GB1590627A - Anchor - Google Patents

Description

(54) ANCHOR (71) We, SIR ROBERT McALPINE & BR< SONS LIMITED, a British Company, of 40 Bernard Street, London WC1N 1LG, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to anchors.

The traditional forms of steel fluke anchor have been found to be insufficiently reliable and to drag excessively under load when anchoring heavy mooring lines on certain types of soft sea bed, e.g. silt, mud or soft clay.

According to the present invention an anchor is of open-ended box section having upper and lower open ends and defined by peripheral walls of reinforced or prestressed concrete, one of the walls being a front wall whose external face is inclined forwardly and downwardly. In use for anchoring on a soft sea bed the anchor is disposed generally horizontally with the axis of the box section which extends through the open ends thereof being upright, the said external face of the front wall facing directly towards the moored structure; when a load is applied through the mooring line, the said inclined external face acts as a blade so that the anchor cuts into the sea bed to encounter and settle in more substantial bed material, the box section filling with a sea bed dumpling which provides additional anchoring capacity due to its shear resistance.

The inclined external face preferably extends downwardly and forwardly to terminate in a lower cutting edge, preferably projecting below the general level of the remainder of the anchor, this cutting edge assisting the penetration of the anchor into the sea bed.

The currently preferred anchor is of generally rectangular box section in plan view defined by the forwardly and downwardly inclined front wall or blade, a pair of laterally spaced side walls extending rearwardly therefrom, and a rear wall connecting the side walls, but this is not strictly essential and variations in the cross-sectional shape may be made provided that the anchor has its inclined front wall or blade and its hollow section. The precise overall and relative dimensions of any given anchor will depend upon the particular anchoring use to which it is to be put and on the nature of the sea bed where it is to be installed.In any given case the precise shape and dimensions of the anchor will preferably be chosen to provide (a) adequate front wall or blade area to ensure that the anchor cuts into the sea bed efficiently and swiftly whilst developing high shear resistance, (b) adequate stability to ensure its remaining horizontal as it penetrates the sea bed, and (c) adequate area in plan view for the sea bed dumpling contained within the anchor to impart the necessary additional resistance to movement.

The material of which the anchor is fabricated is reinforced or prestressed concrete, the extent of the reinforcement or prestressing being dependent upon the particular requirements for each anchor.

In use, the anchor will be connected to its mooring line or lines by chains or cables, and these may be built into the anchor during its construction-e.g. embedded into the concrete when it is cast. The anchor will normally be positioned at its intended site by heavy lifting gear usually mounted on a floating crane or mooring vessel, and appropriate lifting and lowering bridles, or means for attachment to such bridles, may likewise be cast into the anchor at the time of its construction.

Anchors according to the invention are suitable for the mooring offshore of oil and gas production concrete platforms over soft sea beds where conventional anchors are unsatisfactory. A number of the anchors will normally be disposed around the platform and secured thereto by mooring lines radiating therefrom, the inclined face or blade of each anchor facing along its mooring line to the centre of the mooring pattern. Anchors according to the invention develop their holding capacity with little movement over the sea bed, this being of great importance and advantage in the safe mooring of such large structures as production platforms, e.g.

during their construction.

Currently preferred embodiments of the anchor according to:the present invention will now be described, by way of example only, with reference to the drawings accompanying the provisional specification, in which: Fig. 1 is an isometric view of a preferred form of heavy anchor according to the invention, Fig. 2 is a plan view of the anchor of Fig. 1, Fig. 3 is a side elevation, in section, of the anchor of Fig. 1, and Fig. 4 is an isometric view of a preferred relatively light anchor according to the invention.

Referring firstly to Figs. 1 to 3, the illustrated anchor according to the invention has an inclined front wall or blade 2, laterally spaced side walls or longitudinal members 4 extending rearwardly therefrom, and a lateral member or rear wall 6 connecting the side walls together defining the open box section member constituting the anchor. In the illustrated anchor, the walls 2, 4 and 6 are of prestressed concrete, and are cast integrally with one another. During construction, chains 8 are embedded in the side walls 4, the concrete being cast around them, these chains constituting or being attachable to a lifting bridle by means of which the anchor may be lowered to and placed horizontally on the sea bed by lifting gear at the surface.

Chains 10 similarly embedded in the concrete of the anchor constitute, along with the monkey face 12 to which they are attached and the swivel shackle 14 connected to the monkey face, the mooring bridle for the anchor. The mooring line is connected to the swivel shackle and extends therefrom to the platform or other moored structure.

As indicated most clearly in Fig. 3, the blade 2 extends downwardly to terminate in a cutting edge 3 which projects below the level of the remainder of the anchor. This cutting edge may optionally be extended downwards in structural steelwork.

Anchors of the type shown in Figs. 1 to 3 may for example weigh up to 250 tons.

The construction shown in Fig. 4 is more suitable for lighter anchors, e.g. of 10 or 15 tons. Apart from the form of the side walls 4, which are not cut away as in the anchor of Figs. 1 to 3, and the positioning of the lifting attachments 8 (which may as indicated be constituted by embedded rods rather than chains), the embodiment of Fig. 4 is essentially as described above with reference to Figs. 1 to 3, and like parts are identified by like reference numerals.

WHAT WE CLAIM IS: 1. An anchor of open-ended box section having upper and lower open ends and defined by peripheral walls of reinforced or prestressed concrete, one of the walls being a front wall whose external face is inclined forwardly and downwardly.

2. An anchor according to claim 1 in which the inclined external face terminates in a lower cutting edge.

3. An anchor according to claim 2 wherein the lower cutting edge projects below the general level of the remainder of the anchor.

4. An anchor according to any of claims 1 to 3 which in plan view is of generally rectangular box section defined by the inclined front wall, a pair of laterally spaced side walls extending rearwardly therefrom, and a rear wall connecting the side walls.

5. An anchor according to any preceding claim having a mooring line or lines cast into the concrete thereof.

6. An anchor according to any preceding claim having a lifting and lowering bridle or bridles, or means for attachment thereto, cast into the concrete of the anchor.

7. An anchor substantially as hereinbefore described with reference to Figures 1 to 3 of the drawings accompanying the provisional specification.

8. An anchor substantially as hereinbefore described with reference to Figure 4 of the drawings accompanying the provisional specification.

9. A floating construction moored by one or more anchors according to any one of claims 1 to 8.

10. A floating construction moored by a number of anchors according to any one of claims 1 to 8, the anchors being disposed around the construction and secured thereto by mooring lines radiating therefrom, the inclined face of each anchor facing along its mooring line to the centre of the mooring pattern.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. sea bed, this being of great importance and advantage in the safe mooring of such large structures as production platforms, e.g. during their construction. Currently preferred embodiments of the anchor according to:the present invention will now be described, by way of example only, with reference to the drawings accompanying the provisional specification, in which: Fig. 1 is an isometric view of a preferred form of heavy anchor according to the invention, Fig. 2 is a plan view of the anchor of Fig. 1, Fig. 3 is a side elevation, in section, of the anchor of Fig. 1, and Fig. 4 is an isometric view of a preferred relatively light anchor according to the invention. Referring firstly to Figs. 1 to 3, the illustrated anchor according to the invention has an inclined front wall or blade 2, laterally spaced side walls or longitudinal members 4 extending rearwardly therefrom, and a lateral member or rear wall 6 connecting the side walls together defining the open box section member constituting the anchor. In the illustrated anchor, the walls 2, 4 and 6 are of prestressed concrete, and are cast integrally with one another. During construction, chains 8 are embedded in the side walls 4, the concrete being cast around them, these chains constituting or being attachable to a lifting bridle by means of which the anchor may be lowered to and placed horizontally on the sea bed by lifting gear at the surface. Chains 10 similarly embedded in the concrete of the anchor constitute, along with the monkey face 12 to which they are attached and the swivel shackle 14 connected to the monkey face, the mooring bridle for the anchor. The mooring line is connected to the swivel shackle and extends therefrom to the platform or other moored structure. As indicated most clearly in Fig. 3, the blade 2 extends downwardly to terminate in a cutting edge 3 which projects below the level of the remainder of the anchor. This cutting edge may optionally be extended downwards in structural steelwork. Anchors of the type shown in Figs. 1 to 3 may for example weigh up to 250 tons. The construction shown in Fig. 4 is more suitable for lighter anchors, e.g. of 10 or 15 tons. Apart from the form of the side walls 4, which are not cut away as in the anchor of Figs. 1 to 3, and the positioning of the lifting attachments 8 (which may as indicated be constituted by embedded rods rather than chains), the embodiment of Fig. 4 is essentially as described above with reference to Figs. 1 to 3, and like parts are identified by like reference numerals. WHAT WE CLAIM IS:

1. An anchor of open-ended box section having upper and lower open ends and defined by peripheral walls of reinforced or prestressed concrete, one of the walls being a front wall whose external face is inclined forwardly and downwardly.

2. An anchor according to claim 1 in which the inclined external face terminates in a lower cutting edge.

3. An anchor according to claim 2 wherein the lower cutting edge projects below the general level of the remainder of the anchor.

4. An anchor according to any of claims 1 to 3 which in plan view is of generally rectangular box section defined by the inclined front wall, a pair of laterally spaced side walls extending rearwardly therefrom, and a rear wall connecting the side walls.

5. An anchor according to any preceding claim having a mooring line or lines cast into the concrete thereof.

6. An anchor according to any preceding claim having a lifting and lowering bridle or bridles, or means for attachment thereto, cast into the concrete of the anchor.

7. An anchor substantially as hereinbefore described with reference to Figures 1 to 3 of the drawings accompanying the provisional specification.

8. An anchor substantially as hereinbefore described with reference to Figure 4 of the drawings accompanying the provisional specification.

9. A floating construction moored by one or more anchors according to any one of claims 1 to 8.

10. A floating construction moored by a number of anchors according to any one of claims 1 to 8, the anchors being disposed around the construction and secured thereto by mooring lines radiating therefrom, the inclined face of each anchor facing along its mooring line to the centre of the mooring pattern.