Abstract:
Power generating equipment comprising a buoyant power generating apparatus and a support structure for the power generating apparatus, which support structure, in use, is disposed on the bed of a body of water, the power generating apparatus comprising a motor-driven winching device having a tether which is connectable at its free end to the support structure whereby retraction of the tether causes the power generating apparatus to be drawn downwardly through the body of water into engagement with the support structure, the power generating apparatus and the support structure being provided with means for aligning and latching the power generating apparatus with respect to the support structure upon engagement with the support structure.

Description:
[0001]    This invention relates to the deployment and retrieval of submerged power generating apparatus designed to generate electricity from tidal currents or ocean waves. 
         [0002]    A key aspect in the design of water current and wave generating equipment is the method by which the power generating apparatus (PGA) is periodically accessed for inspection and maintenance, and deployed again for operation. This can have a significant effect on the cost of maintaining the entire machine over its lifetime. 
         [0003]    Installations in which the PGA is fully submerged during normal operation and must on occasion be brought to the surface for inspection/maintenance can be divided into two categories:
       1. Schemes employing a surface breaking support structure (such as a mono-pile). The structure itself is used as a guide to allow the PGA to be raised to the surface using some form of mechanical lifting device (e.g. hydraulic rams, surface mounted winch) or possibly using the force of buoyancy.   2. Schemes where the support structure lies submerged (or mostly submerged) during normal operation. Such devices may consist of:
           a PGA mounted directly to a seabed foundation;   a PGA attached by cables or long articulated arms to foundations on the seabed, wherein the level of the PGA in the water column during operation is determined by its own positive buoyancy;   a PGA mounted on a submerged lattice or tubular framework structure that supports the PGA mid water column.   
               
 
         [0009]    Water current or wave generating devices are, by their very nature, deployed in extreme marine environments where weather and sea state conditions can often make marine operations difficult. The speed and simplicity of the method of deployment and retrieval of the PGA, and in particular the tolerance of this method to moderately bad weather and sea state conditions are therefore particularly important. 
         [0010]    Generally speaking, it is considerably more difficult to deploy a submerged PGA than it is to retrieve it. The deployment requires accurate alignment and attachment of the PGA with the support structure underwater, where it is difficult to see what is happening and where accurate control of the PGA position may be difficult to achieve. 
         [0011]    Retrieval of the PGA is easier because, once detached, the PGA can be raised off the support structure quickly without the need for careful control of its alignment. 
         [0012]    Known methods for deploying and retrieving submerged PGAs to/from the surface comprise:
       a crane or winch mounted on a surface vessel is used to lower a negatively buoyant (i.e. heavier than water) PGA onto a support structure, with no additional guiding apparatus. Cameras, remotely operated vehicles (ROVs) or divers may be used to monitor the position of the PGA as it is lowered. The PGA is raised in a similar manner. The vessel required to lower/raise a heavy PGA is expensive and may not be available at short notice. The constant motion of the vessel on the surface can pose problems for the position control of the PGA, particularly during its deployment.   A telescopic tube normally housed inside the support structure can be raised to the surface to allow the PGA to be fitted to it for deployment. This telescopic tube guides the negatively buoyant PGA straight up and down, to and from the surface. The weight of the PGA and the telescopic tube is supported by a deck-mounted winch/crane on a surface vessel. This method addresses the issue of position control of the PGA, allowing the lifting and lowering operations to be performed more quickly and easily. The scheme still requires vessel-mounted heavy lifting equipment, and there is a considerable overhead associated with the telescopic tube, which is only used on an occasional basis. There is also the possibility of the telescopic tube jamming, with the associated risk that the machine is left in a potentially dangerous state should weather conditions deteriorate.   Guide wires running from a surface vessel down to a support structure are used to guide the PGA as it is lowered and raised through the water column. The correct tensioning of these guide wires from the surface vessel requires accurate position holding and possibly heave compensation equipment, which is expensive. In the case of a water current generating machine where there is only a short period of relatively slack water available, the deployment and recovery of the guide wires is a time-consuming operation and there may be insufficient time.   A buoyant but submerged PGA, attached to the seabed by mooring cables, is raised and lowered using a crane/winch mounted on the deck of a surface vessel. A tether runs down from the crane, passes through a seabed anchor point/pulley block, and connects to one or more of the seabed mooring cables. By paying out/pulling in the tether from the surface vessel the seabed cable moorings are slackened/tightened, allowing the PGA to rise/fall to/from the surface under action of its own buoyancy. This scheme requires careful detail design of the seabed anchor point/pulley block, as well as the means by which the seabed mooring cables are attached to the tether and fed through the block. These issues pose significant practical difficulties in the underwater environment, particularly in terms of accessing such equipment for maintenance.       
 
       STATEMENT OF INVENTION 
       [0017]    In accordance with a first aspect of the invention, there is provided power generating equipment comprising a buoyant power generating apparatus and a support structure for the power generating apparatus, which support structure, in use, is disposed on the bed of a body of water, the power generating apparatus comprising a motor-driven winching device having a tether which is connectable at its free end to the support structure whereby retraction of the tether causes the power generating apparatus to be drawn downwardly through the body of water into engagement with the support structure, the power generating apparatus and the support structure being provided with means for aligning and latching the power generating apparatus with respect to the support structure upon engagement with the support structure. 
         [0018]    In a second aspect of the invention, there is provided a method of deploying a submersible buoyant power generating apparatus onto a support structure submerged beneath a body of water, the power generating apparatus comprising a winching device and a tether which is retractable by the winching device, the method comprising the steps of attaching the free end of the tether to the submerged support structure; and operating the winching device to winch the power generating apparatus towards the submerged support structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
           [0020]      FIG. 1  shows power generating equipment attached to a support structure by a tether; 
           [0021]      FIG. 2  corresponds to  FIG. 1  but shows an alternative embodiment; 
           [0022]      FIG. 3  corresponds to  FIG. 1  but shows a third embodiment; 
           [0023]      FIGS. 4   a  to  4   d  illustrate a method of deploying the power generating equipment of  FIG. 1 ; and 
           [0024]      FIGS. 5   a  to  5   d  illustrate a method of retrieving the power generating equipment of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The power generating equipment shown in  FIG. 1  comprises a buoyant power generating apparatus  1  (PGA). In this instance, the PGA  1  is a water current generating device; however, it is to be understood that the invention is equally applicable to any form of power generating apparatus. Also shown in  FIG. 1  is part of a support structure  7 , which is positioned on the bed of a body of water. The support structure  7  comprises the female part  5  of an alignment means, while the PGA  1  comprises the male part  3  of the alignment means. It will be appreciated that the female and male parts of the alignment means could equally be positioned in the opposite orientation without affecting operation of the invention. 
         [0026]    The term “alignment means” as used in this specification is intended to encompass any means by which the PGA  1  and support structure  7  may be brought together in a defined alignment. For example, such alignment means may not have male and female parts as described herein, but two parts which mutually fit together in a predefined relationship. 
         [0027]    The positive buoyancy of the PGA  1  may either be inherent to the PGA  1  itself, or, if the PGA  1  is not inherently buoyant, buoyancy aids may be added to the PGA  1 . Positioned on the male part  3  of the alignment means is an electrical connector  11 . A power cable  13  runs from an electrical generator within the PGA  1  and terminates at the electrical connector  11 . A complementary electrical connector  6  is positioned in the female part  5  of the alignment means, with a further power cable  8  running from the complementary electrical connector  6  to shore. In this way, when the PGA  1  and support structure  7  are brought together, as will be described in more detail below, the power generated by the PGA  1  may be supplied to shore. There may also exist certain other connections between the male and female parts  3 ,  5 , for example, fibre optic connectors or hydraulic connectors. 
         [0028]    Also shown attached to the PGA  1  is a winching device  4 . The winching device  4  is powered by a motor drive  2 . In  FIG. 1 , the winching device  4  is shown permanently attached to the PGA  1 , and the motor unit  2  is shown as being detachably connected to the winching device  4 . However, the winching device  4  may be detachable from the PGA  1 , and the motor unit  2  may be permanently attached to either the PGA  1  itself or the winching device  4 . An advantage of permanently attaching the motor unit  2  is that it reduces the number of operations that are required to deploy the PGA  1 . However, advantages of detachably connecting the motor unit  2  are that only one motor unit  2  may be required to deploy multiple PGAs, and the motor unit  2  will be easier to maintain. 
         [0029]    A tether  10  extends from the winching device  4 , through guides  12  in the PGA  1 , to a tether connection  9  which is connected to the support structure  7 . The tether  10  can be any flexible length of line, rope, cable, chain, or similar means. 
         [0030]    The centre of buoyancy A of the PGA  1  is positioned in line with the central mating axis B of the alignment means  3 ,  5 . The guides  12  may also be positioned in line with the central mating axis B of the alignment means  3 ,  5 . This has the advantage that the PGA  1  will be lowered in a stable manner, ensuring that the PGA  1  remains suitably orientated in terms of pitch, roll and lateral position. 
         [0031]    In the embodiment shown in  FIG. 2 , the tether  10  is made up of two falls which pass through a pulley block  14  mounted on the support structure  7 . The end of the tether  10  is attached to a hard point  15  on the PGA. Further falls may be added if required. By using two or more falls, the mechanical advantage can be increased, allowing for a reduction in the power requirement of the motor unit  2 . 
         [0032]    In the embodiment shown in  FIG. 3 , a pair of winching devices  4 , each with a respective tether  10 , guide  12  and tether connection  9 , is used. Further winching devices may be used as necessary. The plurality of winching devices could each be powered by a corresponding plurality of motor units, or by a single motor unit via a connecting shaft or gearbox. By using more than one winching device, the stability and orientation control of the PGA could be improved during deployment. Further, extra winching devices provide extra power to pull the PGA down through the water column. 
         [0033]      FIG. 4   a  shows the first step in deploying the PGA  1  of  FIG. 1  on the support structure  7 . The PGA  1  is initially floating in a body of water, and is towed to the required position above the support structure  7  by a surface vessel  16 . The PGA  1  is connected to the vessel  16  by a line  17 . 
         [0034]    In this example, a motor unit  2  is detachably connected to the winching device  4 , and linked with the surface vessel  16  via a second line. This second line may be used to power or control the motor unit  2 , or it may simply provide a means for retrieving the motor unit  2  after operation. 
         [0035]    A remotely operated vehicle (ROV)  18  is used to take the tether connection  9  down through the water column and attach it to the top of the support structure  7 , whilst the winching device  4  pays out the tether  10  ( FIG. 4   b ). Divers, or any other suitable means, could also be used to perform this task. 
         [0036]    In another embodiment of the invention, the support structure  7  may comprise an automatic release buoy (not shown), which on activation deploys a separate tether to the surface for attachment to the winching device  4 . This has the advantage that a ROV  18  or diver will not be required to attach the tether connection  9  to the support structure  7  itself. 
         [0037]    At this stage, the PGA  1  is cast off from the surface vessel  16  by releasing the line  17 . 
         [0038]    The motor unit  2  is then used to power the winching device  4 , thereby pulling the PGA  1  down through the water column towards the support structure  7  ( FIG. 4   c ). 
         [0039]      FIG. 4   d  shows the PGA  1  after it has mechanically mated with the support structure  7 . The male  3  and female  5  parts of the alignment means connect together, automatically aligning the PGA  1  in its correct orientation, and the two latch together so that the PGA  1  is retained on the support structure by the latched alignment means. In this example, the electrical connections  6 ,  11  (and fibre optic and hydraulic connections if there are any) are also made automatically upon mating of the male  3  and female  5  parts of the alignment means. This may be through the use of stab-plate type underwater mate connectors. Alternatively, electric, hydraulic, and/or fibre optic connections may be performed by divers or ROVs. 
         [0040]    As shown in  FIG. 4   d , the motor unit  2  is detached from the winching device  4  and retrieved by the surface vessel  16 . Because the PGA  1  is retained on the support structure  7  by the latched alignment means, the tether  10  is relieved of tension and so the removal of the motor unit  2  will not result in unwinding of the tether  10  from the winching device under the buoyancy of the PGA  1 . 
         [0041]    The subsequent retrieval of the PGA  1  is achieved by the steps shown in  FIGS. 5   a  to  5   d.    
         [0042]      FIG. 5   a  shows the PGA  1  attached to the support structure  7  as described above. A remotely operated vehicle (ROV)  18  is deployed from the surface vessel  16  to release the latch holding the PGA  1  and the support structure  7  together. Alternatively, this action could be performed by a diver or other suitable means. 
         [0043]      FIG. 5   b  shows the situation after the latch between the PGA  1  and the support structure  7  has been released. Under its own positive buoyancy, the PGA  1  rises towards the surface whilst the winching device  4  pays out the tether  10 . The tether  10  remains connected to the support structure during this stage. However, the motor unit  2  may be used to power the winching device  4  if necessary. The tether connection  9  preferably remains attached to the support structure  7  during this stage, to avoid the PGA  1  rising uncontrollably under its own buoyancy. 
         [0044]      FIG. 5   c  shows the situation when the PGA  1  has reached the surface. The PGA  1  is made fast to the surface vessel  16  by the line  17 . 
         [0045]    As shown in  FIG. 5   d , the ROV  18  is then used to disconnect the tether connection  9  from the support structure  7 . Again, a diver or other suitable means may be used to perform this task. The motor unit  2  then powers the winching device  4  to retract the tether  10 . 
         [0046]    Although some inspection or maintenance of the PGA  1  may be performed while the PGA  1  floats above the support structure  7 , it will be more usual for the PGA  1  to be towed by the vessel  16  to shore or to a suitably equipped ship for such maintenance. 
         [0047]    The invention has the particular advantage that no large surface vessel with a heavy lift crane/winch is required to deploy the PGA  1  onto the support structure  7 , or subsequently to retrieve it. This obviates the need for heavy vessel lifting equipment and eliminates problems associated with the motion/heave of the vessel pulling in an unpredictable manner on the PGA. Consequently, it is possible to deploy or retrieve the PGA  1  in moderately severe weather conditions. 
         [0048]    A further advantage is that no mechanical guide arrangement is required whilst lowering the PGA down through the water column, since the natural buoyancy of the PGA keeps the tether pulling in a predominantly vertical direction. 
         [0049]    A further advantage is that no seabed-mounted anchor/pulley block apparatus or complex tether connection devices are required. 
         [0050]    A further advantage is the accessible location of the winch and tether on the PGA where they can be serviced whenever the PGA is retrieved. 
         [0051]    A further advantage is that deployment or retrieval can be safely aborted at any time by releasing the tether from the support structure or cutting it, thus allowing the PGA to float freely to the surface and be recovered by a surface vessel.