Advancements in Guidance for the Specification and Assessment of Mooring Steel Wire Rope

Prior to the SCORCH JIP, industry guidance for steel wire ropes specified only a maximum service life for various rope constructions, providing no insight into the corrosion degradation processes affecting mooring ropes at different locations in the mooring line and under different working conditions, or to potential differences in manufacture. One of the objectives of the SCORCH JIP was to address this gap in knowledge by providing design guidance on the specification of mooring wire ropes and analytical tools to estimate the design service life of mooring wire ropes for particular site conditions, especially in tropical waters. In the wire rope component of the JIP, field recovered specimens were examined and field tests on wires and wire ropes were performed to quantify the effects of external factors driving degradation of wire ropes in marine environments, including: protective coating; blocking compound, temperature, flow velocity, oxygenation and dynamic loading. Failure modes for wire rope were analysed based on the examination of recovered samples from in- service wire ropes, inspection records from different floating production units (FPUs), reported experiments in the field and experiments in literature. Experimental testing (both field tests and laboratory tests) of individual wire strands and complete wire rope constructions were carried out at a number of locations with varying environmental conditions. These data were then analysed to derive a phenomenological model that represents the observed behaviour of both ropes and individual wire layers. Amongst the key findings of the SCORCH JIP investigation was that the corrosion endurance of steel wire rope mooring lines is largely driven by the longevity of the galvanizing or other galvanic protecting coating, and the blocking compound, which forestall the direct corrosion loss of metallic area of the relatively small steel wires. Two major mechanisms by which zinc oxidation occurs and the major environmental contributors to the acceleration of wire degradation were identified; and efficacy of various rope protection methods were assessed. A predictive model was derived which characterised the rate of zinc dissolution, performance of blocking compound and corrosion rates for different exposure and working conditions. The outcome of the wire rope component of the SCORCH JIP was a significant advancement in industry knowledge on the corrosion behaviour of steel mooring wire ropes.