Methods for Fatigue Assessment of Critical Ship Details

Fatigue assessment, as part of ship design, was not required until about 10 years ago, due to the increasing use of high strength steel and demand for minimizing structure weight. As early as 1993, ABS (American Bureau of Shipping) published Guidance on fatigue strength assessment of tankers (the latest version of the Guide can be found in [1]). Since then, ship design has been developed with more optimization involved, as a balance of cost, steel weight and safety. Consequently, fatigue assessment has become a more and more important factor in ship design. There are various methods, design codes and recommended practices on fatigue design and assessment of welded structures. Industries have tried to organize Joint Industry Projects (JIPs) to further develop the method or see the possibility of harmonizing the codes or practices. Individuals have also made their efforts to provide information from view points of designers, operators or regulators. This paper addresses some of the available methods applicable for critical ship details through the case studies of bilge knuckle joint of an oil tanker and a side longitudinal/frame connection of an FPSO. Various methods of hot spot stress calculation including mesh size schemes and element type selection will be evaluated and comparison of fatigue lives from the hot spot stress methods with associated S-N curves will be made with the experimental data.