WO2013068410A1

WO2013068410A1 - Installing underwater structures

Info

Publication number: WO2013068410A1
Application number: PCT/EP2012/072044
Authority: WO
Inventors: James Sheppard
Original assignee: Tidal Generation Ltd
Current assignee: Tidal Generation Ltd
Priority date: 2011-11-10
Filing date: 2012-11-07
Publication date: 2013-05-16
Anticipated expiration: 2014-05-10
Also published as: GB2496412A; GB201119398D0; GB2496412B

Abstract

A method of installing a structure (2) on a bed (4) of a body (5) of water comprises deploying a structure (2) to be installed to a bed (4) of a body (5) of water from a deployment vessel (6), operating an installation unit (10) to secure the structure (2) to the bed (4), wherein the method further comprises using the structure (2) as a tether point for the deployment vessel (6) during operation of the installation unit (10).

Description

INSTALLING UNDERWATER STRUCTURES

The present invention relates to installing underwater structures and, in particular, to installing a structure on a bed of a body of water.

BACKGROUND OF THE INVENTION It is becoming increasingly important to secure structures to sea beds and riverbeds in areas of high flow. For example, tidal flow electricity generating turbines must be installed in relatively high flow areas in order to be able to generate the desired electrical power.

Installation of underwater structures in such areas of high flow has conventionally been achieved using expensive jack-up rigs, having extendible legs which engage with the sea bed, and which carry a deployment rig clear of the water surface. An alternative technique is to use a dynamic positioning (DP) vessel which makes use of sophisticated location finding and positioning techniques, for example using the global positioning system (GPS) and multiple thrusters, to maintain the vessel within a close range of a desired position. Such techniques then make use of traditional drilling techniques for enabling the structure to be fixed to the sea bed using one or more piles.

However, jack-up rigs and DP vessels are expensive to deploy. It is, therefore, desirable to reduce dependence upon expensive deployment vessels.

UK Patents Nos. GB2431 189 and GB2448358 describe techniques in which less

sophisticated, and hence less costly, vessels can be used to deploy and install underwater structures. Such techniques make use of flexible umbilical cables for supplying power and control signals to drilling rigs installed on the structure itself. The structure and drilling rigs are lowered to the bed from the vessel, and then the drilling operations take place under the control of the vessel. Such flexible umbilical cables enable the installation vessel to operate over a greater range of position compared to previous techniques. However, even with these previously-considered techniques, maintaining a vessel on station can be difficult, particularly when unfavourable weather conditions prevail. Also, if an installation operation takes longer than expected, for example if the operation is delayed into a spring tide rather than a neap tide, overall tidal flow begins to increase, in which case holding position becomes increasingly difficult. It is, therefore, desirable to provide a technique which can overcome the disadvantages of the previous installation techniques. SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method of installing a structure on a bed of a body of water, the method comprising deploying a structure to be installed to a bed of a body of water from a deployment vessel, operating an installation unit to secure the structure to the bed, wherein the method further comprises using the structure as a tether point for the deployment vessel during operation of the installation unit.

Such a method provides a relatively fixed point around which the deployment vessel moves. Having the structure itself providing such an anchorage point for the vessel has the advantage that the relative distance between the installation unit mounted on the structure, and the deployment vessel remains within a predefined region around the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 to 5 illustrate a method according to one example of the present invention;

Figure 6 is a flowchart showing steps in the method of Figures 1 to 5;

Figure 7 illustrates a method according to another example of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 illustrates a water current turbine 1 installed on a bed 4 of a body of water 5. The water current turbine 1 comprises a support structure 2 secured to the bed 4, and a turbine 3 installed on the support structure 2. Although the present invention will be described with reference to such a water current turbine, it will be appreciated that embodiments of the present invention can be applied to deployment of any underwater structure, whether that be used for power generation, or for any other purpose. For example, offshore wind turbine towers, water current turbines, tidal stream turbines, or oil and gas equipment, could be secured to the sea bed using a technique embodying the present invention.

In order to deploy the current turbine 1 on the bed 4 of the body of water, the support structure 2 must first be attached to the bed 4. Figure 2 illustrates a deployment vessel 6 on the surface of the water 5 carrying the structure 2 prior to deployment of that structure 2 to the bed 4. The vessel 6 is equipped with a crane or hoist 7 for deploying the structure from the vessel 6. In Figure 2, the structure 2 has been fitted with a series of installation units 10, each of which includes a pile 1 1. The installation units 10 are mounted on a frame 14 which itself is mounted on the structure 2. A power and control unit 16 is provided on the vessel 6, and operates to provide power and control signals to the installation units 10 via a flexible umbilical 15.

Figure 3 shows the structure 2 (and associated installation units 10) in place on the bed 4, having been lowered from the vessel 6 using the crane 7 and a cable 9. The flexible umbilical 15 connects the installation units 10 with the power and control unit 16 on the deck of the vessel 6. In one example, the umbilical 15 does not carry any load itself, and so a tether cable 20 serves to tether the vessel 6 to the structure 2. The tether cable can be attached to the structure whilst the structure is on the deck of the vessel 6, or could be attached using an ROV when the structure is underwater. In another example, the tether cable 20 is provided integrally with the umbilical 15, and is arranged to be load bearing within the umbilical. The crane hoist 9 is removed from the structure. As the vessel 6 moves around on the surface of the body of water 5, the tether serves to hold the vessel 6 in place relative to the structure 2, as shown in Figure 4. The tether 20, when under tension as shown in Figure 4, defines a maximum extent of movement of the vessel 6 relative to the structure 2. The vessel 6 is free to move within that range of position. In the example where the tether 20 is separate from the umbilical, in order that the umbilical 15 does not experience undesirable tensile loading, the tether 20 is shorter than the umbilical 15. The umbilical may be supported or carried by the tether 20 in order to resist drag loads experienced by the umbilical. In the example where the tether 20 is integral with the umbilical 15, the tether is arranged to carry the loading, without undue loading being experienced by the umbilical. The tether is shown attached to the stern of the vessel 6, but it will be readily appreciated that the tether is suitable for attachment to any convenient point on the vessel 6. The tether 20 may be a single cable, or may be provided by a suitable number of cables.

The installation units 10 are then operated under control from the power and control unit 16, via the umbilical cables 15. The installation units 10 form holes into which the piles 1 1 are located. The installation units 10 may utilise any suitable hole-forming technique such as rotary drilling or percussion drilling, and are supplied with electrical, hydraulic, pneumatic or other power via the umbilical 15. Each pile 1 1 is secured in place in its associated hole using a grout, in accordance with known practice. When the piles 1 1 have been grouted in the holes, the structure 2 is effectively secured to the bed 4.

During the installation procedure, the tether 20 serves to retain the vessel 6 within a suitable operating distance of the structure 2, such that the umbilical cable 15 does not carry any undesirable tensile load.

This arrangement is particularly suited to enabling the vessel to maintain a heading directly into the flow of the water. Positioning of a vessel in a relatively high flow area is made more difficult when the vessel encounters perturbations in the relative direction of the flow. For example, a wind current may cause the vessel to rotate, thereby causing the heading of the vessel to change slightly with respect to the flow direction. This change in direction results in the flow encountering the side of the vessel, thereby imparting higher turning loads on the vessel. Using the structure 2 as a tether anchor point enables the vessel to maintain its heading into the oncoming flow. In addition, the single tether point enables the vessel to maintain a position downstream of the structure, even as the flow direction deviates or swings around.

Once the structure 2 has been secured to the bed 4 by installation of the piles 1 1 , the installation units 10 are recovered to the vessel 6, as illustrated in Figure 5. The structure is now secured to the bed 4.

Figure 6 shows a flowchart of the steps illustrated in Figures 2 to 5. At step 101 , the structure is located on the vessel for transportation. In the example shown in Figures 2 to 5, the installation units are attached (step 102) to the structure before the structure is lowered to the bed. It will be readily appreciated, however, that the installation units could be lowered onto the structure following deployment of the structure to the bed.

The tether and umbilical are attached to the structure and to the installation units

respectively (step 103). If required, an underwater power cable is attached to the structure (step 104). Such a power cable will be used for export of power from a turbine mounted on the structure.

The structure and installation units are lowered to the bed (step 105). The hoist cable is removed, and the installation process then proceeds so as to install the piles into the bed (step 106). The installation units are then recovered to the vessel 6 (step 107) Figure 7 illustrates an alternative method embodying the present invention, in which the vessel 6 uses tethers 21 and 22 connected to previously installed structures 23 and 24. The tethers 21 and 22 are connected to respective structures 23 and 24 using a remotely operated vehicle (ROV), and serve to maintain the vessel in an appropriate position for the deployment of another structure 25, in a manner similar to that described above. The structure 25 is secured to the bed 4 using piles and installation units mounted on the structure. The installation units are powered and controlled from the vessel via a flexible umbilical cable 15.

It will be appreciated that any number of previously-installed structures can be used to provide tether anchor points for the installation vessel, using just the pre-installed structures, or in combination with the structure being installed. It will also be appreciated that a single structure may provide more than one tethering point.

The previously-installed structures provide convenient tethering points for the installation vessel, and this technique is useful in situations when an array of structures is being deployed. For example, tidal current turbines are intended to be installed in an array across an area of the seabed. In one practical example, it is envisaged that the support structure for the first turbine to be deployed in an array could be installed using the first example technique of the present invention, with subsequent installations making use of the first as a tether anchoring point. Techniques embodying the present invention serve to reduce the need for complex and expensive installation vessels, by providing at least one tether anchor point using the structure being installed, or by using previously-installed structures.

Claims

CLAIMS:

1. A method of installing a structure on a bed of a body of water, the method comprising deploying a structure to be installed to a bed of a body of water from a deployment vessel, operating an installation unit to secure the structure to the bed, wherein the method further comprises using the structure as a tether point for the deployment vessel during operation of the installation unit.

2. A method as claimed in claim 1 , wherein the structure is a support structure for a tidal flow turbine, and the installation unit is powered and controlled from the deployment vessel via a flexible umbilical cable.

3. A method of installing a plurality of support structures for tidal flow turbines on a sea bed, the method comprising installing a first structure using a method as claimed in claim 1 or 2, and then installing subsequent structures on the sea bed using at least one installed structure as a tether point for the deployment vessel.

4. A method as claimed in any one of the preceding claims, wherein the tether point serves to maintain the deployment vessel at a heading substantially into a flow.

5. A method of installing a structure on a bed of a body of water substantially as hereinbefore described before with reference to the accompanying drawings.