U.S. patent number 3,842,776 [Application Number 05/419,760] was granted by the patent office on 1974-10-22 for anchoring system.
This patent grant is currently assigned to Skagit Corporation. Invention is credited to Donald J. Wudtke.
United States Patent |
3,842,776 |
Wudtke |
October 22, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
ANCHORING SYSTEM
Abstract
An anchoring system suitable for an offshore drilling platform
is shown in which both wire rope and a length of anchor chain are
used. Chain alone becomes too heavy in the lengths needed, and the
wire rope used retains substantial residual forces which interfere
with proper operation of the anchor unless a substantial weight of
chain is interposed between the wire rope and the anchor. An end
connector member which joins the chain and wire rope cooperates
with a special fairlead sheave having three concentric grooves--one
for wire rope, one for chain, and an outside groove designed to
mate with a curved surface on the end connector to assure that the
wire rope, end connector and chain may all pass smoothly over the
failead sheave without imposing kinking forces on the wire rope or
excessive bending forces on the chain links. The wire rope is
stored on a drum driven by a winch. When the wire rope is
essentially all on the drum and the chain is adjacent the drum, the
load is transferred to a devil's claw or similar device, the chain
is disconnected from the wire rope and end connector and is then
connected to a lead chain extending from a windlass which transfers
chain in and out of a chain locker. In this manner, both the wire
rope and the chain are stored on the platform and are easily
connected and separated for storage in separate facilities.
Inventors: |
Wudtke; Donald J. (Burlington,
WA) |
Assignee: |
Skagit Corporation
(Sedro-Woolley, WA)
|
Family
ID: |
23663641 |
Appl.
No.: |
05/419,760 |
Filed: |
November 28, 1973 |
Current U.S.
Class: |
114/294;
114/230.23 |
Current CPC
Class: |
B63B
21/10 (20130101); B63B 21/22 (20130101); B63B
21/50 (20130101) |
Current International
Class: |
B63B
21/00 (20060101); B63B 21/22 (20060101); B63B
21/50 (20060101); B63b 021/50 () |
Field of
Search: |
;114/230,26R ;9/8R,8P
;254/15R,175.7 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Blix; Trygve M.
Assistant Examiner: Goldstein; Stuart M.
Attorney, Agent or Firm: Smith; Robert C. Thornton; William
F.
Claims
I claim:
1. An anchoring system for anchoring a structure comprising
an anchor,
a chain attached to said anchor,
an end connector attached to said chain of generally frusto-conical
configuration with a convex curvilinear surface at its larger end
and means in said larger end for receiving and retaining a link of
said chain,
a wire rope having one end secured within the smaller diameter end
of said end connector, and
a fairlead shave supported on said structure, said sheave including
an inner groove for receiving said wire rope, an intermediate
groove, and an outer groove having inner surfaces substantially
mating with said convex curvilinear surface, said grooves being
concentric along radii of said sheave.
2. An anchoring system as set forth in claim 1 wherein said
structure includes a drum and drum means for storing said wire
rope,
a chain locker for storing said chain and a chain hoist with a
length of lead chain,
means for transferring the anchor load from said wire rope to a
chain link, and
means including a detachable link for enabling said anchor chain to
be detached from a link near said end connector and attached to
said lead chain.
3. An anchoring system as set forth in claim 1 including a
detachable link in said lead chain and a detachable link in said
anchor chain, and wherein alternate links of said chain are at
least partially supported in said inner groove.
4. A system for anchoring a structure in the sea including an
anchor, an anchor chain, a wire rope and means connecting said wire
rope to said anchor chain, drum and winch means for storing,
retrieving and paying out said wire rope, a chain locker and a
windlass for retrieving and paying out chain from said chain locker
including a length of lead chain, and a fairlead sheave for guiding
said wire rope and said chain toward said drum, characterized in
that said connecting means comprises an end connector of generally
frusto-conical configuration with a convex curvilinear surface at
its larger end and means in said larger end for receiving and
retaining a link of said chain and with one end of said wire rope
fastened to its smaller end,
said sheave including an inner groove for receiving said wire rope,
an intermediate groove to at least partially support links of said
chain, and an outer groove having inner surfaces substantially
mating with said convex curvilinear surface, said grooves being
concentric along radii of said sheave.
5. A system for anchoring a structure as set forth in claim 4
wherein a load-engaging member is suspended from said
structure,
and at least one detachable link is incorparated into said chain
near said end connector whereby said anchor load may be transferred
from said wire rope to said load-engaging member, said detachable
link may be transferred to said lead chain, said windless may be
actuated to take up the load from said load-engaging member
allowing said member to be disconnected from said chain, and said
chain may then be drawn into said chain locker.
6. A system for anchoring a structure as set forth in claim 4
wherein said curvilinear surface is substantially spherical.
7. A system for anchoring a structure as set forth in claim 4
wherein a load-retaining member is fastened to said structure for
attachment to said anchor chain and a detachable link is placed is
said anchor chain near said end connector such that the load of
said anchor chain may be transferred to said load-retaining member
and said detachable link may thereby be unloaded and opened to
permit said anchor chain to be connected to said lead chain.
8. An anchoring system for anchoring a structure in deep water
comprising
an anchor;
a substantial length of chain attached to said anchor;
an end connector member having a generally frusto-conical
configuration with a convex curvilinear surface at its larger end
and a socket in said larger end, said socket including means for
retaining a link of said chain;
a substantial length of wire rope fastened at one end of the
smaller diameter end of said end connector, and
a fairlead sheave supported on said structure, said sheave
including a large diameter groove having inner surfaces of
configuration substantially mating with said convex curvilinear
surface of said end connector member, an inner diameter groove of
cross-section sufficient to support said wire rope and to provide
at least partial support for alternate links of said chain and of
diameter at least sufficient to avoid excessive bending stress in
said wire rope, and an intermediate diameter groove at least
partially supporting other alternate links of said chain, said
groove being concentric such that said chain, end connector and
wire rope are retained in alignment in a single plane as they pass
over said sheave.
9. An anchoring system as set forth in claim 7 wherein said
cirvilinear surface is essentially a spherical segment.
10. In an anchoring system for anchoring a structure including an
anchor, an anchor chain, a wire rope, an end connector connecting
said wire rope to said anchor chain, a drum and drum drive means
for storing said wire rope, a chain locker and chain hoist means
including a length of lead chain for storing said anchor chain,
a method for retrieving and storing said anchoring system on said
structure comprising the steps of
1. winding said wire rope on said drum,
2. attaching a link of said anchor chain to a load-retaining member
fastened to said structure,
3. unwinding said wire rope slightly to remove the load from a few
links adjacent said end connector, one of said links being
detachable,
4. detaching said anchor chain from said end connector and
attaching it to said lead chain,
5. operating said chain hoist means to unload said retaining
member,
6. detaching said retaining member, and
7. operating said chain hoist means to load said chain into said
chain locker.
11. A method of retrieving and storing an anchoring system as set
forth in claim 9 wherein first and second detachable links are used
on said anchor chain near said end connector and on said lead
chain, respectively, so that said anchor chain is removed from said
first detachable link leaving said link with said end connector and
said anchor chain is connected to said second detachable link on
said lead chain.
Description
BACKGROUND OF THE INVENTION
Structures such as offshore drilling rigs or platforms are located
at various locations in the ocean where they may be exposed to
heavy seas. It is necessary for such structures to be very firmly
anchored to avoid excessive lateral drift relative to the
associated drilling structure. The anchoring structure used has
consisted of a substantial number to conventional anchors having
flukes which dig into the bottom attached by a length of chain.
Because of the weight of the chain relative to its strength, this
attachment means has limited the depth of such installation to
about 500-600 feet. Wire rope has better characteristics with
respect to strength per unit length, but does not, alone, form a
good suspension emans for an anchor of the type referred to above
because it tends to impose upward forces on the anchor when
unloaded. Recent computer studies have indicated that an optimum
suspension arrangement should include a length of chain attached to
the anchor combined with a length of wire rope. This has been tried
but has involved complications in retrieving the anchor and
suspension means.
The chain is normally stored in a chain locker and is reeled in by
means of a windlass. The wire rope is wound on a reel or drum by
means of a winch, and it does not appear practical to store both on
the same drum or storage means. Also, efforts to guide through the
same fairlead structure have frequently subjected the wire rope to
kinking or sharp bends not conducive to long life.
One approach to recovery of the anchoring structure has involved
operating a winch on the platform to reel in substantially all of
the wire rope, hooking on to the anchor or anchor chain by means of
a hook or grapple suspended from a work boat and pulling the end of
the chain or anchor up to the work boat. The chain is then loaded
into a chain locker on the work boat, and when the joint between
the chain and wire rope is reached, this joint is disconnected and
the remaining wire rope is reeled onto the drum on the platform and
the remaining chain stored in the chain locker on the work boat.
This arrangement is quite cumbersome since each of four corners of
the drilling platform may be tied down with three such anchor
structures, making a total of twelve to be so recovered before the
platform can be moved.
SUMMARY OF THE INVENTION
From the foregoing it will be appreciated that a considerable
saving in crew time and facilities expense could be effected if the
platform contained means capable of reeling in and storing both the
cable and chain structures smoothly and without requiring
excessively complex gear. Applicant has devised a systen whereby
this may be done. A key element in this system is a fairlead sheave
which aligns the wire rope/chain assembly for loading onto he
platform and which cooperates with a special end connector device
to assure a smooth transition as the wire rope pulls through the
sheave followed by the end connector and the chain, said transition
being accomplished without subjecting the wire rope to bends
sharper than that required to follow the sheave.
The sheave is also designed to provide good support for the chain
links and specifically to avoid substantial bending forces on the
links. This fairlead sheave has an inner groove which is of minimum
diameter to limit the curvature of the wire rope to that which
imposes acceptable stresses. The determination of such diameter is
well within the skill of the art. A second or intermediate groove
has a broad base such that it essentially supports the links which
lie flat against it, alternate links being oriented in the plane of
the sheave and extending into the inner groove. It is desirable for
the vertical links to bear on the wire rope groove if the
proportions of the diameter of the wire rope and the diameter of
the chain link are such that they are compatible. In this case the
forces carried in the vertical links become largely tension forces
which are more easily accommocated than the bending forces carried
by links lying flat against the sheave.
An outer groove has a larger cross-section and is curved to mate
with a bell-shaped end connector to which the chain is attached at
its large end and the wire rope at its smaller end. The large end
has a surface in the form of a spherical segment which lies in the
outer groove as it rotates around the sheave and prevents excessive
bending of the wire rope at the point of contact with the end
connector until the rope begins to wrap around the sheave.
The wire rope is reeled onto a drum by means of a winch mounted on
the platform, thereby pulling the chain up adjacent the drum. A
load-retaining device in the form of a devil's claw or similar
device is then attached a few links below the end connector and the
winch operated to unload these few links, one of which is
detachable, and transfer the load to the devil's claw. The
detachable link is then disconnected and remains with the end
connector, and a second detachable link at the end of a lead chain
is lowered by means of a chain hoist and is connected to the upper
link of anchor chain. Alternatively, a single detachable link is
disconnected from near the end connector and attached to the lead
chain. The chain hoist is then operated to wind in the chain,
storing it in a chain locker on the platform, except for the length
extending from the hoist to the anchor which is normally stored on
a fixture on the side of the column just below the fairlead
sheave.
The reverse process is used to deploy the anchor, chain and wire
rope assembly. The chain hoist winds the chain out until the
detachable link is just above the devil's claw, and the devil's
claw is attached to the chain. The hoist is then operated to cause
the load to be taken up by the devil's claw and removed from the
detachable link. The detachable link is then opened and either
remains with the lead chain or is attached to the end link of the
few links extending from the end connector, if there is not a
second detachable link attached thereto. The winch is then pg,5
operated to remove the load from the devil's claw, the devil's claw
is disconnected, and the wire rope/chain assembly then reeled out
as needed to cause the anchor to reach the ocean bottom. As set
forth above, there may be as many as 12 such anchor assemblies on a
platform.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a typical offshore drilling
platform and anchoring means;
FIG. 2 is a side view of a winch and chain hoist assembly mounted
on one column of the above drilling platform, including a wire
rope, anchor chain and fairlead assembly;
FIG. 3 is a view, partly in section and partly in phantom, of a
fairlead sheave according to my invention;
FIG. 4 is a sectional view taken along line 4--4 of FIG. 3; and
FIG. 5 is a sectional view taken along line 5--5 of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a perspective view of an offshore drilling platform 10 of
the type for which anchoring systems incorporating my invention are
designed. A deck 12 of substantial area, such as approximately 260
feet by 320 feet, supports one or more buildings 14 for housing men
and machinery, a number of cranes 16, and a drilling tower 18.
Drilling may take place in water up to 1500 feet deep or more with
a drill shaft 20 extending vertically downwardly from the tower 18
to a location on the ocean bottom which should be essentially
directly below the tower. Flotation for the platform is provided by
a pair of large pontoons 22 which are ballasted such that they are
submerged to a depth where the wave action is much less than at the
surface. The deck is supported on the pontoons by means of a
plurality of columns 30. The corner columns may be about 32 feet in
diameter. Extending from the corner columns are shown a plurality
of anchors 24 attached by means of wire ropes 40, end connectors
54, and a length of anchor chain 60 connected to anchors 24.
Although only two such anchor assemblies are shown in detail, it
will be appreciated that to maintain the position of platform 10
over the drilling location, at least two or three such anchor
assemblies will usually extend from each corner column arranged to
resist forces in all directions.
Referring now to FIG. 2, one of the columns for the offshore
platform is shown at numeral 30. At the top of this platform is a
radially extending annular shelf structure 32 which is supported at
various places by means of a plurality of trusses 34 fastened to
the side wall of column 30. Mounted on the top surface of the
annular member 32 is a drum and chain hoist assembly built on a
frame 36 which supports a large winchdriven drum 38 upon which is
reeled a substantial length of wire rope 40. Also mounted on frame
36 is a chain hoist 42 including a wildcat 44 driven by suitable
motor means and which carries a length of lead chain 46. Chain
extending from the left side of sheave 46 extends down into s chain
locker, not shown. Also attached to the frame 36 is a boss 48
supporting a short length of wire rope 50 which in turn is attached
to a devil's claw 52.
Wire rope 40 is firmly secured through a zinc poured socket or by
other suitable means to a bell-shaped end connector 54 having an
opening at its larger end and a cross-member for supporting a chain
link 56. Attached to link 56 is a detachable link 58 which is, in
turn, attached to the anchor chain 60. As shown, the devil's claw
52 is affixed below one of the links of chain 60 such that any
significant movement downwardly of the chain would cause the chain
load to be transferred from the wire rope and the upper links of
the chain to the devil's claw 52. The remaining length of chain 60
extends downwardly where it is directed around a fairlead sheave 62
and from there downwardly and outwardly to an anchor structure, not
shown. Fairlead 62 is pivotally attached to column 30 by means of a
bracket 64 attached to the side wall of column 30.
FIG. 3 is a view, partly in section, of the fairlead sheave 62
showing the arrangement whereby the chain 60, the end connector 54
and the wire rope 40 may all smoothly and conveniently pass across
the fairlead sheave 62 without danger of either kinking the wire
rope 40 or imposing substantial bending stress on the links of
chain 60. In this view, the wire rope 40, the end connector 54, and
the chain 60 are shown in various positions typical of those which
they assume as the assembly passes the fairlead sheave 62. As the
cable/chain assembly is retrieved, the wire rope 40 will first
reach the sheave 62 where it falls naturally and smoothly into the
inside groove 66 of said sheave. Member 54, as shown in section,
has just made contact with the larger diameter groove 70 of the
sheave, and the chain, including link 56, is shown pulling straight
to the right as viewed in this figure. In the sectional view, FIG.
4, it will be appreciated that the sheave 62 actually incorporates
three separate grooves, on of which is the inside groove 66
designed to accommodate the wire rope 40, an intermediate groove 68
having a broad base for supporting links of chain 60 which are
oriented perpendicularly to the plane of the sheave and a third
groove 70 which is larger in diameter and has a curved
configuration to mate with the spherical segment portion at the
large diameter end of end connector 54.
Referring again to FIG. 3, as end connector 54 makes contact with
the internal surface of groove 70, it will be carried by said
groove around the periphery of the sheave 62 as shown in the
successive phantom views. There is essentially a line contact
between the spherical surface of member 54 and each side of the
groove 70 until the end connector approaches the position shown in
the upper right of the drawing where it is about to separate from
the sheave. Just before member 54 separates itself from sheave 62,
the line contact becomes shorter and diminishes to a point on each
side of the groove 70.
It will be observed that in the first of the several positions, the
wire rope 40 is pulled directly toward the right from the smaller
end of member 54 and is not caused to bend until it begins to wrap
around the groove 66. In the next position, the connector member 54
has been carried a substantial distance around the periphery of the
sheave 62 and assumes a quite different angle with respect to the
sheave; however, the wire rope 40 still pulls directly outwardly
from the small diameter end of member 54 and suffers no bending
effect until it reaches the edge of groove 66. In the third
position of member 54 as it is rotated farther to the right, member
54 is now at a postion and oriented at an angle such that wire rope
40 is completely outside the periphery of sheave 62, and yet the
force on it is such that it pulls straight away from the small
diameter end of member 54. In the fourth position of member 54,
this connector member is just at the point of being separated from
contact with sheave 62 and wire rope 40 is still pulling directly
outwardly from the small diameter end of member 54. Thus, member 54
always assumes a position which permits the wire rope 40 to pull
directly outwardly from the small diameter end thereof and never
permits rope 40 to be subjected to a sharp bend or kink adjacent
its point of entry into the member 54.
The large diameter end of member 54 includes an open chamber for
receiving one end of link 56 which is secured to member 54 by means
of a cross bar 72 whose center is even with the end of member 54.
FIG. 5 shows a sectional view of member 54 taken along line 5--5 of
FIG. 3 such that the section shown in FIG. 5 is perpendicular to
that shown in FIG. 3. In this section, cross bar 72 is shown as
extending across the center of the opening in the large diameter
end of member 54 and shows the manner in which end link 56 is
captured by bar 72. This arrangement limits, to some extent, the
angle at which link 56 can pull against member 54 without imparting
bending forces to the link 56. This angle is normally in the order
of 45.degree. and provides adequate flexibility for the system.
Should this not be the case in any particular installation, those
skilled in the art can modify the dimensions of the parts to
provide the necessary clearance. The location of the cross bar 72
is of some importance, since the point of contact with link 56
against said cross bar should be very close to the point at which
the large diameter portion of member 54 tends to rotate in groove
70. If cross bar 72 were to be moved substantially outwardly or to
the left as shown in FIG. 3, a moment would be created causing
member 54 to rotate around a pair of contact points in the groove
70, thereby tending to swing the small diameter end of member 54
outwardly with resultant risk of kinking the wire rope 40.
While only a single embodiment is shown and described herein,
modifications may be made within the scope of the present
invention. Thus, while the large diameter segment of end connector
54 is a spherical segment, other convex curvilinear shapes will be
operable. A conical shape is not recommended because of a tendency
to wedge in the groove. The arrangement of detachable links may
vary as discussed above, or a disconnecting device may be combined
with end connector 54.
* * * * *