Fatigue Damage Analysis For Ships Based On Semi-analytical Approach Of Wave-induced Loads And Ship Responses

Fatigue damage is one of the most significant destruction forms for ships and offshore structures. It is thus necessary and mandatory to conduct fatigue strength assessment during their lifetime.Normally, the stress range of fatigue damage is calculated through two methods: the one with empirical formulae from class society, the other is through strip theory or three-dimensional hydroelasticity method. The former method is quite simple, but the calculation results seems too conservative while it can not reflect multiple parameters influences, as well as that it is not suitable for the design assessment of new type ships. The latter gives quite an accurate results, but it is quite complex and require much more effort in computer time, while it is not suitable for the strength safety assessment of the conceptual design phase as well.Accordingly, this thesis mainly concerns with the following areas:Firstly, based on the classical strip theory and ship motion equations, sectional hydrodynamic parameters were simplified, and consequently a semi-analytical approach is given for wave-induced loads and ship responses in frequency domain. The Approach mainly concerns with the following parameters: main dimensions: length, breadth, draught, block coefficient and bow flare coefficient, in addition to speed and heading, and the sea state parameters significant wave height (Hs) and zero upcrossing period (Tz).Secondly, the frequency response function of wave-induced bending moment amidships for box-shape ships is modified by considering forward speeds and hull forms. After that, concerning with the non-linear influences for bending moment, including the bow flare, skewness and whipping, the previous expression is modified to the extent as a semi-analytical expression for the probability distribution function of the wave-induced bending moment amidships. Lastly, some classical ship types, which are out of the prediction of the regular strip theory methods, are selected to validate the semi-analytical approach. The results indicate that this approach can give quite a quick and reasonable prediction for wave-induced loads and ship responses, as well as short-term and long term analysis.Finally, basic method and procedure for fatigue damage assessment is given, while factors that have great influence for fatigue damage are being studied in details as well, such as the S-N curve, stress points selection, ship operational profiles and sea states. Lastly, a full calculation procedure for fatigue assessment of a given chemical tanker is given. The result indicates that it is reasonable and accurate to conduct fatigue damage assessment with this approach.