Position-keeping performance of a turret-moored tanker assisted by dynamic positioning system

Abstract

In this paper, a study has been made on the position-keeping performance of a tanker exposed to seas. A turret mooring system with dynamic positioning (DP) is being frequently adopted for shuttle tankers and FPSOs mainly due to its excellent performance even in heavy weathers. A mathematical model for a single-point moored tanker is derived. Environmental loads due to wave, wind and current are considered. For current load, towing tests are carried out and the result is compared with the widely used data of OCIMF. The linear and nonlinear wave load are calculated based on the Green function method. The nonlinear equations of motion are linearized for designing the DP system. In order to improve the efficiency of control system, high- and low- frequency motions are separated by Kalman filter. The control gains are estimated with help of the fuzzy and the optimal control theories and their results are compared in terms of performance. The numerical simulation shows the weathervaning capability and the enhanced performance of the turret mooring, when it is assisted by DPS.