Apparatus for retrieving anchors

In the placement of an anchor in the sea bed for mooring of a vessel a chaser in the form of a loop or hook is attached to a service line and the chaser is positioned on the anchor to support the anchor. A service vessel with the service line secured thereto pulls the anchor and anchor chain from the parent vessel to be moored and simultaneously lowers the anchor to the sea bed via the service line. The anchor is subsequently set in the sea bed for mooring and the chaser can then be removed from the anchor chain or returned up the chain to the moored vessel. To retrieve the anchor, the chaser is run down the anchor chain until it engages the anchor, and the service vessel then pulls the anchor from the sea bed by means of the service line and takes the anchor aboard. The anchor is then returned to the parent vessel. The present invention provides a chaser wherein the chain engaging surface of the chaser is defined by an arc of radius R not less than twice the chain diameter (D) with the longest chord contained by the arc of length not less than 2.9D. Additionally the chaser includes burial surfaces to produce burial forces transverse to the direction of sliding of the chaser. This arrangement facilitates movement of the chaser along the anchor chain when in the sea bed, and also reduces wear on the chaser caused by rubbing on the chain.

The present invention relates to a chaser or retrieval device for placement 
or recovery of a marine anchor at the sea bed. 
At present, mobile floating oil drilling platforms and drilling vessels 
generally use anchor and cable mooring systems wherein placement and 
recovery of each anchor is achieved by means of a pendant or service line 
fastened at one end to a rear portion of the anchor and at the other end 
to a flotation buoy. 
The pendant is passed from the drilling vessel to an anchor handling ship 
which draws the anchor out from its rack on the drilling vessel as the 
anchor cable is paid out. When the anchor handling ship is over the anchor 
placement location, additional standard lengths of pendant wire are 
shackled together as the anchor is lowered to the bottom to give a 
combined length slightly in excess of the water depth. Finally, the 
flotation buoy is shackled on the pendant line and heaved overboard prior 
to setting the anchor by tensioning the anchor cable with the 
corresponding mooring winch on the drilling vessel. The placement cycle of 
anchoring is now complete. 
Recovery of the anchor is achieved by lassoing the flotation buoy, hauling 
it on board the supply vessel and detaching it from the pendant line. The 
pendant line is then shackled on to the anchor handling winch on board and 
heaved in once the anchor cable has been slackened off by the drilling 
vessel. Heaving in the pendant line breaks the anchor out of the sea bed 
and permits the anchor handling ship to remove the added lengths of 
pendant wire. Once the final length is stoppered off, the drilling vessel 
heaves in the anchor cable until the anchor is hove home on the anchor 
rack with the anchor handling ship being pulled towards the drilling 
vessel in the process. The pendant line is then transferred to the 
drilling vessel and the recovery cycle of anchoring is complete. 
This process involves repeated stoppering-off and shackling operations with 
heavy wire ropes, typically 21/4 inch diameter, and the handling of a very 
large heavy buoy. The size of the buoy and the length and weight of the 
pendant wires increases with water depth and hence so do the costs. The 
concomitant handling problems additionally place a limit to the depth of 
water which can be worked and to the wave-heights permissible for safe 
working. 
In view of these disadvantages of the above anchoring procedure, an 
alternative method of anchoring without using buoy-supported pendant lines 
has been proposed for use in the offshore drilling industry. This involves 
the use of a cable riding device, generally known as a "chaser" or anchor 
retrieval device, attached by a wire rope to an anchor handling ship, and 
the method is described in U.S. Pat. Nos. 3,927,636, 3,929,087 and 
3,921,782. 
Usually, the chaser has the form of an open or closed loop of steel which 
encircles the anchor cable and is stowed adjacent the anchor when the 
anchor is racked. Attached to the chaser is a wire rope pendant line which 
is passed to the supply ship as before. The supply ship draws the chaser 
on to the anchor shank so that the anchor is drawn out as the anchor cable 
is paid out until the anchor placement location is reached. A long towing 
line is shackled on to the chaser pendant and paid out until the 
chaser-borne anchor rests on the sea bed below the anchor-handling ship. 
The drilling vessel then tensions the anchor cable until the anchor digs 
into the sea bed (carrying the chaser with it) and provides a pull of 
about 50 tons. The anchor handling ship next turns round and returns along 
the anchor cable towards the drilling vessel so that the chaser is hauled 
off the anchor shank and rides the cable back to the drilling vessel. The 
towing line is heaved in and unshackled from the chaser pendant which is 
passed back on board the drilling vessel. The chaser is then hove into a 
stowed position against the anchor cable fairleader as the anchor is 
finally tensioned up. Anchor placement is now complete. 
Recovery of the anchor is effected by the anchor handling ship drawing the 
chaser back down the taut anchor cable until it re-engages on the shank of 
the anchor. The anchor cable is then slackened off and the anchor broken 
out of the sea-bed by a pull from the anchor handling ship. Heaving in on 
the towing wire then allows the chaser pendant to be stoppered off on the 
anchor handling ship whereupon the drilling vessel heaves in the anchor 
cable until the anchor (and chaser) is hove home on the anchor rack with 
the supply ship being pulled towards the drilling vessel in the process. 
The pendant line is then transferred to the drilling vessel and tensioned 
to stow the chaser on the anchor cable against the anchor cable 
fairleader. The cycle of anchor placement and recovery is now complete 
without having used a buoy and with only a single shackle connecting 
operation having occurred for each round trip of the chaser. 
However, the design of prior devices has given rise to problems in the 
operation of this method of anchoring which are presently preventing the 
general adoption of the system by the offshore industry. The main problem 
is failure of the chaser to negotiate the buried portion of an anchor 
chain cable leading to a deeply buried anchor or failure to negotiate the 
anchor shackle connection of a deeply buried anchor. This results in the 
anchor having to be broken out of the sea-bed by pulling up on a bight in 
the anchor cable whereby very high stresses are induced in the chaser 
pendant, chaser, anchor cable and anchor with considerable risk of failure 
of any or all of these. Additionally, the broken out anchor is likely to 
be in an attitude unsuitable for reliable re-laying and for heaving on 
board the supply vessel. 
It is an object of the present invention to obviate or mitigate this 
disadvantage. 
In particular it is an object of the present invention to provide an anchor 
chaser of improved form enabling more efficient and effective operation, 
and especially enabling the chaser to move without snagging along the 
anchor chain to a deeply buried anchor. 
The present invention provides a chaser comprising an anchor grappling 
member including a loop shaped portion serving to catch and support the 
anchor for anchor retrieval or placement, said grappling member being 
adapted for constrained movement along the chain to or from the anchor, a 
lower part of the loop shaped portion having an inner surface for sliding 
engagement with the chain during said constrained movement of the 
grappling member along the chain while an upper part of the loop shaped 
portion includes means for attachment of the grappling member to a pendant 
line, said inner surface of the loop shaped portion defining in transverse 
cross-section, an arcuate line, which arcuate line includes a leading 
portion and a following elongate portion having a radius of curvature 
substantially greater than than of said leading portion, said following 
elongate portion serving to engage two successive similarly orientated 
links of the chain during movement of the grappling member on the chain 
towards the anchor, the chord subtended by said following portion of the 
arcuate line forming an obtuse angle with the line from the attachment 
means to the point on the arcuate line separating said leading portion 
from the following portion. 
Preferably the material of the member is harder than the material of the 
chain and of the anchor.

Referring to FIGS. 1 to 5 an anchor retrieval device or chaser 1 comprises 
an annular member 2 having a base portion 6 and a crown portion 3 bearing 
a lug 4 with a shackle hole 5 for attaching the chaser 1 by means of a 
shackle to a service cable or pendant (not shown), the annular member 2 
being dimensioned to permit the passage therethrough of any of the anchor 
chain, joining links, swivels, anchor attachment shackle, or anchor shank 
of the anchor system. The chaser 1 is cast from a suitable wear resistant 
steel having a hardness well in excess of that of either the chain or the 
anchor material. The base portion 6 and crown portion 3 of the annular 
member 2 are formed with curved surfaces 7, 8 adapted to slide in contact 
with stud-link chain 9 (FIGS. 3 and 4) of diameter D. The medial 
cross-section of the curved sliding surface 7 of the base is bounded by an 
arc of radius R = 12D (FIG. 5) cut off by a chord of length x = 4.66D and 
adjoining a semi-circle of radius R.sub.1 chosen to be not less than D, 
and in this embodiment radius R.sub.1 = 1.33D, with the remainder of the 
cross-sectional shape of the member bearing the sliding surfaces being 
defined by a semi-circle R.sub.2 of radius 0.75D and a straight line 11 
joining the extremities of the two semi-circles R.sub.1 R.sub.2. The 
centre of the shackle hole 5 of the attachment lug 4 lies on a straight 
line 12 which passes through the intersection of the arc of radius R = 12D 
and the semi-circle of radius R.sub.1 = 1.33D and forms an angle .beta. 
(FIG. 5) of 143.degree. with the chord length x measured on the side of 
the chord remote from the centres of curvature of the sliding surfaces. 
The medial cross-section of the crown portion 3 bearing sliding surfaces 
is defined by a straight line 14 (FIG. 4) of length 3.36D having each 
extremity joined by a sequence of tangentially joined circular arcs of 
radii R.sub.A, R.sub.B, R.sub.C, R.sub.D, R.sub.E of 0.75D, 8.8D, 1.33D, 
5D and 1.33D respectively. The plane C transverse to the direction of 
motion of the chaser 1 and containing the beforementioned straight line 
12, on which the lug shackle hole 5 centre is located, is referred to 
hereinafter and in the claims as the "plane of the chaser" and contains 
the centre of curvature of the arc of radius R.sub.c of the section of the 
crown portion 3 of the annular member 2. Line 14 of this section lies at 
an angle .alpha..sub.c (see FIG. 4) of 60.degree. to the plane of the 
chaser. 
The shackle hole centre 5 is 21.6D distant from the intersection of the 
plane C of the chaser with the sliding surface 7 on the base portion 6 of 
the annular member 2. The smallest distance X in the plane of the chaser 
separating the crown portion sliding surface 8 from the base portion 
sliding surface 7 is 15.6D (see FIG. 4). 
The side limbs 16, 17 of the annular member 2 joining crown portion 3 to 
base portion 6 having parallel facing inner surfaces 18 13.12D apart and 
are of truncated triangular cross-section T (FIG. 2) approximately 5D 
long, 1.3D wide at the trailing edge 19 and 0.5D wide at the leading edge 
20. This shape T of cross-section provides opposed forwardly converging 
external surfaces 21 with an angle of convergence of 18.degree.. These 
external surfaces 21 provide transverse forces due to soil interaction 
which have a stabilising effect on the chaser 1 by virtue of the resultant 
transverse forces from the two surfaces 21 combining to produce a 
restoring moment about the roll axis R the line joining the shackle hole 
centre 5 to the contact point 22 between chaser 1 and chain 9 when they 
are horizontal in the plane C of the chaser deviates from right angles 
with the vertical plane containing the axis 13 of the chain 9. 
The crown 3 and base 6 portions of the annular member 2 are also formed 
with burial surfaces 23, 24 (see FIG. 1) arranged such that line 
intercepts 14, 15 (FIG. 4) of the surfaces 23, 24 with planes parallel to 
line 12 and at right angles to the plane C of the chaser are inclined at 
angles .alpha..sub.B, .alpha..sub.c of 60.degree. to the plane of the 
chaser 1. These surfaces 23, 24 are located uppermost on each of the crown 
3 and base 6 portions of the annular member 2, adjacent each side of the 
lug 4 on the crown portion 3 and adjacent each side of the curved sliding 
surface 7 on the base portion 6, and the surfaces 23, 24 lie within the 
end planes EP of the annular member 2. The curved sliding surface 7 of the 
base portion blends by transition curves into the adjacent planar burial 
surfaces 24 which form a shallow V with an included angle .beta..sub.v 
(FIG. 1) of 140.degree. when viewed in the direction of line 15 (FIG. 4). 
This V encourages the chain 9 to ride only on the sliding surface 7 
located at the apex of the V. The burial surfaces 23 on the crown portion 
3 of the annular member form an inverted V having an included angle 
.beta..sub.c (FIG. 1) of 96.degree. when viewed in the direction of line 
14 (FIG. 4) whilst the sliding surface 8 underneath is blended by 
transition curves along an arc of radius 18D to merge with the parallel 
surfaces 18 of the side limbs of the annular member 2. 
Although the burial surfaces 23, 24 thus described are substantially 
planar, they could be curved so that the V configuration would be better 
described as a U configuration. 
Further, although the burial surfaces 23, 24 have been described as 
integral with the annular bar member 2, they could be located on a 
separate member flexibly joined to the annular member 2 so that the bar 
member 2 would have the sliding surfaces 7, 8 whilst the separate member, 
functioning as a cable depressor, would have at least one of the burial 
surfaces 23, 24. Combinations of these arrangements are also envisaged 
together with the possibility of the annular member 2 being replaced by a 
U-shaped or V-shaped member. 
FIG. 6 shows a previous chaser 1A fully buried in the sea bed while 
attempting moving along the inverse catenary curve of a deeply buried 
anchor cable 9. The chaser 1A comprises a loop of steel having a constant 
circular cross-section. FIG. 6 shows the forces acting at the point of 
contact with the attendant moments M.sub.d for drag and M.sub.p for cable 
pull in balance. Soil drag forces on chaser and pendant line combine to 
tilt the chaser 1A up from the cable 9. The resultant force, RF, is the 
sum of the increased drag force, d, and the upwards inclined pendant 
force, p. In FIG. 6 .theta. max. is the angle between a line parallel to 
the axis 13 of the chain passing through the point of chaser contact and a 
line through the point of contact mutually perpendicular to the sliding 
surface known as the "normal" to the point of contact. By experiment it 
has been found that the inclination to the horizontal of a chain at the 
shackle of a deeply buried anchor may be as high as 20.degree. whilst the 
inclination of a chaser wire-rope pendant of diameter equal to the 
wire-bar diameter of the chain may be as high as 10.degree.. For a chaser 
having R = 12D, .theta. max will work out at 78.81.degree.. With the chain 
inclined downwards at 20.degree., the normal at the contact point between 
chaser and chain will therefore be at 58.81.degree. to the horizontal. If 
the friction co-efficient .mu. = 0.8, tan.sup.-1 .mu. = 38.66.degree.. 
Therefore, the resultant force, RF, cannot be inclined more than 
20.15.degree. to the horizontal if sliding is to occur. Assuming that the 
drag force, d, is exerted on the chaser in a direction parallel to the 
axis of the chain and that the pendant force, P, is at 10.degree. to the 
horizontal, the vector diagram of FIG. 6 shows that the magnitude of d 
cannot exceed 26 percent of the magnitude of P if the inclination of RF is 
not to exceed the 20.15.degree. maximum for sliding to occur. Thus, if the 
pendant tension is 50 tons, the drag force on the chaser will be 13 tons, 
the resultant force applied to the taut chain will be 39.6 tons at 
40.15.degree. to the axis of the chain, and the normal reaction force 
exerted by the chain on the chaser will be 30.8 tons. 
The chaser 1 of FIG. 1 greatly reduces the drag force, d, the reaction 
force between the chain and chaser at their point of contact and the 
inclination of P at the point of contact. Reduction of d will allow the 
inclination of the pendant force, P, to approach more closely the said 
maximum inclination of the resultant force, RF, so that sliding of the 
chaser can occur at as full development of the inverse catenary of the 
pendant as possible in order to maximise the ability of the chaser to 
penetrate deeply below the sea-bed surface. Reduction of the reaction 
force will decrease the rate of wear at the surfaces in sliding contact. 
Reduction of the inclination of the pendant force applied at the point of 
contact between chaser and chain despite full development of an inverse 
catenary in the pendant will both reduce the reaction force and promote 
sliding at high angles of inclination of the chain cable. 
These improvements are realised in the chaser of FIG. 1 by burial forces 
being generated on the chaser by the passage of sea bed soil over the 
burial surfaces 23, 24 and by minimising the penetration resistance of the 
parts of the chaser 1 in the soil. Both upper and lower burial surfaces 
23, 24 will act to reduce the contact pressure on the under surface of the 
chain whilst the upper burial surface is arranged additionally to depress 
the forward part of the chaser on to or nearer to the chain despite the 
lifting component of the force applied by the inclined pendant wire. 
The chaser 1 operates in the manner described hereinbefore with the 
capability of negotiating steeply inclined chain cable to engage on the 
shank of a deeply buried anchor whereas, from the analysis presented, it 
may be seen that previous chasers can have little or none of this 
capability. Additionally, the reduction of contact pressure between chaser 
and chain produced by the burial surfaces promotes a large reduction in 
the high rate of wear experienced previously in chaser operation. 
It is submitted that the dimensions and shape of the present chaser allow 
it to negotiate the anchor shackle both when engaging and disengaging from 
the anchor shank. 
It will be understood also that the present invention could readily be 
embodied in a hook-shaped member with or without an attached cable 
depressing member bearing a burial surface. The sliding surfaces would be 
located on the central portion of the U-shaped part of the hook and the 
burial surfaces could be located adjacent each side of the central portion 
and extending transversely to the axis of the chain along the arms of the 
U-shaped part.