Patent Publication Number: US-2013243527-A1

Title: Tidal Flow Generation Structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from PCT/GB/2011/052197 filed on Nov. 11, 2011, and GB 1019080.9 filed on Nov. 11, 2010, both of which are hereby incorporated by reference in their entireties. 
     The present invention relates to a tidal flow generation structures. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Tidal energy is to a great extent predictable. At depths below significant wave effects the only basic changes in current flow are due the naturally occurring phases of the moon and sun. Superimposed on this pattern is a variation of flow velocities, some reaching a considerable fraction of the free-stream values, and which are due to intense atmospheric events. 
     The deterministic nature of the availability of power, together with its high density and the implicit absence of visual impact makes tidal energy extraction a very attractive proposition particularly since virtually the whole of the available resources remain untapped. 
     2. Related Art 
     A number of tidal turbine schemes have been proposed with a division being between those which require the setting of sea floor foundations and those which do not. US2011/0206467 describes 5 categories of foundation structures for hydraulic turbines, namely, piles, suction anchors, gravity foundations, floating structures and anchored base plates. US2011/0206467 then goes on to describe an anchored base plate structure employing a very heavy central mooring that may partially sink into the sea bed. The base plate foundation is initially anchored in position before being levelled and the turbine then assembled on the already anchored base plate. 
     Additionally, free standing framework designs have been proposed. An exemplary such structure is disclosed in WO2009/081162. The structure disclosed is deployed to rest on the sea bed and supports multiple turbines. The design benefits from an overarching simplicity of construction and implementation which offers, through the absence of complex failure-prone mechanisms, high inbuilt reliability. 
     An improved arrangement has now been devised. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention provides a sea bed penetrating support foot structure, for a sea bed mounted tidal energy generation structure, the sea bed penetrating support foot structure including a penetrating tip which is pre-assembled with the structure prior to lowering to the sea bed and arranged to penetrate into the sea bed surface under the weight of the structure. 
     Because the weight of the entire structure is used to embed the penetrating foot into the sea bed there is no requirement for the penetrating foot structure itself to be particularly massive or heavy. This benefit is not disclosed in the prior art. 
     According to a further aspect, the invention provides a method of securing a sea bed mounted tidal energy generation structure in position on the sea bed, the method comprising lowering a pre-assembled structure comprising frame connected nacelle structures and a plurality of sea bed penetrating support foot structures pre-assembled with the structure, the support foot structures being fixed in position relative to the structure before lowering to the sea bed and including a penetrating tip which is arranged to penetrate into the sea bed to a significant degree under the weight of the tidal energy generation structure. 
     The prior art does not disclose such a technique in which a pre-assembled structure including frame connected nacelles and included penetrating foot structures are lowered ready assembled onto the sea bed to self secure in position without further adjustment. 
     It is preferred that the penetrating tip has opposed inclined surfaces terminating at an apex. 
     The smaller the apex angle between the opposed inclined surfaces, the more significant is the penetration. Accordingly, the apex angle between the opposed inclined surfaces is preferably 70 degrees or less (more preferably 60 degrees or less). 
     In one embodiment, the penetrating tip preferably comprises a generally triangular plate having an apex. Alternatively an inverted cone shaped structure may be used. A plate may be preferred because it requires displacement of less material for an equivalent depth of penetration. 
     Beneficially, the arrangement is provided with penetration depth limitation means to prevent the tip penetrating beyond a predetermined distance into the sea bed. 
     In certain embodiments penetration depth limitation means is positioned above the penetrating tip and, may comprise a stop, flange disc or plate. 
     In one embodiment the structure comprises a spacer section provided above the penetrating foot. The spacer section provides capture into sedimentary deposits on the sea bed to further secure the structure in position. The spacer section is provided above the penetration depth limitation means. 
     The spacer section preferably tapers from a relatively wide upper portion to a relatively narrower portion adjacent the penetrating foot. 
     According to an alternative aspect the invention provides a module for a sea bed mounted tidal energy generation structure, the module comprising:
         a tidal turbine nacelle structure; and,   pre-assembled with the nacelle structure and deployed with the nacelle structure for setting on the sea-bed in fixed position, a sea bed penetrating support foot structure including a penetrating tip which is arranged to penetrate into the sea bed surface under the weight of the tidal energy generation structure, when deployed on the sea bed.       

     The sea bed penetrating support foot structure is preferably positioned directly below the tidal turbine nacelle structure. 
     In a further aspect, the invention provides a sea bed mounted tidal energy generation structure including one or more sea bed penetrating support foot structures including a penetrating tip which is arranged to penetrate into the sea bed surface under the weight of the structure. 
     The energy generation structure preferably has a generally triangular frame, and the structure is provided with three penetrating support foot structures, preferably spaced at the apexes of the triangular frame. 
     According to a further aspect, the invention provides a method of securing a tidal energy generation structure in position on the sea bed, the method comprising lowering the structure into position on the sea bed such that a respective penetrating tip of one or more support foot structures penetrates into the sea bed to a significant degree. 
     The depth of penetration required will be dependent on the shape of the foot and the type and strength of the seabed material. The significant degree to which the tip penetrates is preferably to a depth of &gt;100 mm. A range of penetration of between 50 mm for medium strong rock (35 MPa) and 540 mm for very weak rock (1.5 MPa) can be expected. 
     In a preferred realisation, the tidal energy generation structure is preferably provided with a plurality of spaced support foot structures, each having a respective penetrating tip of one or more support foot structures penetrates into the sea bed to a significant degree. 
     The invention will now be described in a specific embodiment, by way of example only, and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view representation of a tidal flow turbine structure in accordance with the invention; 
         FIG. 2  is a perspective view representation of the tidal flow turbine structure of  FIG. 1 ; 
         FIG. 3  is a side view of the support foot structure of the side tidal flow turbine structure of  FIGS. 1 and 2 ; 
         FIG. 4  is a close up view of the foot structure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings there is shown a tidal flow energy generation structure  1 , which is required to be deployed and operate in extreme conditions. To be commercially competitive with other forms of power production areas of the seabed of high tidal flow energy concentration need to be utilised. These areas are difficult and dangerous to work in and the structure and its deployment and retrieval need to take into account significant environmental hazards. The current flow, for example, is fast, typically upward of 4 Knots. Areas are often in deep water, which may be deeper than those in which a piling rig can operate. Storm conditions can cause costly delays and postponement. Tidal reversal is twice a day and the time between tidal reversal may be very short (for example between 10 and 90 minutes). Additionally, in such high tidal flow areas, the seabed is often scoured of sediment and other light material revealing an uneven rock seabed, which makes anchorage difficult. In the situations described it may be difficult and risky for divers or remote operated vehicles to operate on the structure when positioned on the seabed. Installation, recovery and service are therefore most conveniently carried out from the surface. To be environmentally acceptable, all parts of the structure and any equipment used in deployment or recovery must be shown to be recoverable. 
     The tidal flow energy generation structure  1  comprises a freestanding structural frame assembly comprising steel tubes  2 . The frame assembly comprises welded tubular steel corner modules  3 . The corner units are interconnected by lengths of the steel tubes  2 . The structure as shown in the drawings is triangular in footprint and this may for certain deployment scenarios be preferred however other shape footprints (such as rectangular) are also envisaged in such arrangements the angular configuration of the corner modules  3  will of course be different to that shown and described in relation to the drawings. The use of a triangular,  3  corner module construction has advantages in the stability of three point contact on the sea bed. 
     The corner modules  3  comprise first and second angled limbs  7 ,  8  extending at an angle of 60 degrees to one another. The respective corner module  3  includes a mount  14  to receive and locate a shaft  20  of a respective nacelle  9 . The corner module  3  and interconnecting tubes  2  include respective flanges for bolting to one another. The tubes  2  may be configured to permit water to flood into and drain out of the tubes  2  which enables in sinking of the structure to deploy on the sea bed and pumping out for lifting. 
     The structure is held in position by its own mass and lack of buoyancy due to flooding of the tubes  2  and end modules  3 . The tubes  2  are positioned in the boundary layer close to the seabed and the structure has a large base area relative to height. This minimises potential overturning moment. Horizontal drag is minimised due to using single large diameter tubes  2  as the main interconnecting support for the frame. As an alternative to hollow tubes, solid frame beams of metal construction may be used. 
     The structure forms a mounting base for the turbines  9  mounted at each corner module  3 , the support shaft  20  of a respective turbine  9  being received within the respective nacelle structure  14  such that the turbines can rotate about the longitudinal axis of the respective support shaft  20 . The turbine shaft mounting bearings and other power generation components can be housed within the nacelle structures  14 , such that it is only required to mount the shaft into the pre-assembled generation structure when positioned on the sea bed. Power is transmitted from the corner mounted turbines  19  to onshore by means of appropriate cable as is well known in the marine renewables industry. 
     Areas of deep water and high current and low visibility are very hazardous for divers. The structure is designed to be installed and removed entirely from surface vessels. The structure is designed to be installed onto a previously surveyed site in the time interval that represents slack water between the ebb and flood of the tide. This time may vary from 10 to 90 minutes. The unit may be restricted from being deployed outside the timeframe as the drag on the structure from water movement could destabilise the surface vessel. 
     In times of extremely high tidal flow velocities, there is a risk with a freestanding structure of this type that the axial loading on the turbines  9  can be so high that the structure could shift on the underlying seabed. This would have numerous undesirable consequences, including tension being placed on cables and the like. 
     In order to ensure that when deployed the structure remains firmly anchored in place, sea bed penetrating support foot structures  15  are utilised including a penetrating tip which is arranged to penetrate into the sea bed to a significant degree under the weight of the tidal energy generation structure, in accordance with the invention. 
     The sea bed penetrating support foot structure  15  as shown in the drawings has a generally triangular tip plate  26  welded to a horizontal disc  31 , the disc  31  being connected below the corner module  3  by welded support stanchions  36  which define a spacing section above disc  31 . The triangular tip plate  26  is braced by support plates  32  welded to the tip plate  26  and the disc  31 . The inclined sides  41 ,  42  of the tip plate  26  are typically inclined at a mutual angle of 60 degrees to meet at a vertex  43 . The smaller the apex angle between the opposed inclined surfaces, the more significant is the penetration. Accordingly, the apex angle between the opposed inclined surfaces is preferably 70 degrees or less (more preferably 60 degrees or less). 
     As has previously been identified, a key aspect of the invention is that the structure can be deployed quickly (for example during slack water) and without extensive preparation to the sea bed. This is achieved by using the structure&#39;s own weight to anchor the structure in place. A large footprint area for the tidal energy generation structure  1  (and triangular footprint) aid in the robust positioning of the structure. The respective foot structures pre-assembled with the structure, the support foot structures being fixed in position relative to the structure before lowering to the sea bed. This requires minimum further adjustment post deployment in the short time available. Furthermore, because the weight of the entire structure is used to embed the penetrating foot  15  into the sea bed there is no requirement for the penetrating foot structure  15  itself to be particularly massive or heavy. This benefit is not disclosed in the prior art, and enables the penetrating tip to be made of a metal plate construction, for example. 
     The sea bed penetrating support foot structure  15  is shown most clearly in  FIGS. 3 and 4 . The sea bed penetrating support foot structure  15  includes a penetrating tip  25  which is designed to penetrate into the sea bed surface under the weight of the structure  1  as the structure  1  is deployed. In the embodiment shown, each corner module  5  is provided with a separate, identical sea bed penetrating support foot structure  15 , providing 3 points of contact. These relatively few points of contact enables sufficient weight to be transferred via the respective sea bed penetrating support foot structure  15  to enable the penetrating tip  25  to break the sea bed surface, and sink into the material of the sea bed, in order to anchor the structure  1 . This design is contrary to the way in which conventional structures are mounted to the sea bed in which prepared foundations are typically utilised and the setting area of the surface foundation would be as large as possible in order to minimise the pressure on the foundation (to avoid degradation of the foundation). 
     The welded support stanchions  36  which define the spacing section above the penetration limitation flange or disc  3 , can sink into sedimentary deposits to provided added fixing of the structure in position. The welded support stanchions  36  which define the spacing section taper from a relatively wider upper portion to a relatively narrower portion at the penetration limitation flange or disc  3 . 
     The design of the sea bed penetrating support foot structure  15  provides that when the tidal energy generation structure has been deployed on the sea bed, the sea bed penetrating support foot structure  15  ensures that the penetrating tip plate  26  breaks the surface and penetrates to a significant degree. The sea bed penetrating support foot structure  15  then provides significant resistance to lateral motion of the structure over the sea bed. The penetration depth is typically at least 100 mm. 
     The inclined nature of the surfaces  41 ,  42 , combined with the reversing directional forces acting as a result of the cyclic tidal flow reversal, mean that over time, the side to side ‘rocking’ of the structure (as a result of the cyclic tidal flow reversal) results in the tip plate penetrating deeper into the sea bed, and hence more securely locking the structure against lateral motion over the sea bed. The plate  31  provides a penetration depth limitation means to prevent the tip penetrating beyond a predetermined distance into the sea bed. Alternatively other means such as a stop, flange or plate could be used for this purpose. Furthermore, as an alternative to the triangular tip plate  26 , an inverted cone shaped structure may be used as a penetrating tip. A plate may however be preferred because it requires displacement of less material for an equivalent depth of penetration. 
     The invention provides for a largely preassembled structure for mounting several turbines, in which the weight of the structure acts to ensure penetration of spaced penetration foot structures sufficiently to mount the structure in place. The pre-assembled structure utilises pre-assembled frame connected nacelle structures provided with respective penetrating feet pre-assembled prior to lowering to the sea-bed.