GB2087819A

GB2087819A - Durable anchorage of floating structures

Info

Publication number: GB2087819A
Application number: GB8135103A
Authority: GB
Original assignee: Blohm and Voss GmbH
Current assignee: Blohm and Voss GmbH
Priority date: 1980-11-20
Filing date: 1981-11-20
Publication date: 1982-06-03
Also published as: JPS57116821A; NL191283B; NO152495C; DE3043755C2; NL191283C; NL8104908A; GB2087819B; NO813905L; DE3043755A1; NO152495B

Abstract

A durable anchorage for a floating structure on the high sea comprises a single-point swivel member 3 interconnecting under water for relative rotation about a vertical swivel axis a plurality of anchor cables 4 and a plurality of securing cables 2 leading to the structure 1. A method of laying such an anchorage involves dropping four sea anchors and towing the anchor cables towards a central point at which the swivel member 3 is positioned for coupling to the anchor cables. <IMAGE>

Description

SPECIFICATION Durable anchorage of floating structures The invention relates to a durable anchorage of floating structures on the high sea in accordance with the principle of the single point mooring.

Floating structures of concrete or steel for the accommodation of factories, gas liquefying plants or oil production plants must be durably positioned on large water surfaces.

For this there exists the possibility of direct anchorage at a plurality of fixed points which are located in a star-shaped manner relatively to the place of location of the structure. Such a direct anchorage which is fastened to individual locations at the periphery of the structure, does not permit rotation of the floating structure relatively to the fixed points of the anchors.

Another positioning is the "single point anchorage" (single-point-mooring). In a known single point achorage the structure is pivotally connected at the narrow side.

A structure rotates about the anchorage point and thus positions itself always with the narrow side against the attacking waves.

In this way the anchor loads are relatively small.

The "single point anchorages" known heretofore are constructed as follows: Anchor cables radiate from a buoy in a star-like manner in at least 3 directions and extend to the anchor points securely located in the ground. In this way the buoy is securely positioned in all directions.

A rigid connection in the form of a V-shaped support structure is pivotally connected between the buoy and the structure. The pivotal connecting points are constructed at the location on the structure in a pivotal manner, at the location of the buoy in a cardanic manner.

The presence of the support structure avoids that the structure, when swinging about the anchor point, hits against the buoy and thus damages the buoy or the structure.

The invention is based on the problem to improve such a single point achorage to the effect that the buoy and the V-shaped support structure may be omitted.

The principle according to which this problem is solved in accordance with the invention, resides in that the centre of rotation is located under water and below the bottom of the floating structure and therefore cannot collide with the structure during the swinging motion thereof.

The means for the solution, starting from a single point anchorage as described above, reside in that a single point swivel member connects together under water a plurality of anchor cables and, substantially symmetrically thereto a plurality of securing cables leading to the structure rotatable relatively to each other about a vertical swivel axis.

In this case the "symmetrical arrangement" of the anchor cables relatively to the securing cables signifies that the tension forces produced in every direction of attack in the securing cables as well as in the anchor cables are to be distributed as uniformly as possible to the loaded anchor cables and securing cables, respectively.

In a further embodiment of the invention it is provided that a single point swivel member comprises a swivel block which runs in plain bearings which are equipped with high grade steel bushes on bronze bearings.

Furthermore anchors on anchor blocks may be used selectively for the anchorage.

Finally the anchorage may be performed by means of the method steps a to c stated in claim 4.

The drawing illustrates an example of the invention and its use and serves for explanation. There are shown in: Figure 1 a side view of the anchorage of a floating structure by means of a single point swivel member according to the invention, Figure 2 the attachment to the structure of the securing cables, Figure 3 details of the anchor cables and the securing cables, Figures 4a and 4b a diagrammatic drawing of the single point swivel member according to the invention, in a perspective view and in section through the axis of the swivel member, Figures 5a and 5b the geometry of the anchorage according to the invention in longitudinal view and side view of the structure, and Figures 6a - dthe individual steps for performing the anchorage according to the invention.

Figure 1 illugtrates the floating structure 1 with the anchor cables 4, the single point swivel member 3 and the securing cables 2 which, according to Figure 2, are guided through hawse holes 5 and over rollers 6 to the fixing point 7 on the structure 1. The float water line of the structure 1 is denoted by 8, and the sea bottom by 9. The latter is engaged by the anchors V - as illustrated in Figure 3. They are attached to the anchor cables 4 which consist of individual lengths 4a and 4b and which are connected at the other ends to the lower eyes 12 (Figure 4) of the single point swivel member 3.

Figure 3 illustrates in detail the division of the anchor cables 4 and the connecting elements of the anchor cables and securing cables 4 and 2. There signify: I a fork cable sleeve II a swivel cable sleeve with shackle Ill a rail-less chain link IV an anchor swivel cable sleeve V an anchor, and VI a stirrup cable sleeve with shackle.

Figures 4a and b illustrate the single point swivel member 3 with the swivel block 10 and the lower eyes 12 for the connecting elements II of the anchor cables 4 and the swivel pin 11 with the upper eyes 13 for the connecting elements VI of the securing cables 2. The high grade steel bushes are denoted by 14, and the swivel axis is denoted by 15.

Figures 5a and b illustrate the use of the anchorage in calm seas in narrow end view (Figure 5a) and in rough seas in side view (FigureS!'). In calm seas the floating structure 1 is held approximately midway between the anchorage points V and the cables 4 are relaxed and lie for the greater part of their length on the sea bed 9. In rough weather the floating structure 1 is swept to one side, the strain being taken by the corresponding cable 4 which lifts off the sea bed 9 as shown as it is tensioned. At the same time the structure 1 swivels so that the narower side thereof is presented to he force of the wind and the waves.

Figure 6, sketches a) to c), illustrates substantially the method steps a to c of claim 4. Namely sketch a) illustrates two anchors V as end points of a part of the cross-shaped anchorage geometry which consists firstly of four anchor cables 4 and a respective anchorVwhich are marked by buoys 17, as soon as the anchors V have been inserted in the ground by means of the anchor towing ship 16 and the anchor cable portions 4b have been severed, as shown in sketch b).After the spacings between the centre point 18 of the cross-shaped anchorage geometry and the buoys 17 have been measured at the inwards pointing ends of the anchor cable portions 4a, the structure 1 with the securing cables 2 in the hawse holes 5 and the single point swivel member 3, as well as four anchor cable portions 4b the lengths of which correspond to the measured spacings and which likewise are fastened to the swivel member 3, are moved into position - as described at c) in claim 4 and illustrated in sketch c) - and the 150 m long anchor cable portions 4a are take up successively by means of anchor towing vessel 16 over a buoy stand not illustrated, extended and connected to anchor cable portions 4b which are dimensioned in accordance with the measurement.

Sketch d) of Figure 6 illustrates that the anchorage system is capable to anchor the structure 1 securely in every direction. The pontoon may rotate about the achorage point in a manner free of problems.

Claims

1. A durable anchorage for a floating structure on the high sea, comprising a single-point swivel member interconnecting underwaterfor relative rotation about a vertical swivel axis a plurality of anchor cables and a plurality of securing cables leading to the structure.

2. An anchorage as claimed in claim 1, wherein the single point swivel member comprises a swivel blockwhich runs in plain bearings fitted with stainless steel bushes on bronze bearings.

3. An anchorage as claimed in claim 1 or 2, wherein selectively anchors or anchor blocks are used.

4. A method of laying an anchorage according to any one of claims 1 to 3, comprising the following steps: a) a centre point and four end points of a crossshaped anchorage geometry are marked by buoys and an anchor is dropped at each end point; b) the four anchor cables are towed in the direction towards the centre point and uncoupled, the coupling points being marked by buoys; the spacings between the buoys and the centre point of the cross are measured; because of differently long penetration paths of the anchors, different spacings are obtained; c) the floating structure with the swivel member, securing cables and anchor cables extended by four anchor cable portions the lengths of which correspond to the measured spacings is moved into position and the anchor cable portions are coupled to the anchor cables on the sea bed.

5. A durable anchorage for a floating structure on the high sea, substantially as herein described with reference to the accompanying drawings.

6. A method of laying an anchorage for a floating structure on the high sea, substantially as herein described with reference to and as illustrated in the accompanying drawings.