Experimental Study on Dynamic Positioning Control for a Semi-submersible Lower-hull Type Offshore Platform

Abstract

Dynamic positioning control tests were carried out for a semi-submersible two-lower-hull type platform model in the simulated environment for offshore current, wind and wave in a ship model basin. Prior to the control tests, hydrodynamic characteistics of the platform and the propulsion systems were measured and computer simulations were made to obtain the basis for the design of the dynamic positioning control system. The results of over-all control tests indicate that the dynamic positioning control is duly practicable at the two-lower-hull type semi-submersible platform. Offshore platforms, floating or semi-submerged, usually encounter all kinds of drifting forces due to current, wind and wave. To keep the platform at the desired position and heading, the anchor-chain/wire system has been known as one of the most reliable means at relatively shallow sea area. However, position holding capacity of the anchor-chain/wire system becomes inadequate when the depth of the sea approaches the order of deeper continental shelves. In order to be able to operate the platform in such deep sea area, therefore, position holding device other than the anchor-chain/wire system will be called for. To keep the position against drifting forces by use of the closed-loop control of hydrodynamic propulsion devices (dynamic positioning system) has been regarded as one of the most effective means to solve the problem. The feasibility of this kind of positioning control has already been verified for drilling ships by use of the systems based on digital or analog computer control methods. In the present paper, fundamental study is made on the applicability of such system to the positioning of a large-size semi-submersible two-lower-hull type platform.