Evaluating the effect of particle intrusion in deep water mooring ropes

Stephen Banfield Tension Technology Int. Ltd.
Arne Ulrik Bindingsbø Norsk Hydro ASA
Vidar Åhjem Det Norske Veritas AS

Abstract

For the purpose of evaluating particle intrusion in fibre ropes, two types of tests were conducted. Firstly, Y-O-Y abrasion tests were conducted on textile yarns to establish effects of different particle sizes, materials and concentrations. Secondly, cyclic loading of small ropes in various concentrations of seabed sediment and measurement of residual strength was conducted.

There is significant reduction in Y-O-Y abrasion life due to small particles penetrating between filaments in a textile yarn. The reduction in Y-O-Y abrasion life was around 26 times when tested in the presence of seabed sediment compared with reference tests without particles. The abrasion increases with increasing concentration of particles. This reduction in life is of a similar order of magnitude whether silica or seabed sediment is used. This infers that the mechanical properties of the abrasive particles are not key parameters for abrasion life.

The key parameter in determining Y-O-Y abrasion life from this study was found to be particle size where the smaller particles, possibly with a limiting size, have the worst effect. This is probably associated with the depth of particle penetration where smaller particles can migrate further into the yarn structure, thereby abrading more of the yarn cross section. Significant strength loss was found on small scale ropes when subjected to cyclic loadings submersed in a tank with a seawater and seabed clay environment.

The results from these tests infer that there could be a significant long term strength reduction due to internal abrasion in permanent fibre mooring ropes if particles penetrate the rope. Rope strength reduction will be dependent on particle size and depth of penetration.

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