Power generating systems

Abstract

Power generating equipment includes a support structure adapted to be disposed on the bed of a body of water having a surface above the bed; and a buoyant power generating apparatus having positive buoyancy releasably connectable to the support structure, the power generating apparatus being adapted to be released from the support structure and to make controlled free ascent to the surface of the water following such release.

Claims

1. Power generating equipment comprising: a support structure adapted to be disposed on a bed of a body of water; a buoyant power generating apparatus having positive buoyancy releasably connectable to the support structure, the power generating apparatus being adapted to be released from the support structure and to make controlled free ascent to a surface of the body of water following such release; a winch releasably mounted on an upper surface of the power generating apparatus, the upper surface of the power generating apparatus being oriented towards a surface of the body of water; and a winch tether having a first portion that is releasably attachable to the support structure and a second portion that is releasably attachable to the first portion and that is coupled to the winch, the first portion extending through the power generating apparatus when the power generating apparatus is attached to the support structure, wherein the winch selectively pays out the first portion and the second portion of the winch tether.

2. Power generating equipment as claimed in claim 1, in which the power generating apparatus is a tidal turbine.

3. Power generating equipment as claimed in claim 1, in which the equipment further comprises an umbilical comprising a first end for attachment to the power generating apparatus and/or support structure and a second end for attachment to a retrieval structure, in which the umbilical is arranged in us to supply electrical power and/or control signals to control the releasable connection between the power generating apparatus and the support structure.

4. Power generating equipment as claimed in claim 1, in which the equipment further comprises an umbilical comprising a first end for attachment to the power generating apparatus and/or support structure and a second end for attachment to a retrieval structure, in which the umbilical is arranged in use to supply electrical power and/or control signals to control the releasable connection between the power generating apparatus and the support structure, and in which the first end of the umbilical is attached to the power generating apparatus.

5. Power generating equipment as claimed in claim 1, wherein the winch is releasable from the power generating apparatus when the power generating apparatus is attached to the support structure by the first portion of the winch tether.

6. Power generating equipment as claimed in claim 1, wherein the winch may selectively pay out the first portion of the winch tether when the first portion is attached to the second portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a submerged power generating system embodying one aspect of the present invention;

(2) FIGS. 2a to 2i illustrate steps in a method of deploying the system of FIG. 1;

(3) FIG. 3 is a flowchart showing the deployment steps illustrated in FIGS. 2a to 2i;

(4) FIGS. 4 to 6 illustrate retrieval of part of the system of FIG. 1; and

(5) FIG. 7 is a flowchart showing steps in a retrieval process illustrated in FIGS. 4 to 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) FIG. 1 illustrates a power generating system that comprises a support structure 1 located on a bed 19 of a body of water having a water surface 18. A power generating apparatus 2 is releasably engaged with the support structure 1. The power generating apparatus 2 is positively buoyant.

(7) Deployment of the power generating apparatus onto a support structure 1 will now be described with reference to FIGS. 2a to 2i and 3, which are schematic illustrations and a flow chart respectively. FIGS. 2a to 2i illustrate steps 100 to 116 of FIG. 3 respectively.

(8) In FIGS. 2a to 2i, the support structure 1 is located on a sea bed 19, and is prepared for the reception of a power generating apparatus 2 to be mounted thereon. In FIG. 2a, the power generating apparatus 2 is towed (step 100) to the location of the support structure 1 by a vessel 20 on the water surface 18. A winching device 3 is mounted on the power generating apparatus 2, such that respective alignment features are aligned, and a latching mechanism is engaged with corresponding features on the power generating apparatus 2.

(9) In FIG. 2b, an umbilical 7 connects the vessel 20 to the winching device 3. A remotely operated vehicle (ROV) 21 is deployed (step 102) from the vessel 20 via a control cable 22, and is used to carry a winch tether 6 to the support structure 1. As the ROV 21 descends, the winching unit 3 pays out the winch tether 6. With the winching device 3 mounted on an upper surface of the power generating apparatus 2, the winch tether 6 passes through the power generating apparatus 2, such that the ROV 21 pulls the tether 6 through an aperture in the power generating apparatus 2 as it descends to the support structure 1 on the bed.

(10) In one example, the tether 6 comprises two connected portions, connected to one another by a coupling device 5.

(11) The ROV 21 connects the winch tether 6 to the support structure 1, as shown in FIG. 2c, such that a first portion 6a of the tether 6 is attached to the support structure and a second portion 6h of the tether 6 is coupled to the first portion 6a and is attached to the winching device 3. The winching device 3 operates to take up any slack in the tether (step 104), and the ROV releases its grip from the tether 6.

(12) FIG. 2d (step 106) illustrates the winching device 3 operating to wind in the winch tether 6. Since the tether 6 is secured to the support structure 1, the winching device 3, and the power generating apparatus 2 are pulled down towards the support structure 1. The coupling device 5 passes through the power generating apparatus 2 prior to the power generating apparatus 2 docking with the support structure 1.

(13) FIG. 2e shows the power generating apparatus 2 docked with the support structure 1.

(14) When docking (step 108) is complete, the winching device 3 releases its connections with the power generating apparatus 2 and pays out winch tether 6 for buoyant ascent of the winching device 3 to the surface 18, as shown in FIG. 2f (step 110). During the ascent of the winching device 3, the first and second portions of the winch tether remain connected to one another, such that the winch is attached to the support structure 1 to enable its ascent to be a controlled manoeuvre.

(15) When the winching device 3 is on the surface, the ROV 21 disengages the coupling device 5 (FIG. 2g, step 112) and the second portion of the winch tether is retracted (FIG. 2h, step 114). The first portion of the winch tether, together with a first part of the coupling device 5 are left attached to the support structure 1. The first portion of the tether 6 remains extended through the aperture of the power generating apparatus 2. In one example, an upper end region of the first portion of the tether 6 is held in place on the power generating apparatus 2 by an engagement means located in an upper region of the power generating apparatus 2. Such and engagement means is arranged to allow the tether 6 to pass back through the power generating apparatus 2 when sufficient load is applied between the tether 6 and the power generating apparatus 2, for example during a retrieval operation. The first portion of the tether may be held in place on the power generating apparatus 2 by any other suitable means.

(16) Preferably, the first portion of the tether, which remains attached to the support structure 1, includes a buoyant element in the vicinity of the upper end region of the tether 6, so that the first portion of the tether remains extended away from the support structure 1 when the power generating apparatus 2 is no longer present in order to prevent the tether 6 falling down onto the support structure 1, thus serving to prevent damage on the support structure 1 by the tether 6.

(17) FIG. 2i shows the completed deployment operation and the winching device 3 is then returned (step 116) to storage or used for further marine operations.

(18) FIGS. 4 to 7 illustrate retrieval of the power generating apparatus from the support structure in accordance with a technique embodying an aspect of the present invention. FIG. 4 shows the power generating system (including the support structure 1 and power generating apparatus 2) ready for retrieval of the power generating apparatus 2 therefrom. It will be readily understood that the power generating apparatus 2 may have been deployed in accordance with the method described above, or by any other suitable method. A retrieval vessel 20 is deployed (step 120) on the water surface 18 to the vicinity of the power generating system located on the bed 19 of the body of water.

(19) The power generating apparatus 2 is instructed to commence a detachment sequence from the support structure 1 (step 122). The instruction or instructions may be delivered to the power generating apparatus in any appropriate manner. In one example, the power generating apparatus has a control and monitoring connection with a central control system (both not shown for the sake of clarity) via a cable linked to the support structure 1. The cable is not shown for the sake of clarity. In another example, a control and power cable (umbilical) may be deployed from the retrieval vessel 20. The exact nature of the delivery of the instruction to detach is not material to the present invention.

(20) Once the power generating apparatus 2 has received the instruction to commence detachment from the support structure, a suitable pre-detachment protocol is performed (step 124). This pre-detachment protocol is performed by a control unit within the power generating apparatus 2 in communication with a central control unit located, for example, on the retrieval vessel, or on another vessel, or at another location such as the shore. In a particular example, the protocol includes the following steps: 1. Determining that there is sufficient energy stored on board the power generating apparatus (for example in batteries or accumulators or similar) to be able to complete the required detachment actions (including disengaging connections from support structure, and releasing the clamping mechanism). The required energy level may include a contingency amount of stored energy to provide a reserve amount to reengage the power generating apparatus with the support structure if the operation needs to be aborted for any reason; 2. Determining that any electricity supply and/or power export connections are not live 3. Disconnecting the electricity supply connections, and any other connections (such as control and hydraulic connections) from the support structure Optionally, the process may include the following steps: 4. Transmitting the results of the protocol steps to the central control unit; and 5. Receiving confirmation of the instruction to detach from the central control unit

(21) Successful completion of these steps means that the power generating apparatus 2 is ready to ascend to the water surface 18.

(22) Further steps may include obtaining early confirmation that the power generating apparatus is still buoyant (i.e. not critically flooded), using, for example, visual checks from on-board cameras, instrumentation such as proximity sensors that change during partial release of the clamp, strain gauges appropriately placed to indicate net upward force and/or water sensors for detecting the presence of water within the power generating apparatus 2.

(23) The power generating apparatus 2 then releases the clamping mechanism, and checks that the clamping mechanism has been released using appropriate position, or other, sensors (step 126). Assuming that the clamping mechanism has released correctly, then the power generating apparatus 2 is in a detached state, and ascends (step 128) to the water surface 18, since the power generating apparatus 2 is buoyant. Sensors, such as proximity sensors and depth sensors can be used to determine that the power generating apparatus has started to ascend. If such ascent does not occur within a predetermined time frame, then the clamp can be reset to a locked position, so that the power generating apparatus 2 is secured once more to the support structure.

(24) In an alternative example, the power generating apparatus 2 may be released by the use of an ROV to mechanically release the clamping mechanism. This is particularly useful for a power generating apparatus 2 with a failure that prevents use of the systems described above.

(25) Once the power generating apparatus 2 reaches the water surface 18, the retrieval vessel 20 attaches a tow line 23 to the now floating power generating apparatus 2, and places the power generating apparatus 2 under control for recovery to an appropriate location (steps 130 and 132).

(26) In accordance with the method embodying the present invention, the power generating apparatus 2 ascent is a controlled free ascent. The power generating apparatus 2 is designed to have positive buoyancy with mass and buoyancy distributions such that the ascent of the power generating apparatus 2 to the water surface 18 occurs with desirably low roll and pitch. The net buoyancy distribution is designed to take into account the effects of expected drag that will be experienced by the power generating apparatus 2 during ascent, of partial flooding of the power generating apparatus 2, and of marine growth on the power generating apparatus 2.

(27) The ascent of the power generating apparatus 2 is free in the sense that there is no load bearing tether, or winch line, which lifts the power generating apparatus 2 or restricts the ascent of the power generating apparatus 2. In one example, at least one tag line is secured to the power generating apparatus 2 from the vessel 20 prior to release (this could be the same item as the control umbilical if it was mechanically strong enough but this is not preferred), in order to restrict the horizontal movement of the power generating apparatus 2. That is, a tag line may be attached in advance of the ascent of the power generating apparatus 2, in order that currents, tides, or other conditions do not carry the power generating apparatus 2 away from a desired area of retrieval. The tag line does not restrict or control the vertical movement of the power generating apparatus 2.

(28) The term controlled is used herein to refer to the power generating apparatus 2 being stable during the free ascent, for example having limited or reduced, for example minimal, roll and pitch during the free ascent without the application of external guiding threes. Preferably, the roll of the power generating apparatus is no more than a predetermined maximum value. Preferably, the pitch of the power generating apparatus is no more than a predetermined maximum value. Stabilisers may be provided on the power generating apparatus prior to ascent, in order to maintain the desired levels of roll and pitch. The stabilisers may be already part of the power generating apparatus, and may be moved from a stored position to a deployed position before ascent. Alternatively, fixed stabilisers may be provided on the power generating apparatus 2. In a further alternative, such stabilisers may be fitted to the power generating apparatus 2 when in its underwater position, for example by ROV operation. It will be appreciated that any appropriate number and type of stabilisers may be provided. The stabilisers may be passive, in the sense that they are fixed in position, or may be active in the sense that they adjust and move dependent upon conditions of the ascent.

(29) The roll and pitch of the power generating apparatus during the free ascent to the surface may be measured in an appropriate manner, for example by the use of one or more inclinometers. The stability of the power generating apparatus 2 during the free ascent advantageously reduces the risk of damaging the support structure 1 and the power generating apparatus 2 or the risk of inversion of the power generating apparatus 2.

(30) Allowing a large buoyant body such as the power generating apparatus to ascend freely to the surface is counter-intuitive for marine engineers, who are specifically concerned about stability of the apparatus, horizontal flight, g-forces, collision risk for vessels, damage of interfacing parts, and damage to the apparatus and/or support structure or other infrastructure elements.

(31) However, as described above, the techniques embodying the present invention enable the free ascent to be controlled, without the use of winch tethers, or lifting cables. Removing the need for such lifting or winching tethers speeds up the process by which the power generating apparatus can be retrieved from the support structure. Such a decrease in the amount of time taken to retrieve the apparatus has significant beneficial effects on the cost of retrieval.

(32) In an alternative example, the speed of ascent of the power generating apparatus 2 is controlled by the power generating apparatus 2 being arranged to ascend up a line having a predetermined level of resistance to that motion, in order to provide a controlled ascent on the line (e.g. a buoyed line to the surface with a sliding stop that the power generating apparatus 2 pushes up).

(33) It will be readily appreciated that systems and techniques embodying the present invention are able to provide significant advantages over previously considered solutions for deployment and retrieval of underwater power generating apparatus.

(34) Although aspects of the invention have been described with reference to the embodiment shown in the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiment shown and that various changes and modifications may be effected without further inventive skill and effort.