This invention relates to means for protecting marine installations such as offshore drilling and production installations in polar waters from large floating ice formations.
More specifically it relates to an energy absorbing system, which converts the kinetic energy of large ice structures into heat and power; the power being stored and used to energise, monitor and control, so that difficulties of control can be overcome.
A number of ways exist for absorbing the kinetic energy of ice formations in polar waters. One way is to construct a gravel island on the seabed, with a long sloping beach, up which the ice formations ground.
The gravel island can be improved by containing the top section within a heavy caisson so as to minimise the quantity of filling material.
Another method consists of fabricating a number of large, hollow concrete horizontal slabs which can be floated into position and sunk onto the seabed stacked on top of each other. The finished face of the outer wall is shaped so that the advancing ice is forced upward until the horizontal motion becomes vertical and is absorbed by gravity.
A fourth method is to create underwater obstacles, sometimes in the form of a vertical wire fence, anchored to the seabed, against which the advancing ice keels will collide.
A fifth method is by creating an underwater circle of drag elements (as described in my U.S. Pat. No. 4,547,093 of Oct. 15, 1985) which are attached to seabed anchorages by long anchor lines, the drag elements having teeth which penetrate the underside of the ice and absorb the kinetic energy of the ice formation by their shearing action.
All these methods suffer from very real disadvantages. The artificial islands are very expensive and take a long time to construct even in shallow water. Removal is an even greater problem. The caisson retained top helps to minimise the cost, but there are still time and cost problems. Concrete slabs stacked on top of each other have the advantage of re-use on a number of different sites in different seat water depths, but they become very large and expensive in deep water especially if they have to resist the collision impact force from a large ice island. Sea obstructions can be destroyed too easily by a large ice keel, and repair is not possible during the winter season. Drag elements can be very effective with fast moving ice but they suffer from the disadvantages that the drag force cannot be controlled, the teeth-freeze into very slow moving ice, and there can be a tendency for following pack ice to overslide ice slabs held on the drag elements.