Multiaxial fatigue analysis of offshore mooring chains, considering the effects of residual stresses and corrosion pits

Abstract

The integrity of the mooring lines used to anchor offshore facilities and floating platforms has been always desired while its continuous achievement has been challenging. Many mooring line failures have occurred in the last decades and the failure rate has been higher than the industry expectations. Several joint industry projects have been conducted and many mechanisms and phenomena have been identified to be influential on the fatigue life of mooring chains, which are the most susceptible elements in mooring lines. In this thesis, an overview of the knowledge and experimental studies on the phenomena and mechanisms influencing the fatigue life of mooring chains has been made and the gaps in the state-of-the-art have been identified. The effects of corrosion pits and residual stresses on the fatigue crack initiation life of pitted mooring chains have been closely studied. A comprehensive study on the fatigue behavior and cyclic plasticity of the mooring chain high strength steel grade R4, which is broadly used in the offshore industry, has been conducted. The material’s fatigue and cyclic plasticity parameters have been identified and calibrated against the experimental data. A rapid cyclic softening behavior has been observed from the material, which is of great importance when fatigue crack initiation and propagation are of interest. Multiaxial fatigue performance of the material has also been studied experimentally. A quick material hardening followed by softening has been observed when the material is subjected to biaxial (both proportional and non-proportional) loading. An advanced material model based on non-linear kinematic hardening combined with isotropic hardening has been calibrated to the experimental data obtained from small-scale uniaxial fatigue tests and used in the established finite element models of mooring chains to predict the residual stresses due to proof loading. Artificial hemispherical pits at the fatigue critical locations on a typical studless mooring chain were introduced to the FE models and strain localization at the pit sites was studied for different service load levels. Further, the residual stress redistribution due to corrosion pitting and cyclic service loads applied to large mooring chains has been investigated both numerically and experimentally using two different measurement techniques. Ultimately, fatigue crack initiation from the critical pit at the chain crown was assessed using a strain-energy based fatigue damage parameter and a critical plane approach to account for the stress multiaxiality, residual stresses, and corrosion pits effects. The predicted crack initiation lives from the two approaches were comparable and in the range of 15-40 % of the (total) experimental lives depending on the applied load level.