Wave Tank Test of a DP Drilling Ship Model With Active Environmental Force Emulation

Model testing is essential to understand the behavior of physical phenomena and, therefore, to develop and validate mathematical models. In this paper, we analyze small-scale model tests of a DP drilling ship (length 2.75 m, scale 1:80), subject to environmental forces (wave, wind and current). We present a hybrid model test setup, where the vessel is tested in a 14 m x 14 m wave tank with 156 active wave generators. The wind and current effects are emulated using four force actuators attached to the model. The actuators consist of servomotors controlling the force in lines connected to the vessel, and, together, they apply the desired force and moment in the horizontal plane (surge, sway and yaw). The drilling ship model has 6 azimuthal thrusters and a DP system with Bias and Weathervane functions. First, the force actuator system was tested with a six degree-of-freedom load cell, where the controller (super-twisting sliding mode control) and force allocation algorithm were validated. The system could apply forces of up to 2 N, with a maximum frequency of 1 Hz and an accuracy of 90 %. Then, we performed decay tests of the vessel, with the four forces actuators connected, to measure the force controller capacity to compensate for movement disturbances. The results indicated that the force actuators did not affect the natural periods of the vessel and only slightly increased damping. Finally, we did a final validation of the complete setup, with station-keeping tests of the model subject to wind, wave and current.