U.S. patent application number 10/572438 was filed with the patent office on 2007-10-18 for mooring system.
Invention is credited to Michael Collee, Edward Maycock, Martin Shaw, Richard Yemm.
Application Number | 20070240624 10/572438 |
Document ID | / |
Family ID | 29227247 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070240624 |
Kind Code |
A1 |
Collee; Michael ; et
al. |
October 18, 2007 |
Mooring System
Abstract
A mooring system for a floating aquatic structure includes a
harness, a primary weight, and a first anchor. The harness has
first and second ends. The first end is pivotally attached to the
structure. The primary weight is removably attached to a connecting
line and suspended from the second end of the harness. The anchor
is removably attached to the primary weight to resist movement of
the structure in a first direction.
Inventors: |
Collee; Michael; (Edinburgh,
GB) ; Maycock; Edward; (Edinburgh, GB) ; Shaw;
Martin; (Edinburgh, GB) ; Yemm; Richard;
(Edinburgh, GB) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 S. WACKER DRIVE, SUITE 6300
SEARS TOWER
CHICAGO
IL
60606
US
|
Family ID: |
29227247 |
Appl. No.: |
10/572438 |
Filed: |
September 17, 2004 |
PCT Filed: |
September 17, 2004 |
PCT NO: |
PCT/GB04/03997 |
371 Date: |
December 26, 2006 |
Current U.S.
Class: |
114/230.23 ;
114/230.24 |
Current CPC
Class: |
B63B 21/50 20130101 |
Class at
Publication: |
114/230.23 ;
114/230.24 |
International
Class: |
B63B 21/50 20060101
B63B021/50 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2005 |
GB |
0321768.4 |
Claims
1. A mooring system for a floating aquatic structure, the system
comprising: a harness having first and second ends, the first end
being adapted to be pivotally attached to the structure; at least
one primary weight removably attached to a connecting line, and
suspended on the connecting line from the second end of the
harness; and at least one first anchor removably attached to said
primary weight by at least one first anchor line, wherein the first
anchor is adapted to resist movement of the structure in a first
direction.
2. The mooring system of claim 1, further comprising at least one
second anchor removably attached to the connecting line by at least
one second anchor line, wherein the second anchor is adapted to
resist movement of the structure in a second direction
substantially opposite to the first direction.
3. The mooring system of claim 2, further comprising a support line
having one end adapted to be removably attached to the structure
and its other end removably attached to the second anchor line, the
support line adapted to restrict yaw motion of the structure.
4. The mooring system of claim 2, further comprising one or more
secondary weights attached to one or more of the first and second
anchor lines.
5. The mooring system of claim 2, wherein the system comprises at
least two first anchors removably attached to said primary weight
by respective first anchor lines, wherein each first anchor is
adapted to resist movement of the structure in the first
direction.
6. The mooring system of claim 1, wherein the at least one primary
weight comprises a frame member and one or more chain links
suspended from said frame member.
7. The mooring system of claim 6, wherein the frame member includes
at least two first mechanical attachment means adapted to removably
attach the connecting line and at least one first anchor line
thereto.
8. The mooring system of claim 7, wherein the first mechanical
attachment means are universal joints having detachable pin
members.
9. The mooring system of claim 1, further comprising: first and
second primary weights attached to first and second connecting
lines, respectively, and suspended on the connecting lines from the
second end of the harness; and a pair of first anchors having
respective first anchor lines, one of the first anchors removably
attached to the first primary weight by one first anchor line and
the other first anchor being removably attached to the second
primary weight by the other first anchor line, wherein the first
anchors are adapted to resist movement of the structure in a first
direction.
10. The mooring system of claim 9, wherein the system further
comprises a third primary weight attached to a third connecting
line and suspended on the third connecting line from the second end
of the harness; and at least one second anchor removably attached
to the third connecting line by at least one respective second
anchor line, wherein the second anchor is adapted to resist
movement of the structure in a second direction substantially
opposite to the first direction.
11. The mooring system of claim 10, further comprising a support
line having one end adapted to be removably attached to the
structure and its other end removably attached to the second anchor
line, the support line adapted to restrict yaw motion of the
structure.
12. The mooring system of claim 9, wherein the system further
comprises a pair of second anchors each having respective second
anchor lines, one of the second anchors being removably attached to
the first primary weight by one second anchor line and the other
second anchor being removably attached to the second primary weight
by the other second anchor line, wherein the second anchors are
adapted to resist movement of the structure in a second direction
substantially opposite to the first direction.
13. The mooring system of claim 12, further comprising first and
second support lines, each support line having one end adapted to
be removably attached to the structure and its other end removably
attached to one of the second anchor lines, the support lines
adapted to restrict yaw motion of the structure.
14. The mooring system of claim 1, wherein the harness comprises
first and second arm members and a shaft member, wherein the arm
members have first and second ends and the shaft member is
connected between the respective first ends of the arm members to
form the first end of the harness, and wherein the shaft member has
an axis of rotation about which the harness pivots.
15. The mooring system of claim 14, wherein the harness is adapted
to apply roll restraint to the structure.
16. The mooring system of claim 14, wherein said shaft member and
at least one of said arm members are hollow, the harness further
comprising at least one conduit that is at least partially located
in the hollow arm member and the shaft member.
17. The mooring system of claim 16 wherein the harness further
comprises connecting means for connecting one or more utility
supplies to the harness.
18. The mooring system of claim 17, wherein said connecting means
comprises a junction box having one or more connectors therein, the
junction box being in communication with the at least one conduit,
and the conduit carrying utility supply through the harness to the
structure.
19. The mooring system of claim 1, wherein the harness further
comprises retrieval means to facilitate retrieval of the harness
from a body of water.
20. The mooring system of claim 19, wherein the retrieval means
comprises one or more slots provided on the harness.
21. The mooring system of claim 20, wherein the retrieval means
further comprises one or more roller members provided on the
harness.
22. The mooring system of claim 1, wherein the harness further
comprises a second mechanical attachment means for removably
attaching the connecting line thereto.
23. The mooring system of claim 22, wherein the second mechanical
fixing means is a universal joint having a detachable pin
therethrough.
24. The mooring system of claim 2, wherein the at least one
connecting line includes a linking plate member, and wherein the at
least one second anchor line is removably connected to the
connecting line by the linking plate member.
25. The mooring system of claim 24, wherein the linking plate
member includes: retrieval means adapted to allow the mooring
system to be retrieved from the water; and stabilising means
adapted to hold the plate member in substantially the same plane
during retrieval.
26. The mooring system of claim 25, wherein the linking plate
member further includes locking means adapted to secure the plate
member to a marine vessel.
27. The mooring system of claim 1, wherein the floating aquatic
structure is a wave energy converter.
28. A weight for a mooring system, the weight comprising a frame
member and at least two chain links suspended from said frame
member.
29. The weight of claim 28, wherein the weight comprises a
plurality of chain links suspended from the frame member.
30. The weight of claim 28, wherein the frame member includes at
least two mechanical attachment means adapted for removably
attaching components of a mooring system thereto.
31. The weight of claim 30, wherein the mechanical attachment means
are universal joints having detachable pin members.
32. A recovery apparatus for a marine vessel, the apparatus
comprising: a rotatable drum; a recovery line having a first end
adapted to be removably attached to a portion of an object to be
recovered and a second end adapted to be removably attached to the
rotatable drum; a retention means comprising a base member and a
retaining member attached to the base member by a spherical
bearing, the retaining member being adapted to have a portion of
the recovered object removably attached thereto; and a guide means
adapted to guide the recovery line over the retaining member during
winding and unwinding of the line.
33. The recovery apparatus of claim 32, wherein said guide means is
a ramp having an inclined surface lying in a ramp surface plane,
and wherein the retention means is housed on the ramp.
34. The recovery apparatus of claim 33, wherein the base member is
adapted to permit longitudinal movement of the retention means
relative to the ramp.
35. The recovery apparatus of claim 34, wherein the ramp further
comprises a guide track in which the base member is located.
36. The recovery apparatus of claim 35, wherein the guide track has
first and second portions, wherein the first portion lies at a
greater angle relative to the horizontal than the second
portion.
37. The recovery apparatus of claim 36, wherein the first guide
track portion lies at an angle relative to the ramp surface plane
and the second guide track portion is substantially parallel to the
ramp surface plane.
38. The recovery apparatus of claim 35, wherein the guide track
includes locking means adapted to lock the retention means in one
or more positions on the guide track.
39. The recovery apparatus of claim 32, further comprising a
tensioning roller member adjacent a first end of the guide means
for tensioning the recovery line.
40. The recovery apparatus of claim 32, wherein a second end of the
guide means terminates adjacent the stem of the marine vessel, and
the apparatus further comprises a recovery roller member adjacent
the stern of the vessel, the recovery roller member adapted to
assist in the recovery of an object onto the guide means.
Description
[0001] The present invention relates to a system for the mooring
and retrieval of floating aquatic installations and structures.
[0002] Known mooring systems for floating structures comprise a
number of anchors, weights and associated tether lines which are
attached to the installation or structure. The anchors can take
different forms, but each is intended to hold its associated tether
line on the bed of the body of water upon which the structure is
floating. Additional weights can be added to the anchor lines in
order to counter the forces on the structure (e.g. wind, waves or
tidal currents) and maintain the position of the structure on the
water. Additional mooring and restraint lines can also be
incorporated depending on the conditions which the structure is
expected to encounter.
[0003] One problem with such known mooring systems is that they
present problems when the structure has to be retrieved from the
water. With the number of tether and restraint lines attached to
the structure, it is necessary for these all to be disconnected
before the structure can be removed from the water. This normally
requires human intervention, with an operator having to climb onto
the structure in order to detach the various lines before the
structure can be retrieved. In particularly heavy seas or bad
weather, this cannot sometimes be done for safety reasons. The
retrieval of the structure can therefore be delayed, for days at a
time in some instances. Such delay can have serious time and cost
implications, with maintenance or installation schedules being
affected because of the delays.
[0004] A further disadvantage of the known systems is that they are
normally intended to moor only one structure per system. If a
number of structures are to be installed at the same location,
separate anchor points, weights and tether lines are needed for
each structure. As a result, the structures need to be a certain
distance from one another to avoid fouling the various lines.
Having to space the structures from one another in this manner is
not an effective use of space. It is often desired to place a
number of structures in as small a space on the water as possible,
normally for economic, aesthetic or environmental reasons.
[0005] Another disadvantage is that known mooring systems do not
make efficient use of the available water depth. The excursion
required to cope with extreme waves and currents require large
ranges of motion before the mooring lines become taut. If mooring
lines become taut loads rise extremely fast--a process known as
line snatching. This problem becomes serious if the structure is to
be moored in shallow water and large waves and/or strong currents.
Also, known systems do not make efficient use of the suspended
weight of the mooring lines or components. Suspended weight costs
money and may itself impart loads on the structure that may be
large or undesirable.
[0006] Yet another disadvantage is that known mooring systems do
not provide for ready, protected connection of the structure to one
or more utility supplies such as power, fluid or water. Typically
such connection must be made on or near the water surface,
independently of mooring line connections.
[0007] In addition, a lot of known mooring systems are limited in
that they are only intended to encounter loading from a single
direction (e.g. a marine structure which is only intended to
encounter waves from one direction). Such systems can lack
stability if they encounter forces from an unexpected direction.
Such forces can lead to rotational or yaw movements of the
structure which can cause damage to both the structure and the
components of the mooring system.
[0008] It is an aim of the present invention to obviate or mitigate
one or more of the disadvantages of these known mooring
systems.
[0009] According to a first aspect of the present invention, there
is provided a mooring system for a floating aquatic structure, the
system comprising: [0010] a harness having first and second ends,
the first end being adapted to be pivotally attached to the
structure; [0011] at least one primary weight removably attached to
a connecting line, and suspended on the connecting line from the
second end of the harness; and [0012] at least one first anchor
removably attached to said primary weight by at least one first
anchor line, wherein the first anchor is adapted to resist movement
of the structure in a first direction.
[0013] Preferably, the mooring system further comprises at least
one second anchor removably attached to the connecting line by at
least one second anchor line, wherein the second anchor is adapted
to resist movement of the structure in a second direction
substantially opposite to the first direction.
[0014] Preferably, the mooring system further comprises a support
line having one end adapted to be removably attached to the
structure and its other end removably attached to the second anchor
line, the support line adapted to restrict yaw motion of the
structure.
[0015] Preferably, the mooring system further comprises one or more
secondary weights attached to one or more of the first and second
anchor lines.
[0016] In one preferred embodiment the system comprises at least
two first anchors removably attached to said primary weight by
respective first anchor lines, wherein each first anchor is adapted
to resist movement of the structure in the first direction.
[0017] Preferably, the at least one primary weight comprises a
frame member and one or more chain links suspended from said frame
member. The frame member includes at least two first mechanical
attachment means adapted to removably attach the connecting line
and at least one first anchor line thereto. Most preferably, the
first mechanical attachment means are universal joints having
detachable pin members.
[0018] In a further preferred embodiment, the mooring system
comprises: [0019] first and second primary weights attached to
first and second connecting lines, respectively, and suspended on
the connecting lines from the second end of the harness; and [0020]
a pair of first anchors having respective first anchor lines, one
of the first anchors removably attached to the first primary weight
by one first anchor line and the other first anchor being removably
attached to the second primary weight by the other first anchor
line, wherein the first anchors are adapted to resist movement of
the structure in a first direction.
[0021] Preferably, the system further comprises a third primary
weight attached to a third connecting line and suspended on the
third connecting line from the second end of the harness; and
[0022] at least one second anchor removably attached to the third
connecting line by at least one respective second anchor line,
wherein the second anchor is adapted to resist movement of the
structure in a second direction substantially opposite to the first
direction.
[0023] Preferably, the system further comprises a support line
having one end adapted to be removably attached to the structure
and its other end removably attached to the second anchor line, the
support line adapted to restrict yaw motion of the structure.
[0024] In a still further preferred embodiment, the system further
comprises a pair of second anchors each having respective second
anchor lines, one of the second anchors being removably attached to
the first primary weight by one second anchor line and the other
second anchor being removably attached to the second primary weight
by the other second anchor line, wherein the second anchors are
adapted to resist movement of the structure in a second direction
substantially opposite to the first direction.
[0025] Preferably, the system further comprises first and second
support lines, each support line having one end adapted to be
removably attached to the structure and its other end removably
attached to one of the second anchor lines, the support lines
adapted to restrict yaw motion of the structure.
[0026] Preferably, the harness comprises first and second arm
members and a shaft member, wherein the arm members have first and
second ends and the shaft member is connected between the
respective first ends of the arm members to form the first end of
the harness, and wherein the shaft member has an axis of rotation
about which the harness pivots. The harness is adapted to apply
roll restraint to the structure.
[0027] Preferably, said shaft member and at least one of said arm
members are hollow, the harness further comprising at least one
conduit that is at least partially located in the hollow arm member
and the shaft member. Preferably, the harness further comprises
connecting means for connecting one or more utility supplies to the
harness.
[0028] Preferably, said connecting means comprises a junction box
having one or more connectors therein, the junction box being in
communication with the at least one conduit, and the conduit
carrying utility supply through the harness to the structure.
[0029] Preferably, the harness further comprises retrieval means to
facilitate retrieval of the harness from a body of water.
[0030] Preferably, the retrieval means comprises one or more slots
provided on the harness. Most preferably, the retrieval means
further comprises one or more roller members provided on the
harness.
[0031] Preferably, the harness further comprises a second
mechanical attachment means for removably attaching the connecting
line thereto. Most preferably, the second mechanical fixing means
is a universal joint having a detachable pin therethrough.
[0032] Preferably, the at least one connecting line includes a
linking plate member, and wherein the at least one second anchor
line is removably connected to the connecting line by the linking
plate member.
[0033] Preferably, the linking plate member includes: [0034]
retrieval means adapted to allow the mooring system to be retrieved
from the water; and [0035] stabilising means adapted to hold the
plate member in substantially the same plane during retrieval. Most
preferably, the linking plate member further includes locking means
adapted to secure the plate member to a marine vessel.
[0036] Preferably, the floating aquatic structure is a wave energy
converter.
[0037] According to a second aspect of the present invention, there
is provided weight for a mooring system, the weight comprising a
frame member and at least two chain links suspended from said frame
member.
[0038] Preferably, the weight comprises a plurality of chain links
suspended from the frame member.
[0039] Preferably, the frame member includes at least two
mechanical attachment means adapted to removably attaching
components of a mooring system thereto. Most preferably, the
mechanical attachment means are universal joints having detachable
pin members.
[0040] According to a third aspect of the present invention there
is provided a recovery apparatus for a marine vessel, the apparatus
comprising: [0041] a rotatable drum; [0042] a recovery line having
a first end adapted to be removably attached to a portion of an
object to be recovered and a second end adapted to be removably
attached to the rotatable drum; [0043] a retention means comprising
a base member and a retaining member attached to the base member by
a spherical bearing, the retaining member being adapted to have a
portion of the recovered object removably attached thereto; and
[0044] a guide means adapted to guide the recovery line over the
retaining member during winding and unwinding of the line.
[0045] Preferably, said guide means is a ramp having an inclined
surface lying in a ramp surface plane, and wherein the retention
means is housed on the ramp.
[0046] Preferably, the base member is adapted to permit
longitudinal movement of the retention means relative to the
ramp.
[0047] Preferably, the ramp further comprises a guide track in
which the base member is located. Preferably, the guide track has
first and second portions, wherein the first portion lies at a
greater angle relative to the horizontal than the second
portion.
[0048] In a preferred embodiment, the first guide track portion
lies at an angle relative to the ramp surface plane and the second
guide track portion is substantially parallel to the ramp surface
plane.
[0049] Preferably, the guide track includes locking means adapted
to lock the retention means in one or more positions on the guide
track.
[0050] Preferably, the recovery apparatus further comprises a
tensioning roller member adjacent a first end of the guide means
for tensioning the recovery line.
[0051] Preferably, a second end of the guide means terminates
adjacent the stern of the marine vessel, and the apparatus further
comprises a recovery roller member adjacent the stern of the
vessel, the recovery roller member adapted to assist in the
recovery of an object onto the guide means.
[0052] Preferred embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0053] FIG. 1(a) shows a perspective view of a first embodiment of
a mooring system used in conjunction with a wave energy
converter;
[0054] FIG. 1(b) shows a schematic plan view of the mooring system
of FIG. 1(a);
[0055] FIG. 2 shows the mooring system of FIG. 1 when applied to an
array of wave energy converters;
[0056] FIGS. 3(a) and 3(b) are side and end elevations,
respectively, detailing a yoke component of the mooring system of
FIG. 1;
[0057] FIGS. 4(a) and 4(b) are side elevations showing the mooring
system of FIG. 1 in operation;
[0058] FIGS. 5(a) and 5(b) are plan views showing the mooring
system of FIG. 1 in operation;
[0059] FIG. 6 is an end elevation of the yoke of FIGS. 3(a) and
3(b) in operation;
[0060] FIG. 7 is an end detail view of the upper part of the yoke
of FIGS. 3(a) and (b);
[0061] FIGS. 8(a) and 8(b) are end and side detail views,
respectively, of the lower part of the yoke of FIGS. 3(a) and
3(b);
[0062] FIG. 9 is an end elevation of a tether weight component of a
mooring system;
[0063] FIGS. 10(a) and 10(b) show the tether weight of FIG. 9 in
operation with the mooring system of FIG. 1;
[0064] FIGS. 11(a)-(c) show detail views of the tether weight of
FIG. 9 in operation;
[0065] FIGS. 11(d) and 11(e) show detail views of a prior art
tether weight in operation;
[0066] FIG. 11(f) shows a graph detailing the comparative loads put
on the tether line by the tether weight of FIG. 9 and the prior art
tether weight of FIGS. 11(d) and 11(e);
[0067] FIGS. 12(a) and 12(b) show detail views of a recovery
apparatus for retrieving the yoke component of a mooring
system;
[0068] FIGS. 13(a)-(c) show end, side and plan views of an
attachment mechanism of the retrieval apparatus;
[0069] FIGS. 14(a) and 14(b) schematically show the various
components of the recovery apparatus shown in FIGS. 12(a) and
12(b);
[0070] FIGS. 15(a)-(d) show the retrieval apparatus in operation;
and
[0071] FIGS. 16(a)-(h) show the steps for retrieving the yoke
component of the mooring system of FIG. 1 from the water;
[0072] FIGS. 17(a) and 17(b) show alternative steps for retrieving
the yoke component of the mooring system of FIG. 1;
[0073] FIG. 18 shows a schematic view of a second embodiment of a
mooring system;
[0074] FIG. 19 shows a detail view of a link plate used in the
mooring system shown in FIG. 18;
[0075] FIGS. 20(a) and 20(b) show schematic views of the mooring
system of FIG. 18 being retrieved to a vessel;
[0076] FIG. 21 shows a schematic view of a third embodiment of a
mooring system; and
[0077] FIG. 22 shows a schematic view of a fourth embodiment of a
mooring system.
[0078] A first embodiment of a mooring system for a floating
aquatic structure in accordance with the present invention is shown
in FIGS. 1(a) and 1(b). The mooring system is intended for use with
any structure, marine vessel or installation which requires
securing in water at a particular location. However, in the
illustrated embodiment, the mooring system is securing a wave
energy converter (WEC), which is itself the subject of
International Patent Application No PCT/GB99/03204 to the same
Applicant. The WEC will therefore not be described further
here.
[0079] As seen in FIG. 1(a), the WEC 1 floats on the surface of the
water and is provided with a pivoting harness or yoke 2 at its
front end 3--that is, the end facing in the direction of the
anticipated wave action or current. Attached to the yoke 2 below is
a connecting or tether weight line 11 to which is secured a primary
or tether weight 10. First or front anchor lines 16 are attached to
the tether weight arrangement 10 and have first or front anchors 15
of a known type at the ends of the front anchor lines 16 remote
from the tether weight arrangement 10. Although, the embodiment
shown in FIGS. 1(a) and 1(b) is provided with three front anchors
15 and front anchor lines 16, the system need only utilise one
front anchor and front anchor line. Additional front anchors and
front anchor lines can be added depending on the operational
requirements of the system. Optional secondary or clump weights may
also be provided on the front anchor lines 16 to provide additional
friction if desired.
[0080] Also connected to the yoke 2 is a second or rear anchor line
13 and its associated second or rear anchor 18, again of a known
type. A support or yaw line 14 is also connected between the WEC 1
and the rear anchor line 13 to help stabilise the WEC 1, as will be
described below. Again, it should be understood that the rear
anchor and yaw lines are optional components which may be used in
certain applications. FIG. 1(b) shows a schematic plan view of the
WEC 1 and front and rear anchor lines 16,13 when under the action
of waves coming from the direction shown by arrow 4.
[0081] One of the advantages of the mooring system shown in FIGS.
1(a) and 1(b) is that it is particularly suitable for securing
aquatic structures in an array. As shown in FIG. 2, a number of
WECs or other structures can be held by shared anchor points on the
sea bed and can also share electrical connections and such like.
Where an array of WECs 1 is to be moored at the same location, they
can be arranged in rows, as shown in FIG. 2. Each WEC 1 has
substantially the same mooring system as that described with
reference to FIGS. 1(a) and 1(b). However, where the mooring system
differs in an array is that the front anchors 15 are shared by
adjacent WECs 1. Thus, the front anchor lines 16 of adjacent WECs 1
share the same front anchor 15. Where there is more than one row of
WECs/structures, the front anchors 15 of each row also act as the
rear anchors of the adjacent row. As shown in FIG. 2, the rear
anchor line 13 of the WEC 1 is connected between the tether line 11
of the WEC 1 and the front anchor 15 of the WEC in the next row. In
addition to sharing anchors, the adjacent WECs also can also share
a variety of utilities via one or more umbilical utility supply
lines 27, which in the illustrated embodiment are electrical
cables. As will be explained further below, the umbilical lines 27
are adapted so that they may continue to supply utilities around
the array, even if one WEC is temporarily removed for some
reason.
[0082] Furthermore, as the umbilical lines are connected
"mid-water" (i.e. they lie under the surface of the water but not
on the floor of the body of water), only a main supply cable 80 to
the array need be on the floor. This avoids the need to have a
variety of cables or umbilical lines lying on the floor in addition
to the lines of the mooring system.
[0083] In the particular embodiment shown in FIG. 2, electrical
switchgear is provided in each WEC 1 in order to act as circuit
breakers. Thus, if there is a fault in one of the electrical cables
27 connected between two WECs 1 in the array, the circuit breakers
in the associated WECs trip in order to isolate the faulty cable.
Thus, the array can continue to operate whilst the faulty cable is
replaced.
[0084] FIGS. 3(a) and 3(b) show the yoke 2 of the mooring system in
more detail. The yoke 2 is designed to pivot relative to the WEC 1
in a substantially vertical plane (when viewed in FIGS. 3(a) and
3(b)) through a large angle range 5, both above and below the
surface 9 of the water. This allows the yoke 2 to pivot about the
front end 3 of the WEC 1 so that the lower end of the yoke 2 can be
recovered to the surface 9 without removing any attachments, as
will be explained below. The yoke 2 is also adapted to pivot in
order to allow the WEC 1 to move on its moorings without the yoke 2
and the WEC 1 coming into contact.
[0085] Referring now to FIG. 3(b), it can be seen that the yoke 2
comprises a pair of arm members 7 which are connected together at
their lower ends. The first or upper ends of the arm members 7 are
also connected to respective ends of a hollow shaft member 8, such
that the yoke 2 is substantially triangular in shape. The yoke 2
may also be provided with one or more crossbrace members 6 which
allow the yoke 2 to spread the mooring loads. The crossbrace
member(s) 6 strengthen the yoke 2 whilst still allowing it to pivot
about the front 3 of the WEC 1.
[0086] FIGS. 4(a) and 4(b) show how the mooring system provides
restorative forces to the WEC 1 when it is subjected to wave
action. As described above, the lower end of the yoke 2 is
connected to the tether weight 10 by the tether line 11. The rear
anchor line 13 and rear anchor 18 are connected to the tether line,
preferably by a link plate 12, although they may be attached by any
other suitable means. The tether weight 10 is linked to the front
anchors 15 using the front anchor lines 16. At rest, the system is
arranged such that the tether weight 10 is suspended just off the
sea bed 17. The rear anchor line 13 is connected to the rear anchor
18 at the rear of the WEC 1 and mooring system to provide restraint
should the WEC 1 and system move forwards.
[0087] FIG. 4(a) shows how the system generates a restoring force
R.sub.H for rearward displacement of the WEC 1 under wave action
from the direction illustrated by arrow 4. During the rearward
displacement, the yoke 2 and tether line 11 adopt an angle .phi. to
the vertical which generates a restoring force R.sub.H back to the
rest position due to the effect of the tether weight 10. The system
remains linear in behaviour for angles .phi. up to approximately 30
degrees (approximately half water depth). After this, the restoring
force R.sub.H becomes more non-linear with displacement but
`snatching` is avoided for angles less than approximately 60
degrees (approximately 85% of water depth) As will be understood by
those skilled in the art, snatching occurs when a slack mooring
line is suddenly subjected to a high load. An example of snatching
occurs when the tether weight is lying on the seabed and the slack
tether line is subjected to a high load by movement of the
structure. As the line goes taught, the tether weight will be
lifted from the seabed, the inertial and possible suction forces on
the weight as it lifts from the sediment on the bed can also
increase the load on the tether line. These sudden loads on the
line are known as snatch loads.
[0088] FIG. 4(b) shows how the system generates a restoring force
R.sub.H for forward displacements towards the direction 4 of the
waves. The rear anchor 18 and rear anchor line 13 act on the bottom
of the yoke 2 to generate an angle on the yoke. This leads to a
restoring force R.sub.H as shown.
[0089] The mooring system allows the WEC 1 to yaw about the
vertical axis in response to changes in the incident wave
direction, as shown in FIGS. 5(a) and 5(b). The available range of
motion is limited by the yaw line 14 acting in conjunction with the
rear anchor line 13. The lengths of these lines 13,14 can be chosen
to limit the yaw motion if necessary. The yaw limit would typically
be .+-.90 degrees relative to the expected direction 4 of incoming
waves for a coastal location. FIG. 5(a) shows a typical limiting
angle for waves coming from the starboard direction shown by arrow
19. FIG. 5(b) shows a typical limiting angle for waves coming from
the port direction shown by arrow 20. If more yaw angle is required
the mooring system can be provided with a swivel to allow it to
rotate fully, and electrical/other interconnection would be through
slip rings or similar means.
[0090] A further function of the mooring system is to provide
greater rotational stability about a particular axis. For the WEC
illustrated, the mooring system is used to provide additional roll
stability. The yoke 2, tether line 11 and tether weight 10 combine
to give the WEC additional roll stability about its longitudinal
axis, as shown in FIG. 6. With the tether weight 10 attached to its
lower end, the yoke 2 behaves in the same manner as the keel of a
yacht, producing a counteracting force to counter the roll action
of the WEC.
[0091] FIG. 7 shows a detail view of the upper part of the yoke 2.
The two arm members 7 of the yoke 2 are connected to respective
ends of a hollow shaft 8. The shaft 8 provides a structural link
between the arm members 7, which ensures that the yoke 2 is a
rigid, structurally efficient frame able to withstand large mooring
loads while remaining relatively lightweight. The shaft 8 is
located in a housing at the front end 3 of the WEC 1 and bearings
21 are provided between the shaft 8 and the housing. These bearings
21 allow low friction rotation 22 of the yoke about the rotational
axis 23 of the shaft 8. Conduits 24 are also provided which are
located within either one or both of the arm members 7 and the
shaft 8. The conduits 24 allow electrical and communications
wiring, and/or hydraulic or other lines to run between the upper
and lower ends of the yoke 2. The conduits 24 exit the shaft 8 at
one or more exit apertures 25 with the flexible end portions 26 of
the conduits 24 connecting to the WEC 1. The flexible end portions
26 are designed to allow the yoke 2 to pivot freely about the
rotational axis 23 of the shaft 8 while minimising strain on the
internal wires or other lines. This allows all internal/external
connections to the yoke 2 and machine 1 to be made at the lower end
of the yoke 2. As a result, the connection/disconnection of the WEC
1 from the mooring system can be done without needing human
intervention on the WEC 1 whilst in the water. The arrangement also
protects the wiring and other connections from the water.
[0092] FIGS. 8(a) and 8(b) show details views of the lower end of
the yoke 2. The lower ends of the arm members 7 meet to form the
corner or apex of the substantially triangular yoke 2. All of the
mechanical, electrical and/or other connections to the machine are
housed at this lower end of the yoke 2. These connections typically
comprise flexible umbilical utility supply lines 27 which usually
contain electrical, communication, hydraulic or other lines
interconnecting adjacent WECs in an array such as that shown in
FIG. 2, or connecting the WEC to main supply lines on the sea bed.
In the embodiment shown, the umbilical lines 27 between adjacent
WECs 1 are made "mid-water". In other words, the umbilical lines 27
lie below the surface of the water, but are not in contact with the
sea bed. These umbilical lines 27 are connected via sealed
connectors 28 to a connecting means in the form of a junction box
29 mounted on the yoke 2. The junction box 29 is linked to the
conduits 24 to provide a path for the various lines to the upper
part of the yoke 2 and then subsequently into the WEC 1, as shown
in FIG. 7. If two umbilical lines 27 are used, the connectors 28
are designed to be a male/female mating pair so that they can be
connected together in order to maintain continuity for an array of
WECs if this particular WEC 1 and yoke 2 are off-site for any
reason.
[0093] The lower part of the yoke 2 also includes the main
mechanical attachment to the tether line 11. In the illustrated
embodiment, this is provided by a universal joint 30 to allow free
motion about two perpendicular rotational axes and to minimise wear
of the connection in service. This connection is made using a
single pin 31 to allow straightforward and rapid attachment and
removal of the tether line. In addition, back-up lines (not shown)
may be provided to maintain integrity of the mooring system in the
event of failure of any of the components.
[0094] The lower part of the yoke 2 is also provided with a
retrieval means to allow easier retrieval of the yoke 2 onto the
rear of an installation/removal vessel, as will be described
further below. This retrieval means comprises profiled slots 32 or
may alternatively comprise a ball and socket joint (not shown) or
any similar connection. In addition, to facilitate simple, safe
recovery of the yoke 2 onto the vessel, skids or rollers 33 are
also provided. These allow the system to be easily and safely
pulled up over stern of the vessel and guide the end of the yoke 2
into a latching means. Further guides, rollers, or latches may also
be added to aid the safe and rapid recovery of the yoke 2.
[0095] A tether weight 10 which is used in the mooring system is
shown in FIG. 9. The tether weight 10 comprises a frame member 34
for attachment to the tether line 11 and the front anchor lines 16
and any other ancillary or back-up lines (not shown). The
attachment of the various lines is again made using universal
joints 30 to allow full and free motion and minimise wear of the
connection. Loops or lengths of chain links 35 are suspended from
the frame 34 to provide the weight required to generate the mooring
reaction forces, as will be described below.
[0096] The operation of the tether weight 10 is shown in FIGS. 10
and 11(a)-(c). The mooring system is arranged such that when it is
not experiencing any displacement forces from the water, the tether
weight 10 is positioned as close as possible to, or partially
resting on, the sea bed 17. In large waves, the vertical motion or
pitch of the WEC 1 may be great, as shown in FIGS. 10(a) and 10(b).
When the front 3 of the WEC 1 is on top of a wave crest, as shown
in FIG. 10(a), the tether weight 10 will be lifted away from the
sea bed 17. Conversely, when the front of the machine is in a wave
trough, as shown in FIG. 10(b), the tether weight 10 will fall onto
the sea bed 17 unless sufficient clearance is provided.
[0097] With known tether weights, it is undesirable for the weight
to touch the sea bed. However, the clearance required from the sea
bed would seriously compromise the linear range of motion if the
tether weight is not allowed to touch down on the sea bed. The
tether weight 10 shown in FIG. 9 allows a progressive pick-up from
the sea bed, as shown in FIGS. 11(a)-(c). The chain links 35 are
progressively picked up by the frame 34, ensuring a smooth increase
in the mass of the tether weight 10 as it rises from the sea bed
17.
[0098] The problem with known solid tether weights touching down on
the sea bed is illustrated in FIGS. 11(d)-(f). When a solid tether
weight 36 hits the sea bed, the tether line 11 goes slack. When the
next wave crest comes along, the tether line 11 tightens again as
the tether weight 36 is lifted back off the sea bed, and the
resultant accelerations involved are very high, putting large
snatch loads on the line 11. As the solid tether weight 36 is
lifted from the sea bed, it can also be subjected to a suction
effect by the mud and sediment on the sea bed, which worsens the
aforementioned snatch load on the tether line 11. Thus, with known
solid weights 36 an extreme snatch load on the tether line 11 will
be encountered as the tether weight is lifted from the sea bed
under the wave action.
[0099] FIG. 11(f) shows how the load in the tether line would vary
with time for both types of tether weight during the passage of a
large wave. The time represented by point 38, the tether weights
are resting fully on the sea bed. At the time represented by point
39, the tether line tightens as the weights start to be lifted back
off the sea bed. The large snatching load 40 with the solid tether
weight is caused by the rapid vertical acceleration of the moored
structure during the wave action and the suction effect of the sea
bed sediment on the tether weight. This snatching load is many
times larger than the normal loads intended for the tether line to
encounter in service, and can lead to damage and, in extreme cases,
failure of the tether line. In using a tether weight as shown in
FIG. 9, the weight rises from the sea bed more progressively, and a
much smaller snatch load 41 is placed on the tether line, as shown
in the graph of FIG. 11(f).
[0100] One of the advantages of the mooring system as described
above is that the moored machine or structure can be disconnected
whilst out of the water. In this way, there is no need for an
operator to either be in the water or on the structure during the
connection or disconnection.
[0101] This is facilitated thanks to the yoke, as it houses the
connections to the utility umbilical lines and the tether weight
and anchors. As will be explained below, the yoke remains attached
to the machine/structure when the umbilical line(s) and connecting
line to the tether weight are disconnected, thus allowing the
remainder of the mooring system to remain in the water when the
structure and yoke are retrieved.
[0102] In order to permit the connection and disconnection of the
mooring system out of the water, a recovery or retrieval apparatus
is required for retrieving the yoke from the water. An example of
such an apparatus is shown in FIGS. 12(a) and 14(b). The retrieval
apparatus 45 is installed on the deck 46 of a support vessel and is
arranged to hold the end of the yoke 2, as shown in FIG. 12(b),
during connection or disconnection of the remainder of the mooring
system. The structure and yoke can then be towed to a suitable
location for the maintenance work, or else they can be lifted onto
a barge or the like for maintenance or transportation.
[0103] The retrieval apparatus is provided with a ramp 47 onto
which the lower end of the yoke 2 is recovered, as has been
previously described with reference to FIGS. 12 and 13. The ramp 47
has an inclined surface which lies in a ramp surface plane. The
rear end of the ramp is inclined at an angle to the ramp surface
plane in order to help with the recovery of the yoke. Furthermore,
either the rear of the vessel itself or the ramp 47 is provided
with a recovery roller or drum 44 to ensure the smooth recovery or
deployment of the various lines and umbilicals attached to the yoke
2. A tensioning roller 48 is also provided at the top end of the
ramp 47 to ensure that the recovery line 42 is pulled onto its drum
(not shown) at the proper angle and tension. Guide plates 49 are
located on either side of the ramp 47 to guide the yoke 2 onto a
retention means 50 of the apparatus 45.
[0104] As seen best in FIGS. 13(a)-(c), the retention means 50
comprises a retaining member or block 51 which has a pair of
coaxial bars 52 on either side thereof. The block 51 is mounted on
a spherical bearing 53 which ensures that the block 51 can move in
all three axes (i.e. move in any roll, pitch or yaw motion). The
bearing 53 and block 51 are mounted on a base member comprising a
pedestal 54 mounted on a carriage 55, as shown in FIGS. 14(a) and
16(b).
[0105] The carriage 55 has a number of rollers 57 which are housed
within a set of guide tracks 56 so that the whole retention means
may move longitudinally relative to the ramp 47. As can be seen in
FIGS. 14(a) and 16(b), the guide tracks 56 have first and second,
or lower and upper, portions 58,59 which are at different angles
relative to the horizontal. The lower portion 58 is at a steeper
angle to the ramp surface plane in order that the attachment block
51 lies generally in the ramp surface plane when the yoke 2 is
pulled onto the ramp 47. The upper portion 58 of the guide track 56
is substantially parallel to the ramp surface plane.
[0106] FIGS. 15(a)-(d) show the steps of the yoke 2 being
retrieved. In FIG. 15(a), the yoke 2 has been pulled up onto the
ramp 47 as the recovery line 42 is wound onto its drum (not shown).
With the carriage 55 in position in the lower portion 58 of the
guide track 56, the lower end of the yoke 2 and the attachment
block 51 come into contact. The coaxial bars 52 of the block 51
locate in the previously described slots 32 of the yoke 2. With the
lower end of the yoke 2 now attached to and supported by the
attachment block 51 and carriage 55, the carriage 55 moves up the
guide track 56 as the winding recovery line 42 continues to pull
the yoke 2 up the ramp 47. This stage is shown in FIG. 15(b), as
the carriage 55 moves from the lower portion 58 to the upper
portion 59 of the guide track 56.
[0107] In each of FIGS. 15(a)-(d), it can also be seen that the
connecting line 11 of the primary weight 10 is still connected to
the yoke 2 and running up over the stern roller 44. The retrieval
apparatus 45 may also be provided with a catch 60 on the ramp 47 in
order to secure the connecting line 11 once it is disconnected from
the yoke 2, as shown in FIGS. 15(c) and 17(d). In this instance,
the connecting line 11 is provided with a sleeve 61 located at a
point on the line 11 such that when the carriage 55 and yoke 2
reach the uppermost point of the guide track 56, the sleeve has
been pulled up past the catch 60. The recovery line 42 can then be
slackened a little so that the sleeve 61 hangs on the catch 60, as
shown in FIG. 15(d). By doing this, the load of the connecting line
11 and primary weight 10 are then removed from the mechanical
attachment joint 30 of the yoke 2. The attachment pin 31 and
connecting line 11 may then be easily and safely removed from the
yoke 2.
[0108] In addition, locking pins 63 may be located in holes 62 in
the sides of the guide track 56 in order to lock the carriage 55 in
position. Once this has been done, the recovery line 42 can also be
disconnected from the yoke 2, and the WEC or other structure will
then be held or towed directly through the block 51 and bearing 53.
Any umbilical utility lines may also be disconnected from the yoke
2 at this point. Finally, the yaw line 14 can be disconnected from
the WEC or structure using either a surface buoy or boat hook, as
described below with reference to FIG. 16, or by some other
remotely operated means. The yoke 2 and WEC 1 are now fully
separated from the remainder of the mooring system. The yoke 2 and
WEC are now ready for towing, as shown in FIG. 12(b). In order to
reconnect the yoke 2 and the remainder of the mooring system, the
steps described above are simply undertaken in reverse.
[0109] FIGS. 16(a)-(h) illustrate one method of recovering the yoke
2 and WEC 1 to a support vessel 100. As seen in FIG. 16(a), the WEC
1 is held by the mooring system as previously described. To aid
recovery without having to have operators entering the water, a
recovery line 42 is permanently attached to the lower end of the
yoke 2 when the yoke 2 and WEC 1 are moored. The end of the
recovery line 42 remote from the yoke 2 is held by a buoy 102. The
recovery line may be provided with one or more clump weights 104
and/or buoyancy aids 106, as required. During this initial stage,
the support vessel 100 arrives at the buoy 102. The remote end of
the recovery line 42 is retrieved from the buoy 102 and is attached
to its winding drum (not shown) on the vessel 100.
[0110] FIG. 16(b) shows the next stage of the recovery procedure.
Once the recovery line 42 has been attached to the drum, the buoy
102 is retrieved and placed on the deck of the vessel 100. As the
recovery line 42 is wound in, the clump weight 104 is lifted from
the sea bed and rises towards the vessel 100. If fitted, the
buoyancy aids 106 will also rise to the surface as the recovery
line 42 is wound in. Once the recovery line 42 has been wound in
enough to take up any slack in the line 42, it will then begin to
raise the lower end of the yoke 2 towards the vessel 100. As the
line 42 raises the lower end of the yoke 2, the yoke 2 will pivot
about the axis of rotation 23 of the shaft 8, as shown in FIG.
16(c). With the pivoting of the yoke 2, the yoke 2 will raise the
tether line 11 and associated tether weight 10. As seen in FIG.
16(d), the recovery line 42 will continue to be wound in until such
time as the yoke 2 is recovered to the deck of the vessel 100,
which is the state previously described with reference to FIGS.
12(b) and 15(c).
[0111] As illustrated in FIG. 15(d), the tether line 11 can be
disconnected from the yoke 2 along with any other lines or
umbilicals once the yoke is attached to the attachment mechanism
50. To prevent the tether line 11 sinking to the sea bed once it
has been disconnected, it is attached to the buoy 102 and the buoy
is replaced in the water, which is the state shown in FIG.
16(e).
[0112] At the same time as the vessel 100 is undertaking the steps
shown in FIGS. 16(a)-(e), a second vessel 110 is used to detach the
rear anchor and yaw lines 13,14 from the WEC 1. This is shown in
FIGS. 16(f) and 16(g), which illustrate schematic end views of the
mooring system and WEC 1. A second recovery or chaser line 112 is
attached by a first end to the WEC 1 and by a second end to the yaw
line 14. The second vessel 110 recovers the first end from the WEC
1 and winds in the chaser line 112, as shown in FIG. 16(f). As the
chaser line 112 is wound onto the second vessel 110, the rear
anchor and yaw lines 13,14 are also recovered onto the second
vessel 110, as illustrated in FIG. 16(g). Once on the second vessel
110, the rear anchor and yaw lines 13,14 can be disconnected and
replaced in the water attached to a further buoy 114, as shown in
FIG. 16(h).
[0113] FIGS. 17(a) and 17(b) show an alternative arrangement, in
which the recovery line 42 is clipped to the yoke 2. By clipping
the recovery line 42 to the yoke 2, there is no need for the
recovery line 42 to be provided with clump weights or buoyancy
aids. It can simply be unclipped from yoke 2 and recovered in the
same way as described with reference to FIG. 16.
[0114] FIG. 18 shows a second embodiment of a mooring system in
accordance with the present invention. The mooring system in FIG.
18 shares a number of components with the mooring system previously
described, and these features will be assigned the same reference
numerals, but with a 2--prefix. Further description of certain
components is not considered necessary given their description
above.
[0115] As with the previously described embodiment, this second
embodiment of a mooring system is shown in FIG. 18 mooring a WEC
201. The mooring system has a pivoting harness or yoke 202 at the
front end 203--that is, the end facing in the direction of the
anticipated wave action or current--of the WEC 201. Where the
second embodiment differs from the previous embodiment is that
attached to the yoke 202 below are first and second connecting or
tether weight lines 211A,211B, rather than a single connecting
line. Connecting together the first and second connecting lines
211A,211B is a link plate 212, which will be described in more
detail below. The primary or tether weight 210 is hung from the
second connecting line 211B. As before, first or front anchor lines
216 are attached to the tether weight arrangement 210 and have
first or front anchors (not shown) of a known type at the ends of
the front anchor lines 216 remote from the tether weight
arrangement 210.
[0116] A recovery line 242 incorporating a suitable float is also
connected to the link plate 212 to facilitate recovery of the WEC
201. Also connected to the link plate 212 is a second or rear
anchor line 213 and its associated second or rear anchor (not
shown), again of a known type. A support or yaw line 214 is also
connected between the WEC 201 and the rear anchor line 213 to help
stabilise the WEC 201.
[0117] The link plate arrangement of the second embodiment is shown
in detail in FIG. 19. As can be seen, the link plate 212 has a
generally triangular shape and has four connection points for
connecting together the first and second connecting lines
211A,211B, rear anchor line 213 and recovery line 242. Rear anchor
line 213 is connected to the link plate 212 via a connection fork
150. Stabiliser bars 152 are also provided either side of the link
plate 212 to ensure it remains stable when removed from the
water.
[0118] The link plate 212 makes the connections between all the
main lines in the mooring system. Thanks to the generally
triangular shape of the plate 212 and the stabiliser bars 152, the
plate 212 can be easily retrieved over the stern or stern roller of
an installation/removal vessel. As well as the stabiliser bars 152,
each side of the link plate 212 is also provided with a channel
154. By providing the plate 212 with substantially identical
channels 154 on either side, the plate 212 can be locked in KARM
forks or similar.
[0119] The link plate 212 and rear anchor line connection fork 150
give a clean load of wires back over a stern roller. The first
connecting line 211A is provided with a hang-off pad-eye 158 to
allow tow loads to be taken off the connection pin 156. The
connection pin 156 is extracted remotely using a wire or hydraulic
ram to avoid having personnel operating near the mooring system as
the pin 156 is removed.
[0120] FIGS. 20(a) and 20(b) show schematically how the second
embodiment of the mooring system is retrieved onto the deck of a
support vessel. As with the previously described embodiment, the
recovery line 242 is first retrieved, preferably via a float, and
is wound onto a drum or the like on the vessel. In winding in the
recovery line 242 the link plate 212 and other lines 211A,211B,213
are also pulled towards the vessel as well. As the link plate 212
reaches the vessel, as seen in FIG. 20(a), it slides onto the deck
of the vessel. To assist the retrieval, the vessel may have a
recovery roller 244 at the stern.
[0121] Once on the deck of the vessel, as shown in FIG. 20(b), the
generally triangular shape of the link plate 212 and stabiliser
bars 152 provided thereon allow the link plate 212 to slide across
the deck with relatively little friction and no twisting, thus
ensuring that the various lines 211A,211B,213 do not become
entangled with one another. The deck of the vessel is provided with
KARM forks 160 or a similar locking arrangement to hold the link
plate 212 on the deck so that the lines 211A,211B,213 can be
detached. As shown best in FIG. 19, the link plate 212 has channels
154 either side thereof which receive the KARM forks 160.
Additionally, guide rollers 162 can also be provided on the deck to
ensure that the link plate 212 passes over the KARM forks 160 when
being retrieved.
[0122] FIG. 21 shows a third embodiment of a mooring system in
accordance with the present invention. This third embodiment shares
a number of components with the previously described first and
second embodiments and as before is shown mooring a WEC 301. The
mooring system has a pivoting harness or yoke 302 at the front end
303--that is, the end facing in the direction of the anticipated
wave action or current--of the WEC 301. Where this third embodiment
differs from the previous embodiments is that attached to the
second end of the yoke 302 are first, second and third connecting
or tether weight lines 311A,311B,311C. At the end of each
connecting line 311A,311B,311C is attached a respective primary or
tether weight 310A,310B,310C. Attached to the first and second
tether weights 310A,310B are first or front anchor lines 316 which
have first or front anchors 315 at the remote ends thereof.
Attached to the third tether weight 310C is the second or rear
anchor line 313 and its associated second or rear anchor 318, again
of a known type. A support or yaw line 314 is also connected
between the WEC 301 and the rear anchor line 313 to help stabilise
the WEC 301.
[0123] A fourth embodiment of the mooring system of the present
invention is shown in FIG. 22. As before, this fourth embodiment
shares a number of components with the previously described
embodiments and as before is shown mooring a WEC 401. The mooring
system has a pivoting harness or yoke 402 at the front end
403--that is, the end facing in the direction of the anticipated
wave action or current--of the WEC 401. The fourth embodiment
differs from the previous embodiments in that attached to the
second end of the yoke 402 are first and second connecting or
tether weight lines 411A,411B. At the end of each connecting line
411A,411B is attached a respective primary or tether weight
410A,410B. Attached to the first and second tether weights
410A,410B are first or front anchor lines 416 which have first or
front anchors 415 at the remote ends thereof. Also attached to each
tether weight 410A,410B are respective second or rear anchor lines
413A,413B and their associated second or rear anchors 418. Support
or yaw lines 414A,414B are also connected between the WEC 401 and
the rear anchor lines 413A,413B for stabilising the WEC 401.
[0124] The present invention allows a large array of machines to be
installed on the same site. This helps reduce costs as the WECs or
other structures can share anchor points. In particular, it is
desirable for WECs to be spaced closely. Thus, a mooring system
such as that of the present invention which has a small spread
between anchor points is advantageous. In addition, the mooring
system of the present invention provides a large dynamic range of
motion which is sufficient to cope with extreme waves while
minimising extreme loads on the system and anchors.
[0125] The mooring system can also provide restraint to other forms
of motion such as yaw. This allows the system to respond to the
incident waves while avoiding complete rotation about the mooring
axis which would require expensive and potentially unreliable
assemblies for electrical (or other) interconnection.
[0126] The mooring system also combines a number of reaction means
on the seabed to address different wave- or current-induced loads.
Wave loading is characterised most of the time by small, steady and
unsteady drift loads due to wave action on the
machine/installation. However, in very large and extreme seas very
large loads occasionally may be experienced. Small lightweight
solid secondary or clump weights provide sufficient reaction for
small normal loads accounting for the vast majority of operating
conditions and are cheap and easy to deploy in large numbers. The
reaction provided by these small weights can be effectively
augmented for extreme conditions by the use of conventional
embedment anchors, conventional or suction piles, larger clump
weights, or by the interlinking of smaller clump weights.
Alternatively a weighted suction pile provides both means of
reaction where the sea bed is of a suitable composition--the mass
of the pile provides adequate reaction for small waves and under
the action of extreme loads the suction of the pile in the seabed
provides large resistance to short-term large loads.
[0127] A further benefit of the present invention comes from the
use of a pivoting harness/yoke assembly or linking plate member to
provide a means of mechanical connection and disconnection of the
structure to/from the mooring system. This allows the structure to
be connected or disconnected in rough seas without having to have
personnel board the structure itself. The pivoting yoke mechanism
or linking plate can be recovered to the deck of an
installation/recovery vessel by retrieval of the recovery line
remote from the machine and the various connections can then be
attended to from the comparative safety of the vessel. Such a
provision is vital to allow intervention activities to be carried
out safely in large seas, thereby reducing the weather dependence
of these operations and the associated cost and risk. The yoke
assembly also provides restraint to the WEC or structure in
combination with the suspended tether weight(s). As illustrated in
FIG. 6, and in particular for a WEC, the harness and tether weight
provide additional roll restraint to the moored structure. This is
beneficial as it reduces the requirement for ballast on or in the
structure to provide roll stability. The reduction in the ballast
required can reduce the size, mass and volume of the structure,
directly reducing the cost thereof.
[0128] Furthermore, the provision on the yoke of the electrical or
other (e.g. hydraulic or pneumatic) utility interconnections
alongside the mechanical attachment means ensures that these can be
connected or disconnected at the same time as the mechanical
attachment without personnel having to board the WEC or other
marine/offshore installation. This means that adjacent machines may
be connected in an array `mid-water`. That is to say that a the
umbilical lines connect adjacent machines via the lower ends of the
yokes without contact with the seabed. This greatly eases the tasks
of installing, inspecting and repairing the electrical or other
interconnection, reducing cost and increasing reliability.
[0129] A benefit is also provided in connecting the rear anchor
line to the lower end bottom of the yoke or the linking plate
member. In this way, connection or disconnection of the rear anchor
line can be done at the same time as the connection or
disconnection of the main connecting line(s). For the WEC machine
of the illustrated embodiments, the rear anchor line acts as a
surge restraint to prevent the system surging forward under the
action of small waves from an offshore direction, or tidal currents
when waves are small. Sufficient slack in the complete system is
provided to allow the lower end of the yoke or linking plate to be
recovered to the deck of a support vessel without moving or
recovering any of the anchor/reaction points.
[0130] The use of heavy suspended tether weights hanging from the
end of the yoke and linked to the main mooring lines is also
advantageous, as this is what provides the means of restraint to
the system. The combination of the yoke, tether line(s) and tether
weight(s) behaves like a pendulum to provide location restraint to
the system being moored. The tether weights are effectively fixed
in location by the main mooring lines. The system provides a `soft`
or low-rate spring restraint which stops `snatching` of the mooring
lines under extreme motions.
[0131] The new arrangement of tether weight used in the present
invention also has advantages of known arrangements. The tether
weight of the present invention can descend to, rest on and rise
from the sea bed with progressive loss and recovery of suspended
weight. This is done thanks to the suspended and interlinked chain
segments. The new tether weight can rest close to (or even
partially on) the seabed, maximising the length of the tether
assembly and, as a result, its range of motion for a given range of
tether/yoke angle to the vertical. This allows the mooring system
of the present invention to be used in siting WECs or other
installations in shallower water depths near the shore/coastline
and in energetic wave regimes.
[0132] The provision of the retrieval apparatus on the back of the
support vessel ensures that the lower end of the yoke or linking
plate can be recovered onto the back of the support vessel without
direct human intervention. This allows the mechanical connections
between the yoke or linking plate and the remainder of the mooring
system and also the umbilical utility lines to be attached or
detached in safety from the deck of the support vessel. Thus,
personnel do not need to board the WEC or other
structure/installation in heavy seas or similarly adverse
conditions.
[0133] Although the tether weights of the illustrated embodiments
are shown suspended off the sea bed in the rest position, they may
alternatively be partially resting on the sea bed in the rest
position. It should also be understood that additional clump
weights or any other form of anchorage such as suction anchors or
piles may be used in addition to or instead of the conventional
anchors described above. As regards the recovery procedure
illustrated in FIG. 16, the rear anchor and yaw lines may be
disconnected prior to the disconnection of the tether line. In this
way, the same vessel could do both tasks and there would be no need
for a second vessel. Finally, the recovery lines used in the
recovery procedure can be attached to the WEC or structure by
release means which are remotely controlled. The lines can
therefore be remotely released from the WEC and then recovered to
the vessel without intervention required on the machine. These and
other modifications and improvements may be made without departing
from the scope of the invention.
* * * * *