WO2005023636A1

WO2005023636A1 - Mobile offshore nit, mooring apparatus, foundation for an offshore vessel, and a method for mooring a vessel

Info

Publication number: WO2005023636A1
Application number: PCT/NO2004/000262
Authority: WO
Inventors: Jan VATSVÅG
Original assignee: Vatsvaag Jan
Priority date: 2003-09-09
Filing date: 2004-09-07
Publication date: 2005-03-17
Also published as: EA200600555A1; NO331994B1; EA008959B1; NO20033979D0; NO20033979L

Abstract

A mobile offshore unit with a vessel (1) and an anchoring includes a mooring apparatus with a receiving area (7) on the vessel (1), shaped as an, in the relation to the sea surface, a horizontal U defining a limited area towards the sea surface. Fenders (2) are placed along the periphery of the receiving area (7) and a foundation (10) for being lowered to the seabed from the vessel (1) forms the anchoring. The foundation is shaped with a cylindrical surface (14) for being received in the receiving area (7) with the fenders (2). A launch system for foundation (10) on the vessel (1) includes a tilt frame (20) supported for being pivoted about a vessel side for launching the foundation (10). A method is also claimed.

Description

Mob i l e offshore unit, mooring apparatus, foundation for an offshore vessel, and method for mooring a vessel.

The present invention concerns a mooring apparatus for a vessel, a movable offshore unit, a foundation, and a method. The mooring apparatus is particularly adapted for use offshore in connection with wells for producing hydrocarbons. The mooring apparatus according to the invention includes a substantially cylindrical foundation anchored to seabed and a receiving area secured to the vessel for receival of and anchoring to the foundation. The anchoring system allows movement of the vessel about, in addition to up and down along this foundation. The method describes how the moveable offshore unit may be moved from one location to another. Furthermore, the invention concerns a moveable offshore unit for a semipermanent, rigid anchoring for use during production of or exploration for hydrocarbons. The unit includes a foundation or a silo adapted to be submerged onto the seabed, and a vessel with equipment to launch and take up the foundation from the seabed, in addition to anchoring the vessel to the foundation when the foundation is placed on the seabed. During exploration for and/or production of hydrocarbons offshore, the vessel must in many cases be anchored to the seabed, and be connected to equipment on the seabed or elsewhere for loading of the hydrocarbons. This anchoring muβt be such that the vessel that is to be anchored can be positioned in relation to waves, ocean currents and wind direction to avoid considerable loads on the mooring. Furthermore the vessel must be able to move vertically to accommodate for movement due to waves, tide and loading or unloading. The vessel must also be connected to equipment for transferral of hydrocarbons in a favourable way. From US Patent 5,447,114, it is shown an example of a mooring system for a tanker where the hydrocarbons can be transferred between the mooring and the ship in the open sea. The mooring is placed by the bow part of the ship and allows pivotal movement of the ship in addition to a certain movement caused by waves and loading. The system shows a subsea buoy anchored such that this can go into corresponding mating elements in the bottom part of the hull. This system, however, requires a fairly complicated installation of a loading buoy and is unsuited for use in shallow water. US 4,927,296 describe a vessel for installation of a platform structure with jacket legs. The vessel is a twin hull vessel with an opening between the 5 hulls on the forward part of the vessel that the structure can be tilted down between. The structure is tilted about a pivot point at one side of the opening. However, the system is not adapted for anchoring of the structure after launch. Accordingly, there is a need for a system or a unit that combines easy launch of a substructure, mooring of the substructure and mooring of the vessel ιo to the substructure, and that is adapted to be used in fairly shallow waters. Shallow waters in this connection are intended to include waters down to a depth of 20-30 meters. Accordingly the present invention concerns a mooring apparatus, a mobile offshore unit, a foundation and a method as defined in the independent i5 claims. The vessel is equipped with a receiving area for receival and mooring of the vessel to the foundation. The area is placed at one of the ends of the vessel, preferably in the bow area of the vessel and is shaped for receival of fenders along the periphery of the area. The area is placed at the end of the 2o vessel to allow the vessel to pivot about the foundation in relation to weather impact and current conditions as a "wind vane". The area can for instance be defined by two projections creating a U-shaped bow on the vessel, or can be made of a discrete structure that can be secured to the vessel as an accessory. This structure can for instance include three beams creating an open u-shaped 25 frame that is equipped with means for secural at the end of the vessel. The frame can in this case be bolted or welded to the vessel. Alternatively, the structure may be hinged, such that it can be tilted up onto the deck when it not is in use. The open end of the frame must include fasteners such that one or several obstruction elements can be secured to these fasteners to secure the 30 frame to the foundation. The frame must also include fender elements between the frame and the foundation. The fenders along the circumference of the area are placed such that these can absorb the forces between the vessel and the silo. The fenders can be revolvably secured to reduce the transferral of torque to the foundation to reduce the abrasion on the fenders. The fenders can revolve about an axis parallel to the foundation, or normal to the foundation. The fenders are preferably made of a tough and elastic material adapted to resist the conditions in questions, for instance a rubber material or an elastomer. The area that is created by two projections and that preferably is shaped as a "U" has an open end the foundation can be led into. When the foundation is led into the receiving area, it is secured to the vessel by leading locking elements with fenders towards the foundation and locking the foundation in a position limited by the size of the receiving area and the locking element or elements. The locking may be achieved by for instance leading a beam over the open end of the U-shaped area, or through claw elements with thereto secured fenders that can be led against the foundation with one or several actuating elements, and can be locked with one or several locking elements. The actuating element(-s) may include hydraulic cylinders, pneumatic cylinders, propulsion screws, rack-and-pinions, or similar driving actuating elements. If claw elements are used, these may be locked in position by locking elements for the claw and may include a locking bar holding the claw in a predetermined locked position. The locking bar may in an alternative embodiment be placed between the claws. The claws will be pivotally secured in the receiving area. Furthermore, the vessel may be equipped with equipment for taking up and launching the foundation to create a mobile unit primarily for use within the area production of hydrocarbons. The equipment may include a skidding track on the deck of the vessel for being able to push or pull the foundation back and forth, means for leading the foundation along the skidding track, to a tilt frame or tilt beam at the edge of the receiving area. The tilt frame is journaled in a point of rotation by this edge, is adapted to temporarily being secured to the silo or foundation, and cooperates with means for controlling the angle of the tilt frame in relation to the vessel the axis of rotation being substantially perpendicular to the longitudinal axis of the barge. The tilt frame can be rotated about the edge, at least 90 degrees by means of hydraulic jacks, winches or other mechanical devices that are adapted to rotate the foundation to a vertical position. The temporary securing of the silo can be performed by fasteners or releasable fastening means such that the foundation can be held in position in relation to the tilt frame during installation and be released only when the load is in the desired position. The releasable fastening means can preferably be controlled electrically or hydraulically. The launch can be performed by letting the tilt frame that is built as hydraulic telescopic beam, lower the inner telescopic beam such that the foundation will skid on the outer beam and down to the seabed. The fasteners that hold the foundation can thereafter be released such that the tilt frame is released and can be rotated back to its initial position. Alternatively, the launching can be performed by means of winches lifting the foundation off the fasteners and lowering the foundation in a controlled manner down to the seabed. Furthermore, the vessel may be equipped with ordinary elements for production of oil and gas, such as separation elements, pumps and valves. The foundation is preferably shaped as a silo with at least a cylindrical part which always will be above the surface for contact with the fenders of the receiving area. Accordingly, the length of the foundation must be such that it can extend from the seabed and to a distance above the sea surface. The foundation is preferably made of steel or concrete and is adapted to be anchored to the seabed, preferably by placement directly onto the seabed. It can be equipped with suction anchors or posts and post hammers for anchoring the foundation to the seabed. The foundation may also be equipped with equipment for adapted for producing hydrocarbons or for search/drilling purposes. The foundation must be dimensioned to resist those loads that are caused by the vessel that is to be anchored thereto. The foundation or silo may be hollow, may contain different equipment and may in some cases be adapted to be emptied foV water such that easy access to the equipment may be achieved. However, the unit or system could also be used for floating foundations that are anchored through anchor chains or similar devices to the seabed, but it is important that the foundation can maintain a substantially stable, vertical axis to allow the anchoring system for the vessel to function properly. The foundation may be equipped with a swivel and a well head if the vessel is to be used for oil production. The swivel allows the vessel to rotate without ruining pipe connections and similar equipment. Alternatively, a "drag chain" system may be used. The vessel is equipped with cranes and similar equipment for all the gear that is to be installed on the silo. When the unit with vessel and foundation shall move to the next location, the vessel itself may lift the foundation and bring it along. If the vessel not is self propelled, assistance from a tug will be required. The unit or system is limited by water depth, and 20 m is considered to be a realistic upper limit due to the forces that will act on the foundation that has to be secured to the seabed and the foundation may become too heavy and bulky. However, there is nothing wrong with dimensioning the unit for use at other depths. The advantage of allowing the vessel to rotate in the wind is that this will substantially reduce the forces acting on the vessel and the foundation. Without turning, the system would not be able to work properly in all the weather conditions in question. If for instance oil is unloaded from the vessel, if the vessel is used as a storage barge, the tankers can lay aside the barge without getting waves in from the side. The vessel can be anchored to the foundation and revolve about the foundation as the wind and weather changes direction, and will be equipped with all necessary equipment for oil production, drilling or similar activities. The vessel can slide up and down vertically along the silo. The vessel can, after installation, be moored to the foundation as a support function to the activities on the foundation or silo, such as a supply ship in connection with drilling operations, storage of oil, for accommodation purposes, etc. The foundation or silo may act as a foundation for a drilling rig. In this connection, it is achieved reductions in connection with drilling equipment because the need for heave compensation or submerged blow-out preventers "BOP" is reduced. The vessel can be equipped with modules to cover several functions, for instance drilling and production. The foundation must be dimensioned to withstand considerable loads, for instance loads from ice present in the northern Caspic Sea during the winter. The cylindrical shape of the foundation is advantageous to resist loads from ice. The receiving area of the vessel can be defined in that one end is equipped with two bow projections such that receiving area is defined between these bow projections. The foundation can be lowered between these projections. The area may typically have a size of 20 m x 30 m. These projections are advantageous because they will define deck area that eases access to the foundation and results in increased buoyancy during launch and lifting of the silo. Short description of the enclosed figures Figure 1 is a perspective view of a vessel, a mooring apparatus and a silo according to the invention; Figure 2 is a partly cut through perspective view of an embodiment of a foundation according to the invention; Figure 3 is a perspective view of a vessel, a mooring apparatus and a foundation according to the invention where a swivel carrier is clearly shown; Figure 4 corresponds to figure 3, but where a swivel assembly on the foundation is clearly shown; Figure 5 is a perspective view in detail of a receiving area of the vessel where fenders are clearly shown; Figure 6 corresponds to figure 5, but where the claw elements are clearly shown; Figure 7 corresponds to figures 5 and 6, but where the vessel is shown moored to a foundation; Figure 8 is a perspective view of how a foundation can be moved in a substantially horizontal position on a vessel; Figure 9 corresponds to figure 8, but where the foundation is in a position during launch to a seabed; and Figure 10 corresponds to figure 9, but where the foundation is placed in a vertical position for further being lowered, placed and secured to the seabed.

Detailed Description of an Embodiment of the Invention Figure 1 shows a vessel according to the invention in form of a barge 1. Barge 1 includes a large deck area with equipment for production of hydrocarbons. The barge 1 includes two projections creating a U-shaped bow 6 defining a receiving area 7 for a foundation 10 shaped as a silo. The foundation 10 creates a mooring for the barge 1. The foundation 10 also acts as a place to install different equipment adapted to the purpose of the combination of the foundation 10 or silo and the vessel or barge 1. The foundation 10 is equipped with a swivel that allows the barge to revolve without pipes and other connections being twisted and ruined. Alternatively, a "drag chain" system may be used. The barge is equipped with cranes and similar contraptions for all the equipment that is to be installed on the foundation. From the figure it is also shown fenders 2 for absorption of forces between the foundation 10 and barge 1 in addition to two claw elements 3 for locking the foundation 10 in the receiving area 7. The mooring apparatus is shown as an integrated part of the barge 1. Figure 2 shows how the foundation can be equipped with suction elements 12 that are provided to allow the foundation 10 to be fixed by suction to the seabed in an ordinary way. The suction elements 12 are connected to a vacuum pump (not shown). The suction elements 12 are adapted for securing the foundation 10 in loose seabed conditions. Furthermore, the foundation is equipped with posts 13 that may be drifted down into the seabed by means of post hammers (not shown) in a conventional way. By means of this equipment, the foundation 10 can be secured to the seabed in hard seabed conditions. In this way, the foundation 10 may be adapted for being placed on seabeds of different kind. The foundation 10 includes a cylindrical outer surface 14 that is adapted to the mooring system with a receiving area, the fenders and the locking elements of the vessel. A swivel 11 is shown for connecting the foundation to the vessel or barge 1. Figure 3 shows how crane equipment on the barge 1 can be used to install equipment on the foundation, for instance a swivel carrier 15. A support vessel is shown moored to the barge, and because the barge will revolve in relation to the direction of the wind and waves, the support vessel lying alongside the barge will also be placed in a favourable position. Figure 4 shows how crane systems on the barge 1 can lift a swivel 11 in place on a foundation 10 and where it is clearly shown how the claw elements 3 with fenders 2 and locking elements 4 for the claw elements 3 hold the barge 1 to the foundation 10. Figure 5 shows a detail of the receiving area 7 of the barge 1 and where the fenders 2, a claw element 3 and an actuating element 5 for the claw 3 are clearly shown. The claw 3 is shown in a retracted position. In this position, the vessel 1 can be released from a foundation, or a foundation can be led into the receiving area 7. From the figures, it is also clearly shown how a U-shaped bow provides a deck area on the side of the receiving area 7. Figure 6 corresponds to figure 5, but shows the claw elements 3 in a position for securing a foundation. From the figure, it is clearly shown a fender fastener or bracket 8 that can be made such that the fenders 2 can revolve about its own axis to prevent transferral of torque between the barge 1 and the foundation. At the same time this will reduce the wear between the fenders and the foundation. Locking elements 4 are placed between rigid points along the peripheral part of the receiving area and the claw elements 3, to hold these elements in a locked position when the barge 1 is to be moored to the foundation 10. Figure 7 corresponds to figure 6 but also shows how a foundation 10 with a cylindrical outer surface 14 is placed in a locked position in relation to the barge 1 by means of claw elements locked by the locking elements 4 for the claw elements. Figures 8, 9 and 10 show in three sequences how a foundation 10 may be placed on a seabed. The lifting of the foundation will be performed as a reversed process of the launch. Figure 8 shows a mobile offshore unit comprising a barge 1 with a tilt frame 20 for launch of a foundation 10. In this position, the foundation 10 is secured to the tilt frame 20 and the foundation 10 is lying in a substantially horizontal freight position such that it is out of the way when the barge 1 is to be moved. The tilt frame 20 includes means for securing and releasing the foundation 10 and a mechanism for moving the foundation 10 in relation to the frame 20. This mechanism may include hydraulically driven telescopic frame elements, screw or rack and pinion systems, or winches. Alternatively, the tilt frame 20 may be equipped with a top frame 21 with a lifting system including winches or hydraulic cylinders secured between the top frame and the holding arrangement 23 for the foundation. ^' The tilt frame 20 with a top frame 21 is pivotally secured to the end of the barge 1 such that the tilt frame 20 with the top frame 21 and the foundation 10 temporarily secured, can tilt about the end edge of the barge 1 and be moved or launched into the water. This tilt can be actuated by moving the centre of gravity of the tilt frame 20 in relation to the pivot point, be actuated by hydraulic cylinders, be actuated by means of winches, or with a combination of these mechanisms. Figure 8 also shows an obstruction beam 22 for the tilt frame 22 holding the tilt frame in place and securing the tilt frame during transport of the barge. Figure 9 corresponds to figure 8 but shows that the obstruction beam 22 for the tilt frame 20 is removed and the tilt frame 20 with the foundation 10 is partly tilted for launch of the foundation 10 and is held by a holding arrangement 23. Figure 10 corresponds to figures 8 and 9, but the tilt frame is tilted to a substantially vertical position for lowering the foundation 10. From figure 10 the holding arrangement 23 for the foundation 10 suspended in the top frame 21 on the tilt frame 20 is also clearly shown. The holding arrangement 23 can be lifted up and down in relation to the top frame 21 for lifting or lowering the foundation 10. In this way the barge can lower or lift a silo 10 from a seabed and bring this along to a new location. The lifting and lowering of the foundation is preferably performed through the receiving area 7 such that the foundation is placed on the seabed and is ready for being anchored. A method for using the mobile offshore unit will include leading the unit to a desired position by tugs or by its own power, removing the obstruction beam 22 from an obstructing position below the tilt beam 20, tilting the tilt beam 20 about the pivotal support at the end of the barge 1 with the foundation 10 while it is secured to the holding arrangement 23, until the foundation 10 is placed substantially vertically, lowering the foundation with the lifting system that may include winches or hydraulic cylinders secured between the top frame and the holding arrangement 23 for the foundation, anchoring the foundation 10 to the seabed by means of posts 13 or suction anchors 12, securing barge 1 to the foundation 10 by activating the claw elements 3 with the actuation elements 5, locking the claw elements 3 with the locking elements 4 to moor the barge 1 to the foundation 10, and releasing the holding arrangement 23 from the foundation 10. Following this procedure, different equipment may be installed on the foundation 10 and various connections between the foundation 10 and the barge 1 can be connected. When the mobile offshore unit is to be moved, this method is reversed, and the unit can be moved to a new location.

Claims

P a t en t Cl a i m s :

1. A mooring apparatus adapted to be placed at an end of a floating vessel (1), with two projections (6) creating a substantially inner U-shaped receiving area (7) with sides, an open end and fender elements (2) c h a r a c t e r i s e d i n that the fender elements (2) are placed alongside all the sides of the inner U- shaped receiving area (7) such that the apparatus is adapted for receiving and mooring to a substantially cylindrical foundation body (10) anchored to a sea- or ocean bed and extending a height above a sea surface; and at least one obstruction element at the open end of the receiving area (7) with further fender elements, for releasable and pivotal securing of the cylindrical foundation body to the apparatus.

2. The mooring apparatus according to claim 1 , wherein the apparatus forms an integrated part of the bow or stern of the floating vessel (1).

3. The mooring apparatus according to claim 1 , including two obstruction elements and wherein these includes two claw elements (3) that in one end are pivotally journaled in the internally U-shaped receiving area (7) and an actuator element (5) for actuating each of the claw elements (3) inn and out of a locking position with the cylindrical body (10).

4. The mooring apparatus according to claim 1 , wherein the fender elements (2) are cylindrical, are pivotally journaled and are secured with a central axis substantially parallel with the central axis of the foundation body (10) during mooring.

5. The mooring apparatus according to claim 1 , wherein the fender elements (2) are cylindrical, are pivotally journaled and are secured with a central axis substantially perpendicular to the central axis of the foundation body (10) during mooring.

6. A foundation (10) for an offshore vessel (1 ) for placement on a seabed, c h a r a c t e r i s e d i n that it further includes: a silo adapted to extend from the seabed and to a height above a sea surface, with at least a part with a cylindrical outer surface (14) along the height above the sea surface when the foundation (10) is placed on the seabed; and posts (13), post hammers and suction anchors (12) with suction pumps for anchoring the foundation (10) to the seabed.

7. A mobile offshore unit including a vessel (1) and a mooring for mooring the vessel to a seabed, c h a r a c t e r i s e d i n that it further includes: a substantially cylindrical foundation (10) with a length such that the foundation can be placed on the seabed and extend a distance above the seabed, and adapted to be lowered onto the seabed from the vessel (1); a mooring apparatus on the vessel, with a receiving area (7) for the foundation (10) formed as a, in relation to a sea surface, horizontal U with an inner periphery and sides that define a limited area of the sea surface when the unit is launched and with fenders (2) placed along the periphery of the receiving area (7); a launching system for the foundation (10) on the vessel (1) with a tilt frame (20) supported for being pivoted about a vessel side from a substantially horizontal freight position to a substantially vertical position for placing and tilting the foundation (10), means for moving the foundation (10) in relation to the tilt frame (20), a holding arrangement (23) for holding the foundation (10) to the tilt frame (20) and means for locking the tilt frame (20) in the substantially horizontal freight position.

8. The mobile offshore unit according to claim 7, wherein the means for locking the tilt frame (20) includes an obstruction beam (22) for being placed below one of the ends of the tilt frame (20) and securing the tilt frame (20) when it is in the substantially horizontal freight position.

9. A method for mooring a vessel, with a unit according to claim 7, c h a r a c t e r i s e d i n that it includes: