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.
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.
In the accompanying, which are not necessarily drawn to scale, like reference characters denote like components; and
The cross section of an anchor 10 embodying the invention is shown in
In
Anchors 10 include an extension 130a, 130b, extending from vertical sections 112 and 114 (outer wall of anchor 10). The extension 130a, 130b, 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
Anchor 10, as shown in the figures, includes an “anti-scouring” skirt 132a, 132b 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 130a, 130b and the base region of the anchor. The anti-scouring skirt prevents scouring by hugging and overlying the sea bed area extending around 130a, 130b 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
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
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:
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
As shown in
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
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
Another significant aspect of the invention pertains to the use of the anchors to hold wave energy converters (WECs) in place. Referring to
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.