Performance of PA6C Plastic Sheaves Used for Mooring Line Installation

Abstract

Mooring lines are important components in floating offshore energy installations such as wind turbines and oil and gas vessels. Efficient installation of the mooring lines is important to reduce costly and energy consuming offshore operations. In the present paper investigations have been performed on the performance of sheaves made from Lamigamid® 319, which is a specific type of PA6C plastic. The sheaves are intended for subsea pull-in operations of mooring lines during mooring line installation. The sheaves are a vital component of a proposed installation system which could limit the time spent on subsea operations. Additionally, the low density of the material, the possibility of using it without bearings and using highly stressed sheaves makes it possible to reduce the necessary material for temporary equipment to a minimum. Initial Finite Element Analyses based on material models from tension tests showed that the designed sheaves would suffer large deformations when subject to the design loads. A full-scale test was performed to corroborate these results but during this test almost no permanent deformation was recorded on the sheave. Further, a test series with compression tests of different shaped samples were initiated. The first results from this test series showed a remarkable strength of the material in compression and raised several questions regarding models for predicting the behavior of Lamigamid® 319 during loading. Hence, further tests were done and modelling techniques for finding structural capacity are discussed.