Pitch motion problem induced by dynamic positioning system for new sandglass-type floating body

Abstract

In this paper, a new concept of sandglass-type floating body, which can overcome the performance limitations of traditional ship-type and cylindrical FDPSO (floating drilling production storage and offloading), is proposed as research subject. In the general design of dynamic positioning systems, it is adequate to deal with a three-degree of freedom problem in the horizontal plane. However, the new floating body with small water-plane area and low metacentric height may cause an unintentional coupling phenomenon between the pitch and surge motions by the thruster system. Therefore, first by numerical boundary element method (BEM) based on wave potential theory and experiments, the pitch motion characteristic of the sandglass-type model is studied and meanwhile the numerical method of this paper is validated. Furthermore, two dynamics models with and without consideration for the additional pitch moment by the thruster systems are introduced and numerically simulated, which can show the problem of pitch motion induced by the positioning thrusters. Then by the mass-spring-damper systems with two-degree of freedom, the influence of additional pitch moment by the thrusters on the pitch and surge motion performances is theoretically analyzed. Finally, based on the essential reason of pitch motion problem, a control law considering pitch inertia effect has been used and proven to be effective to decrease the pitch motion response.