GB2123778A - Anchoring assembly - Google Patents

Abstract

An anchoring assembly for maintaining an acceptable position of a floating vessel such as a drilling platform 10 over a fixed point at the sea bottom comprises one or more mooring elements 22 having a positive buoyancy with respect to seawater. Each buoyant element 22 extends in use in a substantially inverted catenary arc from an anchor 24 to the platform 10. The mooring elements 22 provide restoring forces against the forces of wind, wave and current action which tend to offset the floating vessel from the desired mooring point. <IMAGE>

Description

SPECIFICATION Anchoring assembly This invention relates to anchoring systems, and, more particularly, to an anchoring assembly adapted to maintain the positioning of a moored, floating body which is subjected to lateral foces resulting from, for example, high wind, wave and current action.

Floating offshore structures, particularly those used in the drilling and production of fluid fossil fuels from the ocean floor, utilize a number of means for fixing the position of the vessel relative to a structure located on the water bottom such as a well-head, drilling template and the like. Drill ships and, particularly, drilling and production platforms are often held in position by a plurality of radially extending catenaries in the form of anchor chains secured between the bottom of the body of water and the structure of the floating vessel. In a tension leg platform, a plurality of tenstoned mooring legs extend generally vertically from the bottom of the body of water to the floating platform, the legs being maintained in tension by buoyant forces of the platform.In such tension leg platforms, it may be desirable or necessary to further include catenary mooring elements as an additional aid to the maintenance of the platform in its proper position. Catenary mooring elements of this type are described in U.S Patent Nos. 3 822 663, 3 952 684 and 4226555.

U.S. Patent No. 3 295 489 describes a compound catenary for anchorage and/or cable support which is comprised of alternate sets of floating elements and ballast elements interconnected by chain. A sinusoidal connecting assembly allows for a mooring element linkage which can compensate for wide variations in relative depth and extensive movement of a moored body.

The depth of water in which a floating vessel can be moored utilizing a catenary is limited by the extreme weight of a long catenary chain which must be overcome by the buoyancy of the floating vessel. In extremely deep waters, the floating vessel must be of extremely large size in order to be buoyant against the extreme weight of catenary mooring elements used to hold it in position. In shallower water, restoring forces tending to hold a moored vessel in a fixed position against the forces of wind, current and wave action acting against the moored vessel and tending to offset it from its desired position are limited to the weight of the catenary mooring elements or to the tension forces that the structures of the catenary, vessel and/or anchoring means can withstand.Otherwise, complex motor driven thrusters must be employed to maintain the desired position over a well head or a subsea drilling/production template.

Viewed from a first aspect the invention provides an anchoring assembly for connecting between a vessel floating in a body of liquid and a fixed anchor disposed at the bottom of said liquid, the assembly comprising at least one mooring element having a positive buoyancy in said liquid and extending in use in a substantial inverted catenary arc from said anchor to said vessel.

Viewed from a second aspect the invention provides an anchoring system for maintaining an acceptable position of a vessel floating in a body of liquid relative to a fixed point located on the bottom of said body of liquid, comprising a plurality of anchoring means disposed radially around said fixed point on said bottom and a plurality of mooring elements, each mooring element connecting between said floating vessel and one of said plurality of anchoring means wherein at least one of said mooring elements has positive buoyancy in said liquid and extends in a substantial inverted catenary arc from said floating vessel to one of said anchoring means.

A marine anchoring assembly in accordance with the invention enables the restoring forces for the positioning of a moored body two be selected.

Thus, the anchoring assembly may provide greater restoring forces for maintaining the position of a floating vessel against wind, wave and current forces than is possible with a simple catenary mooring system. Furthermore, the anchoring assembly may be used in water of various depths without consideration of the size and/or buoyancy of the moored vessel relative to the weight of the anchoring assembly.

In accordance with a preferred embodiment of the invention, the aforementioned mooring element comprises a chain including a selectively variable number of regularly spaced buoyant links extending from the anchor to the moored vessel.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: FIGURE 1 is a schematic view of an anchoring system utilizing by buoyant mooring elements of the present invention; FIGURE 2 is a fragmented, perspective view with parts shown in phantom of one link used in the buoyant mooring element of the present invention, and FIGURE 3 is a fragmented, perspective view of an alternative chain link structure which may be used in the buoyant mooring element of this invention.

Referring now to the drawings wherein the showings are for the purposes of illustrating the preferred embodiment of the invention only and not for the purpose of limiting same, Figure 1 shows a floating vessel 10 disposed on the surface 12 of a body of liquid 14 such as a lake or ocean, the body of liquid 14 having a bottom 16.

The floating vessel 10 may be a semi-submersible oil drilling/production platform as shown or any of several other vessels which require relatively stable positioning over a fixed point on the bottom 16 such as a drill ship, tension leg platform, and the like.

It is desired to the greatest extent possible, to maintain the floating vessel 10 in position directly above a subsea structure 18 such as a drilling/production template, well head or the like to which an element 20 extends from the floating vessel 10. The element 20 may be any of several elements common in oil drilling or production use such as a drill string, production riser, flow line and the like.

In order to maintain the floating vessel 10 in the desired position above the subsea structure 18, at least one, and usually a plurality of mooring elements 22 are provided which are attached to the floating vessel 10 and extend to an anchor 24 disposed on the seabottom. The anchor 24 may be a temporary anchor which is retrievable or an anchor which is fixed to the seabottom 1 6 such as a cemented structure, piling or other similar structure.

The mooring element 22 comprises a plurality of buoyant links 26 disposed at regularly spaced intervals along the length of the mooring element 22. As can be clearly seen in Figure 1, the buoyant links 26 cause the mooring element 22 to assume the shape of an inverted catenary in an upwardlyconvex arc between the anchor 24 and the floating vessel 1 0.

Also shown in phantom in Figure 1 is a displaced floating vessel 10' which has been moved away from its desired position above the subsea structure 1 8 by any of a number of environmental forces such as wind, wave and/or current action acting in the direction of arrow A.

The lateral displacement a of the displaced floating vessel 10' has caused a condition which is unsuitable for continued operation because of excessive distortion of the element 20'. The unacceptable degree of lateral displacement a of the displaced floating vessel 10' has also caused a pulling of the mooring element 22' in its extent between the fixed anchor 24 and the displaced floating vessel 1 0'. Buoyant forces of the displaced mooring element 22' apply a restoring force to the displaced floating vessel 10'. With proper selection of the mooring element sizes, the restoring force will balance the environmental forces, thus maintaining the displaced vessel 10' at an acceptable position over the subsea structure 1 8.

For purposes of simplicity and clarity, only one buoyant mooring element 22 is shown in the figures. It will be understood however, that several mooring elements are commonly employed disposed radially from the floating vessel 10. Any or all of such radially disposed mooring elements may be a buoyant mooring element 22.

It may be possible, or desirable, particularly in water of shallow depths, to provide both buoyant and normal catenary mooring elements to position the floating vessel 10 over a fixed subsea structure 1 8. In such an arrangement of buoyant and normal catenary mooring elements, the buoyant element would desirably be positioned so that its superior restoring forces would act against prevailing wind, wave and/or current forces which are anticipated to act against the floating vessel 1 0. The increased restoring forces of a buoyant mooring element 22 over that of a normal catenary would assure better positioning of the floating vessel 10 than would be possible with the system in which all mooring elements were of the normal catenary type.Alternatively, all of the radially disposed mooring elements may be of the buoyant type in accordance with this invention.

The buoyant mooring element 22 is made buoyant by providing a number of buoyant links 26 at regularly spaced intervals along its length. It can be seen that the number and spacing of the buoyant links 26 may be varied over a wide range to provide a wide range in the buoyancy of, and thereby the restoring forces induced by the buoyant mooring element 22. The buoyant links 26 may be constructed so as to have increased buoyancy in each link and/or the number of buoyant links 26 in the mooring element 22 may be increased to increase the buoyancy thereof.

The buoyant links 26 may take any of a number of forms one of which is illustrated in Figure 2 in which a linking bar 28 extends through a buoyant body 30. The linking bar 28 includes a linking eye 32 on each end outside the buoyant body 30. In the preferred form shown in Figure 2, the buoyant body comprises a generally spherically shaped, hollow body having a hollow interior space 34 through which the linking bar 28 passes. It will be understood that, while a spherical buoyant body 30 is preferred because of its structural advantages under extreme pressure, other shapes such as ovate, ellipsoidal, cylindrical, or any other three-dimensional hollow form may be used. The interior space 34 may be empty or a cellular structure or material may be provided in the interior space 34 to give added rigidity to the buoyant body 30.

A buoyant body 30 may be constructed of any suitable material which is resistant to the corrosive environment in which it is used and which can withstand the high pressures associated with deep water applications.

As shown in Figure 2, the buoyant link 26 may be connected through its linking eyes 32 to either another buoyant link 26 as shown to the left of the figure or, alternatively, to one or more chain links 36 as illustrated to the right of the figure. It will be further understood that through the use of a number of buoyant links 26 with or without interconnecting chain links 36, the overall buoyancy of the completed mooring element 22 may be varied over a wide range.

Illustrated in Figure 3, is an alternative embodiment of a buoyant link 126 also having linking eyes 132 disposed at the ends thereof in a manner similar to the linking eyes 32 on the linking bar 28 (Fig 2). The buoyant link 126 is generally cylindrical in form having tapered end portions 1 34 tapering from the central cylindrical portion 1 36 to the linking eyes 1 32. The central cylindrical portion 1 36 comprises a generally hollow body having an interior space 1 38 formed by the cylindrical outer walls 140 and a transverse web wall 142 located at the ends thereof.The tapered end portion 1 34 may also enclose a generally conical interior space 144 formed by the transverse web wall 142 and outer conical walls 146 as shown at the right end of the buoyant link 126 in the figure. The hollow interior spaces 138, 144 may be empty as with the interior space 34 of the buoyant body 30 as shown in Figure 2 or, similarly filled with a cellular foam material as desired.

An alternative to the conical tapered end portion 134 of the buoyant link 126 is shown on the left side of Figure 3. In this embodiment, a pair of linking legs 148 extend from the central cylindrical portion 1 36 to the linking eye 32. This embodiment would obviously provide less buoyancy to the buoyant link 126 than if the tapered end portion 1 34 having a closed conical interior space 144 were provided.

From the foregoing, it can be clearly seen that the present invention provides a marine anchoring assembly which results in selectively variable restoring forces for positioning of a moored body 10 which are greater than those of a non-buoyant catenary mooring system. The present invention also provides a mooring assembly which may be used in water of any depth without consideration of the size and/or buoyancy of the moored vessel 10 relative to the weight of the mooring assembly.

It has been shown that in accordance with an embodiment of the invention, a marine anchoring assembly for connecting between vessel 10 floating on the surface 12 of a body of liquid 14 and an anchor 24 disposed at the bottom 1 6 of the body of liquid 14 comprises a mooring element 22 having positive buoyancy in the liquid and extending in a continuous, inverted catenary arc from the anchor 24 to the vessel 10.

It has been further shown that in a preferred embodiment of the invention, the mooring element 22 comprises a chain including a selectively variable number of regularly spaced buoyant links 26, 126 extending along the mooring element 22 from the anchor 24 to the moored vessel 1 0.

The buoyant mooring element 22 used for positioning the floating vessel 10 in the body of liquid 14 may be used in any length, irrespective of the depth of the body of liquid byt any sized floating vessel 1 0. The buoyant mooring element 22 has greater restoring forces for maintaining the position of the floating vessel 10 against forces acting in the direction of arrow A resulting from wind, wave and current action against the floating vessel 10 than is possible with a simple, catenary mooring system.

Claims (6)

1. An anchoring assembly for connecting between a vessel floating in a body of liquid and a fixed anchor disposed at the bottom of said liquid, the assembly comprising at least one mooring element having a positive buoyancy in said liquid and extending in use in a substantial inverted catenary arc from said anchor to said vessel.

2. An anchoring assembly as claimed in claim 1 wherein said mooring element comprises a plurality of regularly spaced, buoyant links.

3. An anchoring assembly as claimed in claim 2 wherein said buoyant links comprise a buoyant body having linking eyes disposed thereon.

4. An anchoring system for maintaining an acceptable position of a vessel floating in a body of liquid relative to a fixed point located on the bottom of said body of liquid, comprising a plurality of anchoring means disposed radially around said fixed point on said bottom and a plurality of mooring elements, each mooring element connecting between said floating vessel and one of said plurality of anchoring means wherein at least one of said mooring elements has positive buoyancy in said liquid and extends in a substantial inverted catenary arc from said floating vessel to one of said anchoring means.

5. An anchoring system as claimed in claim 4 wherein all of said plurality of mooring elements have positive buoyancy in said liquid and each of said mooring elements extend in an inverted catenary arc from said vessel to one of said anchoring means.

6. A marine anchoring assembly substantially as herein described with reference to any of the accompanying drawings.