In this paper CFD results are presented for the thruster-hull interaction effects for a drillship with 6 azimuthing thrusters. The results using different approaches to model or simulate the propeller are compared and their advantages and disadvantages are discussed. The approaches investigated are the so-called Frozen Rotor approach, where the propeller rotation is modeled, the Actuator Disk approach with prescribed body forces and the unsteady Sliding Interface approach where the motion of the propeller is simulated in time.
First, open-water calculations for a tilted thruster are carried out using the Frozen Rotor approach. The open-water calculations are repeated using the Actuator Disk prescribing the propeller thrust and torque distribution obtained from the Frozen Rotor calculations. The results with Actuator Disk are very similar for the unit thrust and nozzle thrust compared to the results using the Frozen Rotor approach. Furthermore, the results using the Frozen Rotor or the Actuator Disk are very close to the experimental results for the nozzle thrust.
The thruster-hull interaction of one active thruster under the drillship is investigated using the Actuator Disk approach, the Frozen Rotor Approach and the Sliding Interface approach. A comparison to experimental results is presented for the thruster-hull interaction coefficients. Using the Actuator Disk a good agreement with the experiments is obtained. The results using the Actuator Disk and Sliding Interface are very similar to each other, but the computational costs for the Sliding Interface method are at least a factor 20 higher. The results using the Frozen Rotor deviate due to an unphysical wake behind the thruster.
Based on the results presented in this paper we conclude that, using the steady-state approach with the Actuator Disk, CFD can be a cost-efficient and accurate method to determine the thruster-hull interaction effects at bollard pull conditions for a typical offshore vessel.