A novel hybrid frequency-time domain method for the fatigue damage assessment of offshore structures

• A novel hybrid frequency-time domain method is proposed for fatigue damage assessment of offshore structures.
• The new method is compared with the traditional fatigue damage assessment methods.
• The effectiveness of the new method is validated with a numerical model.

The cumulative fatigue damage analysis plays a key role during offshore structural design. Although many methods have been developed, there is not yet a method that can predict fatigue life effectively and efficiently. A hybrid frequency-time domain method, which can be considered to be a hybrid of the spectral method and the time-domain analysis method, is proposed in this paper. In the newly developed method, the spectral density function of structural stress is first obtained in the frequency domain and then converted into a stress time history by using an improved signal conversion approach. With this methodology, the fatigue damage of structures can be assessed easily with the rain-flow counting method and Palmgren–Miner rule. In contrast with the traditional fatigue damage assessment methods, the Rayleigh distribution assumption of the stress range, which is associated with narrow band random process and is commonly used in the spectral method, is not required for the hybrid frequency-time domain method. At the same time, the newly developed damage assessment method can avoid the complicated coupled dynamic analysis used in the time domain method, greatly reducing computation time. To verify the effectiveness of the present method, a case study for a monopile platform was conducted. Numerical results illustrate that the fatigue life assessed by the new method is more accurate than that by the spectral method, and the new method takes much less computational time than the time-domain method.