Abstract Offshore wind turbines ara a complex mechanical systems located in severe environmental conditions. The calculation of design loads in offshore wind turbine structures is a complex undertaking involving the integration of different wind and wave load simulation methods. The aim of the thesis is to investigate thew influence of the environmental conditions on the aerodynamic and hydrodynamic loads acting on fixed offshore wind turbine structurs using the in-house bondary element method (BEM) code panMare. So the geric NREL 5MW offshore wind turbine is chosen. In the investigations, three different turbines support structure typologies are considered: monopile, tripod and jacket. The applied BEM code based on three-dimsional first order panel method, which can be applied for investigation of various aerodynamic and hydrodynamic flow problems. Furthermore, for verification of BEM results, RANSE simulations are carried out using the ANSYS CFX solver that is based on a finite volume method. Before simulating the flow on the complete offshore wind turbine structure, test simulations are conductet for three different configurations(OWT rotor only, land wind turbine and monopile) using the mentioned two different methods. The simultation results are use to enhance the simulation models. The results of the first simulation allow the verification of the global values such as torque and thrust as well as information on the local flow field such as the pressure distribution on the different blade sections. The tower is added to the rotor in a second simulation and the unsteady forces due to the interaction between the tower and the rotor blades are calculated. The results of the third simulation (a monopile in wave) are used to improve the accuracy of BEM code by calculating the hydrodynamic loads. The second part of the study focuses on the prediction of the influence of environmentel conditions on the desgn loads, which arre one of the most important factores regarding the safety and reliability of the system. The unique treatment of the combined aerodynamic and hydrodynamic loads is carried out by coupling two different solvers within the BEM code. The developed solution method enables changing the wind and wave parameters independently during the simulation. The calculated forces at the inflow direction on offshore wind turbine in combination with differnt foundations (monoplie, tipod and jacket) are compared with the corresponding CFX results, whre an acceptable deviation between the calculated forces by the BEM and the RANSE methods is found. The results presented the ability of the BEM code to simulate the aerodynamic and hydrodynamic flow on a complex 3D offshore wind turbine.weiterlesen