WO2011042535A1

WO2011042535A1 - Calm buoy

Info

Publication number: WO2011042535A1
Application number: PCT/EP2010/065086
Authority: WO
Inventors: Julien Mazallon; Cédric Stella; Cédric FONTANIEU
Original assignee: Single Buoy Moorings Inc.
Priority date: 2009-10-08
Filing date: 2010-10-08
Publication date: 2011-04-14

Abstract

Mooring buoy (16) comprising a floating body (1) anchored to the sea bed (11) via two or more anchor lines (12, 12', 12' '), the floating body comprising a lower part (13) suitable to be situated below water (20) level and an upper part (18) suitable to be situated above water level, a rotation member (2) rotatably attached to the floating body upper part via a bearing (7), a first conduit (10) extending in a substantial vertical direction from the lower part of the floating body to a swivel (14) on the floating body, along a vertical center line (23) of the floating body, a second conduit (17) extending in a substantial horizontal direction along the rotation member from the swivel to an outboard connecting position (3), a mooring line connector (4) being provided on the rotation member, characterized in that the anchor lines are attached to respective tensioning devices (5,5') at or near a top surface of the floating body upper part, the top surface having a length and a width dimension, the rotation member extending across the top surface (32) from near the connecting position to an access position near a side opposite the connecting position, the width dimension W of the rotation member being smaller than the width dimension of the top surface to allow access to at least one tensioning member via the top surface, for each angular position of the rotation member.

Description

CALM BUOY

FIELD OF THE INVENTION

The invention relates to mooring buoy comprising a floating body anchored to the sea bed via two or more anchor lines, the floating body comprising a lower part suitable to be situated below water level and an upper part suitable to be situated above water level, a rotation member rotatably attached to the floating body upper part via a bearing, a first conduit extending in a substantial vertical direction from the lower part of the floating body to a swivel on the floating body, along a vertical center line of the floating body, a second conduit extending in a substantial horizontal direction along the rotation member from the swivel to an outboard connecting position, a mooring line connector being provided on the rotation member.

Such mooring buoys are well known in the art and can for instance be formed by a catenary anchor leg mooring (CALM) buoy comprising a fixed floating body anchored to the seabed via chain connection means for attaching at least one mooring line to the floating body, a turntable rotatably mounted on the floating body provided with flowline coupling means serving for connection to an offshore structure, and with a mooring platform where mooring lines are received. Alternatively the buoy according to the invention comprises a single point mooring buoy of the wheel and rail type or comprises a turret type buoy. The single point mooring buoys according to the invention may be fixed to the sea bed via anchor lines which may be completely or partly formed by steel or synthetic cables or via synthetic wire rope, or chains or combinations thereof.

CALM buoys are well known systems for importing and exporting of hydrocarbons. They are used for mooring a floating unit, such as a vessel, and for connecting said floating unit via the first and second conduit, mutually interconnected by a swivel, to a submerged pipe structure extending over or into the seabed to any subsea well or the like.

These buoys can be moored in relatively shallow water at near shore locations, often under very harsh environmental (wave and wind) conditions. The use of CALM buoys for offloading crude oil from FPSOs in deep water, is also possible, for example at a number of West of Africa oil fields.

BACKGROUND OF THE INVENTION

CALM buoys are well known as being buoys with buoyancy floating at sea level. Because of said buoyancy floating at sea level, the CALM buoys are affected by severe loads exerted upon the hawser lines that moor the vessel to the buoy by the vessel floating towards and away from the buoy and by forces exerted on the catenary chains anchoring the buoy to the seabed, by the action of waves on the buoyant body of the buoy.

To connect floating units, such as vessels, to a mooring buoy and to a submerged pipe structure extending over or into the seabed to a subsea well or the like, several mooring designs are known from the prior art.

Large semi-submersible mooring buoys are known, for instance from U.S. patent No. 4,784,079. Such large structures are particularly indifferent to the motions of the sea surface because of their size being relatively large compared to the amplitude of the waves. These structures are also indifferent to the motions of a vessel connected thereto because of their size and mass which may be equal or even larger than those of said vessel. In U.S. patent. No. 4,784,079 a rather complex structure for interconnecting the buoyant body and the vessel is shown, primarily to cope with the motions of the floating unit relative to the buoyant body. Said large structures involve high production and maintenance costs.

U.S. patent. No. 4,617,000 discloses a much smaller buoy not being connected rigidly to a vessel, comprising a buoyant body with anchoring chains, a turntable on said body with a mooring point for a vessel. According to said publication the turntable has buoyancy as well and preferably takes part in the water displacement of the buoy. The bearing between turntable and body is preferably above sea level between a central sleeve of the body and the turntable, accessible from above and located in the line from mooring point on the turntable to a chain stopper on the body. The buoy disclosed is a semi-submersible buoy. The structure disclosed in U.S. patent No. 4,617,000 is particularly affected both by the motions of the sea surface because of its size being smaller than or equal to the amplitude of the waves, and by the motions of the vessel interconnected thereto because of its size and mass being smaller than or equal to that of the vessel.

Such motions may still lead to an unexpected overload of the anchor lines or of the connecting arm. In practice, said loads appear to be so high that the maximum allowable tensions in the catenary anchor lines as well as in the connecting arm or in the hinge interconnecting said arm to the buoy, are easily exceeded causing line failure and breaking of the arm or pivot respectively.

When the anchor lines that moor the buoy to the seabed are stretched during use, or after repair and maintenance of the buoy at an onshore dry dock location, followed by reattaching the buoy to its anchor lines at the site of deployment, the anchor lines need to be retensioned. It is an object of the present invention to provide a single point mooring buoy that allows easy access to the anchor lines and which allows easy retensioning. It is a further object to allow easy access to the top part of the buoy with improved safety for personnel. It is another object of the present invention to provide a buoy which is of a lightweight and reliable construction, requiring relatively little maintenance.

The mooring buoy according to the invention aims at minimizing fabrication costs while providing reliable tanker berthing and cargo transfer functions in a safe environment. The mooring buoy of the present invention should allow to employ light weight materials to construct a lighter buoy, a simple design and material more suitable for use in offshore environments.

SUMMARY OF THE INVENTION

Hereto a mooring buoy according to the present invention is characterized in that the anchor lines are attached to respective tensioning devices at or near a top surface of the floating body upper part, the top surface having a length and a width dimension, the rotation member extending across the top surface from near the connecting position to an access position near a side opposite the connecting position, the width dimension of the rotation member being smaller than the width dimension of the top surface to allow access to at least one tensioning member via the top surface for each angular position of the rotation member. Because the rotating deck at the top of the buoy extends only partly across the top surface of the buoy's floating body, the majority of anchor lines are accessible, for instance from a separate work-over vessel orfrom the deck, for different

weathervaning positions of the deck. Hereby safe and easy access to the anchor lines for tensioning is possible. This allows safe and rapid tensioning of the anchor lines upon deployment, during operation or after maintenance of the buoy. The advantages of the buoy design according to the invention are also its light weight and that it is optimised for lower fabrication costs and reduction of the fabrication schedule, also optimised for loading/unloading oil in shallow waters. Moreover as anchoring forces are supported by the overall buoy body, this configuration reduces the global mass of the buoy. Therefore, only one diver is needed to check the chains angle upon installation or after retensioning of the anchor lines.

The rotation member to which the vessel is moored may comprise a personnel platform at the access position. The anchor lines may be accessible for tensioning from the deck position, without the need for personnel to leave the deck and to access the top surface of the buoy with the risk of interference by the rotation member when the vessel that is moored to the buoy weathervanes. A lifting device, such as a winch or a linear jacking device, may be provided on the rotation member of the buoy for lifting the top part of the anchor lines and for tensioning of the anchor lines, preferably at the position of the personnel platform. By rotation of the rotation member relative to the buoy, the lifting device may be positioned successively above each anchor line for tensioning thereof.

In one embodiment, the rotation member is not larger than a quarter of the top surface. In this way a particularly compact and light weight buoy construction is achieved. The buoy body may be of cylindrical shape having a vertical wall, the tensioning devices being situated near a vertical wall of the buoy body for providing a stable mooring configuration.

The mooring lines may at least at their upper parts comprise a chain section, the tensioning device comprising a chain stopper. The chainhawsers may be provided with so-called " flip flap" technology for stoppers located on the buoy body such that chain re-tensioning operations are facilitated.

In a favourable mooring configuration, the mooring line connector extends across the top surface from a mid position of the rotation member to a circumferential mooring position near the perimeter of the top surface. In this way there is sufficient distance between the mooring line connector on the one hand and the personnel platform and the connecting position of the conduit, respectively, on the other hand.

The angle between the second horizontal conduit and a first line connecting the mooring connector and the center line may be less than 90 degrees, whereas the angle between a line connecting the access area and the center line and the first line is larger than 30 degrees, preferably larger than 90 degrees. In this way it can be ensured that the access area is situated downwind from the conduit connection position, such that any fumes which may escape will not reach personnel in the access area.

A preferred construction of a "self-lubricating" bearing allowing low maintenance by having an increased resistance against the harsh marine environment comprises an upper and lower axial slide bearing, and a radial slide bearing situated between the axial bearings, the slide bearings comprising plastic slide padsMaintenance operations on the bushes of the bearing according to the invention are easier as in general just one flange needs to be taken out to be able to lift the turn table, compared with known bearings. Access to bearing slide pads or bushes and replacement thereof is facilitated thereby. BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of a mooring buoy in accordance with the invention will, by way of example, be further elucidated referring to drawings, wherein:

Figure 1 is a side view of a CALM buoy according to the invention,

Figure 2 is a vertical cross-sectional view of the buoy of Figure 1,

Figure 3 is a top view of the buoy of Figure 1, and

Figure 4 is a detailed vertical cross-section view of a bushing element of the rotation member, or turntable, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a side view of a mooring buoy 16, in particular a CALM buoy according to the invention and Figure 2 shows a vertical cross section of the CALM buoy of Figure 1. In these figures, the buoy 16 has a hull or a fixed floating body 1 anchored to the seabed 11 by anchor lines 12, 12'. The upper parts of the anchor lines 12, 12' may comprise respective chain connection means 5 for attaching at least one anchor line 12 via a lower part 13 of the floating body 1. The fixed floating body 1 is circular in plan view (see Figure 3) and has a central moonpool 21, within which a first conduit 10, that is connected to a subsea riser or hydrocarbon transport duct, extends upwards along a vertical center line 23. The central conduit 10 extends to a swivel 14. From the swivel 14 a second, horizontal conduit 17 extends to a flowline connector 3 at an outboard connecting position, serving for connection to an offshore structure. The flowline connector 3 is supported on a rotation member, which is in the particular embodiment of FIG.1 a turntable 2, at an upper part 18 of the buoy 16, situated above water level 20. The turntable 2 is mounted rotatably around the vertical center line 23 on the floating body 1, supported centrally by a self- lubricating bushing system 7.

The floating body 1 also comprises chain connection means 5 for attaching at least one mooring line 12 to the floating body 1. As clearly shown in Figure 2, the chain connection means 5 consists of a stopper having a support flange 5 ' located in the upper part 18 of the floating body 1 in a dry environment for supporting the chain ends of the anchor lines 12 above water level 20. This facilitates chain re-tensioning operations. In fact, as a preferred embodiment, the mooring lines could consist of an upper part made of chain and a lower part made of polyester rope with the chain connection means 5 located above water level which renders the readjustment of the mooring line upper part easier even for deepwater systems. A lifting device, such as a winch 30 is situated on the turntable 2, in the area of the personnel platform 31 for attaching to the upper end of an anchor line which is situated near the lifting device 30. As the length and width dimensions of the turntable 2 are smaller than the top surface 32 of the floating body, the anchor lines 12 are accessible via the top surface 32 of the buoy body 1 at different rotational positions of the turntable 2. Each time, the lifting device 30 can be connected to an upper end of a respective anchor line 12, 12', 12". Moreover, as anchoring forces are taken up by the overall floating body 1, the relatively narrow turntable construction reduces the overall mass of the buoy. It should also be noted that the chain connection means 5 are located in the vicinity of the outer surface wall 15 of the floating body 1.

Figure 3 is a top view of the buoy of Figure 1. The top parts of the anchor lines 12, 12', 12" are accessible via openings 33, 34 and 35 in the turntable 2. In this figure it appears clearly that the turntable 2 that is rotatably mounted on the floating body 1, has a width dimension W that is smaller than the diameter of the top surface 32. The turntable 2 is provided with an area A dedicated to the flowline connector and with another area B dedicated to the mooring line connector 4 situated near the perimeter of the floating body 1, and with an access area C for personnel. The relative position of these areas A, B and C is such that the area B, dedicated to mooring chains connecting a vessel to the buoy 16, is located in between the access area C for personnel and the area A of the flow line connector 3. The access area C may integrate a diver platform and a boat landing area.

By providing the turntable 2 with an area A dedicated to flowline coupling means, with another area B dedicated to the mooring means and an access area C for personnel in such a way that the area A dedicated to flowline coupling is located substantially diametrically opposite the access area C with the area B dedicated to chain connection in between these areas A and C, the access area C is located upwind from the area A such that fumes escaping from the flowline connector 3 cannot reach the personnel in the access area C.

The arrangement on the turntable is such that the three areas A, B and C cover a surface area that corresponds at most to a quarter of the floating body top surface area. The angle a between the center lines defining areas A and B is smaller than 90 degrees, such as for instance 60 degrees, whereas the area β between the center lines defining areas B and C is larger than 90 degrees, such as for instance 150 degrees.

One technical characteristic of an embodiment of a mooring buoy according to the present invention is that the rotating function of the turntable 2 is provided by a bushing system 7 made of a plastic material that offers a lower friction coefficient than bronze and that is more suitable for use in water and in a marine environment, such as for instance a laminated composite material.

As shown in Figure 4, the bushing system 7 is divided into an upper and a lower axial slide bearing 8, 8 ' and an upper and a lower radial slide bearing 9, 9'. According to the present invention, the radial bushing system as well as the axial bushing system is a self-lubricating bushing system comprising pairs of pads of a relatively low coefficient of friction material which slide across each other without requiring external lubrication.

The embodiments shown in FIGs 1 to 4 are limited to a turntable buoy type of mooring buoy. However, it should be noted that the rotation member could also be a rotating part rotating around a geostatic turret in the case of a turret buoy; further the rotation member could also be a rotating bogey platform on wheels i.e. in the case of a wheel and rail buoy.

Claims

1. Mooring buoy (16) comprising a floating body (1) anchored to the sea bed (11) via two or more anchor lines (12, 12', 12" '), the floating body (1) comprising a lower part (13) suitable to be situated below water level (20) and an upper part (18) suitable to be situated above water level (20), a rotation member (2) rotatably attached to the floating body upper part (18) via a bearing (7), a first conduit (10) extending in a substantial vertical direction from the lower part (13) of the floating body to a swivel (14) on the floating body, along a vertical center line (23) of the floating body, a second conduit (17) extending in a substantial horizontal direction along the rotation member (2) from the swivel (14) to an outboard connecting position (3), a mooring line connector (4) being provided on the rotation member (2), characterized in that

the anchor lines (12, 12', 12" ') are attached to respective tensioning devices (5, 5') at or near a top surface (32) of the floating body upper part (18), the top surface having a length and a width dimension, the rotation member (2) extending across the top surface (32) from near the connecting position (3) to an access position near a side opposite the connecting position, the width dimension W of the rotation member (2) being smaller than the width dimension of the top surface (32) to allow access to at least one tensioning member (5, 5 ') via the top surface (32), for each angular position of the rotation member.

2. Mooring buoy (16) according to claim 1 , wherein the rotation member (2) comprises a personnel platform (31) at the access position.

3. Mooring buoy (16) according to claim 2, the rotation member (2) comprising a lifting device (30) at the personnel platform (31) for connecting to the anchor line tensioning devices (5, 5'), for raising or lowering the anchor lines (12, 12', 12").

4. Mooring buoy (16) according to any of the preceding claims, wherein the rotation member (2) is not larger than a quarter of the top surface (32).

5. Mooring buoy (16) according to any of the preceding claims, the floating body (1) being of cylindrical shape having a vertical wall, the tensioning devices (5, 5 ') being situated near a vertical wall of the floating body (1).

6. Mooring buoy (16) according to any of the preceding claims, the upper part of the mooring lines (12, 12', 12") comprising a chain section, the tensioning devices (5, 5 ') comprising a chain stopper.

7. Mooring buoy (16) according to any of the preceding claims, characterized in that the mooring line connector (4) extends across the top surface (32) from a mid position of the rotation member (2) to a circumferential mooring position near the perimeter of the top surface (32).

8. Mooring buoy (16) according to claim 7, wherein an angle (a) between the second horizontal conduit (17) and a first line connecting the mooring connector (4) and the center line (23) is less than 90 degrees, and an angle (β) between a line connecting the access area and the center line (23) and the first line is larger than 30 degrees, preferably larger than 90 degrees, more preferably larger than 120 degrees.

9. Mooring buoy (16) according to any of the preceding claims, wherein the bearing (7) comprises an upper (8) and lower (8') axial slide bearing, and a radial slide bearing (9, 9') situated between the axial bearings (8, 8') the slide bearings comprising plastic slide pads.