Ships – Anchor – Fluke type
Reexamination Certificate
1999-10-28
2001-04-24
Sotelo, Jesus D. (Department: 3617)
Ships
Anchor
Fluke type
Reexamination Certificate
active
06220198
ABSTRACT:
The present invention relates to embedment marine anchors and their embedment means and, in particular to anchors of the types disclosed in International Patent Application PCT/NL92/001144 (published as WO93/03958) and in the present applicants International Patent Application PCT/GB96/01756 (published as WO96/39324).
Embedment marine anchors, embedded by drag embedment means or by direct embedment means, are generally attached to an anchor line for connection to an object to be moored or restrained in a body of water over a mooring bed.
A drag embedment anchor is embedded by pulling (dragging) the anchor horizontally over the mooring bed by use of embedment means comprising an anchor line to cause penetration both horizontally and vertically along a curved embedment trajectory in the mooring bed soil. The drag embedment anchor comprises a planar or curved plate-like or blade-like fluke member and a shank means attached to the fluke member and arranged to provide an attachment point for the attachment of an anchor line, said shank means being adapted such that the anchor provides a line direction from the centroid of the fluke to said attachment point whereby, in relation to a forward edge-wise direction of the fluke member measured in a fore-and-aft plane of symmetry of the anchor, the line direction forms a forward-opening angle with said forward direction of the fluke member whereby a dragging force applied to the anchor at said attachment point by means of the anchor line when said anchor is placed on a sea bed surface causes embedment of the anchor by penetrative movement into the soil of the sea bed substantially in said forward edgewise direction of the fluke member. Such an anchor and its embedment means will hereinafter be referred to as an embedment anchor and its embedment means as hereinbefore described.
A direct embedment anchor, in contrast, is embedded by pushing the anchor into a mooring bed vertically or inclined to the vertical by the use of embedment means comprising a pushing follower member or comprising momentum developed, for example, gravitationally, hydraulically, or explosively. The direct embedment anchor comprises a planar or curved plate-like or blade-like fluke member adapted for accommodating said embedment means and arranged to provide an attachment point for the attachment of an anchor line thereto, said anchor possessing a direction of minimum projected area wherein resistance to penetration of the anchor in a mooring bed soil is minimal, said attachment means providing an attachment point for said anchor line locatable such that a pulling force in the anchor line tends to cause movement of the anchor substantially at right angles to said direction of minimum projected area. Such an anchor and its embedment means will hereinafter be referred to as an embedment anchor and its embedment means as hereinbefore described.
Marine anchors are usually made of steel and often are coated with paint for corrosion protection. Such a coating, being softer than steel, is vulnerable to abrasive removal by hard particles in a sea bed soil ploughing a series of grooves therein which result cumulatively in removal of the coating. This abrasive action increases the range of the coefficient of friction of the coated anchor relative to the material of the sea bed soil particles sliding submerged thereon beyond the usual 0.3 to 0.6 range occurring for an uncoated anchor. The concomitant increase in friction force on the coated anchor increases its resistance to dragging when buried in sea bed soil and so increases its holding capacity but this enhanced holding capacity decreases again to that of an uncoated anchor as the coating is stripped off by abrasion. Thus, other avenues for increasing the holding capacity of anchors have occupied the attention of designers in the past.
Increases in holding capacity of drag embedment anchors have been obtained due to maximisation of fluke area, development of stabilisation means, optimal selection of forward-opening angle to suit particular soils, minimisation of anchor line diameter, and streamlining to avoid stalling of soil flow over the anchor whilst ensuring that the pull applied by the anchor line at the sea bed surface is as close to horizontal as possible.
Increases in the holding capacity of direct embedment anchors have been obtained by maximisation of fluke area, streamlining, minimisation of anchor line diameter, and reduction of said minimum projected area.
It has generally been established that the forward-opening angle of a drag embedment anchor requires to be of a particular magnitude to suit a particular sea bed soil. Thus, a drag embedment anchor may provide maximum holding capacity in sand if the onward opening angle is approximately 45° and maximum holding capacity in soft clay if it is approximately 65°. Beyond these values of forward-opening angle, the holding capacity decreases progressively until the drag embedment anchor fluke ultimately backs out of the sea bed soil after initial penetration. However, when the forward-opening angle is set at 65° for soft clay, negligible holding capacity is obtained in sand. When set at 45° for sand, very low holding capacity is obtained in soft clay. No intermediate angle has been found to provide optimum performance in both sand and soft clay. Although compromise angles in the range 50° to 56° have been used in the past, performance within acceptable deviations from the optima for sand and soft clay has not been realised. The selection of a forward-opening angle to achieve acceptable capacity in a particular soil thus reduces the versatility of the drag embedment anchor for use in a wide variety of soils.
The objectives of the present invention include inter alia increasing the versatility of drag embedment anchors and decreasing the penetration resistance of both drag embedment and direct embedment anchors and their embedment means as hereinbefore described.
According to one aspect of the present invention, an anchor and its embedment means as hereinbefore described is characterised in that at least one of the anchor and its embedment means includes a low friction external surface. In particular, this external surface will have a lower value of coefficient of friction relative to sea bed soil particles sliding submerged thereon than occurs similarly on the external surfaces (usually steel or painted steel) presently known on marine anchors.
Preferably said external surface is formed of a low friction material.
Preferably said external surface is formed of a coating of solid low friction lubricating material adhering to the anchor.
Preferably said coating comprises a layer formed of at least one of the substances polytetrafluoroethylene, ethylene-chlorotrifluoroethylene copolymer, graphite, and diamond, with diamond most preferred.
Preferably said coating comprises a layer formed by at least one of a fluid lubricant and a semi-solid grease-like lubricant.
Preferably said fluid lubricant is at least one of water and a vegetable oil.
Preferably said vegetable oil is at least one of olive oil, rapeseed oil, sunflower oil, coconut oil, and palm oil.
Preferably at least one of said coating, low friction material, and said lubricant includes a hydrophobic substance which prevents adhesion of sea bed soil on a external surface of said anchor.
Preferably said hydrophobic substance comprises a silicone material.
Preferably said lubricant is pumpable and smearable to form said coating.
Preferably said lubricant is at least one of biodegradable and short-term degradable.
Preferably said lubricant contains a delayed-action inhibiting substance whereby the lubricity of the lubricant is considerably reduced after a determinable period of time.
Preferably said low friction external surface comprises a lubricated surface.
Preferably said coefficient of friction of said external surface with respect to sea bed soil particles (e.g. sand) sliding submerged thereon does not exceed 0.27 and, further preferably, does not exceed 0.2.
Preferably said low friction external surface is loca
Arnold & Associates
BruPat Limited
Sotelo Jesus D.
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