Dynamic response and reliability analysis of large offshore structures

Offshore structures, when provided with large deck areas, may be utilized for new purposes other than the conventional oil and gas exploration works. They may be used as airports, power stations, waste treatment plants, etc. Since these structures have very heavy dead loads, reaction forces on the base foundation become severe and necessitate very firm foundation system. If the reaction forces can be reduced by some method, the concept of large offshore structures may become popular. This paper investigates the dynamic response and reliability analysis of such a structure under the action of sea waves, currents and earthquakes. The severe reaction forces on the foundation system have been reduced by adopting large-diameter members of buoyancy-type near the sea bottom. The equation of motion of the structure is obtained by the substructure method. The response analysis is carried out using frequency-domain random-vibration approach. Sensitivity studies are performed to study the effects of parametric uncertainties on the response values. The long-term behavior of the offshore structure is examined with first-passage probabilities of level crossing of extreme responses and reliability indices. (C) 1999 Elsevier Science S.A. All rights reserved.