Development of a New Fatigue Damage Model for Quarter-Modal Spectra in Frequency Domain

Abstract

Floating marine structures are exposed to various environmental loads. When the response of structures caused by these loads can be expressed as a multi-modal spectrum in the frequency domain. To estimate the fatigue damage by these loads, various fatigue damage models have been suggested by many previous researches. The offshore riser system, for example, that is a typical marine structure shows a quarter-modal response. However, existing fatigue damage models are suitable for spectrum with tri-modal in maximum. Consequently, it is necessary to develop a new fatigue damage model that can accurately calculate fatigue damage of quarter-modal spectrum. In this research, the Benasciutti and Tovo model was adopted for developing a new fatigue damage model. And 900 quarter-modal spectra are used to derive a new weighting factor in the Benasciutti and Tovo model by investigating the relationship between the linear weighting factor and the bandwidth parameters. Finally, a new linear weighting factor is derived using the least square method. The suitability of the new fatigue damage model with the derived weighting factor is evaluated by frequency domain and time domain results and their comparison with existing model's results.

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“…Currently, conventional subsea wellhead system studies focus mainly on structural stability analysis and fatigue reliability evaluation. The time-domain and frequency-domain techniques were often used to investigate fatigue damage to critical components of top-hole casings and at the wellhead [2,3]. Grytøyr et al [4] provided an indirect measurement-based technique for estimating bending moments at the wellhead.…”