A marine anchor comprises a fluke with a shank attached to the fluke to enable the anchor to be joined to an anchor cable. Additionally there is provided a soil barrier plate located aft of the rear of the fluke but above the level of the fluke, with a soil passage between the barrier plate and the fluke. The barrier plate is set an angle to the fluke, and the barrier plate and the associated soil passage are arranged so as to function in a manner enabling the anchor to operate effectively and without adjustment in cohesive soils such as mud even when the fluke is set (say at an attack angle 0.degree. of 30.degree.) for optimum operation in non-cohesive soils such as sand, without substantially detracting from the performance of the anchor in non-cohesive (sand) soils.

BACKGROUND OF THE INVENTION 
The present invention relates to fluked burial devices adapted for burying 
into a soil and more particularly to marine anchors, cable depressors and 
such-like fluked devices adapted for burying into submerged soil. 
A marine anchor comprising a shank with a cable attachment point at the 
forward end and a fluke structure attached thereto has a fluke angle 
.theta. defined by the angle between the fore-and-aft central line of the 
fluke structure and the line from the said cable attachment point to the 
rear of the fluke structure measured in the vertical plane of symmetry. Up 
until now, this angle .theta. has been in the range 28.degree. to 
50.degree. with the anchor embedded in the soil. Fluke angles in the range 
28.degree. to 35.degree. have generally been found to give optimum anchor 
performance in granular non-cohesive soils such as sand and gravel, since 
this relatively low fluke angle enables the anchor fluke more readily to 
penetrate the firmer soils formed of sand or gravel. On the other hand a 
fluke angle of approximately 50.degree. has been found necessary to give 
optimum performance in cohesive soils such as soft clay and mud. This is 
due to the fact that in such cohesive soils as mud, the forward end of the 
shank of the anchor tends to tilt upwardly when the anchor is in the fully 
buried condition thereby seriously reducing the actual or effective angle 
of attack of the fluke. Provision of the relatively high fluke angle of 
50.degree. enables this operational disadvantage to be substantially 
overcome and satisfactory anchor holding force maintained. 
For ship use, anchors usually have a fluke angle in the region of 
40.degree. to provide a reasonable compromise performance when used in 
either non-cohesive or cohesive soils. For offshore drilling vessels or 
pipelaying barges using multiple anchor spread moorings, anchors generally 
have means for adjusting the fluke angle to give optimum performance 
according to the soil type in which the anchors are deployed. 
Unfortunately, the nature of the mooring bed soil often is unknown prior 
to deploying anchors and several anchors may be deployed before it is 
realised that incorrect fluke angles have been selected. These anchors 
must then be retrieved for fluke angle adjustment and redeployed. This 
wastes time and consequently incurs high costs. 
It is an object of the present invention to obviate or mitigate these 
disadvantages. 
According to one aspect of the present invention there is provided a fluke 
burial device, particularly an anchor having a burial fluke member 
orientated to provide a positive burial angle for digging into a bed of 
soil when the burial device is in the vertical working burial attitude, a 
cable attachment member attached to said fluke member, soil barrier means 
located substantially above the burial fluke member when the burial device 
is in said vertical working burial attitude such that a straight line from 
a foremost extremity of the fluke member to an upper edge of the soil 
barrier means lies in the range 8.degree. to 24.degree. to the upper 
surface of the fluke member, a major portion of the soil barrier means 
being located aft of the rear edge of the burial fluke member and such 
that the rear of the soil barrier means has a horizontal separation from 
the rear of the burial fluke member not more than half the overall 
longitudinal length of the fluke member, said soil barrier means including 
at least one soil barrier surface which is inclined relative to said fluke 
member, said soil barrier surface having an area less than the upper 
surface area of the fluke member, and passage means associated with said 
soil barrier means to permit escape of non-cohesive soil passing over the 
fluke member. 
According to another aspect of the present invention there is provided a 
fluke burial device, particularly an anchor having a burial fluke member 
orientated to provide a positive burial angle for digging into a bed of 
soil when the burial device is in the vertical working burial altitude, a 
cable attachment member attached to said fluke member, soil barrier means 
located substantially above the burial fluke member when the burial device 
is in said vertical working burial altitude, the major portion of the soil 
barrier means being located aft of the rear edge of the burial fluke 
member and such that the rear of the soil barrier means has a horizontal 
separation from the rear of the burial fluke member not more than half the 
overall longitudinal length of the fluke member, said soil barrier means 
including at least one soil barrier surface which is inclined with a 
forwardly opening acute angle relative to said burial fluke member, said 
soil barrier surface having an area less than the upper surface area of 
the fluke member, and soil passage means associated with said soil barrier 
means to permit escape of non-cohesive soil passing over the fluke member.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1 to 3 show anchors in an operative position which is hereinafter and 
in the claims referred to as "the vertical working burial attitude". In 
this attitude the central symmetry plane of the anchor (i.e. the plane CP 
of FIG. 5) which extends normally from the fluke and on either side of 
which plane CP the anchor is symmetrical, is arranged vertically, and the 
anchor is orientated in this vertical plane so that the fluke is capable 
of digging into the mooring bed soil, the cable attachment end of the 
shank and the toe of the fluke occupying forward positions. 
With reference to FIG. 1, an inclined anchor fluke 2 of a shallow buried 
anchor 1 moving horizontally in non-cohesive soil 3 such as sand causes 
the sand to move relative to the anchor upwards and parallel to the fluke 
into a heap 4 over the fluke whilst a void 5 tends to form under the fluke 
2 and a depression 6 forms in the sand aft of the heap 4. The depression 6 
has forward and after slopes each inclined at an angle of repose of the 
sand which is approximately equal to the angle of internal friction of the 
sand in a loose state, ranging from 28.degree. to 34.degree., and is the 
angle to the horizontal of the slope of a heap produced by pouring sand 
from a small height onto a horizontal plane. Displaced sand, which has 
passed through the heap over the anchor fluke 2, continuously slides down 
the rear slope of the heap and over the rear edge 7 of the fluke 2 to fall 
into the void 5 below in which it slides down another slope at the angle 
of repose prior to making an exit aft by relatively moving in a direction 
opposite to the movement of the anchor. The direction of relative movement 
of sand in the region above and aft of the fluke 2 is thus inclined at an 
angle to the fluke in the range 38.degree. to 64.degree. for anchor 
attitudes giving fluke inclinations to the horizontal in the range 
10.degree. to 30.degree.. A barrier plate 8 located at BC parallel to the 
local direction of relative sand flow should not disrupt the sand flow 
pattern and should not, therefore, inhibit optimal performance of the 
anchor in non-cohesive soil. 
When the anchor fluke becomes more deeply buried in non-cohesive soil, soil 
pressure from the rear slope of the depression 6 alters the direction of 
sand flow off the heap 4 along the angle of repose until ultimately a 
vertical funnel or `pipe` forms from the bottom of the depression to the 
rear of the anchor fluke. Displaced loose sand falls down this pipe into 
the transient void 5 beneath the inclined moving fluke 2 before relatively 
flowing away aft in the direction opposite to that of anchor movement. The 
angle of the barrier plate 8 may therefore be required to be angled as 
much as 120.degree. to the fluke to remain edge-on to sand flow in the 
`pipe` at the rear of the fluke 2. In practice, the pipe of falling loose 
sand will bend round to follow the inclination of the barrier plate 8 with 
the result that a smaller angle between plate and fluke more suitable for 
minimum flow disturbance at shallow burial depth is satisfactory even for 
deep burial. 
With reference to FIG. 2, the anchor of FIG. 1, having a fluke angle 
.theta. of 30.degree., adopts a much smaller fluke inclination to the 
horizontal (i.e. actual angle of attack) when moving in cohesive soil such 
as mud. The cohesion of the soil prevents it from cascading into the 
underfluke void 5 which in consequence, streams out behind the fluke. No 
abrupt change in relative soil flow direction occurs as soil moves into 
the region immediately aft of the fluke. A barrier plate 8 in this region, 
located at BC as before, would be substantially athwart the direction of 
relative soil flow and would therefore greatly disrupt the flow pattern. 
The overall change in the relative flow pattern of mud brought about by a 
barrier 8 at location BC is shown in FIG. 3. On entering the soil, mud 
flows initially parallel to the fluke upper surface until a stalled wedge 
of mud accumulates on the forward face of the barrier plate 8 as indicated 
in section by the dashed triangle BCD. The fluke upper surface and face DC 
of the stalled mud wedge together form a rapidly converging passage 
constituting a choke gap having high resistance to mud flow therethrough. 
This high resistance to flow induces additional mud to dwell over the 
fluke upper surface whereby a dynamically stable and much larger mud wedge 
ABC forms. This large mud wedge effectively moves with the fluke (although 
some mud may flow slowly through the choke gap) and serves to increase the 
fluke angle from the 30.degree. optimum for sand to the desired 50.degree. 
optimum for mud by inducing shearing of the mud along line AB at 
20.degree. to the fluke upper surface. Additionally, deflection of mud 
relative flow by the wedge ABC over the barrier greatly increases the size 
of the void 9 and so increases the suction contribution to horizontal load 
in the anchor line. 
The barrier may be perforated with holes or slots allowing even more mud to 
pass through the barrier but, due to the retardation of mud flow in zone 
ADC, a dynamically stable wedge ABC remains with shearing of the mud still 
occurring along line AB and producing the desired increase in effective 
fluke angle .theta. from 30.degree. to 50.degree. (.theta..sup.1). Such a 
perforated barrier is advantageous for a hinged fluke anchor to permit 
ultimate escape aft of non-cohesive soil falling into the under-fluke void 
which otherwise would be prevented from relatively flowing aft out of the 
void since the barrier would require to be symmetrical about the plane of 
the fluke. 
Referring to FIGS. 4 to 6, a marine anchor 51 comprises a fabricated hollow 
fluke 52 having a substantially planar upper surface 53, and a cranked 
form shank 54 attached to the rear of the fluke 52. The fluke 52 is of 
double-toed form (55) and has a width W greater than the overall 
longitudinal length L of the fluke 52 (by for example 50% approximately), 
the length L being the distance between the front and rear extremities of 
the fluke 52, while the shank 54 has double legs 56, 57 and is in 
accordance with the applicant's European Patent 0020152. The shank 54 
includes transverse strengthening plates 58 and these together with fluke 
surface 53 form non-converging open ended passages 59 in the shank; the 
legs 56, 57 include forward inclined burial portions 56A, 57A while a 
cable attachment hole 60 is at the forward end of the shank. The legs 56, 
57 are of cranked form presenting leg portions 61, 62 and a feature of the 
present shank arrangement is that the medial lines M of these leg portions 
intersect with an acute angle S so that the back of the shank 54 projects 
rearwardly from the rear of the fluke 52. 
The fluke 52 is set at an angle .theta. of approximately 30. For the 
purpose of maintaining an effective fluke angle of attack (or 
alternatively satisfactory fluke forwardly projected area) when the anchor 
is burying in soft cohesive soils, e.g. soft mud, a soil barrier member 63 
is carried by the leg portions 62 of the shank and extends transversely 
relative to the fluke centre line C--C and has a width approximately 28% 
of the fluke length L. The barrier can have a working area of 10% to 65% 
of the fluke area, and preferably 20% to 50% of the fluke area. The 
barrier member 63 can be of steel fabricated hollow construction with a 
triangular cross section, and in this embodiment the leading (working) 
surface 64 is inclined at an angle B to the fluke centre line C--C of 
approximately 45.degree., i.e. negatively (up to 90.degree.) relative to 
the fluke working surface 53, but the angle B could be in the range 
30.degree. to 90.degree.. Further, a soil flow passage 65 is present 
between the barrier member 63 and the fluke 52 and extends laterally from 
the centre plane CP. The width P of the soil flow passage 65 is defined by 
the distance between the soil barrier 63 and the rear edge of the fluke 52 
and in FIG. 4 the width P has a value of approximately 30% of the fluke 
length L, but this could be as high as 40% of length L. 
As can be seen in FIG. 4, the barrier member 63 is located roughly adjacent 
the elbow of the cranked shank 54 but does not extent beyond the back edge 
of the shank: on the other hand, it is a significant feature that the 
barrier member 63 extends beyond the rear edge of the fluke 52. Indeed, in 
this example the member 63 is fully beyond the rear of the fluke 52. In 
particular in this embodiment the axial distance S of the leading edge of 
the member 63 from the fluke rear edge is approximately 8% L but S could 
be in the range 5% to 40% L. With the barrier member 63 located aft as 
shown, there is no part of the anchor construction directly below the 
working surface 64 of the member 63 so that soil deflected from the 
surface 64 can fall vertically without obstruction from any part of the 
anchor. 
A pair of auxiliary fluke devices 66, 67 are formed integrally with the 
ends of the barrier member 63 (the transition is shown dashed in FIGS. 4 
and 5), the fluke devices 66, 67 each having a working surface co-planar 
with the surface 64. It will be noted that the barrier member 64 extends 
substantially over the width of the fluke 52 but does not extend beyond 
the longitudinal extremity lines E--E of the fluke width, while the fluke 
devices 66, 67 on the other hand do extend beyond the lines E--E. The 
auxiliary fluke devices 66, 67 are intended to right the anchor from an 
inverted position on the sea bed surface by rolling when dragged thereover 
and also to provide a degree of dynamic stability when the anchor is 
buried. 
The fluke angle .theta. of 30.degree. is compatible with the fluke angle 
for non-cohesive soils for a conventional anchor. When the anchor 51 of 
FIGS. 4 and 6 is burying in a non-cohesive soil such as sand, the theory 
set out previously in the specification will apply; thus, the barrier 
member 63 will be orientated approximately parallel to the sand repose 
direction R at the rear of the anchor so that the member 63 will not 
substantially disrupt the sand flow and thereby inhibit optimum 
performance of the anchor in sand. When the anchor 51 is burying in a 
cohesive soil, such a soft clay or soft mud (where in a conventional 
anchor a fluke angle .theta. approaching 50.degree. would be desired) the 
flow of cohesive soil reacts with the surface 64 to maintain the effective 
fluke angle, or alternatively maintain the forwarded projected fluke area 
of the anchor in the direction of relative movement of the soil. 
Impingement of soil on the barrier surface 64 will cause the anchor to 
pivot about an axis extending transversely through the cable attachment 
hole 60 to decrease the effective area of surface 64 but increase the 
effective area of fluke surface 53. The total area of the working surfaces 
of the barrier member 63 and the fluke devices, 66, 67 may be 
approximately 0.44.times.the area of the fluke 52. Since the barrier 
member 63 is set at an angle .beta. of 45.degree. to the fluke, the 
projected area of the working surface of items 63, 66, 67 in a direction 
parallel to the fluke is 0.44.times.fluke area.times.sin 45.degree. which 
equals 0.31.times.fluke area. This produces the same forward projected 
area of the anchor as when the angle of the main fluke 53 is increased 
through 18.degree. since sin 30.degree.=0.31. There should be no 
substantial build up of cohesive soil on the fluke surface 53 during 
movement of the anchor and soil impinging on the surface 64 can be 
deflected downwards and rearwardly freely. 
The fluke 53 in the embodiment of FIGS. 7 to 9 is generally similar to that 
of FIGS. 4 to 6 but includes side lugs 68, 69 in accordance with U.K. 
Patent 1356259; these side lugs 68, 69 serve to provide dynamic stability 
in the anchor and may possible also orientate the anchor upright from a 
inverted position. Further, the barrier member 70 in this embodiment is 
set at a positive angle (i.e. greater than 90.degree.) relative to the 
fluke surface 53, the angle B being approximately 127.degree. and the 
fluke devices 66, 67 are not present. The passage 65 in FIG. 9 has a 
smaller width P than that of FIG. 6 and this width may be only 5% to 20% 
L, 10% L is shown, i.e. the passage 65 is substantially of choke gap form. 
Again, the member 70 is located fully beyond the rear of fluke 52, and the 
shank 54 is generally similar to that of FIG. 6. Again, the member 70 does 
not extend beyond the back of the shank. The member 70 will function 
generally in accordance with the theory set out previously in the 
application and this will involve the build up of cohesive soil material 
on the working surface 71 of the member 70. 
It will be understood that the negatively set barrier member 63 of FIGS. 4 
to 6 could be used in place of barrier 70 in FIGS. 7 to 9 and the 
auxiliary fluke devices 66, 67 may or may not be present in this case. 
Also the barrier 70 (or 63) could be joined to upstanding lugs 68, 69 and 
to this end the barrier could be swept forwardly. The anchor of FIG. 10 is 
similar to that of FIGS. 7 to 9, but in this case two separate barrier 
members 70A, 70B are provided with the first set at a greater obtuse angle 
.beta. than the second. The arrangement is such that an additional soil 
passage 65A is provided between members 70A, 70B. Operation is generally 
similar to that of FIGS. 7 to 9. 
FIGS. 11, 12A and 12B show the inventive soil barrier construction of FIGS. 
4 to 6 applied in a pivotal shank (i.e. Danforth) type anchor. To recap, 
the desirable constructional features for the barrier are (1) location 
beyond the rear of the fluke and when the anchor is in the vertical 
working burial attitude always at the upper side of the fluke for 
operation, and (2) no soil flow obstructing structures directly below the 
barrier. The anchor of FIGS. 11 to 12B is designed to have these 
characteristics. 
The anchor of FIGS. 11 and 12 has a spaced double-fluke construction 72, 73 
with the shank 74 located between the flukes 72, 73. The flukes 72, 73 
include edge flanges 75 which blend into a fluke crown portion 76 having 
side cheeks 76A, and the shank 74 is pivotally mounted on a pin 77 in this 
crown portion 76. Crown stop plates 78 limit the pivoting of the shank 74 
by virtue of the shank tail portion 74A abutting against one of these 
plates 78A as shown. Also, the lateral mid-plane M--M through the flukes 
72, 73 is shown in FIG. 12A. 
A soil barrier member 79 carries edge plates 80, 81 which are pivotally 
attached to outer edges of the flukes 72/73 by pins 82, the soil barrier 
member 79 extending laterally only minimally beyond the outer edges of the 
flukes. A mechanism is provided for appropriate pivoting of the member 79, 
this mechanism comprising a slot 83 in the shank tail portion 74A which 
engages a pin 84 carried by lug means 85 on the member 79, the lug means 
85 being pin-jointed to the cheeks 76A via pins 85A which are aligned with 
pins 82. The shank has a part cylindrical portion 86 at the pin 77 
minimising clearance at the plates 80, 81 whereby ingress of soil, e.g. 
sand to block the slot 83 can be substantially avoided. 
In the initial unopened position of the anchor (as shown in FIG. 12A), the 
shank 74 is substantially parallel to the flukes 72, 73 with the pin 84 
located towards the forward end of the slot 83 so that the barrier member 
79 occupies the position shown in FIG. 12A i.e. at right angles to the 
flukes. In setting the anchor in an open operative position as shown in 
FIG. 12B, irrespective of which of the surfaces 53A, 53B constitute the 
fluke upper surfaces, pivoting of the shank 74/(anti-clockwise as shown) 
about the pin 77 (from the position shown in FIG. 12A), for relative 
pivoting apart of the shank 74 and the flukes 72, 73 (as shown in FIG. 
12B), causes the lug means 85 (with the barrier plate 79) to pivot about 
pins 85A in the opposite direction (i.e. clockwise) by virtue of the lug 
means 85 being joined to the shank tail 74A by the pin-and-slot connection 
84, 83. One side of the slot 83 bears on the pin 84 to cause the pivoting 
motion of the lug means 85. The slot 83 in the shank tail 74A permits the 
necessary movement of the pin 84: thus as can be seen by comparing FIGS. 
12A and 12B the pin 84 moves towards the rear end of the slot 83 when the 
shank 74 and the flukes 72, 73 are pivoted apart. Consequently the soil 
barrier member 79 is caused to pivot and take up a position (as shown in 
FIG. 12B) above and aft of the upper surface of the flukes. In this 
position, the barrier working surface 86 will have an angle .beta. of 
45.degree. to the fluke, and the barrier 79 will function similarly to the 
barrier 63 of FIGS. 4 to 6. Further, initially the shank and fluke will be 
fairly aligned (FIG. 12A), (with the barrier in the dashed position in 
FIG. 12B) and soil pressure reaction on the barrier 79 on initial anchor 
drawing will tilt the barrier 79, to force open the flukes 72, 73 and the 
shank 74. The side plates 80, 81 preferably provide anchor stabilising 
surfaces. 
FIG. 12B shows a line joining the front of the flukes 72, 73 to the top 
edge of the barrier member 79 and this line makes an angle of 8.degree. to 
24.degree. with the centre line M--M of the flukes. This angle range 
applies equally in the preceding embodiments. 
It will be understood that the present invention could be applied in other 
forms of anchor, and modification are possible. For example the width P of 
the soil passage could vary along the length of the passage, or may be 
uniform.