Influence of Fibre Stiffness on Deepwater Mooring Line Response

Polyester fibre ropes are now an accepted solution for deepwater mooring of production platforms and a single high tenacity fibre grade is widely used. Few studies of other fibres have been reported but polyesters can be produced with a range of properties by varying drawing parameters, and other stiffer fibres are also available. This paper presents a study of these alternative fibre rope solutions, performed within the French Mooring Line project. First, in order to obtain the input data necessary for mooring line analyses an extensive test program was performed to characterize polyester, improved polyester, PEN, LCP, aramid and HMPE fibre ropes from yarns up to 800 ton break load ropes. Tests at different scales have allowed property transfer to be quantified. Rope modelling has been used in parallel to examine the influence of material and structural parameters. Then, using these data, a series of analyses was run by engineering contractors, which quantified the benefits of higher stiffness for different supports (semi-submersible, production platform and offloading buoy) down to 2500 meter depth. Under certain conditions the stiffer ropes can result in significantly reduced rope diameter and weight.Copyright © 2008 by ASME