The mechanics of a compliant motion suppression system for semisubmersibles

Abstract

This paper describes experimental and theoretical work on a passive motion suppression system for semisubmersible vessels. The system incorporates a pneumatic compliancy which is designed to enhance the wave induced motion characteristics of such a vessel for offshore drilling and production service. The pneumatic compliancy is achieved through the use of open bottom tanks mounted on the vessel. The tanks pierce the water surface and trap a volume of air above their internal water level. During operation wave action on the tanks imparts an oscillating vertical force on the vessel which is 180 degrees out-of-phase with the inertia dominated wave induced heave force. As a result, vessel heave, roll and pitch motions are substantially reduced. Regular and irregular wave tests have been performed on a scale model enabling the motion reduction capabilities of such a system of be evaluated. Test data is compared with a multi-degree of freedom dynamic response calculation in the frequency domain in which the Morison equation is used for calculating wave induced drag and inertia loads on the semisubmersible. The paper is concluded with a discussion on the relative merits and drawbacks of incorporating a pneumatic compliancy into hitherto hydrodynamically rigid semisubmersible designs.