Abstract:
A method of installing a production buoy at a subsea anchoring location is disclosed. The method includes floating a production buoy over a subsea anchoring location. Then, hanging at least a tether off the production buoy such that the or each tether extends from the production buoy towards the subsea anchoring location occurs. The method includes submerging the production buoy to a depth which allows connection of the or each tether to the subsea anchoring location. An apparatus suitable for use with this method is also provided.

Description:
This Application is the U.S. National Phase of International Application Number PCT/GB2011/051223 filed on Jun. 28, 2011, which claims priority to Great Britain Application Number 1010874.4 filed on Jun. 29, 2010. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of installing a buoy, particularly, but not exclusively, a subsea production buoy used in deep water hydrocarbon production facilities employing hybrid riser configurations. The invention also provides apparatus for tensioning a subsea production buoy to an anchoring location, particularly, but not exclusively, an anchoring location provided on a subsea foundation. 
     In deep water production fields, rather than installing a fixed production platform, it is common to anchor a floating production, storage and offloading (FPSO) vessel at a suitable surface location near the field. The produced fluids are recovered from the subsea well(s) to the seabed and then carried along pipelines laid on the seabed to the FPSO. The fluids are processed and stored on the FPSO before being transported, normally by tanker, to an onshore facility for further processing/distribution. 
     The connection between the pipeline laid on the seabed and the FPSO is typically provided by a steel catenary riser (SCR). The SCR is suspended in the water in axial tension by a subsea buoy tethered to the seabed. With such an arrangement, the SCR extends only from the subsea pipeline to the subsea buoy where it is coupled, through a suitable connection, to a flexible riser. The flexible riser then hangs between the subsea buoy and the FPSO. This connection system is sometimes called a “de-coupled system”. Here the heave motions of the surface vessel are de-coupled from the subsea buoy motions and thus the SCRs hanging from it. 
     To meet operational requirements, it is important that such subsea buoys are maintained at an appropriate depth and at an appropriate location in the water. This can be problematic due to the large distance between the surface and the foundation to which the buoy is to be anchored. 
     Another problem is that localised water currents require that the tethers extend from the buoy to the anchoring location at a varying angle. If handled incorrectly, this can cause localised areas of excessive force on the tethers adjacent the connections with the buoy, which can in turn lead to premature failure of the tethers. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of installing a production buoy at a subsea anchoring location, the method comprising the steps of:— 
     floating a production buoy over a subsea anchoring location; 
     hanging at least a tether off the production buoy such that the or each tether extends from the production buoy towards the subsea anchoring location; and 
     submerging the production buoy to a depth which allows connection of the or each tether to the subsea anchoring location. 
     Optionally, the step of submerging the production buoy comprises the step of submerging the production buoy to a first predetermined depth prior to hanging the or each tether off the production buoy. 
     The step of submerging the production buoy may comprise suspending a chain with clump weights from a pair of vessels attached to either side of the production buoy. 
     Optionally, the production buoy comprises a square or rectangular shape and four tethers are hung off the production buoy, one at each corner of the production buoy. Alternatively, the production buoy comprises a triangular shape and three tethers are hung off the production buoy, one at each corner of the production buoy; this triangular shape may provide improved design kinematics. 
     The step of securing the or each tether to the subsea anchoring location may comprise tilting the production buoy to one side in order to secure a pair of tethers at one side of the production buoy to corresponding subsea anchoring foundations at that side of the production buoy, and then tilting the production buoy to the other side in order to secure a pair of tethers at the other side of the production buoy to corresponding subsea anchoring foundations at that side of the production buoy. 
     Optionally, the step of tilting the production buoy is performed by lowering the chain and clump weights further from a vessel attached to one side of the production buoy and then from the other vessel attached to the other side of the production buoy. Alternatively, the step of tilting the buoy is performed by selective flooding of ballast compartments within the buoy. 
     Optionally, the method further comprises attaching the production buoy to the subsea anchoring location with at least a further tether. Optionally, the method comprises attaching the production buoy to the subsea anchoring location with a further four tethers for a square or rectangular buoy or a further three tethers for a triangular buoy. 
     The step of attaching the production buoy to the subsea anchoring location with at least a further tether may comprise the step of lowering the or each further tether until the lower end of the or each tether is adjacent the anchoring location, and an attachment portion, such as a tensioning module, toward the upper end of the tether is adjacent the production buoy, and then attaching the lower end to the anchoring location and the attachment portion to the production buoy. The step of lowering optionally includes lowering the or each further tether from a crane provided on a support vessel. 
     Optionally, the method of installing the buoy further comprises the step of providing tensioning apparatus between the production buoy and the subsea anchoring location. Optionally, the step of providing the tensioning apparatus comprises attaching a support bracket of the tensioning apparatus to the production buoy, securing the tether with respect to the tensioning apparatus with a tether holding arrangement, providing a pivotable articulating member having a tether receiving channel therethrough, the receiving channel having a longitudinal axis aligned with a tether departure axis, and a support socket adapted to pivotably receive the pivotable articulating member such that movement of the tether departure axis out of alignment with the receiving channel longitudinal axis results in corresponding pivotal movement of the pivotable articulating member with respect to the socket. 
     The method of installing the buoy may comprise the steps of selectively actuating the or each tensioning apparatus in order to incrementally adjust the tension held by the or each tether. Optionally, the method comprises substantially equalising the tension held by each tether. 
     According to a second aspect of the present invention, there is provided tensioning apparatus for tensioning a tether extending between a first structure and second structure, the tensioning apparatus comprising: 
     a support bracket for attaching the apparatus with respect to the first structure; 
     a tether holding arrangement for securing the tether with respect to the apparatus; 
     a pivotable articulating member having a tether receiving channel therethrough, the receiving channel having a longitudinal axis substantially aligned with a tether departure axis, 
     and a support socket adapted to pivotably receive the pivotable articulating member such that movement of the tether departure axis away from alignment with the receiving channel longitudinal axis results in corresponding pivotal movement of the pivotable articulating member with respect to the socket. 
     Optionally, the first structure is a subsea production buoy and the second structure is a subsea anchoring location. 
     Optionally, the pivotable articulating member and the support socket are adapted to allow pivotable movement of the apparatus in any direction around a pivot point in order to adjust for corresponding movement of the tether departure axis. Optionally, the pivotable articulating member and the support socket comprise a ball and socket arrangement. 
     The pivotable articulating member may be provided with an elongated extension to facilitate movement of the pivotable articulating member with the tether departure axis. Optionally, the tether holding arrangement is provided at the lower end of the elongated alignment extension. This provides a greater moment force at the interface between the pivotable articulating member and the support socket as the tether departure axis tends to move away from alignment with the receiving channel longitudinal axis. 
     The tether holding arrangement may comprise a pair of locking dogs adapted to engage with chain sections of the tether. The pair of locking dogs may be powered to allow them to open and close independently. This allows lengthening of the tether. 
     Removable bearing pads may be provided between the pivotable articulating member and the support socket. Bearing surfaces of the bearing pads or the pivotable articulating member and/or the support socket may be provided with a low friction coating to facilitate movement relative to each other. The material of the bearing surfaces may also be adapted to minimise wear over the lifetime of the apparatus. 
     Optionally, a bearing sheave may be provided above the pivotable articulating member in order to control a collected portion of the tether having passed through the pivotable articulating member. Optionally, the bearing sheave is provided on an elongated extension arm. 
     The pivotable articulating member may be provided with a jack attachment plate adapted to allow connection to a linear jack. 
     The tensioning apparatus may also be provided with a strain gauge to monitor tension in the attached tether. Optionally, the strain gauge is integrated with the bearing pads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:— 
         FIG. 1  is a schematic side view of an anchor handling tug towing the buoy off of a floating barge; 
         FIG. 2  is a schematic perspective view of the anchor handling tug towing the floating buoy to the desired surface location for subsequent submersion; 
         FIG. 3  is a schematic underside view of the buoy prior to submersion. A pair of anchor handling tugs are connected to the buoy which is located alongside a support vessel; 
         FIG. 4  is a schematic overhead view of the arrangement of  FIG. 3 ; 
         FIG. 5  is a schematic perspective view of the first four tethers approaching foundations provided at the sea bed below the buoy; 
         FIG. 6  is a schematic side view of the submerged buoy tethered to the foundations by the first four tethers, prior to detachment from the anchor handling tugs; 
         FIG. 7  is a schematic perspective view of a fifth tether and associated tensioning apparatus being deployed from the support vessel; 
         FIG. 8  is a schematic perspective view of the fifth tether approaching the foundations provided at the sea bed below the buoy; 
         FIG. 9  is a schematic illustration of the fully tethered subsea buoy; 
         FIG. 10  is a front view of tensioning apparatus according to a second aspect of the invention; 
         FIG. 11  is a side view of the tensioning apparatus of  FIG. 10 ; 
         FIG. 12  is a perspective view of two tensioning apparatus mounted at a corner of the buoy  10 ; 
         FIG. 13  is a front view of the tensioning apparatus of  FIG. 10  with an inclined tether departure axis A-A; 
         FIG. 14  is a detailed view of the ball and socket member of the tensioning apparatus of  FIG. 13 ; and 
         FIG. 15  is cross sectional side view of the tensioning apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Initial Deployment and Tethering of Buoy to Foundations 
     Referring to  FIGS. 1 to 6 , the initial steps involved in installing a subsea buoy  10  at an appropriate sea bed location will be described. At the end of this first deployment phase, the buoy will be tethered to four subsea foundations by four tethers. 
     As shown in  FIG. 1 , the buoy  10  is initially stored on a floating barge  12 . A first tug A is attached to a suitable towing point on the buoy  10  with chain  14 A. Tug A is driven forward to pull the buoy  10  off the barge  12  and onto the surface of the water. Referring to  FIG. 2 , tug A then tows the buoy  10  to the required surface location. 
     As shown in  FIG. 3 , once at the required surface location, tug B is then attached to the opposite side of the buoy  10  with chain  14 B such that the buoy  10  is floating on the surface between the two tugs A and B. The tugs A and B and buoy  10  are adjacent a support vessel V. 
     Tether T 1  and an associated tensioning apparatus  16  (to be described in detail subsequently) are then hoisted from the vessel V by a crane  18  such that the tether T 1  is suspended from a corner of the buoy  10 . This is repeated three more times for tethers T 2 , T 3 , and T 4  until the four tethers are suspended from the four corners of the buoy  10 . At this point, a short length of the chains  14 A and  14 B are in the water. Chain clump weights (not shown) are located on the decks of the tugs A and B. 
     The buoy  10  is provided with certain ballast compartments (approximately 15-20% of the total buoy  10  displacement) that will have enough displacement to float the weight of the buoy  10  plus four tethers T 1  to T 4 , with some reserve buoyancy. All remaining compartments are flooded. These ballast compartments are designed to withstand internal or external over pressure (approximately 5-6 bars). Drop down hoses are fitted to the ballast compartments in order to ensure, before commencing each lowering step, an internal over pressure (2-3 bars) exists. The remaining compartments (approximately 80-85%) will be designed to withstand approximately 3 bar of internal or external over pressure in order to cope with any pressure variations. The displacement of these compartments will provide the buoyancy to carry the entire payload (production fluids, SCR&#39;s, tether and flexible weights) as well as the tether tensions. During installation of the buoy  10  these compartments will be fully open to the sea to avoid any damage due to excessive hydrostatic pressure differential. 
     In order to begin submerging the buoy  10  and attached tethers T 1  to T 4 , the tugs A and B begin to slowly pay out more chain  14 A and  14 B until a series of clump weights  20  ( FIG. 6 ) are deployed off the rear of their decks and into the water. The chains  14 A,  14 B are then paid out further on working wires  15 A,  15 B connected thereto. As a greater and greater length of working wire  15 A,  15 B is deployed, more and more of the clump weights  20  will be suspended by the buoy  10  rather than the tugs A and B. Eventually the combined weight suspended from the buoy  10  will be in balance with the buoyancy of the buoy  10 . The buoy  10  will slowly start to submerge. 
     A short time should then be allowed to pass with the buoy  10  submerged just below the surface, without paying out more working wire  15 A,  15 B from the tugs A, B. This allows all low pressure compartments in the buoy  10  to fully flood ensuring no air bubbles are present. 
     A remotely operated vehicle (ROV) can be used, if required, to inspect “clump weight markings” in order to confirm the buoy  10  buoyancy and thereby determine that all low pressure compartments of the buoy  10  are fully flooded. This is done by identifying (approximately) the lowest link in the clump weights  20 , which will inherently correspond to the weight of the clump weight  20  and chain being carried solely by the buoy  10 . 
     The tugs A, B can then continue to pay out wire  15 A,  15 B in incremental steps of approximately 20-30 m in order to incrementally lower the buoy  10  until it is positioned at approximately the required operational depth below the surface. 
     Referring to  FIG. 5 , this incremental submersion is continued until the foundation connectors  22  of the tethers T 1 , T 2 , T 3 , T 4  are located approximately 5-10 m above the seabed. The tugs A and B are then manoeuvred until the connectors  22  are aligned with suitable anchoring locations on subsea foundations F 1 , F 2 , F 3 , F 4 . 
     Mating of the connectors  22  with the foundations F 1  to F 4  is performed by tilting the buoy  10 . Tilting is achieved by paying out the work wire  15 A from tug A by a relatively small amount until more weight is suspended from that side of the buoy  10  than from the other side of the buoy  10 . This lowers the buoy  10  at that side, while tug B maintains the same length of deployed working wire  14 A, and hence buoy height, at its side. 
     Once this side of the buoy  10  has been sufficiently tilted, the connectors  22  of tethers T 3  and T 4  are close enough to dock with a corresponding connector interface on the foundations F 3  and F 4 . If required, an ROV may be used to assist with any small adjustments in the position of the tethers T 3  and T 4  so that they can be secured to the foundations F 3  and F 4 . 
     With both tethers T 3  and T 4  secured to the foundations F 3  and F 4 , the tug A then hauls in the work wire  15 A until the tethers T 3  and T 4  take a portion of the buoyant load of the buoy  10  away from chain  14 A. 
     Tug A is now held stationary. Tug B then pays out work wire  15 B in order to lower that side of the buoy  10 . Tug B continues to pay out working wire  15 B until the foundation connectors  22  of tethers T 1  and T 2  are close enough to dock with foundations F 1  and F 2  in a similar fashion as previously described for tethers T 3  and T 4 . Now, with both tethers T 1  and T 2  secured to the foundations F 1  and F 2 , and both tethers T 3  and T 4  secured to the foundations F 3  and F 4 , the tug B then hauls in the work wire  15 B until the tethers T 1  and T 2  take a portion of buoyant load of the buoy  10  away from chain  14 B. 
     All four corners of the buoy  10  are now secured to foundations F 1  to F 4  by tethers T 1  to T 4  respectively. The tugs A and B now haul in their work wires  15 A,  15 B until the buoyant load of the buoy  10  is retained only by the tethers T 1  to T 4 . The tugs A and B can now be disconnected from the buoy  10  and recover their chain clump weights  20 , and chains  14 A,  14 B to their respective decks. 
     Installation of Remaining Four Tethers 
     Referring to  FIG. 7 , the buoy  10  is now retained by the first four tethers T 1  to T 4  (one in each corner). In order to accommodate the weight of the following extra four tethers T 5  to T 8 , the buoy  10  may be appropriately de-ballasted (by for example, approximately 600 t; 200 t on each existing tether) prior to the second phase where the remaining four tethers are installed. Spare buoyancy may also be provided (for example, approximately 50 t on each existing tether). 
     An array of the remaining tensioning modules  16  is provided at the side of the vessel V. A foundation connector  22  and depth beacon (not shown) is attached to the first end of each tether prior to deployment from the vessel V. The tether is then passed overboard from the vessel V and paid out until the upper end of the tether is off the reel and on the deck of the vessel V. The length of the tether passed into the water can be monitored using the depth beacon. 
     A top chain  48  (discussed below in more detail) on the tensioning module  16  is adjusted to ensure there will be ample slack during connection to the foundations F 1  to F 4  and the buoy  10 . The top of the tether is then attached to the top chain  48  and connected to the tensioning module  16  and linear jacks  42 . In this way, the remaining tethers T 5  to T 8  can be deployed. 
     To deploy tether T 5 , for example, the crane  18  is attached to the tensioning module  16  and takes the load of the tether T 5 . The crane  18  is then manoeuvred until the load has cleared the side of the vessel V. The tether T 5  and associated tensioning module  16  is now lowered by the crane  18  until foundation connector  22  is a few meters above the seabed (see  FIG. 8 ). The vessel V and/or crane  18  are now manoeuvred, if required, until the foundation connector  22  is close to the required foundation; in this case foundation F 2 . 
     The tether T 5  is now paid out further until foundation connector  22  docks with the foundation (again, an ROV may be used to facilitate docking). 
     At the upper end of the tether T 5 , the vessel V and/or the crane  18  is then manoeuvred to allow mating of the tensioning module  16  with the buoy  10 . As shown in  FIG. 12 , the brackets  24  of the tensioning modules  16  mate with corresponding slots on the buoy  10  to provide a secure attachment thereto. The crane  18  can now be disconnected from tether T 5 . The remaining tethers T 6  to T 8  are deployed in a similar fashion. 
     The tethers T 1  to T 8  are therefore deployed around the buoy  10  in pairs where there is a first tether (deployed in the first phase) and a second tether (deployed in the second phase) at each corner of the buoy  10 . Although the second tether of each pair (tethers T 5  to T 8 ) will be relatively slack at this stage, all of the tethers T 1  to T 8  can subsequently be tensioned such that they hold the same or similar loads as each other, using a tensioning method described in detail below. As shown in  FIG. 9 , the buoy  10  is now secured to the foundations F 1  to F 4  via tethers T 1  to T 8 . 
     Buoy Tether Tensioning 
     Most materials will undergo various phases of extension when subjected to a high degree of tension. Numerous different materials could be used for the presently described tethers; however, sheathed spiral strand wire is commonly available and is utilised in the presently described embodiment. 
     Whilst some extension characteristics are well known and easily predictable using testing, modelling and/or mathematical analysis, some extension characteristics are not accurately predictable. Although these may cause only small inaccuracies in a short length of wire, over longer lengths of say 2000 m, these inaccuracies are large enough to render the overall extension characteristics of the wire sufficiently unpredictable to require addressing. This problem is further compounded by thermal expansion and contraction, extension due to rotation, and extension due to wear of the wire. 
     Furthermore, the anchoring foundations may be at different depths from each other due to the undulation and/or slope of the sea bed. 
     It is therefore not sufficient to make the tethers T 1  to T 8  exactly the same length and assume that they will take equal shares of the load. To accommodate for this it is necessary to have some form of tension adjustment to ensure that each tether shares substantially the same load. The tensioning module  16  of the present invention provides this ability and will now be described in detail with particular reference to  FIGS. 10 to 15 . Operation of the tensioning module is described in the context of tensioning a subsea buoy to subsea foundations; however it could equally be used to tension other tethers and chains. For example, it could be used to tether a surface buoy to a subsea or surface structure, or to pull-in SCR&#39;s, umbilicals or flexible risers. Furthermore, the tensioning modules  16  could be used horizontally on the seabed for e.g. anchor pre-tensioning operations (where two opposing anchor spreads are tensioned against each other to pre-set the mooring by in-bedding drag-type anchors). 
     Tensioning module  16  comprises a support bracket  24 , a tether holding arrangement in the form of chain stops  26 , and a pivotable articulating member  28  supported in a pivotable support socket  30  attached to the support bracket  24 . The pivotable articulating member provides a “ball” member and the support socket  30  provides a “socket” member of a “ball and socket” joint. 
     The ball and socket joint is best illustrated in the cross section of  FIG. 14 . It comprises a ball member  22  having a top collar  32 , a spherical portion  34 , and an elongated lower section  36  having a channel therethrough which receives links  38  of a top chain  48  along a departure axis A-A (which is inclined in  FIG. 14 ). The top collar  32  is provided with jack posts  40  which allow a linear jack  42  to be attached thereto. 
     The socket  30  supports the underside of the spherical portion  34  and is provided with removable bearing pads  44  which provide a bearing surface for the spherical portion  34 . The bearing pads  44  and/or the bearing surface of the spherical portion  34  may comprise a high strength bearing material such as PTFE and/or fluoropolymer materials. 
     The bearing pads may comprise a laminated elastomer material having elastomer layers adhered with metal or composite inserts. This multilayer structure allows the mechanical characteristics of the joint to be adjusted during manufacture in order to suit the particular application. Such laminated elastomers meet the strictest technical specifications in terms of clearances, loads, pressure, operating conditions, environment and service life. In this regard, the size and hence the active bearing surface area between the spherical portion  34  and the socket  30 /bearing pads  44  can be designed during manufacture to withstand a specific bearing pressure dependent on the bearing material chosen. 
     Referring to  FIGS. 10 to 13  and  FIG. 15 , elongated guide members  46  are attached to the bottom of the ball member  22 . These guide members  46  have a pair of chain stops  26  attached between their lower ends. The chain stops  26  together form a ratchet mechanism which engages with links  38  of a top chain  48  connected to a tether wire T (which may be any of tethers T 1  to T 8 ). 
     An upright arm  50  extends from the top collar  32  of the ball member  22  and ends with a chain bearing sheave  52 . A dead weight  60  is attached to the free end of the top chain  48 . 
     The linear jack  42  may be any linear jack capable of operating in a subsea environment and under such loading. In the presently described embodiment, the linear jack  42  has a pair of hydraulic pistons  54  connected to each other at their upper end by a plate  56  which has a pair of locking dogs  58  mounted thereon. 
     As previously described, the tethers T 1  to T 8  are connected in pairs on the buoy  10  (a pair at each of the four corners of the buoy  10 ). Although a linear jack  42  could be connected to every tensioning module  16 , only one linear jack  42  need be provided for each pair, as shown in  FIG. 12 . Alternatively, a linear jack  42  and tensioning module  16  may be provided for each tether; this assists with equalisation of the tether loads since the tension held by one linear jack  42  of the pair can be readily compared with the tension held by the other linear jack  42  of the pair. 
     Each linear jack  42  is connected to a tensioning control manifold (not shown) which has hydraulic jumper hoses connected to the support vessel V. A subsea hydraulic power pack (not shown) may be mounted on the buoy  10  nearby the linear jacks  42 . Alternative/addition electrical power may be supplied by cables from the surface vessel V. A hydraulic power pack can also be provided on an ROV adjacent the buoy  10  if required. 
     The tethers deployed in the second phase (tethers T 5  to T 8 ) need to match the tension of the tethers deployed in the first phase (tethers T 1  to T 4 ) in each pair. The relatively slack second tethers (T 5  to T 8 ) will therefore require tensioning up. This is achieved by stroking the linear jack  42  until the slack tether becomes sufficiently tensioned. In doing this, the locking dogs  58  are engaged with the top chain  48  and the pistons  54  of the linear jack  42  are extended. This causes the top chain  48  to be pulled in which therefore increases the tension on the attached tether T. The locking dogs  58  are then disengaged from the chain  48 , the pistons  54  retracted, and the locking dogs  58  are then re-engaged at a lower point of the chain  48  ready for the next stroke. This is repeated in strokes of approximately two links until the required tension is achieved in the tether T. It is possible to monitor tension in the tether T using the linear jacks  42  by monitoring the hydraulic pressure on the jacks  42  themselves as they approach the predetermined required pressure and tether tension. 
     With the tether&#39;s T 1  to T 8  equally tensioned, the level (depth) and attitude (list and trim) of the buoy  10  can be assessed to determine if any adjustments are required. If adjustments are required, corners of the buoy  10  can be lowered or raised in the water by stroking the linear jacks  42  by incremental amounts until the desired positioning is achieved. 
     Once the final position and orientation of the buoy  10  is achieved, the hydraulic force provided by the linear jacks  42  is relaxed in order to gradually transfer the load onto the chain stops  26 . With the load held by the chain stops  26 , the linear jacks  42  can be disengaged from the top chain  48 . 
     If the buoy  10  floats directly above the anchoring foundations F 1  to F 4  the departure axis A-A of the tethers T 1  to T 8  will be substantially vertical. This situation is depicted in  FIGS. 10 and 11 . However, due to currents within the water, during the operational lifetime of the system (and during the abovementioned tensioning adjustments), the buoy  10  will typically not float directly above the foundations F 1  to F 4 . Instead, the buoy  10  and attached tethers T 1  to T 8  will normally drift away from such alignment such that the departure axes A-A of the tethers T 1  to T 8  are inclined relative to the floating plane of the buoy  10 . This situation is depicted in  FIGS. 12 to 15 . 
     The ball and socket arrangement incorporated into the tensioning apparatus of the present invention allows the tensioning apparatus to adjust position in reaction to such inclinations of the departure axis A-A, as described subsequently. 
     At the buoy end of each tether, the tension load on the tether is held by the engagement between the chain stops  26  and the links  38  of the top chain  48  as previously described. Because the chain stops  26  are provided at the bottom of the elongated guide members  46  any change in inclination of the tether T (due to e.g. a change in water current imparted on the buoy  10 ) will cause the ball member  22  to correspondingly pivot and swivel in the socket  30 . The distance between the chain stops  26  and the ball and socket joint provides a greater moment arm to facilitate such movement. This is desirable since the frictional force between the spherical portion  34  of the ball member  22  and the pads  44  of the socket  30  will be high in view of the magnitude of tension load in the tethers T. 
     This movement of the ball member  22  maintains the apparatus in line with the tether departure axis A-A which thereby ensures that all parts of the top chain  48  are under tension only. There is no kink or bend in the top chain  48  to cause localised overloading or wear over time. The only part of the top chain  48  which is not aligned with the departure axis A-A is the very top end of the top chain  48  that passes over the sheave  52 ; however this is not subjected to the tension of the tether T due to the retaining action of the chain stops  26 . 
     Once the above tensioning adjustments have been made, some predetermined compartments of the buoy  10  may be de-ballasted until the spare buoyancy (net up thrust) is equal, or near to equal, in each corner of the buoy  10 . This can be achieved by connecting down nitrogen hoses from the support vessel V to an “installation ballasting manifold”. 
     Each linear jack  42  is then moved up approximately half a chain link to take the load off the chain stoppers  26  and lock the hydraulic pressure in the linear jacks  42  (to monitor tension in all the tethers T). Pumping of an inert gas, such as nitrogen, into designated compartments is then commenced in stages while monitoring the increase of tension in the tethers T. With the tethers T approaching nominal tension, load sharing and attitude of the buoy  10  is monitored. If required individual tethers can be adjusted for better load sharing prior to fully de-ballasting of the buoy  10 . The buoy  10  is then de-ballasted until all designated compartments have been emptied. The total measured tether tension is then compared to the actual intended tension. If requirements are met, then all valves on the de-ballasted compartments are closed and the ballasting down lines are disconnected. 
     The buoy  10  is now ballasted to nominal operational up-thrust conditions. The buoy  10  depth and attitude can now be finally adjusted and the tether loads optimised as follows:— 
     Ensure all linear jacks  42  are carrying the tether loads, i.e. chain stoppers  26  are not engaged; assess depth of the buoy  10  to determine if requirements are to raise or lower the buoy  10 ; assess trim and list to determine if adjustment of the buoy  10  is required; check individual load sharing at each corner of the buoy  10  and adjust tethers T as required to equalise tension between the tethers T; when complete, relax the linear jacks  42  until the chains  48  are locked-off in chain stoppers  26  and pressure is off the linear jacks  42 ; recover hydraulic down line, manifold and linear jacks  42 . 
     The described system therefore provides an improved method of deploying subsea buoys to an appropriate depth and ensuring they are maintained at that depth regardless of varying degrees of tether extension. Furthermore, the ability of the tensioning apparatus to articulate with changes in tether angle helps to minimise the risk of excessive force on the tethers adjacent the connections with the buoy which can therefore improve the reliability and service lifetime of the tethers and buoy. 
     Modifications and improvement may be made to foregoing without departing from the scope of the invention, for example:— 
     Although, eight tethers in total are used in the embodiment described, the method and apparatus is equally suitable for tethering a buoy using more or less tethers. For example, three or six tethers could be used on a triangular buoy. 
     In the embodiment described, the tensioning modules  16  are mainly used to tension buoy tethers. However, the tensioning modules  16  could be used to tension any elongate member with minimal or no modification. For example, they could be used to pre-tension pipelines laid on the seabed where the pipeline itself comprises a tether. This would be useful to prevent “pipeline walking” (where the thermal expansion and contraction cycle of the pipeline coupled with the topography of the seabed makes such installations prone to an incremental ratcheting movement down the slope of the seabed).