Gravity Effect on the First Natural Frequency of Offshore Wind Turbine Structures

The fatigue limit state is critical for most offshore wind turbine.To minimize the development of fatigue damage,dynamic amplification of the response must be avoided.Thus,it is important to ensure that the first natural frequency of the offshore wind turbine does not coincide with the excitation frequencies related to wind turbine and wave loadings.For evaluating the self-gravity influence on the first natural frequency of wind turbine support structures,the offshore wind turbine system vibration is modeled using an Euler-Bernoulli beam with axial force and horizontal force.Real data from five wind turbines available in the market are considered.The sizes of wind turbines vary from 2.3MW to 6MW,and subsequently,the heights of tubular steel towers vary from 83 mto 100m.Results indicate that the average influence of gravity on the first natural frequency is nearly 1.8%.The first natural frequency is considered ranging from 1P(rototor frequency)to 3P(blade passing frequency).The design procedure requires an accurate evaluation of the first natural frequency.From this perspective,the first natural frequency is reduced since gravity needs to be considered for the design of offshore wind turbine support structures,especially when the first natural frequency of the offshore wind turbine is close to the lower limit,rotor frequency1 P.

The fatigue limit state is critical for most offshore wind turbine. To minimize the development of fatigue damage, dynamic amplification of the response must he avoided. Thus, it is important to ensure that the first nat- ural frequency of the offshore wind turbine does not coincide with the excitation frequencies related to wind turbine and wave loadings. For evaluating the self-gravity influence on the first natural frequency of wind turbine support structures, the offshore wind turbine system vibration is modeled using an Euler-Bernoulli beam with axial force and horizontal force. Real data from five wind turbines available in the market are considered. The sizes of wind turbines vary from 2.3 MW to 6 MW, and subsequently, the heights of tubular steel towers vary from 83 m to 100 m. Results indicate that the average influence of gravity on the first natural frequency is nearly 1.8~. The first natural frequency is considered ranging from 1P （rototor frequency） to 3P （blade passing frequency）. The design procedure requires an accurate evaluation of the first natural frequency. From this perspective, the first natural fre- quency is reduced since gravity needs to be considered for the design of offshore wind turbine support structures, especially when the first natural frequency of the offshore wind turbine is close to the lower limit, rotor frequency 1P