Ships – Anchor
Patent
1994-08-19
1995-12-12
Basinger, Sherman
Ships
Anchor
114301, 114304, B63B 2120
Patent
active
054740150
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to drag embedment marine anchors.
DESCRIPTION OF THE PRIOR ART
A requirement of a drag embedment marine anchor comprising a fluke attached to a shank is an ability to dig deeply into a mooring bed. The holding capacity is directly related to depth of embedment below the surface of the mooring bed. The ability to dig into the mooring bed soil depends on the anchor having a fluke angle appropriate for the particular soil present in the mooring bed. The fluke angle is usually defined as the angle between the forward direction of the fluke and a line connecting the anchor cable attachment point on the shank to a point on the rear edge of the fluke measured in a fore-and-aft plane of symmetry of the anchor. In practice, this angle is about 50.degree. for muds and about 30.degree. for sands. The angle that a straight line containing the cable attachment point and the centroid of the fluke forms with the forward direction of the fluke is correspondingly in the range 60.degree. to 70.degree. for muds and 35.degree. to 45.degree. for sands where the fluke is of triangular or rectangular shapes with a length to breadth ratio in the usual range between 1 and 2. This latter angle may be regarded as the centroid fluke angle.
The angle of friction, .phi., between a marine soil and a smooth steel anchor fluke is usually in the range 22.degree. to 30.degree. for sand and 6.degree. to 14.degree. for mud. Thus, the centroid fluke angle is always made less than (90-.phi.) degrees to ensure that a pulling force applied at the anchor cable attachment point causes the anchor to penetrate by sliding in the soil in the forward direction of the fluke and so bury increasingly below the surface of the mooring bed when pulled horizontally thereon.
A deeply buried marine drag embedment anchor is usually recovered by heaving vertically upwards on the anchor cable attached to the forward end of the anchor shank or by heaving vertically upwards on a pendant cable attached to the anchor at the rear edge of the fluke. This vertical pull first rotates the anchor in the soil until the centroid of the fluke lies vertically below either the cable attachment point on the shank (referred to as the break-out position) or the pendant cable attachment point at the rear edge of the fluke. When heaved up by the anchor cable, following rotation, the anchor simply continues "digging" in the forward direction of the fluke but obliquely to the vertical instead of obliquely to the horizontal until it emerges from the surface of the mooring bed. When heaved up by the pendant cable, following rotation, the anchor moves vertically upwards in the soil since the vertical cable lies in the rotated direction of the fluke.
The breaking-out force is least when heaving up by the pendant cable and greatest when heaving up by the anchor cable. Peak breaking-out force occurs in the anchor cable immediately following rotation of the anchor and just before movement oblique to the vertical occurs. This peak breaking-out force in the anchor cable usually has a magnitude of approximately 20 to 30 per cent of prior peak horizontal embedment force in sands and of the order of 100 per cent in muds. Generally, minimisation of anchor breaking-out force is, inter alia, an objective of drag embedment anchor design.
In contrast, it is an object of the present invention to provide a drag embedment marine anchor and a method wherein the breaking-out force at the break-out position is maximised. It is another object of the present invention to provide a drag embedment marine anchor and a method wherein the holding capacity may be increased at a given depth of embedment in a mooring bed soil.
Yet another objective of the present invention is to provide a method of limiting the load developed by a marine anchor during drag embedment to permit dragging to a desired location at constant load prior to increasing the holding capacity at such desired location.
These objectives are met, in accordance with the present invention, by prov
REFERENCES:
patent: 3407775 (1968-10-01), Lunde
patent: 3685479 (1972-08-01), Bruce
patent: 4369727 (1983-01-01), Fasco
"Marine Engineering and Shipbuilding Abstracts," vol. 2, No. 2 (1972).
Basinger Sherman
BruPat Limited
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