooring device from a resting position below the sea surface towards the turret and connecting the mooring device to the turret,

-removing the service vessel from the floating structure by disengaging the handling frame from the floating structure. The invention further relates to a method for disconnecting a floating structure having a disconnectable turret mooring system, said turret mooring system comprising a turret arranged on the floating structure and a mooring device adapted to be releasably connected to the turret, said method comprising the following sequential steps of

- moving a service vessel comprising a handling frame towards the floating structure,

- connecting the handling frame to the floating structure so that the floating structure and the service vessel are held at a fixed distance along the longitudinal axis B of the handling frame,

- extending a first wire from a winch arranged on the service vessel via a guide pulley towards the mooring device, said guide pulley being attached to the handling frame and/or attached to the floating structure in a fixed position above the turret,

-releasing the mooring device from the floating structure and lowering the mooring device to a resting position below the sea surface,

-removing the service vessel from the floating structure by disengaging the handling frame from the floating structure.

Preferably, the method further comprises the step of pivoting the handling frame between a position where the longitudinal axis of the handling frame is substantially parallel with the sea surface connecting the service vessel to the floating structure, and to a position where the axis through the longitudinal direction of the handling frame is tilted disconnecting the service vessel from the floating structure

Preferable, the floating structure is a floating power generator.

Preferable, the method further comprising the step of connecting a buoyance member to the mooring device prior to the disconnection of the mooring device from the turret.

Preferable, the method further comprises the step of rotating a first handling frame part connected to the service vessel relative a second handling frame part when connected to the floating structure in order to compensate for rolling motion caused by the sea. Preferable the method further comprises arranging the guide pulley in a center position above the turret arrangement so that the wire is adapted to extend in a substantially vertically arranged line between the guide pulley and the mooring device when the wire is connected to the mooring device.

Preferable, the method further comprises a connection between the service vessel and the floating structure being a ball and socket connection where a second wire is extending from the floating structure connection through the handling frame towards a winch adapted to pull in the second wire.

Preferable, the method is repeatedly operable to a number of different floating power generators.

The invention further relates to a mooring system for connecting or

disconnecting a floating structure to or from a mooring device. The mooring system comprising a service vessel having a handling frame shaped as an elongate member, said handling frame being hingedly attached to the service vessel at a first end, said handling frame is adapted to be removably attached to the floating structure when connecting or disconnecting the mooring device to the floating structure at an opposite second end. Said mooring system further comprising a winch system coupled to the service vessel for lifting or lowering the mooring device to or from the floating structure in order to connect or disconnect the floating structure and the mooring device.

Preferable, the handling frame having a fixed length.

Preferable, the winch system comprising a guide pulley being arranged on the handling frame or on the floating structure at a position such that a wire extending from the winch to the mooring device is adapted to extend in a substantially vertical line between the guide pulley and the mooring line.

Preferable, the handling frame comprising a rotary joint.

Preferable, the handling frame comprising a socket connection or a ball connection arranged at the free end of the handling frame. The socket or ball connection is adapted to mate with an oppositely arranged ball or socket connection at the floating structure. Brief description of drawings

The invention will now be in detail with reference to the accompanying drawings, in which:

Figure 1 shows the mooring device fully disconnected from the floating structure and the service vessel. The mooring device is in this figure floating in the sea in a buoyancy equilibrium point.

Figure 2 shows the service vessel in a position close to the floating structure in a position prior to the connection with the floating structure.

Figure 3 shows the service vessel and the floating structure connected to each other through the handling frame.

Figure 4 shows the winch system attached to the mooring arrangement prior to the disconnection or connection of the mooring device to the

turret of the floating structure.

Figure 5 shows the mooring device disconnected from the floating structure. The arrows P indicates that the mooring device could be lifted towards the floating structure.

Figure 6-11 shows another embodiment of the handling frame according to the present invention. In this embodiment, the handling frame is connected to the turret through a ball and socket.

Detailed description of the invention

Figure 1 shows a floating structure in the form of a floating power generator 10. The floating structure 10 comprises a disconnectable turret arrangement 15.

The turret arrangement 15 comprising a turret 5 attached to the floating structure 10 and a mooring device 8 releasably connected to the turret 5.

The details of the turret 5 and the mooring device 6 is further disclosed with reference to fig. 1 where the details are best shown.

The turret 5 comprising a turret bearing 6a for attachment to the floating structure 10. The turret 5 further comprising a turret connector part 6b.

The mooring device 8 comprising a turntable 7a where mooring lines 11 are permanently attached. The mooring lines 11 further extends to the seabed. The opposite ends of the mooring lines 11 are moored to the seabed in conventional ways. The figure shows two mooring lines 11. There could however be one mooring line 11 or more than two mooring lines 11. All these embodiments being in the ambit of the invention.

The mooring device 8 further comprises a mooring connector part 7b.

The turret connector part 6b is adapted to receive the mooring connector part 7b. The turret connector part 6b and the mooring connector part 7b having locking mechanisms (not shown) adapted to lock the parts 6b, 7b together. The locking mechanism could be a snap connection (not shown) or other suitable connections that could both easy lock the mooring device 8 to the turret 5 in a locked position and at the same time disconnect the mooring device 8 from the turret 5 in a released position.

The mooring device 8 having a winch supporting part 9a, 9b. The supporting part 9a, 9b may comprise a bridle 9a and a support line 9b where a first wire or rope 3 from the winch 2 could be connected to the mooring device 8.

When the mooring device 8 is disconnected from the floating structure 10 or the winch 2, the mooring device 8 could be connected to a pick-up assembly 9c, 9d. The pick-up assembly 9c, 9d comprises a floating pick up buoy 9d and a pick- up line 9c that could be connected to the support line 9b.

A buoyancy member 12 is further attached to the mooring device 8. The figure 5 shows two disconnectable buoyancy members 12 that are attached to the mooring device 8 prior to disconnection from the turret 5. The buoyancy 12 could also be permanently attached to the mooring device 8 as a possible embodiment of the mooring device 8.

The figure further discloses a service vessel 1. The service vessel 1 comprising a winch arrangement 2, 3, 4d and a handling frame 4.

The handling frame 4 is hingedly connected to the service vessel 1 through a connection 4a as a typical A-frame and is known per se.

The handling frame 4 is adapted to rotate around the connection 4a.

The handling frame 4 disclosed in the figure 1 is arranged at the stern part of the service vessel 1. The handling frame 4 may however also be arranged at the bow part of the vessel 1 as a possible embodiment of the invention.

The opposite end of the handling frame 4 is adapted to be releasable connected to the floating structure 10.

The handling frame 4 could have a shape as an elongate member. The handling frame 4 has a fixed length so that when the handling frame 4 is connected to the floating structure 10, the distance between the service vessel 1 and the floating structure 10 remains constant. The handling frame 4 further comprises a rotary joint 4c arranged on the handling frame 4.

The handling frame 4 is thus divided into two parts 4e, 4f one part at each side of the rotary joint 4c. The two parts 4e, 4f are rotatable connected to the rotary joint 4c. This result in that each part 4e, 4f is adapted to move independently due to motion in the sea. The service vessel 1 and the floating structure 10 are thus allowed to move freely in a rolling movement due to wind and waves in the sea.

Due to the fixed length, the service vessel 1 and the floating structure 10 are not able to increase or decrease the distance between them when the handling frame 4 is connected to the floating structure 10. The free end of the handling frame 4 is adapted to be connected to the floating structure 10 through a connection 4b, 10a. The connection 4b, 10a could for instance be performed hydraulically by moving two locking pins (not shown) through the connections 4b, 10a and thereby lock the service vessel 1 and the floating structure 10 together. Other locking mechanism suitable for locking the two parts together could also be used. A locking mechanism adapted to permanently lock or releasable lock two parts together such as the handling frame 4 and the floating structure 10 is known per se.

The rotary joint 4c is in the figures positioned in the middle of the handling frame 4 such that each parts 4e, 4f are substantially of equal length. Other positions are however also possible embodiments of the invention.

The service vessel 1 further comprises a winch 2 and a first wire 3. The first wire 3 is adapted to extend from the winch 2 via a guide pulley 4d before the first wire 3 is connected to the mooring device 8. Alternatively, the first wire 3 is attached to the support line 9b.

The first wire 3 is further extending through the turret 5 before attaching to the mooring device 8, depending on the operation being a connecting operation or a disconnection operation.

The guide pulley 4d is attached to the handling frame 4. The guide pulley 4d could be arranged on a frame or bracket 4g protruding from the handling frame

4 so that the guide pulley 4d is positioned a distance from the handling frame 4.

The bracket 4g could have various designs not only the design shown in the figures. The guide pulley 4d is however preferably positioned a distance above the turret 5. The guide pulley 4d is more preferably arranged so that the first wire 3 that is extending from the guide pulley 4d, through the center of the turret

5 and towards the turret arrangement 15, follows a vertical line from the guide pulley 4d towards the turret arrangement 15. In the most preferable

embodiment, the first wire 3 follows a vertical line that is congruent with a vertical center axis A through the turret arrangement 15, as shown in figure 4.

The following figures 2 - 5 discloses the principle of operation of a

disconnection of the floating structure 10 from a mooring device 8. In the figure 2 and 3, the mooring device 8 is connected to the turret of the floating structure

10.

Figure 2 shows the service vessel 1 in a position prior to the connection with the floating structure 10. The part of the service vessel 1 with the handling frame 4, is positioned nearby the turret mooring arrangement 15 on the floating structure 10. In this position, a longitudinal axis B of the handling frame 4 is arranged in a plane inclined relative the sea surface 13 as shown in the figure 2.

The handling frame 4 is then lowered down so that the longitudinal axis B of the handling frame 4 is arranged in a plane being substantially parallel with the sea surface 12. The handling frame 4 could be lowered down by a hydraulic cylinder or by other equipment suitable for performing the lifting or lowering of the handling frame 4. The handling frame 4 is lowered down towards a connection point 10a on the floating structure 10. The free end of the handling frame 4 is then connected to the floating structure 10 through a connection 4b, 10a. The connection could for instance be performed hydraulic by moving two locking pins (not shown) through the connections 4b, 10a and thereby lock the service vessel 1 and the floating structure 10 together. Other locking mechanism suitable for locking the two parts together could also be used. A locking mechanism adapted to permanently or releasable locking two parts together such as the handling frame 4 and the floating structure 10 is known per se.

In another embodiment, the handling frame 4 is designed as a floating quay or member that could be connected to the floating structure 10 through mooring lines (not shown). The mooring lines could be pulled in by winches arranged on the service vessel (1 ). The rotary joint 4c is an optional feature in this

embodiment.

A connection point 10a where the service vessel 1 is connected to the floating structure 10 is shown in figure 3.

Figure 4 shows the position where the first wire 3 is connected to the mooring device 8 or via the support line 9b. The first wire 3 is pulled out from the winch 2. The first wire 3 is further extending over the guide pulley 4d before the first wire 3 is attached to the mooring device 8.

At the same time, the buoyance member or members 12 is connected to the mooring device 8, if the buoyance member(s) 12 is not permanently installed on the mooring device 8.

The guide pulley 4d has the purpose to be a guide for the first wire 3 between the winch 2 and the turret arrangement 15 to compensate for the shifting direction that the first wire 3 must perform to have a suitable position, both in relation to the winch and the mooring device 8.

Figure 5 shows the following step where the mooring device 8 with buoyance members 12 are detached from the turret 5 and lowered by the winch arrangement 2, 3, 4d. When the mooring device 8 is lowered by the winch 2, the buoyancy takes over the weight. When the mooring device 8 finds its

equilibrium, the first wire 3 is disconnected from the support line 9b. The support line 9b could then instead be connected to the pick-up assembly 9c, 9d. The pick-up assembly 9c, 9d represents a connection between the sea surface 13 and the mooring device 8 when the mooring device 8 is resting in its equilibrium position below the sea surface 12. A pick-up buoy 9d makes sure that the mooring device 8 is easy to find when positioned below sea in this resting position. This provides a quick and easy method for obtaining the mooring device 8 for a reconnection with the floating structure 10.

The support line 9b between the first wire 3 or pick-up assembly 9c, 9d and the mooring device 8, is not a necessary feature of the invention. The first wire 3 or pick-up assembly 9c, 9d may however be directly connected to the mooring device 8 as an alternative embodiment. An ROV is then required to connect the first wire 3 or the pick-up line 9c to the mooring device 8. This is however more expensive than using the permanently installed support line 9b.

The support line 9b facilitate the connection of the first wire 3 and the pick-up assembly 9c, 9d to the mooring device 8. The support line 9b makes the connection possible to perform at sea level instead of at the position where the mooring device 8 is positioned. To retrieve the mooring device 8 from the resting position in the sea, the pick-up buoy 9d may be picked up and the pick- up line 9c is pulled in until it is possible to replace the pick-up 9c line with the first wire or rope 3.

The resting position of the mooring device 8 after the disconnection from the floating structure 10 is shown in figure 1.

After the mooring device 8 is fully disconnected from the floating structured, the support vessel 1 may be detached from the floating structure 10. The floating structure 10 may thereafter be moved or towed to the shore for service or repair.

The service vessel 1 may easily repeat the operation on other similar floating structures 10 since all the necessary equipment to perform the operation are arranged on the service vessel 1.

A similar operation for reconnecting the mooring device 8 to the floating structure 10 could also be performed by the reverse action than the above disclosed operation. This operation is also illustrated by the reverse arrangement of the figures.

The figure 1 shows the start position of the operation where the mooring device 8 is fully detached and positioned remotely from the floating structure 10. The mooring device is in its resting position. In this operation, the vessel 1 is connected to the floating structure 10 as previously disclosed. The mooring device 8 is now floating below the sea surface 13 and is in contact with the sea surface 13 surface through the pick-up buoy 9d and the pick-up line 9c. The first wire 3 from the support vessel 1 is then extended towards the support line 9b or the mooring device 8 in the same way as in the disconnecting operation. The pick-up line 9c is then replaced by the first wire 3 and the mooring device 8 is lifted towards the turret 5 of the floating structure 10. This operation is illustrated in figure 5.

The mooring device 8 is then connected to the turret 5 and the first wire 3 is removed. The buoyance member or members 12 may also be removed at the same time. These operations are shown in figure 4 and 3.

The support vessel 1 is further removed from the floating structure 10 by unlocking the connection between the handling frame 4 and the floating structure 10. The handling frame 4 is then lifted to a position where the axis B through the longitudinal direction of the handling frame 4 is tilted compared to the sea surface 13 as shown in figure 2.

The service vessel 1 has completed the operation and may then move away from the now moored floating structure 10. The service vessel 1 may then move to another floating structure 10 for a connection of disconnection of another mooring device 8 to/from the floating structure 10 as disclosed above.

Figure 6-11 shows a different embodiment of the connection between a handling frame 4’ and the floating structure 10 when disconnecting or

connecting of the turret to the floating structure 10.

In this embodiment the features of the invention that are equal, have the same reference numbers.

The figure 6 shows a handling frame 4’ in an inclined position. This position corresponds to the position shown in figure 2. At the free end of the handling frame 4’, there is arranged a socket 4b’. At the top of the handling frame 4’, there is arranged a guide pulley 4d and a bracket 4g. These features are similar to the features disclosed in relation to the first embodiment. The handling frame 4’ has a shape as an elongate member. The handling frame 4’ is also hingedly connected through a connection 4a to the service vessel 1 in a similar way as in the first embodiment.

The floating structure 10 has a floating structure connection 10a’ formed as a semicircular or circular ball. The connection 10a’, 4b’ is arranged near the turret mooring arrangement 15 as shown in the figure 7. The turret mooring

arrangement 15 is equal to the turret mooring arrangement 15 as described in relation to the first embodiment.

It is a further embodiment of the invention that the socket part 10a’ is attached to the floating structure 10 and the circular or semicircular ball 4b' is attached to the service vessel 1 , ie opposite than the connection part as shown in the figures 6-11.

Figure 7 shows a next step in the connection process. In this position, the handling frame 4’ is lowered so that it is parallel with the sea surface as also described in the first embodiment. A second wire 20 is extended between the connection socket 4b’ and the floating structure connection 10a’. The second wire 20 is fixedly connected to the floating structure 10 at the floating structure connection 10a’. The second wire 20 is then led through the handling frame 4’ and coupled to a winch (not shown) on the service vessel 1 by help of a forerunner (not shown).

The second wire 20 is pulled in by the winch and the two connection parts 10a’ and 4b’ are mated together. This is shown in figure 8 and 9.

Figure 10 and 11 shows the disconnection step where the mooring device 8 is disconnected from the floating structure 10. These steps are equal to the steps disclosed in relation to the first embodiment where the first wire 3 is led through the turret 5 and connected to the mooring device 8. The first wire 3 is extended via the guide pulley 4d as shown in the figures. The mooring device is then led to a resting position equal to the position shown in figure 1.

This embodiment may have a rotary joint (not shown) equal to the rotary joint 4c described in the first embodiment, attached to the handling frame 4’. It is however not a necessary feature as the ball and socket connection 4b’, 10a’ allows the free movement of the floating structure 10 and the service vessel 1’ to compensate for the movement due to waves or wind.

The opposite operation by connecting the mooring device is of course possible by reversing the steps from figure 6-11.

The present invention has been described with reference to preferred

embodiments and aspect thereof and related to the accompanying drawings for the sake of understanding only and it should be obvious to persons skilled in the art that the present invention includes all legitimate modifications within the ambit of what has been described hereinbefore and claimed in the attached claims.