EP0045762B1

EP0045762B1 - Improved clump weight for anchor lines

Info

Publication number: EP0045762B1
Application number: EP81900437A
Authority: EP
Inventors: Peter Bruce
Original assignee: Brupat Ltd
Current assignee: Brupat Ltd
Priority date: 1980-02-21
Filing date: 1981-02-20
Publication date: 1984-04-11
Also published as: IE810351L; AU547262B2; WO1981002413A1; AU6778881A; NO153127C; NO153127B; NO813521L; IE50734B1; EP0045762A1

Abstract

Anchor line damping device known as a clump weight; the device is attached to the anchor line 6b) and is located on the sea bed upline of the anchor. The clump weight maintains the anchor line horizontal at high line tension but limits excess line tension to avoid cable breakage. The clump weight of the present invention comprises an elongate flexible sheet metal plate (2) on which are mounted suitable weights (7) with cable attachment means (3) at each end of the plate, and this structure provides economies over previous clump weights which comprised multiple strands of heavy chain or a series of articulated weight members.

Description

The present invention relates to a weighting device for marine anchor lines, such a device being generally referred to as a clump weight.
A clump weight is attached to an anchor line a distance upline from the anchor to cancel uplift of the anchor at high line tensions by tending to maintain the anchor line or cable horizontal at the sea bed at high line tensions. The device firstly assists the anchor to give maximum holding performance, and secondly limits excess tensions in the anchor cable to thereby reduce the risk of cable breakage.
One significant area of use of clump weights is in marine guyed towers designed as offshore drilling and production platforms. These tower platforms provide very considerable advantages, particularly as regards costs, over other forms of offshore platform. The guyed tower generally comprises an elongate trussed-box frame, and has a bottom column portion embedded in the sea bed, with twenty or even more guy lines arranged around the tower and attached thereto for lateral support of the tower. The guy lines are attached to the tower at a selected elevation defined by the centre of pressure of the applied forces so that large overturning moments are not transmitted through the structure to the base, and each line is attached to a clump weight located on the sea bed. A further length of chain stretching along the sea bed connect the clump weight to an anchoring device which may be for example a pile or drag-type anchor. The clump weights serve the important function of providing a stiff guying system for normal sea conditions but soften the system during severe storm conditions by raising from the sea bed, thereby providing compliance in the system. The tower is consequently restricted to a sway of around 2° and has a fundamental period greater than the wave period so that the total wave force is not transmitted to the structure supports.
One previous clump weight has taken the form of a single block weight, but this has had the disadvantage of (a) deep embedment in soft bottoms, with high suction resistance to lifting, and (b) creation of shock loads in the cable when pounding on the sea bottom. Alternative known clump weights have been constituted by multiple block weights articulated in tandem and by arrays of parallel chains (sometimes with added weight blocks) examples of which are shown in US-A-3903705. These have had the disadvantage of (a) significant embedment with high suctional resistance to initial lifting, and (b) wear at the articulated joints or at the contact point between chains links. Additionally the chain clump system is costly.
It is an object of the present invention to provide an improved clump weight obviating or mitigating the above disadvantages.
According to the present invention a clump weight device for use with marine mooring lines comprises a heavy bendable elongate member with means at each end of the member for attaching the member to respective sections of a mooring line. Such a description might be applied to the devices of US-A-3903705; however, the invention is characterised in that the elongate member is made from flexible sheet material.
Preferably the clump weight comprises at least one continuous elongate plate. Ater- natively the elongate member can comprise a plurality of successive plate sections rigidly joined together, e.g. by welding.
Preferably the elongate member is capable of being bowed over its length as much as 20° without suffering permanent bending.
Preferably weights are attached to and spaced along the length of the elongate member, and preferably the weights are located on the upper side of said elongate member.
Preferably the elongate member is sufficiently thick to maintain bending moment stresses less than 20 per cent of the yield stress of the sheet material. The sheet material is preferably steel.
By the present invention there is provided a clump weight device which can have very low ground pressure thereby reducing the instance of deep embedment and suction resistance to lifting. The device can raise from the sea bed by a peeling action at high mooring line tensions in storm conditions with a smooth progression of a water wedge between the clump sheet and the sea bed thereby preventing suction effects. Further, there need be no rubbing or articulated surfaces to cause wear and the device can be relatively cheaply manufactured.
An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a plan view of a clump weight device according to the present invention;
Fig. 2 shows a plan view of a detail to a larger scale;
Fig. 3 shows a transverse section of the device;
Fig. 4 shows an end view of a weight used with the device of Fig. 6;
Fig. 5 shows the use of the clump weight device in an offshore guyed tower structure;
Fig. 6 shows a plan view of a modified form of the clump weight device of Fig. 1; and
Fig. 7 shows a side view of the clump weight device of Fig. 6.

A clump weight 1 (Fig. 1) comprises an elongate plate member 2, having spreader plates 3, at each end and holes 4, 5 to receive respective sections 6a, 6b, of a mooring line. Spaced along the length of the member 2 and attached thereto are transverse weights 7. The plate member 2 is designed to have a desired degree of flexure, and can comprise a long continuous plate or alternatively may comprise a plurality of sheet sections 2a (Fig. 2) joined together by welding chevron form indents 8 between successive sections. British Standard 4360 grade 50B steel can be used for the plate member 2.
The weights 7 (Figs. 3, 4) are preferably simple cast iron elements which are attached to the member 2 by bolts 9 the nuts 9a of which can be locked by welding. As will be appreciated other materials could be used for the weights 7, e.g. concrete or steel. The weight 7 could be welded to plate 2.
Fig. 5 shows the above clump weight device 1 used in a guy line 6 of an offshore guyed tower 10: the line section 6a being attached to the tower 10 and the section 6b to an anchoring device 11 e.g. pile or drag anchor. The figure illustrates the clump device 1 peeling from the sea bed 12 with section 6a tensionted during storm conditions to provide a soft compliant system. Normally the device 1 rests fully on the sea bed to provide a stiff mooring system.
For the tower 10, approximately 20 guy lines may be necessary, and the clump devices 1 are designed to give the desired compliance to the system. By way of example, each member 2 could be 45 m. (150 ft.) long by approx 2.5 m. (8 ft.) wide with a thickness of approx. 3.5 cm. (1.38 ins.), and up to 16 weights 7 could be employed. The total weight of the device 1 could be approximately 180 tons (with each weight 7 approx. 6 ton), and this provides a relatively low gravity pressure on the sea bed of approximately 10.7 kN._M-² (1.75 p.s.i.).
The plate member 2 could be made up of separate sheets of a length between 4 m. and 6 m. (13 to 20 ft.). The member 2 could be non- parallel sided e.g. trapezoidal. The weights 7 are spaced along the member 7 to provide optimum clump characteristics and the spacing can be uniform or non-uniform. The weights 7 are preferably identical and located above the member 2 to give a smooth underside to promote water wedge progression during peeling off the sea bed.
In the embodiment shown in Figs. 6 and 7, the weights 7 are not uniformally positioned on the plate member 2 but in this case the spacing (pitch) between successive weights increases towards the anchor end 5 of the member 2. In particular, it is arranged that the spacing pitch varies between groups of weights; thus the group (four in number) nearest the end 4 remote from the anchor has the smallest pitch (1.2 m. (48 ins.) for a clump size as set out above), whilst the pitch of the remaining four groups have increasing pitch (of 1.57 m. (62 ins.); 1.9 m. (76 ins.); 2.25 m. (90 ins.); and 2.6 m. (104 ins.) respectively). This arrangement will give more effective operation of the clump weight for a given number and weight of weight members.
Any known steps can be taken to minimise corrosion of the member in the sea e.g. coating.
The above clump device 1 has the following beneficial characteristics:-

1) Sufficiently flexible to peel progressively from the sea bed to avoid or minimise suction effects and provide an acceptable transition from a stiff to a compliant mooring system;
2) Provides a sufficient weight per unit length to give satisfactory mooring elasticity (i.e. energy absorption);
3) Provides low pressure on the sea bed to minimise embedment of the device into soft soils;
4) Obviate articulations and rubbing surfaces.
5) Relatively cheap to manufacture.

Where a series of plates are used to form the plate member 2, these plates can be arranged in abutting relationship or alternatively could be arranged to overlap at adjacent edges.
Further modifications are of course possible for example, apertures may be provided in the plate to relieve the water weight during plate lifting but it is preferable that these are restricted in number to reduce the risk of the plate member sinking into a soft mud bed.

Claims

1. A clump weight for use with marine mooring lines comprising a heavy bendable elongate member (1), with means (3) at each end of the member (1) for attaching the member to respective sections (6a, 6b) of mooring line, characterised in that the elongate member (1) is made from flexible sheet material.

2. A clump weight device as claimed in claim 1, characterised in that the elongate member (1) comprises at least one continuous elongate plate (2).

3. A clump weight device as claimed in claim 1, characterised in that the elongate member (1) comprises a plurality of successive plate sections (2a, 2b etc) rigidly joined together.

4. A clump weight device as claimed in any one of the preceding claims, characterised in that the elongate member (1) is capable of being bowed over its length as much as 20° without suffering permanent bending.

5. A clump weight device as claimed in any one of the preceding claims, characterised in that weights (7) are attached to and spaced along the length of the elongate member (2).

6. A clump weight device as claimed in claim 5, characterised in that the weights (7) are located on the upper side of said elongate member (2).

7. A clump weight device as claimed in any one of the preceding claims, characterised in that the elongate member (2) is sufficiently thick to maintain bending moment stresses less than 20 per cent of the yield stress of the sheet material (2).

8. A clump weight device as claimed in any one of the preceding claims, characterised in that the sheet material is steel.

9. A clump weight device as claimed in claim 5 or 6, characterised in that the weights (7) are uniformly spaced along the elongate member (1).

10. A clump weight device as claimed in claim 5 or 6, characterised in that the weights (7) are non-uniformally spaced along the elongate member (1).

11. A clump weight device as claimed in claim 10, wherein the spacing of the weights (7) at one end (5) of the elongate member is greater than at the other end (4).

12. A clump weight device as claimed in claim 11, characterised in that the spacing progressively increases from one end (4) of the elongate member (1) to the other (5).

13. A clump weight device as claimed in any one of claims 10 to 12, characterised in that groups of weights are provided (Fig. 6) the spacing of the weights (7) in a particular group being the same but the spacing between the groups varying.