Study on Practical Fatigue Assessment Considering Hull Vibration Based on the EWP Concept

Onboard multi-position hull monitoring (HM) is carried out on an 8,600 TEU container ship. The relationship between the characteristics of hull vibration (HV) superimposed stress waveforms and the fatigue damage enlargement ratio resulting from HV (Cvib) is studied. Based on this analysis, a methodology for estimating Cvib throughout the entire ship from HM data at a select few reference positions (RP) is proposed. By integrating the proposed whole-ship Cvib estimation technique with the concept of equivalent wave probability (EWP), which allows for the assessment of whole-ship fatigue damage due to wave-induced fluctuating stress (excluding HV) solely via HM data at a limited number of RP, an EWP-based comprehensive ship fatigue assessment approach that accounts for HV is developed. The effectiveness of this developed method is validated using long-term HM data. It is ascertained that the statistical attributes of Cvib throughout the entire ship can be reasonably approximated by those of the RPs with the same structural types (hatch side coaming or bottom of a cargo hold). Furthermore, it is demonstrated that the accuracy of HV superimposed fatigue damage can be improved through the estimation of Cvib’s statistical characteristics using exclusively HM data in the short seas where whipping is significantly occurring.