1. Field of the Invention
The invention relates to anchoring of floating vessels. More particularly, it relates to a retrievable anchor for a floating offshore structure used for drilling or producing hydroacarbons located beneath a body of water.
2. Prior Art
The following discussion of the prior art for illustrative purposes is centered on anchors used with tension leg platforms, Nevertheless, this invention is useful for other floating structures or vessels requiring anchors.
A tension leg platform is sufficiently buoyant so that while floating on the water surface it keeps cables, respectively connected to the platforms and a fixity at the subsea bottom, under tension. One way proposed to provide the fixity adjacent the subsea bottom is through the use of deadweight anchors. Such anchors may include an enclosure with a concrete covering of predetermined thickness and weight; the thickness is a function of the initial ballast required to lower the anchor member to the ocean bottom.
With the anchor on the subsea bottom, ballast material, which may be either a flowable granular material (such as taconite or granular hematite) or sea water, is added to it. But, there are problems associated with each. The problem with granular material is that it is irretrievable and must be wasted at the drilling site. In the case of sea water, the problem is getting a large enough container that is still retrievable to hold sufficient water to provide restraint or resistance to movement as in a sea anchor.
Another way to provide fixity for the cables is to use solid steel or concrete anchors. They, however, upon relocation are either abandoned or lifted up by costly winch equipment.
Needless to say, wasting ballast such as the granular material or a concrete anchor is costly. These costs are eliminated by utilizing the present invention summarized below because it is easily retrievable. Consequently, it is possible to write off the fabrication and installation cost over a period involving several subsequent uses rather than just one use.
Further, the use of the present invention with small boats such as rowboats and sailboats should not be overlooked. The reason is that retrival of it does not require great physical strength of the retriever (for example a person in the boat), since the chain is first raised to the boat and then the empty shell. Other advantages of the invention will become obvious from the Summary of the Invention, Description of the Preferred Embodiment, the Claims and the drawings.