Development of a Numerical Dynamic Mooring Model for Visco-Elastic and Visco-Plastic Deformation of Synthetic Ropes

Abstract

Abstract Synthetic ropes are increasingly being considered for various offshore and marine applications, including for mooring offshore wind turbines and for aquaculture cages. Studies have shown that nonlinear behaviors of a synthetic rope in a dynamic environment can complicate the mooring system analysis. Nonlinear stiffness coupled with time- and load history-dependent characteristics of fibrous materials can allow for over or under estimation of the mooring forces. It is critical that these nonlinear properties are incorporated correctly into a mooring model, especially for studies of structures’ performances in extreme events. The study aims at developing a simulation tool capable of predicting the dynamic behavior of highly extensible synthetic mooring system used in coastal and offshore floating structures. The program employs an implicit finite-difference approach to model the dynamic behaviors of the mooring line subjected to user-defined motions of the fairlead. As opposed to a linear stress-strain relationship typically incorporated in other mooring models, the current program is built with constitutive model of fibrous materials to account for the nonlinearity time- and load-dependent characteristics of synthetic lines. As part of the program, an inverted constitutive stress-strain model, in which stresses are calculated from given strains in stress-based formulas, were presented. Comparisons with published data indicates that the proposed inverted nonlinear stress-strain formulas were successfully integrated with the mooring solver. The coupled nonlinear mooring program predicts accurately both nonlinear reversible and irreversible deformations of synthetic cables.