WO 2005/025978 PCT/GB2004/003997 MOORING SYSTEM 1 The present invention relates to a system for the 2 mooring and retrieval of floating aquatic 3 installations and structures. 4 5 Known mooring systems for floating structures 6 comprise a number of anchors, weights and associated 7 tether lines which are attached to the installation 8 or structure. The anchors can take different forms, 9 but each is intended to hold its associated tether 10 line on the bed of the body of water upon which the 11 structure is floating. Additional weights can be 12 added to the anchor lines in order to counter the 13 forces on the structure (e.g. wind, waves or tidal 14 currents) and maintain the position of the structure 15 on the water. Additional mooring and restraint 16 lines can also be incorporated depending on the 17 conditions which the structure is expected to 18 encounter. 19 20 One problem with such known mooring systems is that 21 they present problems when the structure has to be 22 retrieved from the water. With the number of tether WO 2005/025978 PCT/GB2004/003997 2 1 and restraint lines attached to the structure, it is 2 necessary for these all to be disconnected before 3 the structure can be removed from the water. This 4 normally requires human intervention, with an 5 operator having to climb onto the structure in order 6 to detach the various lines before the structure can 7 be retrieved. In particularly heavy seas or bad 8 weather, this cannot sometimes be done for safety 9 reasons. The retrieval of the structure can 10 therefore be delayed, for days at a time in some 11 instances. Such delay can have serious time and 12 cost implications, with maintenance or installation 13 schedules being affected because of the delays. 14 15 A further disadvantage of the known systems is that 16 they are normally intended to moor only one 17 structure per system. If a number of structures are 18 to be installed at the same location, separate 19 anchor points, weights and tether lines are needed 20 for each structure. As a result, the structures 21 need to be a certain distance from one another to 22 avoid fouling the various lines. Having to space 23 the structures from one another in this manner is 24 not an effective use of space. It is often desired 25 to place a number of structures in as small a space 26 on the water as possible, normally for economic, 27 aesthetic or environmental reasons. 28 29 Another disadvantage is that known mooring systems 30 do not make efficient use of the available water 31 depth. The excursion required to cope with extreme 32 waves and currents require large ranges of motion WO 2005/025978 PCT/GB2004/003997 3 1 before the mooring lines become taut. If mooring 2 lines become taut loads rise extremely fast - a 3 process known as line snatching. This problem 4 becomes serious if the structure is to be moored in 5 shallow water and large waves and/or strong 6 currents. Also, known systems do not make efficient 7 use of the suspended weight of the mooring lines or 8 components. Suspended weight costs money and may 9 itself impart loads on the structure that may be 10 large or undesirable. 11 12 Yet another disadvantage is that known mooring 13 systems do not provide for ready, protected 14 connection of the structure to one or more utility 15 supplies such as power, fluid or water. Typically 16 such connection must be made on or near the water 17 surface, independently of mooring line connections. 18 19 In addition, a lot of known mooring systems are 20 limited in that they are only intended to encounter 21 loading from a single direction (e.g. a marine 22 structure which is only intended to encounter waves 23 from one direction). Such systems can lack 24 stability if they encounter forces from an 25 unexpected direction. Such forces can lead to 26 rotational or yaw movements of the structure which 27 can cause damage to both the structure and the 28 components of the mooring system. 29 30 It is an aim of the present invention to obviate or 31 mitigate one or more of the disadvantages of these 32 known mooring systems.
4 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common 5 general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated 10 element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Preferably, the mooring system further comprises at least one second anchor 15 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. Preferably, the mooring system further comprises a support line having one end 20 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. Preferably, the mooring system further comprises one or more secondary weights 25 attached to one or more of the first and second anchor lines. 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 30 direction.
5 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 5 preferably, the first mechanical attachment means are universal joints having detachable pin members. In a further preferred embodiment, a mooring system for a floating aquatic structure, the system comprising: 10 a harness according to the first aspect of the invention; at least one primary weight removably attached to a tether line and suspended on the tether line from the second end of the frame; and at least one first anchor removably attached to the primary weight by at least one first anchor line. 15 In a further preferred embodiment, the mooring system comprises: 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 20 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 WO 2005/025978 PCT/GB2004/003997 6 1 first anchors are adapted to resist movement of the 2 structure in a first direction. 3 4 Preferably, the system further comprises a third 5 primary weight attached to a third connecting line 6 and suspended on the third connecting line from the 7 second end of the harness; and 8 at least one second anchor removably attached 9 to the third connecting line by at least one 10 respective second anchor line, wherein the second 11 anchor is adapted to resist movement of the 12 structure in a second direction substantially 13 opposite to the first direction. 14 15 Preferably, the system further comprises a support 16 line having one end adapted to be removably attached 17 to the structure and its other end removably 18 attached to the second anchor line, the support line 19 adapted to restrict yaw motion of the.structure. 20 21 In a still further preferred embodiment, the system 22 further comprises a pair of second anchors each 23 having respective second anchor lines, one of the 24 second anchors being removably attached to the first 25 primary weight by one second anchor line and the 26 other second anchor being removably attached to the 27 second primary weight by the other second anchor 28 line, wherein the second anchors are adapted to 29 resist movement of the structure in a second 30 direction substantially opposite to the first 31 direction. 32 WO 2005/025978 PCT/GB2004/003997 7 1 Preferably, the system further comprises first and 2 second support lines, each support line having one 3 end adapted to be removably attached to the 4 structure and its other end removably attached to 5 one of the second anchor lines, the support lines 6 adapted to restrict yaw motion of the structure. 7 8 Preferably, the harness comprises first and second 9 arm members and a shaft member, wherein the arm 10 members have first and second ends and the shaft 11 member is connected between the respective first 12 ends of the arm members to form the first end of the 13 harness, and wherein the shaft member has an axis of 14 rotation about which the harness pivots. The 15 harness is adapted to apply roll restraint to the 16 structure. 17 18 Preferably, said shaft member and at least one of 19 said arm members are hollow, the harness further 20 comprising at least one conduit that is at least 21 partially located in the hollow arm member and the 22 shaft member. Preferably, the harness further 23 comprises connecting means for connecting one or 24 more utility supplies to the harness. 25 26 Preferably, said connecting means comprises a 27 junction box having one or more connectors therein, 28 the junction box being in communication with the at 29 least one conduit, and the conduit carrying utility 30 supply through the harness to the structure. 31 WO 2005/025978 PCT/GB2004/003997 8 1 Preferably, the harness further comprises retrieval 2 means to facilitate retrieval of the harness from a 3 body of water. 4 5 Preferably, the retrieval means comprises one or 6 more slots provided on the harness. Most 7 preferably, the retrieval means further comprises 8 one or more roller members provided on the harness. 9 10 Preferably, the harness further comprises a second 11 mechanical attachment means for removably attaching 12 the connecting line thereto. Most preferably, the 13 second mechanical fixing means is a universal joint 14 having a detachable pin therethrough. 15 16 Preferably, the at least one connecting line 17 includes a linking plate member, and wherein the at 18 least one second anchor line is removably connected 19 to the connecting line by the linking plate member. 20 21 Preferably, the linking plate member includes: 22 retrieval means adapted to allow the mooring 23 system to be retrieved from the water; and 24 stabilising means adapted to hold the plate 25 member in substantially the same plane during 26 retrieval. Most preferably, the linking plate 27 member further includes locking means adapted to 28 secure the plate member to a marine vessel. 29 30 Preferably, the floating aquatic structure is a wave 31 energy converter. 32 WO 2005/025978 PCT/GB2004/003997 9 1 According to a second aspect of the present 2 invention, there is provided weight for a mooring 3 system, the weight comprising a frame member and at 4 least two chain links suspended from said frame 5 member. 6 7 Preferably, the weight comprises a plurality of 8 chain links suspended from the frame member. 9 10 Preferably, the frame member includes at least two 11 mechanical attachment means adapted to removably 12 attaching components of a mooring system thereto. 13 Most preferably, the mechanical attachment means are 14 universal joints having detachable pin members. 15 16 According to a third aspect of the present invention 17 there is provided a recovery apparatus for a marine 18 vessel, the apparatus comprising: 19 a rotatable drum; 20 a recovery line having a first end adapted to 21 be removably attached to a portion of an object to 22 be recovered and a second end adapted to be 23 removably attached to the rotatable drum; 24 a retention means comprising a base member and 25 a retaining member attached to the base member by a 26 spherical bearing, the retaining member being 27 adapted to have a portion of the recovered object 28 removably attached thereto; and 29 a guide means adapted to guide the recovery 30 line over the retaining member during winding and 31 unwinding of the line. 32 WO 2005/025978 PCT/GB2004/003997 10 1 Preferably, said guide means is a ramp having an 2 inclined surface lying in a ramp surface plane, and 3 wherein the retention means is housed on the ramp. 4 5 Preferably, the base member is adapted to permit 6 longitudinal movement of the retention means 7 relative to the ramp. 8 9 Preferably, the ramp further comprises a guide track 10 in which the base member is located. Preferably, 11 the guide track has first and second portions, 12 wherein the first portion lies at a greater angle 13 relative to the horizontal than the second portion. 14 15 In a preferred embodiment, the first guide track 16 portion lies at an angle relative to the ramp 17 surface plane and the second guide track portion is 18 substantially parallel to the ramp surface plane. 19 20 Preferably, the guide track includes locking means 21 adapted to lock the retention means in one or more 22 positions on the guide track. 23 24 Preferably, the recovery apparatus further comprises 25 a tensioning roller member adjacent a first end of 26 the guide means for tensioning the recovery line. 27 28 Preferably, a second end of the guide means 29 terminates adjacent the stern of the marine vessel, 30 and the apparatus further comprises a recovery 31 roller member adjacent the stern of the vessel, the WO 2005/025978 PCT/GB2004/003997 11 1 recovery roller member adapted to assist in the 2 recovery of an object onto the guide means. 3 4 Preferred embodiments of the present invention will 5 now be described, by way of example only, with 6 reference to the accompanying drawings, in which: 7 8 Figure 1(a) shows a perspective view of a first 9 embodiment of a mooring system used in conjunction 10 with a wave energy converter; 11 Figure 1(b) shows a schematic plan view of the 12 mooring system of Figure 1(a); 13 Figure 2 shows the mooring system of Figure 1 14 when applied to an array of wave energy converters; 15 Figures 3(a) and 3(b) are side and end 16 elevations, respectively, detailing a yoke component 17 of the mooring system of Figure 1; 18 Figures 4(a) and 4(b) are side elevations 19 showing the mooring system of Figure 1 in operation; 20 Figures 5(a) and 5(b) are plan views showing 21 the mooring system of Figure 1 in operation; 22 Figure 6 is an end elevation of the yoke of 23 Figures 3(a) and 3(b) in operation; 24 Figure 7 is an end detail view of the upper 25 part of the yoke of Figures 3(a) and (b); 26 Figures 8(a) and 8(b) are end and side detail 27 views, respectively, of the lower part of the yoke 28 of Figures 3(a) and 3(b); 29 Figure 9 is an end elevation of a tether weight 30 component of a mooring system; WO 2005/025978 PCT/GB2004/003997 12 1 Figures-10(a) and 10(b) show the tether weight 2 of Figure 9 in operation with the mooring system of 3 Figure 1; 4 Figures 11(a)-(c) show detail views of the 5 tether weight of Figure 9 in operation; 6 Figures 11(d) and 11(e) show detail views of a 7 prior art tether weight in operation; 8 Figure 11(f) shows a graph detailing the 9 comparative loads put on the tether line by the 10 tether weight of Figure 9 and the prior art tether 11 weight of Figures 11(d) and 11(e); 12 Figures 12(a) and 12(b) show detail views of a 13 recovery apparatus for retrieving the yoke component 14 of a mooring system; 15 Figures 13(a)-(c) show end, side and plan views 16 of an attachment mechanism of the retrieval 17 apparatus; 18 Figures 14(a) and 14(b) schematically show the 19 various components of the recovery apparatus shown 20 in Figures 12(a) and 12(b); 21 Figures 15(a)-(d) show the retrieval apparatus 22 in operation; and 23 Figures 16(a)-(h) show the steps for retrieving 24 the yoke component of the mooring system of Figure 1 25 from the water; 26 Figures 17(a) and 17 (b) show alternative steps 27 for retrieving the yoke component of the mooring 28 system of Figure 1; 29 Figure 18 shows a schematic view of a second 30 embodiment of a mooring system; 31 Figure 19 shows a detail view of a link plate 32 used in the mooring system shown in Figure 18; WO 2005/025978 PCT/GB2004/003997 13 1 Figures 20(a) and 20(b) show schematic views of 2 the mooring system of Figure 18 being retrieved to a 3 vessel; 4 Figure 21 shows a schematic view of a third 5 embodiment of a mooring system; and 6 Figure 22 shows a schematic view of a fourth 7 embodiment of a mooring system. 8 9 A first embodiment of a mooring system for a 10 floating aquatic structure in accordance with the 11 present invention is shown in Figures 1(a) and 1(b). 12 The mooring system is intended for use with any 13 structure, marine vessel or installation which 14 requires securing in water at a particular location. 15 However, in the illustrated embodiment, the mooring 16 system is securing a wave energy converter (WEC), 17 which is itself the subject of International Patent 18 Application No PCT/GB99/03204 to the same Applicant. 19 The WEC will therefore not be described further 20 here. 21 22 As seen in Figure 1(a), the WEC 1 floats on the 23 surface of the water and is provided with a pivoting 24 harness or yoke 2 at its front end 3 - that is, the 25 end facing in the direction of the anticipated wave 26 action or current. Attached to the yoke 2 below is 27 a connecting or tether weight line 11 to which is 28 secured a primary or tether weight 10. First or 29 front anchor lines 16 are attached to the tether 30 weight arrangement 10 and have first or front 31 anchors 15 of a known type at the ends of the front 32 anchor lines 16 remote from the tether weight WO 2005/025978 PCT/GB2004/003997 14 1 arrangement 10. Although, the embodiment shown in 2 Figures 1(a) and 1(b) is provided with three front 3 anchors 15 and front anchor lines 16, the system 4 need only utilise one front anchor and front anchor 5 line. Additional front anchors and front anchor 6 lines can be added depending on the operational 7 requirements of the system. Optional secondary or 8 clump weights may also be provided on the front 9 anchor lines 16 to provide additional friction if 10 desired. 11 12 Also connected to the yoke 2 is a second or rear 13 anchor line 13 and its associated second or rear 14 anchor 18, again of a known type. A support or yaw 15 line 14 is also connected between the WEC 1 and the 16 rear anchor line 13 to help stibilise the WEC 1, as 17 will be described below. Again, it should be 18 understood that the rear anchor and yaw lines are 19 optional components which may be used in certain 20 applications. Figure 1(b) shows a schematic plan 21 view of the WEC 1 and front and rear anchor lines 22 16,13 when under the action of waves coming from the 23 direction shown by arrow 4. 24 25 One of the advantages of the mooring system shown in 26 Figures 1(a) and 1(b) is that it is particularly 27 suitable for securing aquatic structures in an 28 array. As shown in Figure 2, a number of WECs or 29 other structures can be held by shared anchor points 30 on the sea bed and can also share electrical 31 connections and such like. Where an array of WECs 1 32 is to be moored at the same location, they can be WO 2005/025978 PCT/GB2004/003997 15 1 arranged in rows, as shown in Figure 2. Each WEC 1 2 has substantially the same mooring system as that 3 described with reference to Figures 1(a) and 1(b). 4 However, where the mooring system differs in an 5 array is that the front anchors 15 are shared by 6 adjacent WECs 1. Thus, the front anchor lines 16 of 7 adjacent WECs 1 share the same front anchor 15. 8 Where there is more than one row of WECs/structures, 9 the front anchors 15 of each row also act as the 10 rear anchors of the adjacent row. As shown in 11 Figure 2, the rear anchor line 13 of the WEC 1 is 12 connected between the tether line 11 of the WEC 1 13 and the front anchor 15 of the WEC in the next row. 14 In addition to sharing anchors, the adjacent WECs 15 also can also share a variety of utilities via one 16 or more umbilical utility supply lines 27, which in 17 the illustrated embodiment are electrical cables. 18 As will be explained further below, the umbilical 19 lines 27 are adapted so that they may continue to 20 supply utilities around the array, even if one WEC 21 is temporarily removed for some reason. 22 Furthermore, as the umbilical lines are connected 23 "mid-water" (i.e. they lie under the surface of the 24 water but not on the floor of the body of water), 25 only a main supply cable 80 to the array need be on 26 the floor. This avoids the need to have a variety 27 of cables or umbilical lines lying on the floor in 28 addition to the lines of the mooring system. 29 30 In the particular embodiment shown in Figure 2, 31 electrical switchgear is provided in each WEC 1 in 32 order to act as circuit breakers. Thus, if there is WO 2005/025978 PCT/GB2004/003997 16 1 a fault in one of the electrical cables 27 connected 2 between two WECs 1 in the array, the circuit 3 breakers in the associated WECs trip in order to 4 isolate the faulty cable. Thus, the array can 5 continue to operate whilst the faulty cable is 6 replaced. 7 8 Figures 3(a) and 3(b) show the yoke 2 of the mooring 9 system in more detail. The yoke 2 is designed to 10 pivot relative to the WEC 1 in a substantially 11 vertical plane (when viewed in Figures 3(a) and 12 3(b)) through a large angle range 5, both above and 13 below the surface 9 of the water. This allows the 14 yoke 2 to pivot about the front end 3 of the WEC 1 15 so that the lower end of the yoke 2 can be recovered 16 to the surface 9 without removing any attachments, 17 as will be explained below. The yoke 2 is also 18 adapted to pivot in order to allow the WEC 1 to move 19 on its moorings without the yoke 2 and the WEC 1 20 coming into contact. 21 22 Referring now to Figure 3 (b), it can be seen that 23 the yoke 2 comprises a pair of arm members 7 which 24 are connected together at their lower ends. The 25 first or upper ends of the arm members 7 are also 26 connected to respective ends of a hollow shaft 27 member 8, such that the yoke 2 is substantially 28 triangular in shape. The yoke 2 may also be 29 provided with one or more crossbrace members 6 which 30 allow the yoke 2 to spread the mooring loads. The 31 crossbrace member(s) 6 strengthen the yoke 2 whilst WO 2005/025978 PCT/GB2004/003997 17 1 still allowing it to pivot about the front 3 of the 2 WEC 1. 3 4 Figures 4(a) and 4(b) show how the mooring system 5 provides restorative forces to the WEC 1 when it is 6 subjected to wave action. As described above, the 7 lower end of the yoke 2 is connected to the tether 8 weight 10 by the tether line 11. The rear anchor 9 line 13 and rear anchor 18 are connected to the 10 tether line, preferably by a link plate 12, although 11 they may be attached by any other suitable means. 12 The tether weight 10 is linked to the front anchors 13 15 using the front anchor lines 16. At rest, the 14 system is arranged such that the tether weight 10 is 15 suspended just off the sea bed 17. The rear anchor 16 line 13 is connected to the rear anchor 18 at the 17 rear of the WEC 1 and mooring system to provide 18 restraint should the WEC 1 and system move forwards. 19 20 Figure 4(a) shows how the system generates a 21 restoring force RH for rearward displacement of the 22 WEC 1 under wave action from the direction 23 illustrated by arrow 4. During the rearward 24 displacement, the yoke 2 and tether line 11 adopt an 25 angle # to the vertical which generates a restoring 26 force RH back to the rest position due to the effect 27 of the tether weight 10. The system remains linear 28 in behaviour for angles $ up to approximately 30 29 degrees (approximately half water depth). After 30 this, the restoring force RH becomes more non-linear 31 with displacement but 'snatching' is avoided for 32 angles less than approximately 60 degrees WO 2005/025978 PCT/GB2004/003997 18 1 (approximately 85% of water depth). As will be 2 understood by those skilled in the art, snatching 3 occurs when a slack mooring line is suddenly 4 subjected to a high load. An example of snatching 5 occurs when the tether weight is lying on the seabed 6 and the slack tether line is subjected to a high 7 load by movement of the structure. As the line goes 8 taught, the tether weight will be lifted from the 9 seabed, the inertial and possible suction forces on 10 the weight as it lifts from the sediment on the bed 11 can also increase the load on the tether line. 12 These sudden loads on the line are known as snatch 13 loads. 14 15 Figure 4(b) shows how the system generates a 16 restoring force RH for forward displacements towards 17 the direction 4 of the waves. The rear anchor 18 18 and rear anchor line 13 act on the bottom of the 19 yoke 2 to generate an angle on the yoke. This leads 20 to a restoring force RH as shown. 21 22 The mooring system allows the WEC 1 to yaw about the 23 vertical axis in response to changes in the incident 24 wave direction, as shown in Figures 5(a) and 5(b). 25 The available range of motion is limited by the yaw 26 line 14 acting in conjunction with the rear anchor 27 line 13. The lengths of these lines 13,14 can be 28 chosen to limit the yaw motion if necessary. The 29 yaw limit would typically be +/- 90 degrees relative 30 to the expected direction 4 of incoming waves for a 31 coastal location. Figure 5(a) shows a typical 32 limiting angle for waves coming from the starboard WO 2005/025978 PCT/GB2004/003997 19 1 direction shown by arrow 19. Figure 5(b) shows a 2 typical limiting angle for waves coming from the 3 port direction shown by arrow 20. If more yaw angle 4 is required the mooring system can be provided with 5 a swivel to allow it to rotate fully, and 6 electrical/other interconnection would be through 7 slip rings or similar means. 8 9 A further function of the mooring system is to 10 provide greater rotational stability about a 11 particular axis. For the WEC illustrated, the 12 mooring system is used to provide additional roll 13 stability. The yoke 2, tether line 11 and tether 14 weight 10 combine to give the WEC additional roll 15 stability about its longitudinal axis, as shown in 16 Figure 6. With the tether weight 10 attached to its 17 lower end, the yoke 2 behaves in the same manner as 18 the keel of a yacht, producing a counteracting force 19 to counter the roll action of the WEC. 20 21 Figure 7 shows a detail view of the upper part of 22 the yoke 2. The two arm members 7 of the yoke 2 are 23 connected to respective ends of a hollow shaft 8. 24 The shaft 8 provides a structural link between the 25 arm members 7, which ensures that the yoke 2 is a 26 rigid, structurally efficient frame able to 27 withstand large mooring loads while remaining 28 relatively lightweight. The shaft 8 is located in a 29 housing at the front end 3 of the WEC 1 and bearings 30 21 are provided between the shaft 8 and the housing. 31 These bearings 21 allow low friction rotation 22 of 32 the yoke about the rotational axis 23 of the shaft WO 2005/025978 PCT/GB2004/003997 20 1 8. Conduits 24 are also provided which are located 2 within either one or both of the arm members 7 and 3 the shaft 8. The conduits 24 allow electrical and 4 communications wiring, and/or hydraulic or other 5 lines to run between the upper and lower ends of the 6 yoke 2. The conduits 24 exit the shaft 8 at one or 7 more exit apertures 25 with the flexible end 8 portions 26 of the conduits 24 connecting to the WEC 9 1. The flexible end portions 26 are designed to 10 allow the yoke 2 to pivot freely about the 11 rotational axis 23 of the shaft 8 while minimising 12 strain on the internal wires or other lines. This 13 allows all internal/external connections to the yoke 14 2 and machine 1 to be made at the lower end of the 15 yoke 2. As a result, the connection/disconnection 16 of the WEC 1 from the mooring system can be done 17 without needing human intervention on the WEC 1 18 whilst in the water. The arrangement also protects 19 the wiring and other connections from the water. 20 21 Figures 8(a) and 8(b) show details views of the 22 lower end of the yoke 2. The lower ends of the arm 23 members 7 meet to form the corner or apex of the 24 substantially triangular yoke 2. All of the 25 mechanical, electrical and/or other connections to 26 the machine are housed at this lower end of the yoke 27 2. These connections typically comprise flexible 28 umbilical utility supply lines 27 which usually 29 contain electrical, communication, hydraulic or 30 other lines interconnecting adjacent WECs in an 31 array such as that shown in Figure 2, or connecting 32 the WEC to main supply lines on the sea bed. In the WO 2005/025978 PCT/GB2004/003997 21 1 embodiment shown, the umbilical lines 27 between 2 adjacent WECs 1 are made "mid-water". In other 3 words, the umbilical lines 27 lie below the surface 4 of the water, but are not in contact with the sea 5 bed. These umbilical lines 27 are connected via 6 sealed connectors 28 to a connecting means in the 7 form of a junction box 29 mounted on the yoke 2. 8 The junction box 29 is linked to the conduits 24 to 9 provide a path for the various lines to the upper 10 part of the yoke 2 and then subsequently into the 11 WEC 1, as shown in Figure 7. If two umbilical lines 12 27 are used, the connectors 28 are designed to be a 13 male/female mating pair so that they can be 14 connected together in order to maintain continuity 15 for an array of WECs if this particular WEC 1 and 16 yoke 2 are off-site for any reason. 17 18 The lower part of the yoke 2 also includes the main 19 mechanical attachment to the tether line 11. In the 20 illustrated embodiment, this is provided by a 21 universal joint 30 to allow free motion about two 22 perpendicular rotational axes and to minimise wear 23 of the connection in service. This connection is 24 made using a single pin 31 to allow straightforward 25 and rapid attachment and removal of the tether line. 26 In addition, back-up lines (not shown) may be 27 provided to maintain integrity of the mooring system 28 in the event of failure of any of the components. 29 30 The lower part of the yoke 2 is also provided with a 31 retrieval means to allow easier retrieval of the 32 yoke 2 onto the rear of an installation/removal WO 2005/025978 PCT/GB2004/003997 22 1 vessel, as will be described further below. This 2 retrieval means comprises profiled slots 32 or may 3 alternatively comprise a ball and socket joint (not 4 shown) or any similar connection. In addition, to 5 facilitate simple, safe recovery of the yoke 2 onto 6 the vessel, skids or rollers 33 are also provided. 7 These allow the system to be easily and safely 8 pulled up over stern of the vessel and guide the end 9 of the yoke 2 into a latching means. Further 10 guides, rollers, or latches may also be added to aid 11 the safe and rapid recovery of the yoke 2. 12 13 A tether weight 10 which is used in the mooring 14 system is shown in Figure 9. The tether weight 10 15 comprises a frame member 34 for attachment to the 16 tether line 11 and the front anchor lines 16 and any 17 other ancillary or back-up lines (not shown). The 18 attachment of the various lines is again made using 19 universal joints 30 to allow full and free motion 20 and minimise wear of the connection. Loops or 21 lengths of chain links 35 are suspended from the 22 frame 34 to provide the weight required to generate 23 the mooring reaction forces, as will be described 24 below. 25 26 The operation of the tether weight 10 is shown in 27 Figures 10 and 11(a)-(c). The mooring system is 28 arranged such that when it is not experiencing any 29 displacement forces from the water, the tether 30 weight 10 is positioned as close as possible to, or 31 partially resting on, the sea bed 17. In large 32 waves, the vertical motion or pitch of the WEC 1 may WO 2005/025978 PCT/GB2004/003997 23 1 be great, as shown in Figures 10(a) and 10(b). When 2 the front 3 of the WEC 1 is on top of a wave crest, 3 as shown in Figure 10(a), the tether weight 10 will 4 be lifted away from the sea bed 17. Conversely, 5 when the front of the machine is in a wave trough, 6 as shown in Figure 10(b), the tether weight 10 will 7 fall onto the sea bed 17 unless sufficient clearance 8 is provided. 9 10 With known tether weights, it is undesirable for the 11 weight to touch the sea bed. However, the clearance 12 required from the sea bed would seriously compromise 13 the linear range of motion if the tether weight is 14 not allowed to touch down on the sea bed. The 15 tether weight 10 shown in Figure 9 allows a 16 progressive pick-up from the sea bed, as shown in 17 Figures 11 (a)-(c). The chain links 35 are 18 progressively picked up by the frame 34, ensuring a 19 smooth increase in the mass of the tether weight 10 20 as it rises from the sea bed 17. 21 22 The problem with known solid tether weights touching 23 down on the sea bed is illustrated in Figures 11(d) 24 (f). When a solid tether weight 36 hits the sea 25 bed, the tether line 11 goes slack. When the next 26 wave crest comes along, the tether line 11 tightens 27 again as the tether weight 36 is lifted back off the 28 sea bed, and the resultant accelerations involved 29 are very high, putting large snatch loads on the 30 line 11. As the solid tether weight 36 is lifted 31 from the sea bed, it can also be subjected to a 32 suction effect by the mud and sediment on the sea WO 2005/025978 PCT/GB2004/003997 24 1 bed, which worsens the aforementioned snatch load on 2 the tether line 11. Thus, with known solid weights 3 36 an extreme snatch load on the tether line 11 will 4 be encountered as the tether weight is lifted from 5 the sea bed under the wave action. 6 7 Figure 11(f) shows how the load in the tether line 8 would vary with time for both types of tether weight 9 during the passage of a large wave. The time 10 represented by point 38, the tether weights are 11 resting fully on the sea bed. At the time 12 represented by point 39, the tether line tightens as 13 the weights start to be lifted back off the sea bed. 14 The large snatching load 40 with the solid tether 15 weight is caused by the rapid vertical acceleration 16 of the moored structure during the wave action and 17 the suction effect of the sea bed sediment on the 18 tether weight. This snatching load is many times 19 larger than the normal loads intended for the tether 20 line to encounter in service, and can lead to damage 21 and, in extreme cases, failure of the tether line. 22 In using a tether weight as shown in Figure 9, the 23 weight rises from the sea bed more progressively, 24 and a much smaller snatch load 41 is placed on the 25 tether line, as shown in the graph of Figure 11(f). 26 27 One of the advantages of the mooring system as 28 described above is that the moored machine or 29 structure can be disconnected whilst out of the 30 water. In this way, there is no need for an 31 operator to either be in the water or on the 32 structure during the connection or disconnection.
WO 2005/025978 PCT/GB2004/003997 25 1 This is facilitated thanks to the yoke, as it houses 2 the connections to the utility umbilical lines and 3 the tether weight and anchors. As will be explained 4 below, the yoke remains attached to the 5 machine/structure when the umbilical line(s) and 6 connecting line to the tether weight are 7 disconnected, thus allowing the remainder of the 8 mooring system to remain in the water when the 9 structure and yoke are retrieved. 10 11 In order to permit the connection and disconnection 12 of the mooring system out of the water, a recovery 13 or retrieval apparatus is required for retrieving 14 the yoke from the water. An example of such an 15 apparatus is shown in Figures 12(a) and 14(b). The 16 retrieval apparatus 45 is installed on the deck 46 17 of a support vessel and is arranged to hold the end 18 of the yoke 2, as shown in Figure 12(b), during 19 connection or disconnection of the remainder of the 20 mooring system. The structure and yoke can then be 21 towed to a suitable location for the maintenance 22 work, or else they can be lifted onto a barge or the 23 like for maintenance or transportation. 24 25 The retrieval apparatus is provided with a ramp 47 26 onto which the lower end of the yoke 2 is recovered, 27 as has been previously described with reference to 28 Figures 12 and 13. The ramp 47 has an inclined 29 surface which lies in a ramp surface plane. The 30 rear end of the ramp is inclined at an angle to the 31 ramp surface plane in order to help with the 32 recovery of the yoke. Furthermore, either the rear WO 2005/025978 PCT/GB2004/003997 26 1 of the vessel itself or the ramp 47 is provided with 2 a recovery roller or drum 44 to ensure the smooth 3 recovery or deployment of the various lines and 4 umbilicals attached to the yoke 2. A tensioning 5 roller 48 is also provided at the top end of the 6 ramp 47 to ensure that the recovery line 42 is 7 pulled onto its drum (not shown) at the proper angle 8 and tension. Guide plates 49 are located on either 9 side of the ramp 47 to guide the yoke 2 onto a 10 retention means 50 of the apparatus 45. 11 12 As seen best in Figures 13(a)-(c), the retention 13 means 50 comprises a retaining member or block 51 14 which has a pair of coaxial bars 52 on either side 15 thereof. The block 51 is mounted on a spherical 16 bearing 53 which ensures that the block 51 can move 17 in all three axes (i.e. move in any roll, pitch or 18 yaw motion). The bearing 53 and block 51 are 19 mounted on a base member comprising a pedestal 54 20 mounted on a carriage 55, as shown in Figures 14(a) 21 and 16(b). 22 23 The carriage 55 has a number of rollers 57 which are 24 housed within a set of guide tracks 56 so that the 25 whole retention means may move longitudinally 26 relative to the ramp 47. As can be seen in Figures 27 14(a) and 16 (b), the guide tracks 56 have first and 28 second, or lower and upper, portions 58,59 which are 29 at different angles relative to the horizontal. The 30 lower portion 58 is at a steeper angle to the ramp 31 surface plane in order that the attachment block 51 32 lies generally in the ramp surface plane when the WO 2005/025978 PCT/GB2004/003997 27 1 yoke 2 is pulled onto the ramp 47. The upper 2 portion 58 of the guide track 56 is substantially 3 parallel to the ramp surface plane. 4 5 Figures 15(a)-(d) show the steps of the yoke 2 being 6 retrieved. In Figure 15(a), the yoke 2 has been 7 pulled up onto the ramp 47 as the recovery line 42 8 is wound onto its drum (not shown). With the 9 carriage 55 in position in the lower portion 58 of 10 the guide track 56, the lower end of the yoke 2 and 11 the attachment block 51 come into contact. The 12 coaxial bars 52 of the block 51 locate in the 13 previously described slots 32 of the yoke 2. With 14 the lower end of the yoke 2 now attached to and 15 supported by the attachment block 51 and carriage 16 55, the carriage 55 moves up the guide track 56 as 17 the winding recovery line 42 continues to pull the 18 yoke 2 up the ramp 47. This stage is shown in 19 Figure 15(b), as the carriage 55 moves from the 20 lower portion 58 to the upper portion 59 of the 21 guide track 56. 22 23 In each of Figures 15(a)-(d), it can also be seen 24 that the connecting line 11 of the primary weight 10 25 is still connected to the yoke 2 and running up over 26 the stern roller 44. The retrieval apparatus 45 may 27 also be provided with a catch 60 on the ramp 47 in 28 order to secure the connecting line 11 once it is 29 disconnected from the yoke 2, as shown in Figures 30 15(c) and 17(d). In this instance, the connecting 31 line 11 is provided with a sleeve 61 located at a 32 point on the line 11 such that when the carriage 55 WO 2005/025978 PCT/GB2004/003997 28 1 and yoke 2 reach the uppermost point of the guide 2 track 56, the sleeve has been pulled up past the 3 catch 60. The recovery line 42 can then be 4 slackened a little so that the sleeve 61 hangs on 5 the catch 60, as shown in Figure 15(d). By doing 6 this, the load of the connecting line 11 and primary 7 weight 10 are then removed from the mechanical 8 attachment joint 30 of the yoke 2. The attachment 9 pin 31 and connecting line 11 may then be easilt and 10 safely removed from the yoke 2. 11 12 In addition, locking pins 63 may be located in holes 13 62 in the sides of the guide track 56 in order to 14 lock the carriage 55 in position. Once this has 15 been done, the recovery line 42 can also be 16 disconnected from the yoke 2, and the WEC or other 17 structure will then be held or towed directly 18 through the block 51 and bearing 53. Any umbilical 19 utility lines may also be disconnected from the yoke 20 2 at this point. Finally, the yaw line 14 can be 21 disconnected from the WEC or structure using either 22 a surface buoy or boat hook, as described below with 23 reference to Figure 16, or by some other remotely 24 operated means. The yoke 2 and WEC 1 are now fully 25 separated from the remainder of the mooring system. 26 The yoke 2 and WEC are now ready for towing, as 27 shown in Figure 12 (b). In order to reconnect the 28 yoke 2 and the remainder of the mooring system, the 29 steps described above are simply undertaken in 30 reverse. 31 WO 2005/025978 PCT/GB2004/003997 29 1 Figures 16(a)-(h) illustrate one method of 2 recovering the yoke 2 and WEC 1 to a support vessel 3 100. As seen in Figure 16(a), the WEC 1 is held by 4 the mooring system as previously described. To aid 5 recovery without having to have operators entering 6 the water, a recovery line 42 is permanently 7 attached to the lower end of the yoke 2 when the 8 yoke 2 and WEC 1 are moored. The end of the 9 recovery line 42 remote from the yoke 2 is held by a 10 buoy 102. The recovery line may be provided with 11 one or more clump weights 104 and/or buoyancy aids 12 106, as required. During this initial stage, the 13 support vessel 100 arrives at the buoy 102. The 14 remote end of the recovery line 42 is retrieved from 15 the buoy 102 and is attached to its winding drum 16 (not shown) on the vessel 100. 17 18 Figure 16(b) shows the next stage of the recovery 19 procedure. Once the recovery line 42 has been 20 attached to the drum, the buoy 102 is retrieved and 21 placed on the deck of the vessel 100. As the 22 recovery line 42 is wound in, the clump weight 104 23 is lifted from the sea bed and rises towards the 24 vessel 100. If fitted, the buoyancy aids 106 will 25 also rise to the surface as the recovery line 42 is 26 wound in. Once the recovery line 42 has been wound 27 in enough to take up any slack in the line 42, it 28 will then begin to raise the lower end of the yoke 2 29 towards the vessel 100. As the line 42 raises the 30 lower end of the yoke 2, the yoke 2 will pivot about 31 the axis of rotation 23 of the shaft 8, as shown in 32 Figure 16(c). With the pivoting of the yoke 2, the WO 2005/025978 PCT/GB2004/003997 30 1 yoke 2 will raise the tether line 11 and associated 2 tether weight 10. As seen in Figure 16(d), the 3 recovery line 42 will continue to be wound in until 4 such time as the yoke 2 is recovered to the deck of 5 the vessel 100, which is the state previously 6 described with reference to Figures 12(b) and 15(c). 7 8 As illustrated in Figure 15(d), the tether line 11 9 can be disconnected from the yoke 2 along with any 10 other lines or umbilicals once the yoke is attached 11 to the attachment mechanism 50. To prevent the 12 tether line 11 sinking to the sea bed once it has 13 been disconnected, it is attached to the buoy 102 14 and the buoy is replaced in the water, which is the 15 state shown in Figure 16(e). 16 17 At the same time as the vessel 100 is undertaking 18 the steps shown in Figures 16(a)-(e), a second 19 vessel 110 is used to detach the rear anchor and yaw 20 lines 13,14 from the WEC 1. This is shown in 21 Figures 16(f) and 16(g), which illustrate schematic 22 end views of the mooring system and WEC 1. A second 23 recovery or chaser line 112 is attached by a first 24 end to the WEC 1 and by a second end to the yaw line 25 14. The second vessel 110 recovers the first end 26 from the WEC 1 and winds in the chaser line 112, as 27 shown in Figure 16(f). As the chaser line 112 is 28 wound onto the second vessel 110, the rear anchor 29 and yaw lines 13,14 are also recovered onto the 30 second vessel 110, as illustrated in Figure 16(g). 31 Once on the second vessel 110, the rear anchor and 32 yaw lines 13,14 can be disconnected and replaced in WO 2005/025978 PCT/GB2004/003997 31 1 the water attached to a further buoy 114, as shown 2 in Figure 16(h). 3 4 Figures 17(a) and 17(b) show an alternative 5 arrangement, in which the recovery line 42 is 6 clipped to the yoke 2. By clipping the recovery 7 line 42 to the yoke 2, there is no need for the 8 recovery line 42 to be provided with clump weights 9 or buoyancy aids. It can simply be unclipped from 10 yoke 2 and recovered in the same way as described 11 with reference to Figure 16. 12 13 Figure 18 shows a second embodiment of a mooring 14 system in accordance with the present invention. 15 The mooring system in Figure 18 shares a number of 16 components with the mooring system previously 17 described, and these features will be assigned the 18 same reference numerals, but with a 2-- prefix. 19 Further description of certain components is not 20 considered necessary given their description above. 21 22 As with the previously described embodiment, this 23 second embodiment of a mooring system is shown in 24 Figure 18 mooring a WEC 201. The mooring system has 25 a pivoting harness or yoke 202 at the front end 203 26 - that is, the end facing in the direction of the 27 anticipated wave action or current - of the WEC 201. 28 Where the second embodiment differs from the 29 previous embodiment is that attached to the yoke 202 30 below are first and second connecting or tether 31 weight lines 211A,211B, rather than a single 32 connecting line. Connecting together the first and WO 2005/025978 PCT/GB2004/003997 32 1 second connecting lines 211A,211B is a link plate 2 212, which will be described in more detail below. 3 The primary or tether weight 210 is hung from the 4 second connecting line 211B. As before, first or 5 front anchor lines 216 are attached to the tether 6 weight arrangement 210 and have first or front 7 anchors (not shown) of a known type at the ends of 8 the front anchor lines 216 remote from the tether 9 weight arrangement 210. 10 11 A recovery line 242 incorporating a suitable float 12 is also connected to the link plate 212 to 13 facilitate recovery of the WEC 201. Also connected 14 to the link plate 212 is a second or rear anchor 15 line 213 and its associated second or rear anchor 16 (not shown), again of a known type. A support or 17 yaw line 214 is also connected between the WEC 201 18 and the rear anchor line 213 to help stabilise the 19 WEC 201. 20 21 The link plate arrangement of the second embodiment 22 is shown in detail in Figure 19. As can be seen, 23 the link plate 212 has a generally triangular shape 24 and has four connection points for connecting 25 together the first and second connecting lines 26 211A,211B, rear anchor line 213 and recovery line 27 242. Rear anchor line 213 is connected to the link 28 plate 212 via a connection fork 150. Stabiliser 29 bars 152 are also provided either side of the link 30 plate 212 to ensure it remains stable when removed 31 from the water. 32 WO 2005/025978 PCT/GB2004/003997 33 1 The link plate 212 makes the connections between all 2 the main lines in the mooring system. Thanks to the 3 generally triangular shape of the plate 212 and the 4 stabiliser bars 152, the plate 212 can be easily 5 retrieved over the stern or stern roller of an 6 installation/removal vessel. As well as the 7 stabiliser bars 152, each side of the link plate 212 8 is also provided.with a channel 154. By providing 9 the plate 212 with substantially identical channels 10 154 on either side, the plate 212 can be locked in 11 KARM forks or similar. 12 13 The link plate 212 and rear anchor line connection 14 fork 150 give a clean load of wires back over a 15 stern roller. The first connecting line 211A is 16 provided with a hang-off pad-eye 158 to allow tow 17 loads to be taken off the connection pin 156. The 18 connection pin 156 is extracted remotely using a 19 wire or hydraulic ram to avoid having personnel 20 operating near the mooring system as the pin 156 is 21 removed. 22 23 Figures 20(a) and 20(b) show schematically how the 24 second embodiment of the mooring system is retrieved 25 onto the deck of a support vessel. As with the 26 previously described embodiment, the recovery line 27 242 is first retrieved, preferably via a float, and 28 is wound onto a drum or the like on the vessel. In 29 winding in the recovery line 242 the link plate 212 30 and other lines 211A,211B,213 are also pulled 31 towards the vessel as well. As the link plate 212 32 reaches the vessel, as seen in Figure 20(a), it WO 2005/025978 PCT/GB2004/003997 34 1 slides onto the deck of the vessel. To assist the 2 retrieval, the vessel may have a recovery roller 244 3 at the stern. 4 5 Once on the deck of the vessel, as shown in Figure 6 20(b), the generally triangular shape of the link 7 plate 212 and stabiliser bars 152 provided thereon 8 allow the link plate 212 to slide across the deck 9 with relatively little friction and no twisting, 10 thus ensuring that the various lines 211A,211B,213 11 do not become entangled with one another. The deck 12 of the vessel is provided with KARM forks 160 or a 13 similar locking arrangement to hold the link plate 14 212 on the deck so that the lines 211A,211B,213 can 15 be detached. As shown best in Figure 19, the link 16 plate 212 has channels 154 either side thereof which 17 receive the KARM forks 160. Additionally, guide 18 rollers 162 can also be provided on the deck to 19 ensure that the link plate 212 passes over the KARM 20 forks 160 when being retrieved. 21 22 Figure 21 shows a third embodiment of a mooring 23 system in accordance with the present invention. 24 This third embodiment shares a number of components 25 with the previously described first and second 26 embodiments and as before is shown mooring a WEC 27 301. The mooring system has a pivoting harness or 28 yoke 302 at the front end 303 - that is, the end 29 facing in the direction of the anticipated wave 30 action or current - of the WEC 301. Where this 31 third embodiment differs from the previous 32 embodiments is that attached to the second end of WO 2005/025978 PCT/GB2004/003997 35 1 the yoke 302 are first, second and third connecting 2 or tether weight lines 311A,311B,311C. At the end 3 of each connecting line 311A,311B,311C is attached a 4 respective primary or tether weight 310A,310B,310C. 5 Attached to the first and second tether weights 6 310A,310B are first or front anchor lines 316 which 7 have first. or front anchors 315 at the remote ends 8 thereof. Attached to the third tether weight 310C 9 is the second or rear anchor line 313 and its 10 associated second or rear anchor 318, again of a 11 known type. A support or yaw line 314 is also 12 connected between the WEC 301 and the rear anchor 13 line 313 to help stabilise the WEC 301. 14 15 A fourth embodiment of the mooring system of the 16 present invention is shown in Figure 22. As before, 17 this fourth embodiment shares a number of components 18 with the previously described embodiments and as 19 before is shown mooring a WEC 401. The mooring 20 system has a pivoting harness or yoke 402 at the 21 front end 403 - that is, the end facing in the 22 direction of the anticipated wave action or current 23 - of the WEC 401. The fourth embodiment differs 24 from the previous embodiments in that attached to 25 the second end of the yoke 402 are first and second 26 connecting or tether weight lines 411A,411B. At the 27 end of each connecting line 411A,411B is attached a 28 respective primary or tether weight 410A,410B. 29 Attached to the first and second tether weights 30 410A,410B are first or front anchor lines 416 which 31 have first or front anchors 415 at the remote ends 32 thereof. Also attached to each tether weight WO 2005/025978 PCT/GB2004/003997 36 1 410A,410B are respective second or rear anchor lines 2 413A,413B and their associated second or rear 3 anchors 418. Support or yaw lines 414A,414B are 4 also connected between the WEC 401 and the rear 5 anchor lines 413A,413B for stabilising the WEC 401. 6 7 The present invention allows a large array of 8 machines to be installed on the same site. This 9 helps reduce costs as the WECs or other structures 10 can share anchor points. In particular, it is 11 desirable for WECs to be spaced closely. Thus, a 12 mooring system such as that of the present invention 13 which has a small spread between anchor points is 14 advantageous. In addition, the mooring system of 15 the present invention provides a large dynamic range 16 of motion which is sufficient to cope with extreme 17 waves while minimising extreme loads on the system 18 and anchors. 19 20 The mooring system can also provide restraint to 21 other forms of motion such as yaw. This allows the 22 system to respond to the incident waves while 23 avoiding complete rotation about the mooring axis 24 which would require expensive and potentially 25 unreliable assemblies for electrical (or other) 26 interconnection. 27 28 The mooring system also combines a number of 29 reaction means on the seabed to address different 30 wave- or current-induced loads. Wave loading is 31 characterised most of the time by small, steady and 32 unsteady drift loads due to wave action on the WO 2005/025978 PCT/GB2004/003997 37 1 machine/installation. However, in very large and 2 extreme seas very large loads occasionally may be 3 experienced. Small lightweight solid secondary or 4 clump weights provide sufficient reaction for small 5 normal loads accounting for the vast majority of 6 operating conditions and are cheap and easy to 7 deploy in large numbers. The reaction provided by 8 these small weights can be effectively augmented for 9 extreme conditions by the use of conventional 10 embedment anchors, conventional or suction piles, 11 larger clump weights, or by the interlinking of 12 smaller clump weights. Alternatively a weighted 13 suction pile provides both means of reaction where 14 the sea bed is of a suitable composition - the mass 15 of the pile provides adequate reaction for small 16 waves and under the action of extreme loads the 17 suction of the pile in the seabed provides large 18 resistance to short-term large loads. 19 20 A further benefit of the present invention comes 21 from the use of a pivoting harness/yoke assembly or 22 linking plate member to provide a means of 23 mechanical connection and disconnection of the 24 structure to/from the mooring system. This allows 25 the structure to be connected or disconnected in 26 rough seas without having to have personnel board 27 the structure itself. The pivoting yoke mechanism 28 or linking plate can be recovered to the deck of an 29 installation/recovery vessel by retrieval of the 30 recovery line remote from the machine and the 31 various connections can then be attended to from the 32 comparative safety of the vessel. Such a provision WO 2005/025978 PCT/GB2004/003997 38 1 is vital to allow intervention activities to be 2 carried out safely in large seas, thereby reducing 3 the weather dependence of these operations and the 4 associated cost and risk. The yoke assembly also 5 provides restraint to the WEC or structure in 6 combination with the suspended tether weight(s). As 7 illustrated in Figure 6, and in particular for a 8 WEC, the harness and tether weight provide 9 additional roll restraint to the moored structure. 10 This is beneficial as it reduces the requirement for 11 ballast on or in the structure to provide roll 12 stability. The reduction in the ballast required 13 can reduce the size, mass and volume of the 14 structure, directly reducing the cost thereof. 15 16 Furthermore, the provision on the yoke of the 17 electrical or other (e.g. hydraulic or pneumatic) 18 utility interconnections alongside the mechanical 19 attachment means ensures that these can be connected 20 or disconnected at the same time as the mechanical 21 attachment without personnel having to board the WEC 22 or other marine/offshore installation. This means 23 that adjacent machines may be connected in an array 24 'mid-water'. That is to say that a the umbilical 25 lines connect adjacent machines via the lower ends 26 of the yokes without contact with the seabed. This 27 greatly eases the tasks of installing, inspecting 28 and repairing the electrical or other 29 interconnection, reducing cost and increasing 30 reliability. 31 WO 2005/025978 PCT/GB2004/003997 39 1 A benefit is also provided in connecting the rear 2 anchor line to the lower end bottom of the yoke or 3 the linking plate member. In this way, connection 4 or disconnection of the rear anchor line can be done 5 at the same time as the connection or disconnection 6 of the main connecting line(s). For the WEC machine 7 of the illustrated embodiments, the rear anchor line 8 acts as a surge restraint to prevent the system 9 surging forward under the action of small waves from 10 an offshore direction, or tidal currents when waves 11 are small. Sufficient slack in the complete system 12 is provided to allow the lower end of the yoke or 13 linking plate to be recovered to the deck of a 14 support vessel without moving or recovering any of 15 the anchor/reaction points. 16 17 The use of heavy suspended tether weights hanging 18 from the end of the yoke and linked to the main 19 mooring lines is also advantageous, as this is what 20 provides the means of restraint to the system. The 21 combination of the yoke, tether line(s) and tether 22 weight(s) behaves like a pendulum to provide 23 location restraint to the system being moored. The 24 tether weights are effectively fixed in location by 25 the main mooring lines. The system provides a 26 'soft' or low-rate spring restraint which stops 27 'snatching' of the mooring lines under extreme 28 motions. 29 30 The new arrangement of tether weight used in the 31 present invention also has advantages of known 32 arrangements. The tether weight of the present WO 2005/025978 PCT/GB2004/003997 40 1 invention can descend to, rest on and rise from the 2 sea bed with progressive loss and recovery of 3 suspended weight. This is done thanks to the 4 suspended and interlinked chain segments. The new 5 tether weight can rest close to (or even partially 6 on) the seabed, maximising the length of the tether 7 assembly and, as a result, its range of motion for a 8 given range of tether/yoke angle to the vertical. 9 This allows the mooring system of the present 10 invention to be used in siting WECs or other 11 installations in shallower water depths near the 12 shore/coastline and in energetic wave regimes. 13 14 The provision of the retrieval apparatus on the back 15 of the support vessel ensures that the lower end of 16 the yoke or linking plate can be recovered onto the 17 back of the support vessel without direct human 18 intervention. This allows the mechanical connections 19 between the yoke or linking plate and the remainder 20 of the mooring system and also the umbilical utility 21 lines to be attached or detached in safety from the 22 deck of the support vessel. Thus, personnel do not 23 need to board the WEC or other 24 structure/installation in heavy seas or similarly 25 adverse conditions. 26 27 Although the tether weights of the illustrated 28 embodiments are shown suspended off the sea bed in 29 the rest position, they may alternatively be 30 partially resting on the sea bed in the rest 31 position. It should also be understood that 32 additional clump weights or any other form of WO 2005/025978 PCT/GB2004/003997 41 1 anchorage such as suction anchors or piles may be 2 used in addition to or instead of the conventional 3 anchors described above. As regards the recovery 4 procedure illustrated in Figure 16, the rear anchor 5 and yaw lines may be disconnected prior to the 6 disconnection of the tether line. In this way, the 7 same vessel could do both tasks and there would be 8 no need for a second vessel. Finally, the recovery 9 lines used in the recovery procedure can be attached 10 to the WEC or structure by release means which are 11 remotely controlled. The lines can therefore be 12 remotely released from the WEC and then recovered to 13 the vessel without intervention required on the 14 machine. These and other modifications and 15 improvements may be made without departing from the 16 scope of the invention.