CLUMP WEIGHTS - FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to marine structures, particularly those structures employed for the purposes of drilling for and/or producing oil and gas in offshore areas. Such structures are typically held in position over a seabed site by use of spread mooring systems, which are comprised of a multiplicity of mooring means arranged in a radial pattern around the perimeter of the structure. Each individual mooring means generally comprises an anchor, a wire cable, and sometimes an anchor chain in combination with a wire cable. Through the use of said mooring means, the structure is maintained in a desired location, thereby allowing oil and gas production and drilling to take place.

When environmental forces, such as wind, current and waves, act against the structure to move it away from its original location, spread mooring systems develop a net horizontal force called the "restoring" force which restrains unwanted movement of the structure and ultimately "restores" the structure to its original location. The restoration force is developed by the increasing tensions in the mooring lines located on the side of the structure experiencing the environmental forces (the "windward" side), as those lines become increasingly taut due to the movement of the structure, coupled with decreasing tension in the mooring lines located on the leeside of the structure.

The structure reaches a maximum lateral offset from its original position when the horizontal vector sum of the mooring line tensions is equal and opposite to the mean horizontal environmental forces acting on the structure. When the environmental forces have subsided, as for example when a storm has passed, the restoration force causes the structure to return to its original position. Spread mooring systems allow maximum lateral offsets in any given direction away from structure's original position, thereby defining a "watch circle" beyond which the structure will not be moved even under the most severe environmental loads which the structure may be expected to encounter.

The use of weights ("clump weights") on the seabed in conjunction with spread mooring systems is a known means of improving the station-keeping properties of a mooring system. U.S. Pat. No. 3,903,705 to Beck discloses the use of clump weights attached to the mooring lines, said clump weights being intended to remain at least partially resting on the seabed under normal environmental conditions. In Beck's device, the clump weights are never removed completely off the seabed except in the event of abnormal environmental forces such as those that may occur in a severe storm.

However, the mooring system disclosed by Beck has certain disadvantages. For one thing, the clump weights of Beck, which normally are at least partially supported on the sea floor, are susceptible to adhesion to the sea floor, as Beck himself recognized. Gradual subsidence and burial of at least a portion of Beck's clump weights, resting on soft bottoms for long periods of time, may prevent the clumps from lifting off bottom in the event of a severe storm as originally intended.

Moreover, Beck's mooring system is intended to provide stiff lateral restraint for a guyed tower having a bottom which rests on and is affixed to the sea floor. Due to the nature of such a bottom-fixed tower, Beck's mooring system needs to contend only with lateral motions of the upper end of the tower. By contrast, the design of spread mooring systems for a floating structure is much more complex, since the mooring must take into account the vertical (heave), rotational (roll, yaw, pitch) and horizontal (surge and sway) motions of floating structures. The stiff behavior of Beck's configuration in "normal" weather, i.e. weather conditions insufficient to cause the clump weights to lift off bottom, would generally have undesirable effects on the motions and natural periods of heave, roll and pitch motions of floating structures.

SUMMARY OF THE INVENTION

The present invention provides a marine structure, whether floating or bottom-fixed, with spread mooring system having improved station-keeping capability and which eliminates the problems associated with adhesion of clump weights to the sea floor. The invention provides a mooring system having clump weights which are positioned above and completely off the seabed under normal conditions, thereby providing a desirably compliant restraint to all the degrees of motions of the structure. According to the invention, when extreme environmental forces move the structure away from its original location and towards the periphery of its "watch circle", the clump weights on leeward mooring lines come to rest on the sea bottom, greatly and immediately increasing the restoring force of the mooring system.

Hence, the unfavorable aspects associated with subsidence and adhesion of clump weights which are ordinarily at rest on the seabed for long periods of time, as in Beck, are eliminated by the invention. Moreover, the large restoring forces required for good station-keeping performance, rather than being generated stiffly by a line pulling against a heavy clump resting at least partially on bottom, as in Beck, are instead generated by the large and immediate reduction in leeside mooring line tension induced by the leeside clumps being lowered to the sea bottom, while maintaining compliant restraint in the windward lines. This increase in restoring force capacity is achieved passively without the need for a marine crew to slacken leeward lines, as would otherwise be required by good marine practice for floating structures experiencing a violent storm.

Additionally, total "pull down" weight or vertical force of the mooring system of the invention is substantially reduced when the leeside clumps are set onto the seabed during storm conditions. This permits a floating structure to passively reduce its draft, thus allowing additional air gap for storm waves to pass under the deck of the floating structure.