WO2011047736A1

WO2011047736A1 - Method for disconnecting a buoy from a vessel and device for use therewith

Info

Publication number: WO2011047736A1
Application number: PCT/EP2009/063987
Authority: WO
Inventors: Richard Leeuwenburgh; Patrizio Giovanni Matia Ottolini
Original assignee: Bluewater Energy Services B.V.
Priority date: 2009-10-23
Filing date: 2009-10-23
Publication date: 2011-04-28
Also published as: EP2490931A1; RU2012118583A; EP2490931B1; CA2778285A1; CA2778285C; DK2490931T3; RU2508222C2

Abstract

A method is provided for disconnecting a buoy from a turret in an arrangement with a vessel of the type comprising a turret mounted rotatably in the vessel and a buoy supporting risers which is lockable to and disconnectable from the turret. The arrangement further comprises mooring lines connecting the buoy with mooring points on the seabed and loading the buoy with mooring forces. Starting before disconnecting the buoy from the turret the mooring lines are manipulated such that at least during the moment of disconnecting the buoy from the turret the mooring forces are lowered or even substantially eliminated. A hold and release device comprises a housing with at least two distanced mooring line gripper means constructed such that an excess length of mooring line may be positioned therebetween and wherein at least one qripper means is adapted to be brought in a state disengaging the mooring line.

Description

Method for disconnecting a buoy from a, vessel and device for use therewith

The invention firstly relates to a method for disconnecting a buoy from a turret in an arrangement with a vessel of the type comprising a turret mounted rotatably in the vessel and a buoy supporting risers which is lockable to and disconnectable from the turret, the arra.nc7em.ent further comprising mooring lines connecting the buoy with mooring points on the seabed and loading the buoy with mooring forces .

Vessels of the type referred to above are used in offshore oil and. gas fields. Under normal circumstances the turret and buoy are kept in a connected state and the vessel is free to weathervane around, the turret under influence of external factors, such as currents, waves and wind. When the need arises the buoy may be disconnected from the turret and the vessel may travel to a different gripper means. Actual disconnection and (re) connection activities can be achieved in relative limited time, whereas the moment of reconnection and sometimes the moment of disconnection is typically constrained by external factors such as current and waves.

One example of a situation in which it may be required to relocate the vessel is the occurrence of pack ice. In view of the long time needed for reconnecting a buoy to a turret after a disconnect (and the concurrent, loss of pro^¬ duction time) one would like to wait as long as possible with the decision to disconnect. It might happen for example that the ice built up in the end does not. proceed as feared and that, consequently, a disconnect is not needed. The present combination of turret, buoy and mooring lines, however, does not allow to wait too long before taking a decision to disconnect. The pack ice pushing against the vessel creates enormous mooring forces in the mooring lines which, at the moment of disconnecting the buoy from the turret, act on the buoy in such a manner that it may be jammed in the turret and/or that uncontrolled movements of the buoy may occur with the risk of damaging parts of the construction, such as for example the risers.

Therefore it is an object of the present, invention to provide an improved method for disconnecting a buoy from

In accordance with the present invention the method is characterised in that starting before disconnect^¬ ing the buoy from the turret the mooring lines are manipulated such that at least during the moment of disconnecting the buoy from the turret the mooring forces are lowered or even substantially eliminated.

Lowering or even substantially eliminating the mooring forces enables to disconnect, the buoy from the turret without adverse effects, even in situations in which initially high mooring forces are present in the mooring lines. This offers the possibility to delay the moment for deciding to disconnect until, for instance, maximum allow^¬ able loads are present.

In a preferred embodiment of the method according to the present invention the manipulation of the mooring lines comprises allowing the mooring lines to increase their effective length, which is defined as the maximum distance between the buoy and a respective mooring point allowed by a mooring line. During such an increase the mooring forces temporarily are reduced/eliminated.

The increase of effective length may be achieved when, in accordance with a special embodiment, each mooring line extends through a hold and release device and is held thereby at two distanced gripper means with an excess length of mooring line between said two distanced gripper means, and wherein the mooring line for the. increase of its effective length is released at one gripper means for giving out the excess length of mooring line. The hold and release device may be anchored to the seabed, wherein the mooring line for the increase of its effective length is released at the gripper means closest to the buoy. However, as an alternative the hold and release device may be mounted to the buoy, wherein the mooring line for the increase of its effective length is released at the gripper means closest, to the mooring point.

Upon increasing the effective length of the mooring lines the mooring forces are reduced dramatically and the vessel temporarily is free to drift (for example along with drifting pack ice) . This situation will last until the excess length of the mooring lines has been given out. completely. This excess length should be sufficiently long to provide a period of time with reduced mooring forces within which the disconnection between the buoy and turret may be achieved.

In accordance with yet another embodiment of the method according to the present invention the buoy comprises a first, buoy part to which the mooring lines are connected and a second buoy part to which the risers are connected and which is provided with auxiliary moorincj lines, wherein pre^¬ vious to manipulating the mooring lines the second buoy part is released from and lowered relative to the first buoy part .

The main mooring forces occur in the mooring lines and the mooring forces in the auxiliary mooring lines are limited. Therefore the second buoy part (with risers) already may be disconnected when the main mooring forces still are present.

Further it is possible that the connection between the turret and buoy, when connected, is water tight and wherein previous to said manipulation of the mooring lines water is pumped into the turret above the buoy, and preferably until a level above sea level. This water generates a large downward force on the buoy assisting in moving the buoy downward after the disconnection has occurred. Use of this water pressure accelerates the increase in distance between the buoy and the vessel and therefore limits the probability of the buoy and vessel to collide directly after separation from each other.

In a second aspect the invention relates to a hold and release device for use in a method for disconnecting a buoy from a turret in an arrangement with a vessel of the type comprising a turret mounted rotatably in the vessel and a buoy supporting risers which is iockabie to and discon- nectabie from the turret,, the arrangement further comprising mooring lines connecting the buoy with mooring points on the seabed, said device comprising a housing with at least two distanced mooring line gripper means constructed such that an excess length of mooring line may be positioned therebetween and wherein at least one gripper means is adapted to be brought in a state disengaging the mooring line.

The gripper means may be constructed in many dif- ferent ways, for example as hydraulically operated grippers. It should be kept in mind that the mooring forces are trans^¬ mitted through these gripper means which therefore have to withstand, large forces.

Depending on the specific embodiment the hold and release device may be adapted to be mounted to the buoy or may be adapted to be anchored to the sea bed.

Hereinafter the invention will be elucidated while referring to the drawing, in which:

Fig. 1 schematically shows a first embodiment, of an assembly for carrying out a method according to the present invention, and

Fig. 2 schematically shows a second embodiment of an assembly for carrying out a method according to the present invention. Referring to figure 1 a vessel 1 is represented of the type comprising a turret 2 mounted rotatably (by bearings 3 and 4} in the vessel and a buoy 5 supporting ris^¬ ers 6. In a manner known per se (e.g. by means of a tapering top section 7) the buoy 5 is lockable to and disconnectable from the turret 2. There further are provided mooring lines 8 connecting the buoy 5 with mooring points 9 (anchors or piles) on the seabed 10 and loading the buoy with mooring forces .

A hold and release device 11 comprises a housing

12 with at least two distanced mooring line gripper means 13 and 14 (for example hydraulically operated grippers) con^¬ structed such that an excess length 15 of mooring line 8 may be positioned therebetween. By means not shown in detail the device 11 is adapted to be anchored to the sea bed 10 (in an alternative embodiment it could be adapted to be mounted to the buoy 5) . The gripper means 13 is adapted to be brought in a state disengaging the mooring line 8,

It should be noted that such hold and release de- vices 11 may need to be provided in each and every mooring line 8.

The assembly operates as follows. When one desires to disconnect the buoy 5 from the turret 2 under circum^¬ stances with high mooring forces in the mooring lines, the mooring lines, starting before disconnecting the buoy from the turret, are manipulated such that at least during the moment of disconnecting the buoy 5 from, the turret 2 the mooring forces are lowered or even substantially eliminated. This is achieved by allowing the mooring lines to increase their effective length, which is defined as the maximum dis^¬ tance between the buoy 5 and a respective mooring point 9 allowed by a mooring line 8. Such an increase occurs when the gripper means 13 closest, to the buoy disengages the mooring line 8 (for example under remote control from the vessel by an umbilical - not shown -) . The process of disconnecting any piping from the risers 6 within the turret 2 will then very likely already take place and will not be described here because it is con sidered standard practise.

When the gripper means 13 has disengaged, the moor ing line 8 the buoy 5 will be disconnected from the turret (for example by releasing locking mechanisms therebetween) and the buoy will drop from the turret. This disconnecting under reduced mooring forces is possible as long as the ex^¬ cess length 15 of mooring line 8 is being handed out by the hold and release device 11.

A corresponding operation is conceivable in an al ternative embodiment in which the hold and release device 1 would be mounted to the buoy 5 and wherein the mooring line 8 for the increase of its effective length is released at the gripper means closest to the mooring point 9.

It is possible that the connection between the turret 2 and buoy 5, when connected, is water tight. Then one can, previous to said manipulation of the mooring lines and disconnection of the buoy, pump water into the turret 2 above the buoy 5, preferably until a level above sea level. The water pressure then assists in the release of the buoy from the turret.

Reconnecting the buoy 5 to the turret may be achieved in the following manner: the mooring lines 8 firstly are repositioned in the hold and release devices 11 between the gripper means 13 and 14 {with the excess length 15 restored) . For example, a wildcat may be associated with gripper means 13 to pay back released mooring line between gripper means 13 and 14.

A hoisting wire connected to a turret mounted winch (not illustrated) will be lowered and connected to th buoy hovering in the water. The buoy 5 is hoisted with its upper section 7 into a corresponding recess of the turret and is locked. Finally the turret is pumped dry and the ris ers are reconnected to corresponding piping within the turret ,

Figure 2 relates to an alternative embodiment in which the buoy 5 comprises a first buoy part 5 ^f to which the mooring lines 8 are connected and a second buoy part 5" to which the risers 6 are connected and which is provided with auxiliary mooring lines 16. The operation of this alterna^¬ tive embodiment is such that, previous to manipulating the mooring lines 8 as explained above with respect to figure 1, the second buoy part 5" is released from and lowered relative to the first buoy part 5' .

The invention is not limited to the embodiments described before which may be varied widely within the scope of the invention as defined by the appending claims. Thus the hold and release device, for example, might be constructed in such a manner that the respective mooring line is severed, fully in case of an emergency situation, for ex^¬ ample by devising gripper means 14 as a break, element.

Claims

1. Method for disconnecting a buoy from a turret in an arrangement with a vessel of the type comprising a turret mounted rotatably in the vessel and a buoy supporting risers which is lockable to and disconnectable from the turret, the arrangement further comprising mooring lines connecting the buoy with mooring points on the seabed and loading the buoy with mooring forces, characterised in that starting before disconnecting the buoy from the turret the mooring lines are manipulated such that at least during the moment of disconnecting the buoy front the turret the mooring forces are lowered or even substantially eliminated.

2. Method according to claim. 1, wherein the ma^¬ nipulation of the mooring lines comprises allowing the mooring lines to increase their effective length, which is defined as the maximum distance between the buoy and a respective mooring point allowed by a mooring line.

3. Method according to claim 2, wherein each mooring line extends through a hold and release device and is held thereby at two distanced gripper means with an excess length of mooring line between said two distanced gripper means, and wherein the mooring line for the increase of its effective length is released at one gripper means for giving out the excess length of mooring line.

4. Method according to claim 3, wherein the hold and release device is anchored to the seabed and wherein the mooring line for the increase of its effective length is released at the gripper means closest to the buoy.

5. Method according to claim 3, wherein the hold and release device is mounted to the buoy and wherein the mooring line for the increase of its effective length is re^¬ leased at the gripper means closest to the mooring point.

6. Method according to any of the previous claims, wherein the buoy comprises a first buoy part, to which the mooring lines are connected and a second buoy part to which the risers are connected and which is provided with auxiliary mooring lines, wherein previous to manipulating the mooring lines the second buoy part is released from and low^¬ ered relative to the first, buoy part,

7. Method according to any of the previous claims, wherein the connection between the turret and buoy, when connected, is water tight and wherein previous to said manipulation of the mooring lines water is pumped into the turret above the buoy.

8. Method according to claim 7, wherein the water is pumped until a level above sea level.

9. Hold and release device for use in a method for disconnecting a buoy from a turret in an arrangement with a vessel of the type comprising a turret mounted rotatably in the vessel and a buoy supporting risers which is iockable to and disconnectable from the turret, the arrangement further comprising mooring lines connecting the buoy with mooring points on the seabed, said device comprising a housing with at least two distanced mooring line gripper means constructed such that an excess length of mooring line may be positioned therebetween and wherein at least one gripper means is adapted to be brought in a state disengaging the mooring 1ine .

10. Hold and release device according to claim 9, wherein it is adapted to be mounted to the buoy,

11. Hold and release device according to claim 9, wherein it is adapted to be anchored to the sea bed.