A response surface approach to fatigue reliability of ship structures

Today ship designers are facing ever more stringent safety and environmental requirements as well as economic ones. With the advancement of rules and procedures in the classification societies and the tailwind of modern computing technology, spectral fatigue based on first principles has become commonplace in ship design. So far the centrepiece of such a process is by and large a deterministic acceptance criterion despite the highly uncertain nature of the problem. As a result, the observed damage in a fatigue prone area is often significantly inconsistent with the predicted one, making client-facing a daunting task. As a small step towards a reliability-based solution, this paper attempts to introduce structural uncertainties into established fatigue design assessment process such as the ShipRight FDA3 procedure and the supporting software by Lloyd's Register. Key design parameters are randomised in a spectral fatigue model, where a pseudo-excitation method is used to reflect the non-linear effect of inertial loads and external wave pressure in the splash zone. A stepwise response surface method is used in tandem with t by t fine mesh finite element analysis to obtain the probability of failure. The calculation is demonstrated for an oil tanker example.