GB2539566A

GB2539566A - Underwater structure and underwater structure installation and removal method

Info

Publication number: GB2539566A
Application number: GB1610060.4A
Authority: GB
Inventors: Elkington Paul; Murphy Martin
Original assignee: Tidal Energy Ltd
Current assignee: Tidal Energy Ltd
Priority date: 2015-06-11
Filing date: 2016-06-09
Publication date: 2016-12-21
Also published as: GB201610060D0; GB201510222D0

Abstract

The submersible structure comprising a hollow floodable frame (2, figure 4) & a plurality of hinged stability tubes 3, 4 mounted to the frame. The tubes are stowed horizontally on the frame when the frame is not immersed in water and raised under their own buoyancy when the frame is being sunk or raised in the body of water. Preferably the stability tubes revert to a horizontal position when the floodable frame is placed on the bed of the body of water. Also disclosed are valves which permit the flow of water when the water is above a predetermined level and impede the flow of water through a valve when the water is below the predetermined level. The invention also provides a method of placing & retrieving a submersible structure using stability tubes.

Description

Underwater Structure and Underwater Structure Installation and Removal Method [0001] This proposal is for a safe and reliable method of lowering large floating structures to the seabed, and their recovery without the need for heavy lift vessels.

[0002] The method is limited to moderate water depths, due to practical limitations; however, there are applications in the marine sector where the high cost and risks of employing heavy lift equipment cannot be justified. Whilst primarily designed for the installation of support structures for tidal energy and wave energy harvesters, there are many other potential applications.

[0003] Return margins for tidal turbines and wave energy capture devices need to be maximised to justify their investment. A floating base frame structure has significant cost benefits in transportation, and for installation, over the traditional methods of mounting it on, or slinging it under a large sea going barge. Barge transportation involves a number of high risk handling activities: from the manufacturing site to the transport barge; from the transport barge to ocean going heavy lift equipment; dynamic loads occur between the structure mass and the lifting vessel during offshore lifting; impact loads occur during placement on the sea bed, or during lift-off from the seabed; difficulty and risks involved in the remote, simultaneous detachment and reattachment of multiple heavy duty lifting connections. Most significant commercial tidal capture sites have water depths in the range -25m to 50m.

[0004] This invention seeks to overcome the forgoing issues in moderate depths where use of the invention is practical and in particular in water depths of 25m to 50m.

[0005] In a first aspect of the invention a submersible structure for use underwater in a body of water comprises a hollow floodable frame having a plurality of hinged stability tubes mounted thereon, said tubes being stowed horizontally on the frame when the frame is not fully immersed in water and raised under their own buoyancy when the frame is being sunk in the body of water.

[0006] In one embodiment of such a floodable frame, the stability tubes revert to a horizontal position when the floodable frame is placed in position on the seabed.

[0007] In one embodiment the frame has one or more float valves, said valves permitting the passage of water through the valve when the water level in the floodable frame is above or below pre-set levels and closed when between the pre-set levels.

[0008] In such an embodiment, the frame ideally has one or more internal partial bulkheads extending upwards internally from the bottom of the frame to a level between the bottom and top of the frame. Each compartment between bulkheads includes at least one float valve. The bulkheads prevent vessel instability that otherwise could result from water surge, whilst maintaining a common internal pressure between compartments.

[0009] In a further embodiment a frame in accordance with the invention comprises an underwater vessel for placing an underwater structure on the bed of the body of water, the underwater structure being detachably mountable beneath, or surrounded by the vessel. This approach also removes the requirement for any frame of the underwater mounted structure to be sealed and floodable itself to provide buoyancy.

[0010] According to a further aspect of the invention a method of locating a structure on the seabed comprises coupling a hollow floodable frame to sources of air and water. Releasing air from the hollow floodable frame and allowing water to enter the floodable frame reduces the frame to a state of near neutral buoyancy, whereupon water is pumped into the frame to further reduce the frame's buoyancy in a controlled manner to lower the frame to the seabed against the progressively increasing buoyancy provided by the stability tubes which pivot upwards from a substantially horizontal stowed position as the frame sinks. Flooding of the hollow floodable frame once it has reached its desired position on the seabed is achieved by releasing the remaining air contained within the frame.

[0011] In such a method the air may be released from the stability tubes when the hollow floodable frame has reached its desired position underwater whereupon the stability tubes pivot back to a horizontal stowed position.

[0012] To raise the hollow floodable frame to the surface the method is reversed, with air pumped into the stability tubes to raise them about their pivots and also into the hollow frame to achieve near neutral stability, then expelling water to further increase its buoyancy so that the hollow floodable frame rises to the surface of the body of water in a controlled manner against the reducing buoyancy provided by the stability tubes, as the stability tubes pivot back to their substantially horizontal stowed position.

[0013] The invention has a number of economic and operational advantages when compared to existing methods of deploying underwater structures, particularly on sea beds at 25m -50m below the sea surface. These can be summarised as follows: no transport barge is required; there are reduced temporary moorings to hold the structure on station during placement; the use of heavy lift equipment is avoided; there is no complex transition from transport mode to installation mode; there is a significant reduction in dynamic loads and seabed impact loads; there are no heavy lifting attachments to surface vessel, hence operational safety risks are significantly reduced; no remote heavy lifting reattachment operations are needed for recovery; the system provides for rapid installation and or recovery, which is particularly beneficial in and around the seas, for example of the United Kingdom when there are often short weather windows of opportunity for such operations; positioning accuracy is improved as the stability tubes, breaking the surface, indicate exactly where the floodable frame is located; reduced vessel costs; and the capability of rapid interventions if the structure needs to be recovered for any reason as small vessels can be maintained on standby for such eventualities.

[0014] Examples of the invention will now be described with reference to the accompanying drawings, in which: [0015] Figures 1 to 8 are schematic side views of a submersible structure according to the invention, and show various steps in its deployment; [0016] Figure 9 is a schematic cross section of a floodable frame of a submersible structure according to the invention showing the operation of float valve; [0017] Figure 10 is a plan view of the submersible structure of figures 1 to 9; and [0018] Figures 11 is a schematic side view submersible vessel according to a further aspect of the invention.

[0019] In figures 1 to 9 a submersible structure 1 according to the invention comprises, a floodable base frame 2 for the support, for example of a tidal turbine, has two or more hinged stability tubes 3 and 4 which initially are sealed and air-filled for transportation to the place of deployment (figure 1). The stability tube(s) pivoting nearer the workboat 5 are labelled 3 and the tubes pivoting further from the workboat are labelled 4. The rear pivoting stability tubes, 4 have small fluid transfer pipes, or similar, attached to connect water and air supplies on the workboat 5 through fluid swivels (examples can be seen as item 10 in figure 10) to the interior of the floodable base 2. The fluid swivels are aligned with the corresponding pivots 7 of the rear pivoting stability tubes, 4 The pivots 7 attach the stability tubes 3, 4 to the floodable base frame 2. The fluid swivels permit air and water transfer between the workboat and the floodable base frame 2. In this case the submersible structure is to be placed at sea on the seabed.

[0020] In figure 2, the submersible structure 1 is roughly positioned in a dying tide 6 by workboat 5 (bow anchored, or dynamically positioned). Air supply/discharge and water supply/discharge hose connections 19 are made from the workboat 5 at the top of the stability tube(s) 4.

[0021] In figure 3, the floodable base frame 2 is partially flooded by releasing air at the workboat end of the hose connections 19 -allowing seawater to enter through float controlled valve(s) 8 (shown in detail in figure 9) mounted along the side of the base frame 2. The float controlled valve(s) 8 are designed to close before neutral buoyancy of the base frame 2 is achieved. A rapid reduction in air pressure indicates all the float valves 8 are closed, the air release valves at the workboat 5 are then closed. As the submersible structure sinks slightly, the stability tubes 3, 4 begin to pivot upwards about their pivots 7.

[0022] Stages shown in figures 4, 5 and 6, are at slack water. Here, seawater is pumped into the base frame 2 via the water connection at the top of the rear stability tube(s) 4, against the remaining internal air pressure in the base frame 2. This reduces the buoyancy of the submersible structure 1 and it will gradually sink in the water against the increasing resistance provided by the submerging stability tubes. Thus, control of the rate at which water is pumped into the base frame 2 enables fine control of lowering the submersible structure 1. The stability tubes 3, 4 gradually rise further from their substantially horizontal initial position, pivoting about their pivots 7 under their own buoyancy.

[0023] Once the base frame has been positioned on the seabed 9 (figure 7), water continues to be pumped into the floodable base frame 2 until the float valve(s) 8 re-open. Air valves connected to the stability tube(s) 4 are then opened allowing the remaining air in the floodable base frame 2 to be exhausted thus providing a fully ballasted submerged structure 1.

[0024] At this stage the water and air hoses 19 are disconnected from the stability tubes 3, 4 and the air in the stability tubes 3, 4 is slowly released. The stability tubes 3, 4 then pivot back about pivots 7 to their horizontal stowed position in base frame. Finally lines from the workboat 5 are released allowing the workboat 5 to leave station (figure 8).

[0025] To recover the submersible structure 1 from the seabed according to the invention, an Unmanned Submersible Vehicle (USV) is used to connect a small compressed air supply to the stability tubes 3, 4. The injected air evacuates water from the stability tubes 3, 4 which causes the stability tubes to rise to the surface where air and water connections can be made through the stability tubes 3, 4 via flexible hosesl 9 to the base frame 2. Air is pumped into the floodable base frame 2 though the hose connections 19. This forces water out of the floodable base frame 2 via the float controlled valve(s) 8, quickly reducing its buoyancy. When the water level inside the submersible structure is just below the neutrally buoyant condition the float control valve(s) 8 are closed. This enables further water to be expelled at the surface allowing the submersible structure to rise in a controlled manner against the resistance of the progressively reducing buoyancy of the stability tubes until the submersible structure is at the water surface. The rate at which the base frame rises is controlled by the rate at which water is pumped out of the base frame.

[0026] Figure 9 illustrates the operation of valve 8. The part of the floodable base frame 2 is shown in section. Extending upwards inside from the bottom of the floodable base frame 2 is a plurality of partial bulkheads 20 occupying approximately the lower half diameter of the floodable base 2. The purpose of the half bulkheads 20 is to compartmentalise the floodable base frame 2 and thus prevent water surge causing instability to the submersible structure 1. To one side of the floodable base frame 2 is one or more float valves 8; the number of these will depend of the size of the submersible structure 1, but normally a number will be provided around the floodable base to enable rapid transfer of water during ballasting operations.

[0027] Each valve 8 is controlled by a float 12 located at the end of a lever 13. An external flag 11 locks the valve closed during transport of the submersible structure to, or from its deployment location; once the submersible structure is in the deployment position the flags 11 on all valves 8 are released to enable the valve 8 to operate. When the water level within the floodable base frame 2 is above the mid height of the floodable base frame 2 (indicated by the double arrow 14) as would be the situation when the submersible structure is fully deployed and ballasted as in figures 8, the valve 8 is open. Likewise when the submersible structure is on the surface prior to sinking or after being raised, the valve is open, (as indicated by the double arrow 16), in this latter situation the external water level is indicated by 17. During sinking or raising of the submersible structure1, as seen in figure 3 to 5, the water level inside the floodable base frame 2 is indicated by the level 18, between the heads of double arrow 15. Here the valve 8 is closed allowing close control of the buoyancy of the floodable base frame by the rate with which water is pumped in, or out through the surface hose connection 19 (in figure 2).

[0028] Moving on to figure 11, a variation of the invention is shown. Here a submersible structure 1 comprises a submersible vessel with a floodable frame 32 in place of the base frame 2. The submersible vessel is used to place on and/or recover from the sea bed 9 a standalone structure 35. Initially the standalone structure 35 is slung below the floodable frame 32. The floodable frame 32 has two or more hinged stability tubes 33 and 34 which initially are sealed and air-filled for transportation to the place of deployment. The stability tubes 33, 34 are connected to the floodable frame 32 with fluid swivels incorporated 10 into pivots 37 at one end of each of the stability tubes 33, 34 to mount the stability tubes 33, 34 on the submersible vessel 30. As before, the fluid swivels 10 permit air and water transfer between a workboat and the floodable frame 32.

[0029] The submersible vessel 1 is positioned and sunk with the standalone structure 35 in an analogous way to the method described with respect to figures 1 to 7. Once on the seabed 9, the submersible vessel 1 is released from the standalone structure 35. The submersible vessel 1 is then brought back to the surface by pumping air back into the floodable frame 32 in an analogous way to that method described with respect to figures 1 to 7. As before the rate at which the submersible vessel 30 sinks or rises is controlled by the rate at which water is pumped into the floodable frame 32.

[0030] A significant issue in the raising and lowering of structures for placement underwater is stability. Ideally, when lowering or raising an item from underwater, the item should be vertically and rotationally stable. Normally, the buoyancy of an item being lowered or raised varies with depth causing its rate of descent or elevation to vary significantly with depth causing control issues. However, often more seriously items being lowered or raised are rotationally unstable. The use of stability tubes as described in this invention virtually eliminates rotational instability and, by allowing the pumping in and out of air and water, enables the buoyancy of the item to be controlled so that heavy lift equipment is not needed.

Part Numbers in the Figures 1. Submersible structure 2. Floodable base frame 3. Front stability tubes 4. Rear stability tubes 5. Work boat 6. Tide 7. Pivots 8. Float valves 9. Seabed 10. Fluid Swivel 11. External flag 12. Float 13. Float lever arm 14. Internal water range -high 15. Internal water range -mid 16. Internal water range -low 17. External water level 18. Internal water level 19. Hose connections 20. Partial bulkhead 32. Floodable frame 33. Front stability tubes 34. Rear stability tubes 35. Standalone structure 37. Pivots

Claims

Claims 1. A submersible structure for use underwater in a body of water comprising a hollow floodable frame having a plurality of hinged stability tubes mounted thereon, said tubes being stowed horizontally on the frame when the frame is not immersed in water and raised under their own buoyancy when the frame is being sunk or raised in the body of water.
2. A submersible structure according to claim 1 in which the stability tubes revert to a horizontal position when the floodable frame is placed on the bed of the body of water.
3. A submersible structure according to claim 1 or 2 in which the hollow floodable frame has one or more valves, said valves permitting the passage of water through the valve when the water level in the floodable frame is above or below pre-set levels and closed when between the pre-set levels.
4. A submersible structure according to claim 1, 2 or 3 in which the hollow floodable frame has one or more internal bulkheads extending upwards internally from the bottom of the frame to a level between the bottom and top of the frame.
5. A submersible structure according to any preceding claim comprising a submersible vessel for placing an underwater structure on the bed of the body of water, the underwater structure being detachably mountable beneath, or surrounded by the submersible vessel.
6. A method of placing a structure underwater comprising coupling a hollow floodable frame with one or more stability tubes and a source of water releasing air from the hollow floodable frame and allowing water to enter the floodable frame, the stability tubes pivoting upwards from a substantially horizontal stowed position as the hollow floodable fame sinks, and fully flooding the hollow floodable frame once it reaches its desired position on the bed of the body of water.
7. A method of placing a structure underwater according to claim 6 in which remaining air in the stability tubes and hollow floodable frame is released from the stability tubes when the hollow floodable frame has reached its desired underwater position and the stability tubes pivot back to a stowed position.
8. A method of recovering a fully submerged structure to the sea surface comprising a hollow floodable frame with one or more stability tubes and a source of compressed air to evacuate water from the hollow floodable frame and stability tubes pivoting upwards from a substantially horizontal stowed position during the rising of the hollow floodable frame under buoyancy provided by the air introduced into the hollow floodable frame.
9. A submersible structure substantially as hereinbefore described with reference to the accompanying drawings.
10. A method of placing and/or recovering a submersible structure underwater substantially as hereinbefore described with reference to the accompanying drawings.