Duringoffshore heavy lift or pipelay operations the station keeping capabilitiesof a DP-vessel have an important influence on operability limitsof these operations. Heerema Marine Contractors has two DP3-class semi-submersiblecrane vessels for these operations; the Thialf and Balder. Anassessment of the thrust efficiency of the DP thrusters ofthese vessels has been made by comparing CFD computations withdedicated model tests. A numerical study using CFD is performedto assess thruster-hull interaction on a semi-submersible vessel. The CFDresults are validated with a series of model tests, includingan open water thruster, single thruster-hull interaction without current andfull thruster-hull interaction with all thrusters active without current. TheCFD calculation results show good agreement with the model testdata. The forces on the semi-submersible as well as onthe individual floaters with active thrusters using CFD are within10% of the model test data. The largest discrepancies arein the bow quartering conditions when the thruster-hull interaction showthe most complex flow pattern due to the location andshape of the stern keel. The comparison between the CFDand model test data demonstrates that CFD is able topredict the relevant force components well within a sufficient accuracyfor engineering purposes. The paper also addresses lessons learnt toimprove the CFD computations as well as practical aspects andlimitations of thrust efficiency modeling using CFD from an engineeringperspective.