WO1998032648A1 - Marine anchor and anchoring method - Google Patents

Abstract

A marine anchor (2) and a method for deploying a marine anchor (2) are described enabling effective burial of the anchor (2) by drag embedment in a sea bed (83) having material layers (84, 86) of different cohesive properties, more especially where a non-cohesive layer (84), e.g. sand or gravel constituting a topmost layer of the sea bed (83) is at an interface (85) with a lower layer with cohesive characteristics (86), e.g. mud. In particular, the present disclosure provides means whereby the anchor (2) can penetrate on drag embedment through the topmost sea bed layer (84) with the anchor shank (5) set at a first angle (8) relative to the anchor fluke (4) (i.e. at a 'sand' angle compatible for a sand sea bed) and at the interface (85) of the two layers, the shank (5) can be adjusted to adopt a second angle (8) relative to the fluke (4) (i.e. at the 'mud' angle) so that there is continued effective penetration of the anchor (2) through the lower (mud) (86) layer on further drag embedment. In a preferred embodiment, restraint means including a latch device (66, 67, 68) serves to maintain the anchor (2) at said first setting whilst operation of the latch device (66, 67, 68) at said layer interface (85) enables the anchor (2) to adopt said second setting. The setting adjustment may be achieved automatically at the interface (85), preferably by having the latch device (66, 67, 68) responsive to the centre of pressure of sea bed material acting on the fluke (4) so that as the position of this centre of pressure alters at the interface (85) release of the latch device (66, 67, 68) occurs to enable the second anchor setting to be achieved.

Description

MARINE ANCHOR AND ANCHORING METHOD

The present invention relates to drag embedment marine anchors.

A drag embedment marine anchor is known comprising a plate-like or blade-like fluke and a shank means attached to the fluke and arranged to provide at least one attachment point for attachment of an anchor cable, said shank means being adapted such that the anchor is capable of providing two directions from the centroid of the fluke to said attachment point whereby, in relation to the forward edge-wise direction of the fluke measured in a fore-and-aft plane of symmetry of the anchor, a first direction forms a first forward-opening angle with said forward direction and a second direction forms a second forward-opening angle with said forward direction whereby a pulling action on the anchor at an attachment point located in said first direction permits drag embedment of the anchor by movement substantially in said forward direction in non-cohesive soils, such as sand or gravel, whilst a pulling action on the anchor at an attachment point in said second direction permits drag embedment of the anchor by movement substantially in said forward direction in cohesive soils, such as silts and soft clays commonly known as mud. Such an anchor will hereinafter by referred to as an anchor of the type hereinbefore described.

Generally, such a drag embedment anchor when dragged horizontally on a sea bed will penetrate the surface of a non-cohesive sand sea bed soil if the first forward- opening angle does not exceed 52° and will penetrate into a cohesive mud sea bed soil if the second forward-opening angle does not exceed 72°. Thus, when the forward opening angle exceeds 52° but does not exceed 72°, the anchor will be capable of penetrating into a mud sea bed but not into a sand sea bed.

When a sea bed comprises a layer of sand overlying mud, the forward- opening angle must not exceed 52° in order to achieve penetration into the sand layer. Despite this constraint, it might be thought that penetration would subsequently continue into the underlying mud, albeit at a very shallow declination angle of penetration due to the forward-opening angle being considerably less than 72°.

However, in practice, the fluke ceases to penetrate into the mud once it has emerged from the underside of the sand layer. Further dragging causes the fluke to move horizontally in the mud just under the sand layer with no increase in cable tension that would arise from continuing increase in depth of penetration. The anchor thus suffers the disadvantage of prematurely limited holding capacity due to the presence of a surface sand layer in a predominantly mud sea bed.

It is an objective of the present invention to obviate or mitigate this disadvantage.

According to the present invention, an anchor of the type hereinbefore described is characterised in that there is provided relocation means for relocating said cable attachment point from said first direction to said second direction from said fluke centroid when the anchor is embedded in a sea bed soil whilst traversing an interface between a non-cohesive layer of soil and an underlying soft cohesive soil.

Preferably said anchor is further characterised in that first releasable restraint means are provided to hold said cable attachment point in said first direction during embedment in non-cohesive soil.

Preferably, second releasable restraint means are provided to maintain temporarily said cable attachment point substantially in said first direction during release of said releasable restraint means.

Preferably, the shank is joined to the fluke forwardly of the rear edge of the fluke and preferably close to the centroid of the fluke to enable release of said first releasable restraint means to be actuated by moment changes due to movement of the centre of pressure of soil on the fluke of the anchor caused by the anchor traversing said interface.

The present invention is also a method of deploying a marine anchor comprising a fluke with a shank means attached thereto in a sea bed having an uppermost layer of non- cohesive material and an adjacent lower layer of cohesive material, said method comprising the steps of drag embedding the anchor with the fluke orientated at a first angular setting relative to the shank means so that the anchor penetrates through said first non-cohesive sea bed layer to an interface of said first layer with the second cohesive sea bed layer, and adjusting the anchor adjacent said interface to provide the anchor with a setting compatible for passage through said second cohesive sea bed layer.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings wherein:

Figure 1 shows a marine anchor in side elevational view with the shank in two positions in accordance with the present invention.

Figure 2 shows a front elevational view of the marine anchor in Figure 1.

Figure 3 to 10 show sectional details of the operating mechanism of the marine anchor of Figure 1.

Fig. 3A shows a pawl mechanism 56 of Fig. 1 to a larger scale. Fig. 3B shows a pawl mechanism 69 of Fig. 1 to a larger scale.

Figure 11 shows a sequence of embedment positions of the marine anchor of Figure 1 in a sea bed with an upper sand layer.

With reference to Figure 1 and 2, a drag embedment anchor 2, generally in accordance with the pivoting shank anchor 2 described in the present applicant's International Patent Application PCT No. GB96/01755 (Publication No. Wo96/39324), is of slim streamlined form to encourage deep burial of the anchor in submerged soils and comprises an anhedral form plate-like fluke 4 connected to one end of a shank 5, the other end of the shank 5 including a shackle hole 6 for attachment of an anchor cable 7. The shank 5 is pivotally connected to the fluke 4 by a pivot-pin 8 whereby the shank 5 can pivot to move the shackle hole 6 from lying in a first direction line 9 extending through the fluke centroid 10 to lie in a second direction line 11 extending through the centroid 10. The first direction line 9 forms a centroid fluke angle γ with a forward edge- wise direction F of the upper surface 4A of the fluke 4, measured in a fore-and-aft plane of symmetry X-X of the anchor 2, while the second direction line 11 forms a centroid fluke angle δ with the forward direction F. Forward direction F is parallel to the intercept line 4B of two planes containing the major upper anhedral surfaces 4A of the fluke 4. Angle γ is in the range of 35° to 50° for operation in sands, but generally will be chosen to be between 40° and 47°. Angle U is greater than angle γ and is in the range of 52° to 72° for operation in soft clay soils, but generally will be chosen to be between 56° and 66°.

As before, restraint and control means for the setting of shank 5 are housed in a substantially enclosed housing 40 while the shank 5 carries a quadrant plate 41 which extends into the housing 40 through an open forward slot 42A. Figures 3 to 9 show the restraint and control means described in International patent publication WO96/ 39324 including rotary pawl 50 and 51, spring 48, and control plate 53 but these items are not described in depth herein.

Referring to Figures 3 to 7, the shank 5 is stopped at angle γ when rotated anti-clockwise by means of a controlled pawl mechanism 56 engaging on detent 19A in edge 41A of quadrant plate 41, the mechanism 56 including compression spring 57, torsion spring 58 and stop plate 59.

As described in WO96/ 39324, to prevent ingress of grit and other solid soil particles into housing 40 and thus risk jamming the mechanisms contained therein, the housing 40 is packed with grease.

The pawl mechanism 56 comprises a pawl 60 carried by an integral shaft 61 journalled to the housing 40, and a spring 57 mounted in a threaded cap 62 in housing 40 and engaging a connecting rod 63 attached via a pin 64 to the integral shaft 61 of pawl 60 to urge the pawl 60 anti-clockwise (arrow D). However, a stop plate 59 pivoting about pin 59A fixed in side walls 42 of housing 40 and biased by torsion spring 58 attached to the stop plate 59 and to a side wall 42 of housing 40 arrests the pawl 60 via detent 65. The integral shaft 61 makes substantial surface contact (part cylindrical) with a recess 61 A on housing 40 so that pawl 60 can withstand substantial loading.

A pivoting latch 66, mounted via pin 67 on stop plate 59 and biased by spring 68, extends into the path of quadrant plate 41 when shank 5 rotates anti-clockwise about pin 8. Latch 66 pivots to allow passage of quadrant plate 41 to permit detent 19A to engage on pawl 60 without dislodging pawl

60 from detent 65 on stop plate 59 as shank 5 rotates anti-clockwise. However, latch 66 also engages on face 19C adjacent detent 19B of quadrant plate 41 (Figure 4) so that subsequent clockwise movement of shank 5 rotates stop plate 59 anti-clockwise and disengages detent 65 from pawl 60 allowing pawl 60 to rotate anti-clockwise (arrow D) urged by spring 57 until it is out of the path of quadrant plate 41 (Figures 5, 6 and 7). As pawl 60 rotates anti-clockwise, it pushes stop-plate 59 further anti- clockwise until stop plate 59 and pivoting latch 66 are also out of the path of quadrant plate 41 (Figure 7).

An auxiliary pawl mechanism 69 is provided to limit clockwise movement of shank 5 (subsequent to detent 19A engaging on pawl 60) by engaging in detent 19D in edge 41A of quadrant plate 41 (Figures 3 to 8)

The pawl mechanism 69 (Figure 3) comprises a pawl 70 carried by an integral shaft 71 journalled to the housing 40, and a compression spring 72 mounted in a threaded cap 73 in housing 40 and pushing on pawl 70 to urge pawl 70 anti-clockwise (arrow E) into engagement with edge 41A of quadrant plate 41. The integral shaft 71 makes substantial surface contact (part cylindrical) with a recess 74 in housing 40 so that pawl 70 can withstand substantial loading.

A ridge 75 upstanding from edge 41 A of quadrant plate 41 serves to rotate pawl 70 clockwise clear of edge 41A when shank 5 is fully rotated anticlockwise (Figure 8) to bear against shank stop 18 on housing 40. A spring stop mechanism 76 is provided, which locks pawl 70 clear of edge 41 A, comprising a pin 77 driven into engagement with a partial flange 78 on integral shaft 71 by compression spring 79 bearing on threaded cap 80 in housing 40. A pinion 81 mounted on a shaft 82 and engaging with teeth on one side of pin 77 serves to withdraw pin 77 from engagement with partial flange 78 for the purpose of resetting pawl mechanism 69 by turning shaft 82 anti-clockwise with a key external to housing 40.

Prior to using anchor 2 in a sea bed 83 (Figure 11) comprising a sand layer 84 with upper surface 27 and lower interface 85 with an underlying mud layer 86, the pawl mechanism 56 and 69 in housing 40 (Figure 3) are made ready for operation by turning shaft 61 clockwise (by means of an externally applied peg spanner) until pawl 60 engages in detent 65 of stop plate 59 and by turning shaft 82 anti-clockwise (by means of an externally applied peg spanner) to draw pin 77 out of engagement with partial flange 78 to allow pawl 70 to rotate into contact with edge 41 A of quadrant 41.

SUBSTITUTE SHEET (RULE 26\ In use (Figure 11), anchor 2 is laid out on the sea bed surface 27 and pulled horizontally thereon by means of its anchor cable 7 to cause fluke 4 to penetrate into sand layer 84. Soil forces acting at a centre of pressure on fluke 4 (Figure 11A) forward of pin 8 cause fluke 4 to rotate clockwise about pin 8 relative to shank 5 until detent 19A (Figure 4) on quadrant 41 engages on pawl 60 and pawl 70 has partially entered detent 19D so that the shank 5 is set at the angle shown in Fig. 4 i.e at angle γ. Further pulling causes anchor 2 to penetrate deeper into sand layer 84 (Figure 11B) until fluke 4 penetrates interface 85 (Figure 11C). As fluke 4 emerges from sand layer 84 into mud layer 86, with sand bearing on the aft portion of fluke 4 and mud bearing on the forward portion of fluke 4, the centre of pressure of the soil on fluke 4 moves aft of pin 8 (Figure 1 ID). This causes fluke 4 to rotate slightly anti-clockwise about pin 8 relative to shank 5 (Figure 5) until stopped by pawl 71 entering fully into and engaging with detent 19D in quadrant 41. At the same time, latch 66 engages on face 19C in quadrant 41 and forces stop plate 59 to rotate anti-clockwise (due to relative movement of shank 5 and quadrant 41) to release pawl 60 from engagement with detent 65 in stop plate 59. This allows spring 57, acting via connecting rod 63 and pin 64, to force pawl 60 to rotate anti-clockwise clear of the path of quadrant 41 (Figure 6) and, in turn, pawl 60 bears on stop plate 59 to cause it to rotate anti-clockwise whilst overcoming spring 58 to carry latch 66 clear of the path of quadrant 41 (Figure 6). As fluke 4 of anchor 2 penetrates fully into mud layer 86 (Figure 1 IE), the centre of pressure of soil on fluke 4 moves forward of pin 8 once more. This causes fluke 4 to rotate clockwise about pin 8 relative to shank 5 until groove 14A in quadrant 41 engages on shear pin 13 in holes 13A in side walls 42 of housing 40 (Figure 7) and a fixed value for centroid fluke angle U (Figure 1) is established suitable for further penetration in mud (Figure 7 and 1 IF). Thus, the mechanism described allows anchor 2 to penetrate through a sand layer 84 at a relatively small centroid fluke angle γ suitable for sand (Figure 11AB) and automatically change to a larger centroid fluke angle ϋ suitable for mud, on traversing an interface (Figure 11CD) between the sand layer and an underlying mud layer, in preparation for burying deeply in the underlying mud layer (Figure 1 1EF). This eliminates the arrestment of penetration at such an interface that has been a problem with drag embedment anchors in the past. As described in the aforementioned International Patent publication W096/ 39324, shear pin 13 shears during operation of anchor 2 to allow shank 5 to rotate into contact with stop 18 on housing 40 (Figure 8) for uplift operation whereupon ridge 75 upstanding from edge 41 A of quadrant plate 41 rotates pawl 70 until pin 77 of spring stop mechanism 76 is driven into engagement with partial flange 78 to hold pawl 70 in a position clear of the path of edge 41A of quadrant plate 41. This prevents pawl 70 from reengaging in detent 19D in quadrant plate 41 to obstruct subsequent rotation clockwise of fluke 4 relative to shank 5 to bring pawl 50 into engagement with detent 19B (Figure 9) or detent 19A (Figure 10) necessary for the anchor retrieval operation described in W096/ 39324.

The anchor of Figs. 1 to 10 is also operable in a mud sea bed (86) where no upper sand layer 84 is present, and in this case pawls 60 and 70 are not cocked to enable the anchor to function as described in W096/39324.

Modifications are of course possible and the invention could be applied in a different form of anchor from that shown in the above described embodiment. In particular, in an anchor where the fluke has a "shank" in the form of cable means attached thereto for drag embedment, for example as shown in International Patent publication WO93/03958, means could be provided to vary the length of sets of cables located respectively towards the front and rear of the fluke during burial of the anchor in a sea bed to thereby alter the angle of the fluke relative to the shank i.e. to alter the setting of the cable attachment point of the anchor line between the first and second directions as above defined to enable continued effective operation of the anchor as it traverses an interface between a non-cohesive layer of soil and an underlying soft cohesive layer.

Claims

1. A drag embedment marine anchor comprising a plate-like or bladelike fluke and a shank means attached to the fluke and arranged to provide at least one attachment point for attachment of an anchor cable, said shank means being adapted such that the anchor is capable of providing two directions from the centroid of the fluke to said attachment point whereby, in relation to the forward edge- wise direction of the fluke measured in a fore-and-aft plane of symmetry of the anchor, a first direction forms a first forward-opening angle with said forward direction and a second direction forms a second forward-opening angle with said forward direction whereby a pulling action on the anchor at an attachment point located in said first direction permits drag embedment of the anchor by movement substantially in said forward direction in non-cohesive soils whilst a pulling action on the anchor at an attachment point in said second direction permits drag embedment of the anchor by movement substantially in said forward direction in cohesive soils, wherein there is provided relocation means for relocating said cable attachment point from said first direction to said second direction from said fluke centroid when the anchor is embedded in a sea bed soil whilst traversing an interface between a non-cohesive layer of soil and an underlying soft cohesive soil.

2. An anchor as claimed in claim 1, wherein first releasable restraint means are provided to hold said cable attachment point in said first direction during anchor embedment in non-cohesive soil.

3. An anchor as claimed in claim 1 or 2, wherein second releasable restraint means are provided to maintain temporarily said cable attachment point substantially in said first direction during release of said releasable restraint means.

4. An anchor as claimed in claim 2, wherein the shank is joined to the fluke forwardly of the rear edge of the fluke to enable release of said first releasable restraint means to be actuated by moment changes due to movement of the centre of pressure of soil on the fluke of the anchor caused by the anchor traversing said interface.

5. An anchor as claimed in claim 4, wherein means are present to provide a limited relative pivoting movement between the fluke and the shank means, the arrangement being such said moment changes causes said relative pivoting movement, and said first releasable restraint means are responsive to said limited relative pivoting movement so as to permit movement of said cable attachment point from the first direction into said second direction.

6. An anchor as claimed in claim 4, wherein the shank is joined to the fluke close to the centroid of the fluke.

7. An anchor as claimed in any one of claims 2 to 6 wherein said first restraint release means includes a latch device operable when the anchor transverses said interface for relocation of the cable attachment point into said second direction.

8. An anchor as claimed in claim 7, wherein means are provided to enable the anchor to function with said latch means inoperative.

9. A method of deploying a marine anchor comprising a fluke with a shank means attached thereto in a sea bed having an uppermost layer of non-cohesive material and an adjacent lower layer of cohesive material, said method comprising the steps of drag embedding the anchor with the fluke orientated at a first angular setting relative to the shank means so that the anchor penetrates through said first non-cohesive sea bed layer to an interface of said first layer with the second cohesive sea bed layer, and adjusting the anchor adjacent said interface to provide the anchor with a setting compatible for passage through said second