EP0076341A1

EP0076341A1 - A single point rigid yoke mooring

Info

Publication number: EP0076341A1
Application number: EP81201122A
Authority: EP
Inventors: Willem Jan Van Heijst; Jacob De Baan
Original assignee: Bluewater Terminal Systems NV
Current assignee: Bluewater Terminal Systems NV
Priority date: 1981-10-07
Filing date: 1981-10-07
Publication date: 1983-04-13
Also published as: NO823349L; BR8205575A

Abstract

A mooring for a vessel (1) floating on the surface (22) of a body of water and anchored to the floor (21) of that body of water having a rotary member (28) attached to a tension riser (6), the rotary member (28) being located above the surface of the body of water and being supported by a remotely positioned submerged buoyancy chamber (4) by means of a support structure (5) and two rigid yokes (2, 3) interconnecting the support structure with the integrated buoyancy chamber and the vessel, is disclosed. The support structure (5) is rotational with respect to the tension riser (6) and the rigid yokes (2,3) are pivotable about horizontal axes at their connection points with the vessel and the support structure with integrated buoyancy chamber. The mooring incorporates means to transfer fluids or gases between a pipeline on the floor of the body of water and the vessel.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to a novel apparatus for mooring a vessel floating on the surface of a body of water to the floor beneath that body of water by means of a single point mooring. More particularly, the present invention relates to an apparatus that comprises of two rigid yokes or frames, pipes or conduits for transferring fluid into and out from a vessel, and a tensioned riser-type anchoring arrangement to moor the vessel permanently at an offshore location in the vicinity of a pipeline or oil field.
For the development of offshore oil fields it is often more economical to store produced oil in the holds of a floating vessel until sufficient oil is produced to enable the economical filling of shuttle vessels, which then transfer the oil stored in the moored vessel to another location. Since the oil production should not be interrupted, the storage vessel is preferably permanently moored. The mooring must have sufficient integrity not to collapse or fail during the most severe sea conditions. In the prior art, most commonly such storage vessels are moored to a single point mooring, allowing the vessels to respond to the combination of wind, waves and current, seeking automatically the position of least resistance.
The most common form of a single point mooring is a buoy floating on the surface of a body of water and anchored to the floor of that body of water at anchor points with a number of anchor chains. A flexible pipe terminates at the buoy and connects the buoy with the pipeline on the floor of that body of water. Typically mounted on top of the buoy is a turntable supported by a bearing. Mounted concentrically with the turntable is a fluid swivel which is coupled to the turntable. The fluid swivel is of course in fluid communication with the terminus of the flexible pipe leading to the source of production. The combination of turntable and swivel can rotate freely around the vertical axis of the buoy.
Normally, the vessel is moored to the buoy with ropes, and a floating hose connects the fluid swivel on the turntable of the buoy with the vessel piping. With such an apparatus, a fluid connection between the pipeline on the floor of the body of water and the vessel piping is established, allowing free rotation of the vessel around the single point mooring without spillage of fluid or gas.
A major problem with this system is the wear and rupture of the floating hoses and ropes; therefore, in some cases the ropes and floating hoses may be successfully replaced by a rigid yoke connection between the vessel at one terminus and the turntable on the buoy at the other terminus, to which the fluid conduits are fixed. Such an arrangement is described in United States Patents No. 3908212 and No. 3823432.
Another prior art approach is disclosed in United States Patents No. 4029039 and No. 4031582. The disclosures describe a rigid yoke single point mooring consisting of a rigid yoke which includes a buoyancy chamber. The rigid yoke can pivot about a horizontal axis at the extremity connected to the vessel. The other extremity of the rigid yoke consists of an attachment point on a tension riser; that is, a rigid, substantially vertical length of pipe or tube maintained under tension. The attachment point of the yoke to the tension riser consists of a universal joint. The lower extremity of the tension riser is connected via a universal joint to an anchor point placed on the floor of the body of water. The riser incorporates an axial rotation point so that the vessel and rigid yoke can freely rotate around the single point mooring. The tension riser is kept under tension by the permanently submerged buoyancy chamber connected to the rigid yoke. The rigid yoke provides a constant vertical force on the riser via the universal joint connection. The resulting horizontal force due to waves, wind and current working on the vessel is the horizontal force component acting on the tension riser connection. The combination of the horizontal and vertical forces acting on the riser determines the inclination of the tension riser, since the riser can only be loaded in tension. A practical application of these systems can be found in the papers presented at the Offshore Technology Conference in Houston - No. OTC.3564 and No. OTC.3142.
In order to limit the riser angles away from the vertical, the tension in the riser must be large. Because the riser is able to withstand large inclinations, relatively large horizontal excursions can be obtained, making this type of rigid yoke single point mooring more suitable in relatively high waves and shallow water bodies. If the buoyancy chamber is connected to the rigid yoke between vessel and riser, the riser attachment point to the rigid yoke can be placed above the surface of the water body, and larger excursions can be facilitated.
This prior art type of rigid yoke single point mooring has certain strong disadvantages. Due to simultaneous inclination of the riser and list of the rigid yoke as a result of the vessel's mooring forces, the universal joint connection between riser and rigid yoke must be capable of rotating through large angles under full load. These angles can reach 60° or more. This requires very complex fluid or gas articulated conduits integrated with the universal joint, or very long flexible hoses. Both solutions involve total exposure to the direct impact of head-on waves, which are very often destructive to the apparatus during storm periods.
The list angle of the rigid yoke can be minimised by increasing the length of the rigid yoke, which results in a heavy structure with large forces on the hinges at the vessel end of the rigid yoke. The inclination angle of the tension riser can be limited by increasing the tension of the riser. The large riser tension requires a strong rigid yoke construction and a large buoyancy chamber.
Since the buoyancy chamber is integrated into the rigid yoke between riser and vessel hinges, the buoyancy force of the chamber which pushes the rigid yoke upwards is divided between the tension riser and the vessel hinges, according to a ratio which is unversely proportional to the horizontal distance between buoyancy chamber and riser and the horizontal distance between buoyancy chamber and vessel hinges. As a result, the buoyancy chamber must be large to provide an adequate riser tension, since part of the buoyancy force of the said buoyancy chamber is lost in unnecessary upward loading of the vessel hinges.
The wave-induced forces on the buoyancy chamber are directly proportional to its displacement, and dynamic loading may be destructive to the apparatus. The single pair of hinges at the vessel end of the rigid yoke must provide all the support needed to counteract the tensional and longitudinal loading on the rigid yoke as a result of wave action and vessel motion.
If this rigid yoke mooring apparatus is installed in deep water the list angles of the rigid yoke become large even under small riser inclination angles, due to the practical limitations of the overall length of the rigid yoke. If the list angle of the rigid yoke increases, the riser tension load decreases, since the horizontal position of the buoyancy chamber shifts towards the vessel hinges and away from the tension riser. To combat this effect an even larger buoyancy chamber must be fitted into the rigid yoke to provide a larger riser tension, and this makes this type of apparatus not very practical in deep water.
It is the general object of the present invention to provide a novel apparatus for mooring a vessel floating on the surface of a body of water to the floor of that body of water which improves the practicability of the tension riser type of rigid yoke single point mooring based on a rigid yoke with integrated buoyancy chamber. It is another object of the present invention to substantially reduce the size of the buoyancy chamber required in prior art installations. It is yet another object of the present invention to substantially reduce any large rotation angles of the universal joint located at the top of the riser, as experienced in prior art installations. It is still yet another object of the present invention to minimise the individual hinge forces at the vessel/rigid yoke interface. Yet another object of the present invention is to minimise the adverse effects of the water depth on the necessary size of the buoyancy chamber.

SUMMARY OF THE PREFERRED EMBODIMENT

OF THE APPARATUS OF THE PRESENT-INVENTION

According to the preferred embodiment of the invention, an apparatus for mooring a vessel floating on the surface of a body of water and anchored to the floor of that body of water is provided, comprising of two rigid yokes which at one end are connected to the vessel by means of two pairs of hinges, usually at vertical intervals, which allow the rigid yokes to pivot about horizontal axes. The other ends of the rigid yokes are connected to an integrated support structure/buoyancy chamber by means of another two pairs of hinges which also allow the rigid yokes to pivot about horizontal axes.
The buoyancy chamber, which is generally submerged, provides an uplifting force that counterbalances the weight of the rigid yokes and the tension load of the riser, which is supported by the support structure at a location clearly above the surface level of the body of water by means of a rotary member and universal joint. The rotary member, or turntable, allows the support structure and buoyancy chamber (and rigid yokes and vessel) to rotate freely about the vertical axis of the tension riser. Generally concentric with the rotary member is a fluid swivel. The tension riser is anchored to the floor of the body of water via a universal joint that allows the riser to incline in any direction. A pipe and flexible hoses, or flexible pipes, are connected to a pipeline on the floor of the water body. These are fitted to the tension riser and terminate at the fluid swivel. The other side of the fluid swivel is connected to the pipeline that is arranged over the rigid yoke towards the vessel connection. Flexible hoses are used to jump the hinges between the support structure and the rigid yoke and between the rigid yoke and the vessel. A number of parallel lines can be assembled according to standard arrangements known to those skilled in the art.
If the rigid yokes are of equal length and if the pairs of hinges are arranged vertically above each other, the buoyancy chamber and support structure will not incline if the vessel is loaded, or if the vessel is drifting away from the mooring point under the influences of waves and wind. As a result, the universal joint which forms the connection between the tension riser and the support structure will rotate in a similar manner as the universal joint located at the lower end of the tension riser. The fitting of the integrated buoyancy chamber/support structure to the rigid yokes, using hinges which freely rotate about horizontal axes, makes sure that the full nett uplift force of the buoyancy chamber is acting on the tension riser, without loading the rigid yokes with an uplift force.
The vertical tension load force component in the tension riser remains basically constant, providing a continuously increasing restoring mooring force as a function of the vessel drift distance away from the neutral mooring point.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives and additional advantages of the present invention will become apparent to those skilled in the art when they consider the following detailed description and accompanying drawings, wherein like elements have been given like numbers, in which:

Figure 1 is an overall three-dimensional view of a preferred embodiment of the apparatus of the present invention.
Figure 2 is a partial-sectional side elevation of the tension riser shown in Figure 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As may be seen in Figures 1 and 2, a vessel (1) is moored to the floor (21) of a body of water (26) by means of a rigid tension riser (6), and support structure (5) with integrated buoyancy chamber (4), and a pair of rigid yokes (2) and (3). This mooring apparatus also comprises a fluid connection (24) connecting the vessel (1) with a fluid pipeline (25) on the floor (21) of the body of water (26).
The tension riser (6) is attached to the floor (21) of the body of water (26) at an anchor structure (10), such as a concrete block which may be piled into the floor (21), or by any other suitable anchoring means designed to resist the horizontal and vertical forces acting upon it. The tension riser (6) is connected to the anchor structure (10) by means of a universal joint (11) which allows inclination of the tension riser (6) in any direction. The fluid pipeline (25) on the floor (21) runs across the anchor structure (10), enters the fluid pipeline (24a) which is raised above the surface of the anchor structure (10), and penetrates the tension riser (6) immediately above the universal joint (11). It then runs upwards through the tension riser (6). A flexible hose (27) is used to jump the universal joint (11).
The tension riser (6) is connected to the support structure (5) with integrated buoyancy chamber (4) by the universal joint (20) and the rotary member (28). This combination allows completely free movement of the integrated buoyancy chamber/support structure (4)(5) with respect to the tension riser (6), including rotation about a vertical axis. This means that the vessel (1) is free to rotate about the tension riser (6) and find the position of least resistance with respect to wind and wave conditions. In detail, it is seen that the rotary member (28) is mounted on the universal joint (20) and the support structure (5) with integrated buoyancy chamber (4) -is rigidly fixed to the rotary member (28).
The support structure (5) with integrated buoyancy chamber (4) includes four rigid struts (23) extending from the rotary member (28) to immediately interior from each of the four corners of the rectangle. The buoyancy chamber(4), normally submerged beneath the water surface (22) and fitted rigidly to the struts (23), is usually of a circular cross-section. The pair of hinges (18)(19) is fitted to the extremities of the said buoyancy chamber (4), the purpose of which is to allow the first rigid yoke (3) to rotate freely about the horizontal axis (T-T). A second pair of hinges (16) (17) is fitted to the upper side of the support structure (5), the purpose of which is to allow the second rigid yoke (2) to rotate freely about the horizontal axis (Z-Z).
The two rigid yokes (2) and (3) connect the support structure (5) with the integrated buoyancy chamber (4) to the vessel (1). Each of the rigid yokes (2) and (3) is connected to the vessel (1) by hinges. The rigid yoke (2) is connected to the vessel by hinges (13) and-(12) to_allow free rotation about the horizontal axis (X-X). The rigid yoke (3) is connected to the vessel by hinges (14) and (15) to allow free rotation about the horizontal axis (Y-Y). The two rigid yokes (2) and (3) are usually strengthened by means of four struts (30) (31) and (32) (33). This arrangement means that motions of the vessel (1) can take place under variable weather and sea conditions, yet the rigid yokes (2) and (3) will remain parallel.
The fluid pipeline (24b) emerges from the tension riser (6) immediately below the universal joint (20). A flexible hose (36) is used to jump the universal joint (20), and the fluid pipeline (24c) then enters the fluid swivel (34) which is concentric with respect to the rotary member (28). This fluid swivel (34) is of a type known to those skilled in the art, and may be capable of carrying several barallel fluid conduits The fluid pipeline (24c) emerges from out of the top of the rotary member (28) and is arranged across the struts (23) and one of the arms of the. rigid yoke (2), ultil it finally reaches the connection point (35) with the vessel (1).

Claims

₁. A mooring for a vessel floating on the surface of a body of water comprising:

- a single anchor point located on the floor of said body of water;

- a rigid tension riser attached to said anchor point and supported by a support structure with integrated buoyancy chamber by means of a swivel member;

- two rigid yokes attached to said support structure with integrated buoyancy chamber by means of hinges pivotable about horizontal axes; said two rigid yokes being connected to said vessel by means of hinges pivotable about horizontal axes;

- means for full rotation of said vessel, said tw: rigid yokes and said support structure with integrated buoyancy chamber around said rigid tension riser about a vertical axis.

2. The mooring of Claim 1 wherein said two yokes can be of different dimensions.

3. The mooring of Claim 2 wherein the horizontal planes of said two yokes may be parallel or otherwise arranged.