Hybrid offshore structure

Abstract

A hybrid offshore structure for conducting petroleum drilling and producing operations in very deep waters is disclosed. The structure consists primarily of a substantially rigid lower section extending upwardly from the bottom of the body of water to a pivot point located intermediate the bottom and the surface of the body of water, a compliant upper section extending upwardly from the pivot point to a deck located above the water surface, pivot means located proximate the pivot point and adapted to permit the compliant upper section to pivot laterally in response to environmental loads, and torsion means adapted to transmit torsional loads from the compliant upper section to the substantially rigid lower section. The lower section may comprise either a conventional trussed steel frame fixed to the bottom of the body of water by a plurality of piles or a concrete or steel gravity base. The compliant upper section may optionally be either a guyed tower or a buoyant tower. A variety of suitable pivot means and torsion means may be used. The pivot point is positioned so that the weight of the hybrid structure is substantially minimized while maintaining the flexural vibration period of the structure within acceptable limits. Generally, the pivot point will be located above the bottom of the body of water a distance of between about 10 percent and about 50 percent of the total depth of the body of water.