Models to explain out-of-plane bending mechanism in mooring chain links

Abstract

In 2002, several mooring chains of Girassol Off-loading buoy which was installed offshore Angola ruptured just after 8 months of service. A new failure mechanism called out-of plane bending(OPB) fatigue in mooring chain links was identified in addition to tension induced fatigue after a series of experiments. Currently, there are no models in literature which can comprehensively explain OPB mechanism in mooring chain links. Models to explain OPB mechanism are required to calculate OPB stresses in chain links for fatigue damage evaluation. These models need to be "simple" and "adequate" in the sense that it should idealize the complicated geometry of chain links while giving accurate predictions of OPB stresses when subjected to rotation and tension, at a fraction of time used by solid finite element models or full scale experiments. The research carried out in this master's project deals with development of such models to explain OPB mechanism in mooring chain links. In this thesis, a new interlink stiffness model is developed to describe the nonlinear hysteritic relationship between OPB moment and interlink angle. The interlink stiffness model is then applied to a system of chain links using two different methods: a physics based approach and a semi-empirical approach. Each of these methods present a new approach to idealize a system of chain links. Then, a case study is presented in which a methodology to calculate OPB stresses for one sea-state is described. This methodology follows a de-coupled approach in which results of coupled floater-mooring analysis are taken as an input for the simplified models of chain links. To verify this methodology, a comparative study is performed at the end in which coupled floatermooring analysis is carried out with and without including the interlink stiffness of chain links.