Offshore wind turbines (OWTs) are exposed to the combination of cyclic wind and wave load, thus causing fatigue damage to the structure. In this paper, time domain model analysis against fluctuating wind and wave for the large-scale OWT has been done to get the stress time histories for analyzing cumulative numbers of stress ranges. The paper concluded that the structural damage becomes rather significant in case of the OWT around 5000–6000kW, clarifying the relation between the first mode natural periods and the fatigue damage ratios for OWTs.
Offshore wind is, compared with the onshore wind, stronger and more stable, which makes offshore wind energy getting more and more promising owing to the development of large scale wind turbines designed for offshore environment. The large-scale offshore wind turbines enable us to generate the power more economically and the optimization of the supporting structure such as towers and foundations is expected to further reduce the construction cost. 5,000kW class prototype wind turbine has actually constructed aiming the large-scale offshore wind farms. However, in the offshore environmental conditions, the wind load and the wave load act on the structure simultaneously and exposed to the severe conditions in terms fatigue damage. Actually the longest operation record of offshore wind turbines is not more than 10 years and, thus, no practical records for verifying the required design life of more than 20 years. On the other hand, using the larger capacity of generator leads to the heavier nacelle and blades and higher hub height that need the larger cross section resulting in the higher stiffness. Considering these situations, focusing on the fatigue damage, structural design aspects of the enlargement of offshore wind turbines is discussed in this paper.