US3685479A

US3685479A - Anchor-cable systems

Info

Publication number: US3685479A
Application number: US886539A
Authority: US
Inventors: Peter Bruce
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-12-24
Filing date: 1969-12-19
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22
Also published as: NL6919447A; CA923379A; NO129894B; NL164816B; ES374918A1; HK49376A; JPS4925713B1; NL164816C

Abstract

An anchor for mooring a vessel to a mooring bed includes at least one fluke attached to a shank part of the anchor and inclined relative thereto to cause penetration of the anchor into the mooring bed on horizontal movement of the anchor. Stabilizer fin (or fins) is mounted on an extension of the shank part and spaced from the fluke such that, on penetration of the anchor into the bed, any pivoting of the shank part to decrease the angle of attack of the fluke relative to the bed is counteracted. Also, the anchor-cable may include at least one fin device adjacent the anchor, the fin of the device being inclined such that burial forces are produced by the device to counteract the vertical penetration resistance of the adjacent cable and to constrain the buried portion of the cable to depart from the usual catenoidal configuration and adopt a linear or adverse curvature configuration.

Description

United States Patent Bruce 51 Aug. 22, 1972 ANCHOR-CABLE SYSTEMS [22] Filed: Dec. 19, 1969 [21] Appl. No.: 886,539

[30] Foreign-Application Priority Data Dec. 24, 1968 Great Britain ..6l,498/68 Nov. 8, 1969 Great Britain ..54,797/69 [52] U.S. Cl. ..l14/206 R, 114/230 [51] Int. Cl. ..B63b 21/24 [58] Field of Search l l4/208, 206 R, 209, 230,

235 R, 114/235 A, 235 B, 235 F, 0.5 R, 0.5 D, 0.5

F; 244/1 TD, 2, 3,113,135 A, 110 R, 110 F, 110 G; 9/8 R, 8 P; 52/155, 159

[56] References Cited UNITED STATES PATENTS 3,134,355 5/1964 Jakosky et al. ..l l4/209 Dl68,999 3/l953 lsaacs ..l l4/235 B X 2,414,480 l/l947 Morrill ..1 14/235 B 3,434,451 3/1969 Brainard ..1l4/235 B Primary Examiner-Milton Buchler Assistant Examiner-F. K. Yee

Attorney-Young & Thompson 57] ABSTRACT An anchor for mooring a vessel to a mooring bed includes at least one fluke attached to a shank of the anchor and inclined relative thereto to cause penetration of the anchor into the mooring bed on horizontal movement of the anchor. Stabilizer fin (or fins) is mounted on an extension of the shank part and spaced from the fluke such that, on penetration of the anchor into the bed, any pivoting of the shank part to decrease the angle of attack of the fluke relative to the bed is counteracted. Also, the anchor-cable may include at least one fin device adjacent the anchor, the fin of the device being inclined such that burial forces are produced by the device to counteract the vertical penetration resistance of the adjacent cable and to constrain the buried portion of the cable to depart from theusual catenoidal configuration and adopt a linear or adverse curvature configuration.

24 Claims, 9 Drawing Figures v I JPa fanfare v PATENTED I972 3.685.479

sum 1 or 3 Inventor A ttorneys PATENTEUMIB 2 2 m3 SHEET 3 BF 3 E w e m6 0 E P By %7 f Attorneys ANCHOR-CABLE SYSTEMS The present invention relates to an anchor-cable for mooring an object to a mooring bed; for example, for mooring a vessel or other floating body, or a standing or floating drilling rig or the like to the sea bed.

The word cable as used hereinafter and in the claims includes wire hawser, chain cable and the like.

By the term working vertical burial attitude as used hereinafter and in the claims, applied to a burying body, such as an anchor or a cable-depressing fin device, is meant the attitude of the body whereby, when dragged wholly immersed in a particulate mooring bed, the resultant force vector on the body due to interaction with the particulate bed lies substantially in a vertical plane and in a direction such as to tend to cause burial of the body. By the term reverse attitude to the working vertical burial attutide as used hereinafter and in the claims, applied to a burying body such as an anchor or a cable-depressing fin device, is meant the attitude the body would have if rotated 180 about its fore-and-aft axis from its working vertical burial attitude. 7

In previously proposed anchor-cable systems, the anchor includes a shank part to which are attached flukes which can be inclines relative to the shank such that, with the anchor on a sea bed of soft material such as for example mud, sand or gravel and with the shank horizontal and connected by an elongate cable to an object to be moored, the flukes dig into the sea bed when the anchor is subjected to a horizontal pull from the cable and so causes burial of the anchor.

It has been found advantageous in previously proposed anchor-cable systems that the anchor be substantially buried to encourage the holding power of the overall system. However, with the previously proposed anchor-cable systems, with increasing depth of burial of the anchor, the buried portion of the cable gives rise to increasing uplifting forces being applied to the anchor and also tends to adapt an approximately catenoidal configuration, resulting in pivoting of the anchor as a whole, whereby the angle of attack of the flukes relative to the horizontal is decreased, and the downwards force acting on the flukes to bury the anchor and cable is consequently also decreased when required to overcome the increasing uplifting force due to the cable. This change of attitude rapidly progresses until the angle of attack of the flukes is so reduced that an equilibrium state is established with the result that no further burial of the anchor and cable is possible.

The operation of a previously proposed anchor-cable system, currently in use, will now be examined to determine its limitations.

In an anchor of the previously proposed type, the maximum angle of attack of the flukes to the horizontal in a working attitude is the greatest angle enclosed by the plane of the flukes and a straight line passing through the intersection of the pivot axis of the cable shackle pin with the extremity of the shank remote from the flukes and passing through the centroid of the flukes.

In a working attitude of the anchor the angle of attack of the flukes may be designated a and the abovementioned straight line referred to as the tangent to the cable at the shackle pin or, for brevity, the tangent.

With such an anchor on the sea bed with the flukes penetrating the surface of the bed and the portion of the cable adjacent the anchor being substantially horizontal, and for horizontal motion of the anchor, the forces developed by the anchor may be resolved into a vertically downward burying component, P a function of a f (a), and a horizontal drag component, D f (a) K, where K is the horizontal component of anchor drag when the anchor orientation is such that the cross-sectional area in the direction of movement is at a minimum. In the above attitude, the tangent to the cable at the shackle pin is horizontal.

As the anchor buries itself together with the adjacent portion of cable, under the action of the horizontal pull imparted by the cable, the buried portion of the cable will tend to adopt a curve approximately catenoidal in form, due to distributed resistive forces acting along its length arising from movement through the resistive medium of the sea bed. The portion of the cable directly adjacent to the shackle pin on the shank of the anchor and, consequently, the tangent thereat will be inclined therefore to the horizontal by some angle, B. The angle of attack of the anchor, a, thus will be reduced by the magnitude of the angle B as the anchor and cable penetrates below the sea bed surface so that now P=f(a-B) and D=f(aB) +K. Thus the forces developed by the anchor tend relatively to decrease with depth of burial due to a progressive decrease of the angle of attack of the flukes caused by the increasing curvature of the cable.

The resistive forces distributed along the buried portion of the cable may also be resolved vertically and horizontally and summed to give an upward lift component opposing the burialforce, P, of the anchor and a horizontal drag component opposing the pull applied to the anchor cable above the sea bed surface.

When all'the components of the forces developed by the buried portion of the cable and the anchor are added, the horizontal drag components may be seen to act together to constitute the horizontal holding power of the anchor-cable system while the vertical uplifting and burial components oppose and are equal in the equilibrium state.

As the depth of burial of the anchor increases, the overlying reaction volume of sea bed material increases and so the burial force deriving from this reaction volume tends also to increase. However, as the angle ,8

- increases, the angle of attack of the flukes decreases with the result that the rate of increase of burial force with increasing depth of burial is reduced to a rate less than the rate of increase of the upward lift component of force developed by the cable. The increasing upward lift component of force ultimately overtakes and equals the burial component of force developed by the anchor and so vertical equilibrium of the anchor cable system is established. In this equilibrium condition, the anchor has reached the maximum depth of penetration allowed by the cable and moves horizontally only. The horizontal holding force developed with the anchor just moving, comprising anchor drag and cable drag, is now a constant and is generally quoted as the maximum holding power of the anchor although a substantial por- 'tion of the holding power of such an anchor-cable system is derived from the buried portion of the cable.

attitude being inclined relative to the cable axis whereby, on penetration .of the portion of the cable including the devices into the mooring bed when a horizontal pull is applied to the cable and anchor, the inclined surfaces of each device produce burial forces capable of burying the device and countering the vertical drag forces developed by the adjacent cable such that the buried portions of the cable is constrained to depart from an approximately catenoida] configuration and tend to adopt a substantially linear or reverse curvature configuration.

Preferably the inclined surface of each cablemounted device is a surface of an inclined fin forming part of the device.

Preferably the center of area of the inclined fin is offset from the axis of the cable.

Preferably'at least three fifths of thetotal burial fin area of each device lies at one side of the cable. axis.

Preferably the fin-bearing portion of each finned deviceis pivotably mounted on the cable to permit the ofiset fins to trail behind the cable and so adopt automatically an efiective working orientation for the burial fins.

Preferably each cable-mounting device has two or more burial fins-inclined relative to each other as well as to the cable axis such that the device is symmetrical about a vertical plane bisecting it when in a working vertical burial attitude, and such that the fins are in an anhedral configuration.

Preferably the disposition of the burial fin surfaces of each' device is such that, when interacting with, the mooring bed surface in a reverse attitude to the working vertical burial attitude, the resultant force vector produced by the portions of burial fin lying at either side of the vertical plane of symmetry bisecting the device, and making contact with the surface material. of the mooring bed, lies on a straight line intersecting the vertical plane of symmetry at a point below the axis of the portion of the cable trailing behindthe device.

Preferably a plurality of devices are mounted on the cable, the angles of inclination relative to the cable axis of the inclined surfaces of all of the devices being equal in magnitude to each other.

The angle of inclination of the inclined surface of the cable-mounting device to the axis of the cable may be at any angle whereby sufficient burial force is developed by the device on engaging the mooring bed in the working vertical burial attitude to bury the device and counteract the resistance of a portion of the adjacent cable to penetration into the mooring bed.

Preferably the angle of inclination of the inclined surfaces of the devices relative to the cable axis is within the range of to 60, and particularly 30 to In a preferred embodiment of the invention, each finbearing device includes a forward clamping collar positionable on a wire hawser cable and having an annular recess on an end well'thereof, a sleeve on which are mounted a pair of offset opposed fins inclined relative to the axis of the collar and to each other and a third fin between the opposed fins and upstanding from the sleeve, the leading end of the sleeve being insertable into the recess of the forward collar while the trailing end of the sleeve is provided with a recess,-and a trailing clamping collar insertable into the recess of the sleeve to locate the sleeve rotatably in position on the wire hawser cable.

Alternatively, these devices may be located on a chain cable whereby each fin device includes a forward fairing carrying a chain ring and having an annular recess on an end wall thereof, a sleeve on which are mounted a pair of offset opposed fins inclined relative to the axis of the fairing and to each other and a third fin between the opposed fins and upstanding from the sleeve, the leading end of the sleeve being locatable into the recess of the forward .fairing 'gand fitting pivotably about a rearwardly projecting cylindrical projection of the fairing which serves as a bearing for the positioned at the rear of the projection and both retaining collar and chain ring secured to the projection, say, by welding.

The device may then be deployed as an integral link in the chain cable.

The opposed fins may be inclined nose down at an angle to the axis of thesleeve in the range of 15 to-, and particularly 30 to 60.

Considering now the approximately catenoidal curve of the cable, developed more fully under the action of the improved anchor of the first aspect of the invention, it will be readily appreciated that the curve is the result of the drag forces distributed along the buried length of the cable and the burial force developed by the anchor located at the end of the cable.

By the devices according to the present invention, the cable is constrained to depart fromthe usual catenoidal curve and take up a substantially linear configuration of fixed slope throughout its length buried below the sea bed surface. The tangent at the shankshackle' pin will have the same slope as the cable and will be collinear with it. The distributed spaced burial forces, due to the devices on the cable, can be arranged to enforce a known fixed slope on the linearized buried cable so that the angle [3 is never large enough to reduce substantially the angle of attack of the terminal anchor from the maximum. The anchor will thus continue to function effectively and produce a substantially constant burial force irrespective of the depth of penetration of the anchor and cable below the sea bed surface.

Furthermore, the finned devices mounted on the cable are each capable of producing a burial force equal to the lifting force developed by the portion of cable forming an interval between adjacent finned devices in addition to the burial force necessary to bury the finned device itself, so no increase of upward lifting force will be impressed on the terminal anchor due to increase of the buried length of the cable with increasing depth of burial of the anchor. Therefore, all the limitations to the depth of burial of an anchor and cable 7 mooring system will be eliminated or substantially mitigated if finned devices of the present invention are distributed by spacing along the cable. The holding power of such an anchor and cable mooring system will depend only on the depth of anchor-penetrable soil beneath the sea bed surface provided that finned devices are mounted on a sufficient length of cable adjacent to the terminal anchor. Given an adequate depth of anchor-penetrable mooring bed, the holding power may be chosen to have a maximum as close to the breaking strength of the anchor cable as desired simply by mounting a sufficient number of finned devices on the cable to allow the terminal anchor to penetrate far enough under the sea bed surface.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which FIG. 1 shows a pictorial view of an anchor for use in the present invention;

FIG. 2 shows a modified form of the anchor of FIG.

FIG. 3 shows a cross-sectional view of the anchor body of the anchors of FIGS. 1 and 2 through the section A-A;

FIG. 4 shows a sectional side view of a fin device for a cable, according to a first embodiment of the present invention;

FIG. 5 shows an oblique underside view of the fin device of FIG. 4;

FIG. 6 shows a front view of the fin device of FIG. 4 looking in the direction of arrow A in FIG. 4;

FIG. 7 shows an oblique top view of the fin device of FIG. 4;

FIG. 8 shows a side view of a fin device according to a second embodiment of the second aspect of the present invention;

FIG. 9 shows schematically the use of a conventional anchor for mooring a ship; and the useof fin devices according to FIGS. 4 or 8 on a mooring cable.

Referring to FIGS. 1, 3 and 9, an anchor 1 (FIG. 1) for mooring a vessel 2 (FIG. 9) to the sea bed 3 includes a deep elongate bar 4 of generally rectangular cross-section (see FIG. 3) but with the top and

bottom edges

5, 6 chamfered to assist penetration of the bar 4 into the sea bed 3 in the manner of edgewise penetration of a knife blade. Behind the nose portion 7 of the bar 4, a pair of opposed flukes 8 are pivotably mounted together on a pivots 9 on opposed side faces 10, 11 of the bar 4 so that they can pivot about an axis normal to the vertical side faces 10, 11 of the bar 4 and the bills 12 of the flukes 8 project forwardly. The nose portion 7 of the bar 4 has sufficient length L forward of the'pivot axis 9 of the flukes 8 to prevent tipping of the anchor l on engaging the sea bed or in the event of the flukes 8 snagging on a rock or similar obstruction.

Stops 13 are provided on the top and bottom of the bar 4 (top one only being shown) to limit the pivoting of the flukes 8 to an angle of between to 40 on either side of the longitudinal axis of the bar 4; but a maximum pivoting angle of up to 55 is envisaged and could be substituted. Also, a shank 14 is pinned to the nose 7 and stops 15 are provided on the nose pan of the bar to limit the pivoting of the shank 14 to a maximum of 90. A pair of opposed stocks 16, normal to the

side walls

10, 11 of the bar 4 are attached to the flukes 8 and maintain the rotational stability of the anchor 1 about the axis of the bar 4 during penetration of the mooring bed surface 3. Also, tripping palms 8A are provided.

At the end 17 of the bar 4 remote from the nose 7, a pair of opposed stabilizer fins 18 are attached to the side faces 10, ll of the bar 4 and project normally therefrom, the plane of the fins 18 being parallel to the axis of the bar 4. The purpose of these fins 18 is to counteract and so mitigate against pivoting of the anchor 1 about a horizontal axis transverse to the longitudinal axis of the bar 4 when the anchor is buried in the sea bed. The function of the stocks 16 may be achieved, alternatively, as a secondary function of these stabilizer fins 18 by suitably increasing their span transverse to the longitudinal axis of the bar 4 and such an anchor is shown in FIG. 2 where the stocks 16 are dispensed with.

When a pull is imparted to the cable 19 due to movement of the ship, the flukes 8 which will have an angle of attack relative to the sea bed 3 equal to the angle of inclination a to the baraxis which may be up to will dig into the sea bed 3. With further horizontal movement-of the anchor 1, further burial of the anchor 1 below the surface of the sea bed 3 will take place. The angle of attack of the flukes 8 relative to the sea bed 3 will tend to change to conform with the definition of angle of attach mentioned earlier. However, the buried stabilizer fins 18 will control the angle of attack of the anchor l and maintain it close to the angle of inclination a of the flukes 8 with respect to the longitudinal axis of the anchor bar 4.

As the anchor 1 thus buries further below the sea bed surface, the buried portion 19A of the cable 19 will adopt an approximately catenoidal configuration. Consequently, the shank 14 will be inclined upwards at an angle [3 relative to the axis 4A of the bar 4, and the pull on the shank 14 will tend to pivot the bar 4 such that the angle of attack of the flukes with respect to the sea bed 3 is decreased. However, the stabilizer fins 18 will counteract and mitigate against such pivoting of the bar 4; and the anchor flukes will therefore tend to maintain their angle of attack and so permit the anchor to continue to operate effectively.

In the anchor l the angle of attack a is maintained by the stabilizer fins 18 despite increase of the angle B Referring to FIGS. 4 to 8 and 9, a cable 20 for attachment to an anchor 21 as above described for mooring a ship to the sea bed has one or more spaced fin devices 22 adjacent to the anchor end thereof. The spacing H of the fin devices 22 may be at some suitable interval such that the burial force generated by each device 22 can bury the device 22 and, in addition, counteract the vertical drag resistance of an interval of cable 20 between adjacent devices 22. For use with wire cable (FIGS. 4 to 7) each fin device 22 includes a rotatable sleeve 23 (FIG. 4) on the cable and a pair of

collars

24, 25 one at each end 23A, 23B, of the sleeve 23 and locked to the cable 20, for locating the sleeve 23 on the cable 20. A pair of ofiset opposed fins 26 (FIGS. 6 and 7) project from the sleeve, are inclined relative to each other (see FIG. 7) and also are inclined relative to the axis of the sleeve 23 such that, with the cable 20 positioned horizontally at the sea bed 3, the

- portion of the adjacent cable is produced as the cable moves through the sea bed soil. The leading collar .24 hasan end wall 27 with an annular recess 28 therein, and the leading end of the sleeve 23A is inserted into i this recess 28; and the other (trailing) end 23B of the sleeve 23 has a recess 29 into which the trailing collar is inserted to position the sleeve 23 on the cable 20.

A rearwardly projecting annular extension 30 of the leading collar 24 serves as a bearing for the sleeve 23. A further fin 31 is provided between the opposed fins 26 and projects normally from the sleeve 23. This third fin 31 is located in a plane containing the axis of the sleeve 23.

Referring to FIG. 8, for use with chain cable, each fin device 22 includes a rotatable sleeve 40 which has its forward end 40A located and supported in a forward fairing 41 furnished with a chain ring 42 and the trailing end 40B of the sleeve 40 accommodates the leading end of a trailing fairing 43. The fairing 41, 43 are linked with the chain and perform the function of a chain link. A pair of offset opposed fins 44 project from the sleeve 40, corresponding to the fins 26 of the FIG. 4 arrange ment, and operate similarly. The forward fairing 41 has an end wall 45 with an annular recess (not shown) therein, and the leading end 40A of the sleeve 40 is inserted into this recess and the other (trailing) end 408 of the sleeve 40 has a similar recess (not shown) into which a retaining collar may be fitted while encircling a rearwards projecting cylindrical projection of the forward fairing 41 which serves as a bearing for the sleeve. The aft chain ring 42 of the forward fairing 41 is mounted at the rear of the rearwards projecting cylindrical projection of the forward fairing 41. As with the device 22 of FIG. 4 a further fin 46 is provided between the opposed fins 44 and projects normally from the sleeve while located in a plane containing the axis of the sleeve 40.

The cable system is intended for use particularly in a soft sea bed of the type hereinbefore described.

ln the operation of the anchor-cable system, say for mooring a ship 1 to a soft sea bed 3, the length of cable 20 used is several times the depth of water at the mooring location so that the portion 20A of the cable adjacent the anchor 21 lies substantially horizontal along the sea bed. When a pull is imparted to the cable 20 due to the movement of the ship 1, the anchor 21 and the portion of the cable 20A adjacent thereto penetrate into the sea bed 3. Penetration of the sea bed surface by the cable 20 under the action of the terminal anchor 21 causes the fin devices 22 to engage the sea bed with the result that burial forces are developed at the

inclined fins

26, 44 of the fin devices 22. These burial forces ing behind the cable due to the action of the

central fin

31, 46 and .the offset disposition of the

burial fins

26, 44.

It will be understood that the fin devices 22 will automatically establish the correct trailing aspect, on engaging the sea bed 3, owing to the separation of the center of area of the

fins

26, 44 from the axis of the cable and the disposition of the fins as a group.

Thus, the angle of inclination of the anchor end of the cable to the horizontal, B is maintained smaller than would be the case if the cable were not fitted with the fin devices and consequently allowed to adopt the approximately catenoidal configuration hereinbefore discussed. This small value of B is invariant with depth of burial of the cable and establishes a determinable reduction. of the angle of attack of any conventional anchor used as a terminal anchor such that-it can-continue to produce a useful burial force at the cable end.

To prevent clogging of the end clearances between the

sleeve

20, 40 and the collars of fairings 24, .25 (or 41) by the material of the sea bed,for' example sand, ample clearances are provided between the sleeve and the collars to permit the sand etc. to pass freely from the clearances. Modifications are, of course, possible in the fin devices, for example, the fins could be simply mounted on a pair of spaced collars on the wire cable and between a sleeve of the cable serving to locate the device. Again, a sufficient degree of freedom of movement (or play) would be arranged between the collarsand the cable/sleeve to prevent clogging by sand or the like. Also, it would, of course, be possible to dispense with the

middle fin

31, 46.

It will be understood that, for permanent mooring applications where an anchorand cable is positioned underwater on the'sea bed by divers, there will be no necessity for pivoting of the flukes of the anchors 1 and of the

burial fins

26, 44 in the devices 22. Thus a fixed vention and as above described can meet the following criteria for satisfactory burial efficiency 1. The terminal anchor must be able to engage and penetrate the sea bed surface with maximum efiiciency.

2. The cable must be able to resist any tendency to increase its slope with depth of burial.

3. The cable must be capable of producing burying forces distributed along its length in order to continuously counteract the penetration resistance of the cable as it is drawn underground.

Whereas, hereinbefore, the present invention has been described for mooring vessels at sea, it will be understood that the invention is equally applicable for mooring vessels in rivers, lakes or like waters. Further,

the cable system could be used with a conventional anchor.

lclaim:

1. An anchor cable, including a cable for attachment at one end to an anchor for mooring an object to a mooring bed, one or more spaced finned burial devices adjacent to the anchor end of the cable and including means of attaching the device co-axially to the cable, at least one finned member on each device including a burial surface which in a working vertical burial attitude of the device is inclined relative to the cable axis and adapted to produce burial forces capable of burying the finned device and counteracting the vertical drag forces developed by the adjacent cable on penetration of the cable including the device into the mooring bed so that the buried portion of the cable is constrained to depart from an approximately catenoidal configuration and tend to adopt a substantially linear or reverse curvature configuration.

2. An anchor cable as claimed in claim 1, wherein the angle of inclination of the burial surface relative to the cable axis is within the range to 60.

3. An anchor cable as claimed in claim 2, wherein said angle of inclination is within the range 30 to 60.

4. An anchor cable as claimed in claim 1, wherein the finned member is an inclined fin forming part of the device.

5. An anchor cable as claimed in claim 4, wherein the center of area of the inclined fin is offset from the axis of the cable.

6. An anchor cable as claimed in claim 4, wherein at least three fifths of the total burial fin area of each device lies at one side of the cable axis.

7. An anchor cable as claimed in claim 5, wherein the fin-bearing portion of each finned device is pivotably mounted on the cable to permit the offset fins to trail behind the cable and so adopt automatically an effective working orientation for the fins.

8. An anchor cable as claimed in claim 4, wherein each burial device has two or more fins inclined relative to each other as well as to the cable axis such that the device is symmetrical about a vertical plane bisecting it when in a working vertical burial attitude, and such that the fins are in an anhedral configuration.

9. An anchor cable as claimed in claim 8, wherein the disposition of the burial fin surfaces of each device is such that, when interacting with the mooring bed surface in a reverse attitude to the working vertical burial attitude, the resultant force vector produced by the portions of burial fin lying at either side of the vertical plane of symmetry bisecting the device, and making contact with the surface material of the mooring bed, lies on a straight line intersecting the vertical plane of symmetry at a point below the axis of the portion of the cable trailing behind the device whereby a turning moment is formed about the axis of the cable which causes the device to be unstable in the reverse attitude to'the working vertical burial attitude and stable when in the working vertical burial attitude.

10. An anchor cable as claimed in claim 4, wherein a plurality of devices are mounted on the cable, the angles of inclination relative to the cable axis of the inclined surfaces of all of the devices being equal in magnitude to each other.

11. A finned burial device for use with an anchorcable for mooring an object to a mooring bed, including means for attachment of the device to an anchor cable so that the longitudinal axis of the device is coaxial with the cable and at least one finned member including a burial surface which in a working vertical burial attitude of the device is inclined relative to the Iongitudinal axis of the device and adapted to produce burial forces capable of burying the finned device and counteracting the vertical drag forces developed by the adjacent cable on penetration of the device into the mooring bed.

12. A finned device as claimed in claim 11, wherein the angle of inclination of the burial surface relative to the longitudinal axis of the device is within the range 15 to 60.

13. A finned device as claimed in claim 12, wherein said angle of inclination is within the range 30 to 60.

14. A finned device as claimed in claim 11, wherein the finned member is an inclined fin forming part of the device.

15. A finned device as claimed in claim 14, wherein each burial device has two or more fins inclined relative to each other as well as to the cable axis such that the device is symmetrical about a vertical plane bisecting it when in a working vertical burial attitude, and such that the fins are in an anhedral configuration.

16. A finned device as claimed in claim 15, wherein the disposition of the burial fin surfaces of each device is such that, when interacting with the mooring bed surface in a reverse attitude to the working vertical burial attitude, the resultant force vector produced by the portions of burial fin lying at either side of the vertical plane of symmetry bisecting the device, and making contact with the surface material of the mooring bed, lies on a straight line intersecting the vertical plane of symmetry at a point below the axis of the portion of the cable trailing behind the device whereby a turning moment is formed about the axis of the cable which causes the device to be unstable in the reverse attitude to the working vertical burial attitude and stable when in the working vertical burial attitude.

17. A finned device as claimed in claim 14, wherein at least three fifths of the total burial fin area of each device lies at one side of the longitudinal axis of the device.

18. A finned device as claimed in claim.l4, wherein the center of area of the inclined fin is offset from the longitudinal axis of the device.

19. A finned device as claimed in claim 14, wherein the fin-bearing portion of each finned device is pivotably mounted on the cable to permit the offset fins to trail behind the cable and so adopt automatically an efiective working orientation for the fins.

20. A finned device as claimed in claim 19, wherein there are provided a forward clamping collar positionable on a wire hawser cable and having an annular recess on an end wall thereof, a sleeve on which are mounted a pair of offset opposed fins inclined relative to the axis of the collar and to each other and a third fin between the opposed fins and upstanding from the sleeve, the leading end of the sleeve being insertable into the recess of the forward collar whilst the trailing end of the sleeve is provided with a recess, and a trailing clamping collar insertable into the recess of the sleeve to locate the sleeve rotatably in position on the wire hawser cable.

21. A finned device as claimed in claim 19, and for location on a chain cable, wherein there are provided a forward fairing carrying a chain ring and having an annular recess on an end wall thereof, a sleeve on which are mounted a pair of offset opposed fins inclined rela- ,tive to the axis of the fairing and to each other and-a I fitted while encircling the rear end of the cylindrical projection of the forward fairing,

23. A finned device as claimed in claim 22, wherein a chain ring is positioned at the rear of the projection and both retaining collar and chain ring secured to the projection.

for mooring an object to a mooring bed, an anchor cable attached at oneend to the anchor, one or more spaced finned burialidevices adjacent to the anchor end of the cable, and including means for attaching the device co-axially to the cable, at least one finned member on each device including a burial surface which in a working vertical burial attitude of the device is inclined relative to the cable axis and adapted to produce burial forces capable of burying the finned device and counteracting the vertical dragforces developed by the adjacent cable on penetration of the cable including the device into the mooring bed so that the buried portion of the cable is constrained to depart from an'approximately catenoidal configuration and tend to adopt a substantially linear or reverse curvature configuration.

Claims

2. An anchor cable as claimed in claim 1, wherein the angle of inclination of the burial surface relative to the cable axis is within the range 15* to 60* .

3. An anchor cable as claimed in claim 2, wherein said angle of inclination is within the range 30* to 60* .

9. An anchor cable as claimed in claim 8, wherein the disposition of the burial fin surfaces of each device is such that, when interacting with the mooring bed surface in a reverse attitude to the working vertical burial attitude, the resultant force vector produced by the portions of burial fin lying at either side of the vertical plane of symmetry bisecting the device, and making contact with the surface material of the mooring bed, lies on a straight line intersecting the vertical plane of symmetry at a point below the axis of the portion of the cable trailing behind the device whereby a turning moment is formed about the axis of the cable which causes the device to be unstable in the reverse attitude to the working vertical burial attitude and stable when in the working vertical burial attitude.

11. A finned burial device for use with an anchor-cable for mooring an object to a mooring bed, including means for attachment of the device to an anchor cable so that the longitudinal axis of the device is co-axial with the cable and at least one finned member including a burial surface which in a working vertical burial attitude of the device is inclined relative to the longitudinal axis of the device and adapted to produce burial forces capable of burying the finned device and counteracting the vertical drag forces developed by the adjacent cable on penetration of the device into the mooring bed.

12. A finned device as claimed in claim 11, wherein the angle of inclination of the burial surface relative to the longitudinal axis of the device is within the range 15* to 60* .

13. A finned device as claimed in claim 12, wherein said angle of inclination is within the range 30* to 60* .

18. A finned device as claimed in claim 14, wherein the center of area of the inclined fin is offset from the longitudinal axis of the device.

19. A finned device as claimed in claim 14, wherein the fin-bearing portion of each finned device is pivotably mounted on the cable to permit the offset fins to trail behind the cable and so adopt automatically an effective working orientation for the fins.

21. A finned device as claimed in claim 19, and for location on a chain cable, wherein there are provided a forward fairing carrying a chain ring and having an annular recess on an end wall thereof, a sleeve on which are mounted a pair of offset opposed fins inclined relative to the axis of the fairing and to each other and a third fin between the opposed fins and upstanding from the sleeve, the leading end of the sleeve being locatable in the recess of the forward fairing and fitting pivotably about a rearwardly projecting cylindrical projection of the fairing which serves as a bearing for the sleeve.

22. A finned device as claimed in claim 21, wherein the trailing end of the sleeve is provided also with an annular recess into which a retaining collar may be fitted while encircling the rear end of the cylindrical projectiOn of the forward fairing.

24. An anchor-cable system, comprising an anchor for mooring an object to a mooring bed, an anchor cable attached at one end to the anchor, one or more spaced finned burial devices adjacent to the anchor end of the cable, and including means for attaching the device co-axially to the cable, at least one finned member on each device including a burial surface which in a working vertical burial attitude of the device is inclined relative to the cable axis and adapted to produce burial forces capable of burying the finned device and counteracting the vertical drag forces developed by the adjacent cable on penetration of the cable including the device into the mooring bed so that the buried portion of the cable is constrained to depart from an approximately catenoidal configuration and tend to adopt a substantially linear or reverse curvature configuration.