Numerical Methods for Interlink Stiffness Formulations and Parameters Sensitivity of Out-of-Plane Bending in Mooring Chains

Abstract

Mooring components used for offshore floaters are conventionally designed only to resist axial loads with minimum resistance to bending loads. However, the unprecedented failure of four mooring lines of the Girassol Buoy followed by new modifications of similar buoys exposed the gaps in the existing methodology for failure assessment. The root cause of this failure was attributed to the critical role of out-of-plane (OPB) bending induced fatigue which reduced the fatigue life by 95%. The methodology to incorporate OPB fatigue for failure assessments involves a complex process due to numerous parameters required in the formulations and variability of mooring configurations. One of the most critical steps required to simplify methodology is the formulation of the interlink stiffness, contact stiffness and global stiffness of the chain segment. Currently, the interlink stiffness is derived from full-scale laboratory testing which is expensive and has limitations in generating data for a range of configurations. This paper focuses on producing the interlink stiffness using numerical simulations based on non-linear FE analysis to capture the complex interlink contacts mechanism at the mating surface, elastic-plastic material properties considering non-linear isotropic and non-linear kinematic behaviors during OPB response modes, and compare the numerical models based on available experimental data. The numerical model developed for this research are designed to replicate real case OPB scenarios which induces both rotation and vertical displacements at the mooring connection points. This is different from models studied so far that induces only vertical displacements to study OPB responses which produces conservative results. Further to this, an exhaustive analysis of the key OPB inducing parameters like chain diameter, types, pre-tension, instantaneous tensions, proof loading, residual stress, material properties, boundary condition etc. are required for understanding the underlying failure mechanism. This research also investigates the key OPB parameters and analyze their inter-dependencies, proportionalities and relative sensitivities to understand their overall contribution to OPB failures. This paper presents the first part of this research work which focuses on some of these key aspects to generate the simplified methodology using numerical methods. The findings of this research can be used to generate a database of interlink stiffness for application to a range of mooring configurations and develop mathematical formulations for carrying out a direct assessment of OPB fatigue in combination with tension-tension fatigue failures and proposes potential mechanisms for improving the fatigue life.