Modelling of Polyester Mooring Ropes for Ultra-Deep Water

Abstract

The purpose of this thesis was to review the state of the art modelling of polyester mooring ropes, for semi‐submersible oil platforms at ultra‐deep water. Ten material models were implemented in the in‐house analysis software CASH. The motions of the platform and the tensions in the mooring lines were found from analyses performed in the time domain. In order to implement the material models a new subroutine was developed, for calculating the internal forces in the mooring lines. A platform model, with a displacement of 125 000 tonnes, was used as a reference for all material models. It was modelled with a few governing parameters to enhance future platform modelling. The platform was subjected to environmental loading described by Ma et al (2010), taken from measures during three hours of hurricane Gustav in the Gulf of Mexico in September 2008. In order to compare the different material models they were calibrated against experimental rope data. Two sets of rope data were used; one with monotonic loading and one with cyclic loading. The ability to mimic the experimental rope data, as well as the results from the analyses in CASH, was used as a basis when the models were evaluated. Looking at the motions on the platform from the analyses with the different material models, the differences were small. In the context of the present study it is concluded that the classical Hooke model is a rational alternative when compared to the more sophisticated models.