Abstract:
A method of securing a load bearing structure to the bed of a sea, river or estuary. The method comprises placing the load bearing structure on the bed; forming a pile-receiving hole in the bed by means of drilling equipment which is connected to a surface vessel by flexible lines only so that the weight of the drilling equipment is supported by the bed, the drilling equipment being guided by the load bearing structure, and the load bearing structure resisting any non-vertical loads imparted to the drilling equipment; and withdrawing the drilling equipment and installing an attachment pile within the formed hole.

Description:
This invention relates to a method of securing a load bearing structure to the bed of a sea, river or estuary, and is particularly, although not exclusively, concerned with the installation of anchorages for structures such as power generating equipment using water current energy. The present invention also relates to a load bearing structure in combination with drilling equipment for use in securing the load bearing structure. 
     BACKGROUND 
     It is becoming clear that many sites for water current energy systems comprise hard or rocky beds. Existing methods of drilling underwater rock sockets typically require a fixed drilling platform such as a jack-up vessel. This becomes expensive in water depths of more than approximately 30 m because it can no longer be carried out using relatively cheap and available near-shore construction jack-up vessels, but instead requires expensive specialist offshore vessels such as mobile drilling units (MDUs) or dynamically positioned (DP) drilling ships. It is possible that dedicated installation jack-ups could be developed for ocean energy installations; however, these would need to be capable of standing in water up to 70 m deep in order to capture a significant proportion of the UK tidal energy resource. This is a significant extension of current operating envelopes, and there are currently no indications that this could be achieved cost-effectively. 
     It is known to drill into rock using rotary drilling or percussive drilling. 
     The problems associated with using a conventional large diameter rotary drill are firstly that there is little if any of such equipment capable of operation underwater, and secondly that the equipment is large and relatively complex, and requires to be mounted on a structure which can react the large torques generated. The drill string is heavy and the vessel required to deploy it is correspondingly large. 
     Fluid-operated percussive drilling equipment is existing technology and has been used for onshore and offshore drilling. In the offshore environment, however, it has only previously been deployed using surface-breaking rigid drill stringers operated from stable drilling stringers. 
     There is therefore needed a cost-effective means of securing anchorages, moorings or foundations to the bed of a body of water. 
     SUMMARY OF INVENTION 
     In a first aspect of the present invention, there is provided a method of securing a load bearing structure to the bed of a sea, river or estuary, the method comprising the steps of: placing the load bearing structure on the bed; forming a pile-receiving hole in the bed by means of drilling equipment which is connected to a surface vessel by flexible lines only so that the weight of the drilling equipment is supported by the bed, the drilling equipment being guided by the load bearing structure, and the load bearing structure resisting any non-vertical loads imparted to the drilling equipment; and withdrawing the drilling equipment and installing an attachment pile within the formed hole. 
     In a second aspect of the present invention, there is provided, in combination, a load bearing structure for installation on the bed of a sea, river or estuary, and drilling equipment for use in the installation of the load bearing structure, the drilling equipment being adapted for deployment from a surface vessel by flexible lines only, the load bearing structure being provided with guide means for guiding the drilling equipment during a drilling operation on the bed, the guide means and the drilling equipment cooperating with each other to permit vertical displacement of the drilling equipment relative to the guide means but to resist non-vertical loads imparted to the drilling equipment. 
     A method in accordance with the present invention provides a cost-effective means of carrying out drilling in cases where beds are hard or rocky. Such a method can be employed to install a wide range of piles, from shallow-embedment “shear keys” to deeper pile embedments capable of carrying uplift forces. The technique is generic and therefore suitable for any type of seabed anchorage or mooring; however, the main applications discussed below are foundations for ocean and water current energy systems, for example wave, tidal stream and offshore wind energy conversion systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS. 1   a  and  1   b  show a method of deployment of a drilling system and attachment pile to secure a support structure to the seabed; 
         FIGS. 2   a  to  2   c  show the support structure at different stages of the securing process; and 
         FIGS. 3   a  to  3   c  correspond to  FIGS. 2   a  to  2   c  but show an alternative securing process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1   a , a support structure is positioned on the bed of a body of water. The support structure comprises a central column  30  stabilized by support feet in the form of hollow members  1 . In an alternative embodiment there may be only one hollow member, which may be positioned substantially centrally within the support structure. 
     A workboat  8  lowers a drill string down into one of the hollow members  1 . The drill string is an axial assembly comprising all the equipment necessary to operate the drill, such that all equipment may be recovered by the workboat  8  in a single lift after drilling. This may include, but is not restricted to, weights, drive motors for slow indexing of the drill bit, power swivel to receive the power for the drill from an umbilical whilst allowing the drill to index around, and guidance channels to control the exhaust fluid velocity to ensure the removal of drillings. 
     In the embodiment shown in  FIG. 1   a , the drill string is a percussive drilling system. Thus the drill string is made up of a percussion drill  2 , a weight  3 , an air swivel  4  and a motor  5  to rotate the percussion drilling system. 
     Percussive drilling techniques make use of the inertia of an axially oscillating heavy piston striking the drill bit to provide the crushing forces necessary to chip off and remove rock fragments. The drill strings required are much lighter than rotary drilling equipment of equivalent power, they do not require significant torque reaction, and are designed for operation underwater. 
     As it is lowered, the full weight of the drill string is supported by a cable  32  from a deck-mounted crane  7  on the workboat  8 . Pneumatic, hydraulic, electric or any other connections necessary for the drill string to operate correctly are made between the drill string and the workboat  8  by flexible umbilicals  6 . The umbilicals  6  are fitted with helical vortex-induced vibration suppressors and/or joined together at intervals in a manner so as to provide damping against loads induced by currents. 
     As will be described in more detail below, the drill string is lowered through the guide provided by the hollow member  1  and comes to rest on the bed of the body of water. Thus, during drilling, the load of the drill string is supported by the bed of the body of water. 
     Once a hole of the required depth has been drilled as described above, the drill string is retrieved from the bed. As shown in  FIG. 1   b , an attachment pile  9  is lowered from the workboat  8  into the hole through the hollow member  1 . 
     Referring to  FIG. 2   a , the hollow member  1  has been cross-sectioned to show the drill string in position on the bed, ready to drill. 
     The percussion drill  2  rests on the bed such that the weight of the drill string is taken off the crane hook  10 . Additional weight  3  has been attached on top of the percussion drill  2  to enhance the percussive effect of the drill. The drill string is turned by the motor  5 , and air to power the percussion drill  2  is provided by the air swivel  4  from one of the umbilicals  6 . Another of the umbilicals  6  may comprise an electric cable to supply power to the motor  5 . 
     The drill string has torque arms  12  that engage with guide vanes  11  on the inside of the hollow member  1 . The guide vanes  11  support the reaction torque of the motor  5 , so that the drill operates correctly. 
     In the condition shown in  FIG. 2   b , the drill  2  has completed a hole in the seabed. As the depth of the hole increases, the motor torque arms  12  slide down the guide vanes  11 . The drillings are ejected out of the top of the hollow member  1  to the environment. Once the required depth of hole has been drilled, the drill string can be lifted out by the deck-mounted crane on the surface (see  FIG. 1   a ). 
     Referring to  FIG. 2   c , the drill string has been removed and the attachment pile  9  has been lowered into the hole from the surface (see also  FIG. 1   b ). The attachment pile  9  is a steel cylinder fitted with grout tubes  14  which are attached to grout lines running to the surface. Grout is pumped down these tubes to fill the annulus between the attachment pile  9  and the hollow member  1  on the one hand, and the annulus between the attachment pile  9  and the inside of the hole on the other hand. Grout is also supplied to the inside  13  of the attachment pile. The grout lines are then detached and retrieved to the surface vessel  8 . 
     In an alternative embodiment, the attachment pile  9  is attached to the support structure by mechanical means, for example bolts, welding or expanding mandrels. 
       FIGS. 3   a  to  3   c  show an alternative process, although the same reference numbers have been used for similar components as in  FIGS. 2   a  to  2   c . Referring to  FIG. 3   a , the hollow member  1  has been cross-sectioned to show a different drill string arrangement resting on the seabed, ready to drill. The percussion drill  2  rests on the seabed, with the weight taken off the crane hook  10 . In this embodiment, the attachment pile  9  forms part of the drill string, and fits inside the hollow member  1 , aligning the drill string vertically. The attachment pile  9  is attached to the percussion drill  2 , such that as the drilling process takes place, the attachment pile  9  is pulled into the hole at the same time as the hole is created. 
     The attachment pile  9  is attached to the percussion drill  2  at its base using a remotely detachable fitting  17 . This fitting  17  could be a hydraulically or pneumatically actuated pin which engages with a female fitting fixed to the inside wall of the attachment pile  9 . Those skilled in the art will be able to think of alternative fittings that are to be considered within the scope of the present invention. 
     The percussion drill  2  incorporates an under-reamer  16 , which can be deployed for drilling, but is radially retractable in order to allow the drill string to be removed. Thus, during drilling the under-reamer  16  extends the diameter of the drilled hole, so that the attachment pile  9  can fit easily inside it. After drilling, the under-reamer  16  retracts radially so the drill string can be removed from inside and underneath the edges of the attachment pile  9 . 
     As in the process described with reference to  FIGS. 2   a  to  2   c , additional weight  3  is attached to the percussion drill  2 . Guides  20  keep the drill string aligned vertically within the attachment pile  9 . As in the first embodiment, the drill string is turned by the motor  5 , and air to power the percussion drill  2  is provided via the air swivel  4  from an umbilical  6 . Again, a further umbilical  6  can be used to power the motor  5  as necessary. 
     In order to prevent the hollow member  1  from being excessively tall, a releasable extension  18  is used to extend the height of the hollow member  1 . The extension  18  is attached to the top of the hollow member  1  using a tapered fitting  21 . The torque arms  12  engage with guide vanes  11  on the inside of the extension  18 . The guide vanes  11  are fitted with stops  15  at the top of the extension  18 , so that when the drill string is removed from the hollow member  1 , the torque arms  12  abut against the stops  15  and the extension  18  is retrieved along with the drill string. This embodiment is especially useful when a deep embedment hole is required. 
     Referring to  FIG. 3   b , the drill string has made a hole in the seabed by turning the motor  5  and powering the percussion drill  2 . As the drill string makes the hole the motor torque arms  12  slide down the guide vanes  11  on the inner wall of the extension  18 . The drillings are ejected out of the top of the attachment pile  9 , from where they are ejected to the environment either out of the top of the hollow member  1  or through holes  19  made in the wall of the extension  18 . 
     Once the required depth of hole has been drilled, the under-reamer  16  is retracted and the remotely detachable fitting  17  is released. The drill string can then be lifted out vertically by a deck-mounted crane on the surface. As described above, the extension  18  is also lifted out by virtue of the stops  15 . 
     Referring to  FIG. 3   c , the drill string has been removed, leaving the attachment pile  9  in place. Grout is pumped into the annulus between the attachment pile  9  and the inside of the hollow member  1  and the inside of the hole, also flowing inside the attachment pile itself. This could be achieved using a similar grout-tube arrangement as shown in  FIG. 2   c . Alternatively grout could be pumped through grout tubes that run inside the tubular framework of the support structure through the wall of the hollow member (not shown). One skilled in the art may be able to think of further alternatives without departing from the scope of the invention. 
     There is therefore described a method and equipment for installing underwater anchorages to the bed of a body of water. The present invention has several advantages:
     1. it can be used to install piles of sufficient embedment to carry significant uplift forces as well as shear and other loads, and is therefore suitable for all types of anchorages and moorings, including foundations for many types of ocean and water current energy converters;   2. it can be deployed in any water depth;   3. the drill string is deployed using flexible umbilicals instead of a rigid drill string, which eliminates the need for an expensive drill ship, DP vessel, or heave-compensated crane. Installation can be achieved using smaller, cheaper, more readily available non-specialist vessels, which makes the process considerably quicker and more flexible;   4. the drill string is relatively light (typically only 10 tonnes or so for holes up to 1.2 m diameter), which makes for ease of handling and rapid deployment. The deployment vessel requires a crane with sufficient capacity only to lower the drill string to the bed;   5. it makes use of the foundation structure as a drilling template, removing the need for separate templates or alignment devices. This is achievable because of the low reaction loads and minimal guidance requirements of the drill;   6. owing to the deployment of the drill string inside the template (for example inside a hollow member or an attachment pile) it is well protected from water currents and, unlike a conventional surface-breaking stringer, is not exposed to drag loads and vortex-induced vibration (VIV). This technique is therefore particularly advantageous for installing foundations for water current energy systems and wave energy systems;   7. if necessary, the drilling operation can quickly be aborted at any stage in the process simply by raising the drill string to the surface. Drilling can subsequently be restarted by lowering the drill string back down into the hole and using the template guides to pick up on the previous drilling.