The development of FOWTs (Floating Offshore Wind Turbines) is going into the commercial stage from the demonstration one. One of the key issues for success in the commercial stage is how to reduce the construction cost. For example, watertight bulkheads in narrow space of FOWTs become obstacles to the cost reduction from the perspective of structural complexity and manufacturing. On the other hand, new IEC TS 61400-3-2 standard describes the unnecessity of the damage stability requirement under certain conditions. To secure the sufficient safety without the damage stability requirement according to the IEC TS 61400-3-2, an examination method with a probability theory and the FEM analysis is proposed in this paper. The probability of collision with cruising ships, P1, around a FOWT is estimated to use by the AIS data, and the probability of structural total loss, P2, due to the ship collision is indicated according to the total loss curve which is delivered from the FEM collision analysis. The joint probability which is represented by the product of P1 and P2 is compared to the target damage probability, 10-4 per year. The detailed information about this method and the results of trial analyses for the FOWT off the coast of Nagasaki Prefecture are also introduced in this paper.
