US20130074758A1

US20130074758A1 - Anchoring apparatus for wave energy converters

Info

Publication number: US20130074758A1
Application number: US13/200,585
Authority: US
Inventors: William Blaine Powers; Wei Ming Chi
Original assignee: Ocean Power Technologies Inc
Current assignee: Ocean Power Technologies Inc
Priority date: 2011-09-26
Filing date: 2011-09-26
Publication date: 2013-03-28
Also published as: WO2013048915A1

Abstract

An anchoring enclosure embodying the invention includes a chamber whose buoyancy can be controlled by pumping a gas (e.g., air) or a liquid (e.g., sea water) into the chamber. The anchoring enclosure includes a bottom extension for embedding the anchoring enclosure into the seabed and an anti-scouring skirt extending about the perimeter of the lower portion of the structure and outward from the anchoring enclosure for resting along the seabed and preventing water movement from disturbing the embedded bottom extension. The anchoring enclosure may be rendered sufficiently buoyant to be easily towed to a selected site of a body of water without the need for large and expensive equipment and to then be rendered less buoyant so as to sink, in a controlled manner, to the seabed.

Description

BACKGROUND OF THE INVENTION

This invention relates to anchoring apparatus and, in particular, to anchoring apparatus suitable for mooring wave energy converter devices (WECs) and holding them in place at offshore sites.
A variety of anchors for mooring devices in the ocean are known, and each of these anchors has its own set of design features aimed at particular end use requirements. However, there are significant problems in meeting the anchoring needs for large wave energy converter (WEC) structures in the ocean. To meet some key requirements of the wave energy converter business, anchors must be: (a) very large with high holding power, (b) cost effective both in their manufacture and also their deployment, (c) have low environmental impact, (d) able to function in a variety of sea-bed soil conditions, (e) provide anchoring to multiple wave energy converters in a matrix arrangement, and (f) able to be raised from the sea-bed to the water surface in a controlled and inexpensive manner for maintenance, relocation, and or final project site remediation if necessary.
The problem may be better appreciated when it is noted that an anchor to perform the desired holding function to hold a WEC(s) in place may, for example, have to weigh in the neighborhood of 300 to 500 tons, or more, and be 5-15 meters in diameter and 5-15 meters in height. The deployment of such structures requires very large barges, cranes and ships to transport the structures out to a deep sea site.
The costs of the anchoring system alone can become a sizable portion of the cost of the total WEC system. Building and deploying such structures to hold a WEC(s) in place can present significant economic challenges.
Offshore vessels and equipment used to install large anchoring systems need to be very large and are very expensive. An anchor is needed which can achieve the holding power requirement, provide anchoring to multiple WECS, and significantly reduce the size and cost of the installation equipment.
Employing larger and larger equipment for the deployment of such anchors results in reduced number of available days to work offshore to complete equipment installation due to the tight window of workable marine conditions for such lifts. Project delays based on unsuitable marine conditions can cost projects large amounts of money.
Given the significant cost associated with the building and deployment of the anchors for the WECs, it is also important, where numerous WECs are to be deployed in a given area, to minimize the number of anchors per WEC by having WECs share anchors.

SUMMARY OF THE INVENTION

In accordance with the invention, an anchor embodying the invention includes an anchoring enclosure, having at least one chamber, whose buoyancy can be controlled by pumping a gas (e.g., air) or a liquid (e.g., sea water) into the chamber. When a gas (e.g., air) is pumped (or blown) into the chamber the anchoring enclosure is made more buoyant. When a liquid (e.g., sea water) is pumped into the chamber the anchoring enclosure is made less buoyant. By controlling the amount of gas or liquid pumped into the chamber of the anchoring enclosure it may be rendered sufficiently buoyant to be easily towed to a selected site of a body of water and to be deployed at the selected site without the need for large and expensive offshore equipment. The anchoring enclosure can then be rendered less buoyant so as to sink, or be lowered in a controlled manner, to the bottom of the body of water. The anchoring enclosure includes a bottom extension extending around the underside of the anchoring enclosure for embedding the anchoring enclosure into the bottom of the body of water. The anchoring enclosure also includes an anti-scouring skirt extending about the perimeter of the lower portion of the structure and extending about the anchoring enclosure for resting along the bottom of the body of water and preventing water movement from disturbing the embedded bottom extension. The anchoring enclosure also includes mooring attachment points for enabling the connection of the anchoring enclosure to one, or more, wave energy converters (WECs) generally disposed along the surface of the body of water.
Embodiments of the invention may include more than one chamber to provide greater control for the lowering and raising of the anchoring enclosure. Each chamber includes piping and/or tubing means for pumping a gas or a liquid into, or out of, each chamber.
The anchoring enclosure when submerged is of sufficient mass to enable multiple WECs to be connected to the enclosure and maintain the WECs in position.
Anchors embodying the invention provide a solution to the cost problem of deployment of anchors while providing a mooring structure which can prevent the drifting of devices such as WECs.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying, which are not necessarily drawn to scale, like reference characters denote like components; and

FIG. 1 is a simplified cross-sectional diagram of an anchor embodying the invention riding along the surface of a body of water;

FIG. 2 is a simplified cross-sectional diagram of the anchor of FIG. 1, lying along the sea bed;

FIGS. 3A and 3B are isometric drawings of anchors embodying the invention;

FIGS. 4A and 4B are top views of anchors embodying the invention;

FIG. 5 is an idealized drawing of vessels for towing an anchor out to a deep water site and providing the means for lowering or raising an anchor embodying the invention to, or from, the ocean floor; and

FIG. 6 is an idealized diagram illustrating the connection of a WEC to a mooring buoy connected to an anchor, in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The cross section of an anchor 10 embodying the invention is shown in FIGS. 1 and 2. The anchor may have any suitable shape as shown, for example, in FIGS. 3A, 3B, 4A and 4B. That is, the anchor 10 may be square shaped, rectangular, cylindrical, domed or be an “n” faceted polygon or any like structure, as shown in the figures. The anchor may be formed of any suitable material (e.g., concrete, steel). The choice of concrete may be dictated by the economics (concrete is generally very cheap and is also well suited from environmental conditions and other factors). The dimensions and weight of the anchors may vary over a wide range depending on the size of the WEC(s) to be held in place and the forces which need to be counteracted. By way of example, referring to FIGS. 3A and 4A the length “L” and width “W” may range from less than 1 meter to more than 10 meters and the height “H” may also range from less than 1 meter to more than 10 meters. In a similar manner referring to FIGS. 3B and 4B, the diameter “D” may range from less than 1 meter to more than 10 meters and the height “H” may also range from less than 1 meter to more than 10 meters. And, as already noted, the weight of the anchor may vary over a wide range from, for example, less than 1000 tons to more than 500,000 tons.
In FIG. 1, the anchor 10 is shown to have a basic box-like or cylindrical frame 110 with outer vertical wall sections 112 and 114 and an outer upper horizontal section 116 and an outer lower horizontal section 118. Horizontal section 116 is the top most section and horizontal section 118 is the bottom most section. In FIGS. 1 and 2, three (3) tanks/chambers (T1, T2, T3) are formed within the box/block separated by wall 120 and 122. In practice, more, or less, chambers/tanks can be formed. The chambers may be filled with a fluid (e.g., a gas such as air or a liquid such as water) to vary the weight (and buoyancy) of the anchor. The contents of each tank may be controlled or determined by means of pipes which extend from a point external to the tank to a point within the tank. Thus, it is shown that tank T1 has a pipe P1 a and a pipe P1 b, tank T2 has a pipe P2 a and a pipe P2 b and Tank T3 has a pipe P3 a and a pipe P3 b. The Pipes P1 a, P2 a, and P3 a may be coupled via tubing (see FIG. 5) to a source of liquid (which could be sea-water or any other suitable liquid) to selectively fill the tanks with the liquid (e.g., water). Likewise, the pipes P1 b, P2 b, and P3 b may be coupled via tubing (see FIG. 5) to a source of fluid (which could be air or any suitable gas) to selectively push the water out of the tanks and fill them with the fluid to render the structure more buoyant. As illustrated in the figures, the pipes shown and/or any a number of different or additional pipes (Pia, Pib) may extend from a surface of the anchor and be available to access one or more chambers formed within the anchor.
Anchors 10 include an extension 130 a, 130 b, extending from vertical sections 112 and 114 (outer wall of anchor 10). The extension 130 a, 130 b, is intended to dig into the sea bed and provide additional grabbing into the sea bed. Note pipes P10 and P11 extend from above the anchor to a point below the lowest horizontal member 118. Pipes P10 and P11 may also be coupled via tubing (see FIG. 5) to a source of air or water which can be blown underneath horizontal member 118 of the anchor 10 to aid the anchor to rise above the sea bed (when so desired).
Anchor 10, as shown in the figures, includes an “anti-scouring” skirt 132 a, 132 b which functions to ensure that the anchor 10 remains in place by preventing the seabed material/sediment from being pulled or washed away from extension 130 a, 130 b and the base region of the anchor. The anti-scouring skirt prevents scouring by hugging and overlying the sea bed area extending around 130 a, 130 b and preventing the soil/sediment at the anchor's base from being washed away.
The anti-scouring skirt is an important feature of anchor's embodying the invention. Water motion about the base of an anchor resting on the ocean floor may eat away at the soil/sand surrounding the anchor base. If sufficient soil/sand is removed, the anchor may topple over. To overcome this problem, anchors embodying the invention include an anti-scouring skirt which extends from the base of the anchor and rests on the ocean floor to prevent soil/sand from being removed or lost from around the base of the anchor. As shown in FIGS. 1, 3A, 3B, 4A and 4B, the anti-scouring skirt may take several different forms and be of several different materials. For example, a layer of concrete or cement surrounding the anchor base and extending outwardly from there may be used to protect the sea bed sediment structure around the anchor base and prevent scouring.
The size and dimensions and shape of the anti-scouring skirt 132 (a, b) depends on the amount of water motion and the type of sediment at the anchor site. The anchor base will face varying amounts of scouring action. As a general rule, the greater the propensity to scour out, the more the need for protection and a larger area is preferred. In FIG. 3A, the scouring skirt is indicated to be 5 meters greater than the length “L” of the anchor. However, this is by way of illustration only and the scouring skirt may be less than 5 meters and significantly greater than that value.
The anti-scouring skirt may be formed of steel or concrete or any other suitable material. It may be formed as part of the anchor when the anchor is originally constructed. Alternatively, the anti-scouring skirt may be formed as sub assemblies added on to the anchor after fabrication of the anchor.
The anti-scouring skirt may be:

- i. Pre-fabricated mats consisting of reinforced loosely woven heavy duty fabric with concrete blocks pre-attached that can be cut away or rolled up or pulled out from the anchor; or
- ii. Articulated concrete slabs held together with ropes or wire that can be cut away or rolled or pulled out away from the anchor; or
- iii. A concrete form(s) that can unfold from the anchor sides and fold down like ramps extending from the sides of the anchor's outer walls.

Two mooring line attachment points 140 are shown extending from the outer walls of the anchor. It should be appreciated that many more mooring posts can be formed and attached to the outer walls of the anchor 10.
In practice, to tow an anchor 10 to a site, it would be made sufficiently buoyant by pumping a required amount of gas in the chambers to allow the unit to float. The assembled anchor 10 would be sufficiently buoyant to float on the surface of the water. It would then be towed out to a site and subsequently lowered to the sea bed (ocean floor). The towing operation and the lowering of the anchor to the sea bed would be as accomplished follows. The “buoyant” anchor can be towed out via a relatively small vessel (e.g., 89 or 90 in FIG. 5, or an auxiliary vessel) and positioned at a site to be lowered into the water.
As shown in FIG. 5, tubes may be connected between each one of the pipes and a corresponding source of fluid (e.g., 92, 94) mounted on a vessel 90. The sources of fluid (e.g., 92, 94) can pump air or water into or out or the anchor chambers. The amount and type of fluid blown into or out of the tanks/chambers of the anchor determine its buoyancy. To float the anchor some, or all, of its chambers would be filled with a gas (e.g., air). To lower the anchor, one of the chambers (e.g., T2) would be filled with water to cause the anchor to reach a state so it is slightly negatively buoyant and the anchor would start sinking. The rate of descent may be controlled by the use of a crane 87 on a vessel 89. Since the anchor is initially buoyant, the crane and the vessel may be relatively “small”. The buoyancy characteristic of the anchor during the lowering process is controlled by adjusting the amount of liquid pumped into chamber T2, or any of the other chambers. The controlled lowering of the anchor is desirable to prevent any breakage and to control the positioning of the anchor. Once the anchor is lowered to the seabed and placed in its desired location, the other chambers (e.g., T1, T3) are filled with water and this extra mass increases the anchor holding power to its specified value. Once the anchor is along the sea bed bottom the tubing may be disconnected from the anchor and retracted to the surface attending vessel.
Alternatively, the tubing(s) may be left attached at their bottom ends to the anchor 10 with their top ends left floating along the water surface to have the tubing available if, and when, there is a need to raise or move the anchor for additional positioning. Also the tubing(s) may be left attached at their bottom ends to the anchor 10, and the surface ends can be lowered into containment bins located on top of the anchor.
The final positioning of an anchor on the sea bed is shown in FIGS. 2, 5 and 6. Note that anti-scouring skirt (132 a, 132 b) lies along the surface of the sea bed preventing soil erosion about the base of the anchor and its extension 130 a, 130 b.
There are many occasions where it is desirable to raise the anchor. For example it may need maintenance, it may be desirable to move it to another location or upon a project completion and the anchor may need to be removed for site remediation. Referring to FIGS. 1, 2 and 5, note that tubing is shown attached between the anchor 10 and sources of fluid, such as air compressor 92 and water pump 94 mounted on vessel 90. Air can be blown through pipes P10, P11 to provide break suction and lift to the anchor by pushing up on horizontal member 118. Then, air can be pushed into selected ones (or all) of the chambers T1, T2 and/or T3 causing water to be evacuated from the chambers and for the anchor to start rising. The amount of air pushed in can be controlled to assure a smooth and continuous ascent.
Another significant aspect of the invention pertains to the use of the anchors to hold wave energy converters (WECs) in place. Referring to FIG. 6, there is shown a WEC 20 which includes a spar 22 and a float 24. The submerged end of the spar is connected to heave plate 26 which adds mass to the spar and keeps it generally in place and stabilized. The float and spar move relative to each other and their relative motion is converted into electrical energy via power take off device (not shown). An anchor 10 is connected via a line 52 to a mooring buoy 54 which is connected via line 56 and bridles 58, 60 to the spar 22 of a WEC 20. The mooring buoy 54 provides damping action for the forces tending to push the WEC away from the anchor.
An anchor embodying the invention may be used to hold more than one WEC in place. A multiplicity of WECs may be interconnected via mooring buoys to their respective anchors. In addition, a multiplicity (farm) of WECs can be interconnected and share anchors to minimize the number of anchors needed to hold the multiplicity of WECs in place. The anchors of the invention are highly suited to enable a multiplicity of WECs to be held in place.

Claims

What is claimed is:

1. An anchoring system comprising:

an anchoring enclosure, having at least one chamber, whose buoyancy can be controlled;

piping apparatus connected to the chamber for enabling a gas or a liquid to be pumped into or out of the chamber; when a gas is pumped into the chamber the anchoring enclosure is made more buoyant, when a fluid is pumped into a chamber the anchoring enclosure is made less buoyant; said anchoring enclosure being rendered sufficiently buoyant to be towed to a selected site of a body of water and to be deployed at the selected site and said anchoring enclosure being selectively rendered less buoyant so as to sink or be lowered, in a controlled manner, to the bottom of the body of water;

said anchoring enclosure including a bottom extension extending around the underside of the anchoring enclosure for embedding the anchoring enclosure into the bottom of the body of water; and

an anti-scouring skirt extending about the perimeter of the lower portion of the anchoring enclosure and extending about the anchoring enclosure for resting along the bottom of the body of water and preventing water movement from disturbing the embedded bottom extension.

2. An anchoring system as claimed in claim 1 further including mooring attachment points formed along the anchoring enclosure for enabling the connecting of the anchoring enclosure to a wave energy converter (WEC).

3. An anchoring system as claimed in claim 2, wherein said anchoring enclosure is designed to be positioned on and to hug the ocean floor and the WEC is designed to be lie generally about the surface of the ocean to respond to the ocean waves and the anchoring enclosure prevents the WEC from drifting from a selected site.

4. An anchoring system as claimed in claim 1, wherein said anchoring enclosure includes more than one chamber.

5. An anchoring system as claimed in claim 1, wherein the anchoring enclosure has sufficient mass, when submerged, to be connected to a multiplicity of WECs and hold the WECs in their designated space.

6. An anchoring system as claimed in claim 3, wherein each WEC has at least one mooring buoy and wherein the anchoring enclosure is connected to the WEC's mooring buoy.

7. An anchoring system as claimed in claim 6, wherein the WEC is connected to its mooring buoy via a bridle line.

8. An anchoring system as claimed in claim 3, wherein the anchoring enclosure is connected to a WEC having a float and a spar which move relative to each other in response to the waves.

9. An anchoring system as claimed in claim 2, wherein said anchoring enclosure is of cylindrical shape, having a diameter D and a height H, and wherein each of D and H may range from less than 1 meter to more than 10 meters.

10. An anchoring system as claimed in claim 2, wherein said anchoring enclosure is of a generally rectangular shape, having a length “L”, a width “W” and a height “H”, and wherein each of L, W and H may range from less than 1 meter to more than 10 meters.

11. An anchoring system as claimed in claim 1 wherein said anti-scouring skirt extends from less than 1 meter to more than 5 meters from the side of the anchoring enclosure.

12. An anchoring system as claimed in claim 1 wherein said anti-scouring skirt is formed of metal, or concrete.

13. An anchoring system as claimed in claim 1 wherein said anti-scouring skirt is formed of pre-fabricated mats consisting of reinforced loosely woven heavy duty fabric with concrete blocks pre-attached to the anchor and arranged so they can be cut away or rolled up or pulled out from the anchor.

14. An anchoring system as claimed in claim 1 wherein said anti-scouring skirt is formed of articulated concrete slabs attached to the anchor, the concrete slabs being held together with ropes or wire that can be cut away or rolled or pulled out away from the anchor.

15. An anchoring system as claimed in claim 1 wherein said anti-scouring skirt is formed of a concrete form that can unfold from the anchor sides and fold down like ramps extending from the anchor's external walls.