Environmental Effects on Fatigue Strength of Armour Wire for Flexible Risers

Fatigue of armour wires is in many cases a limiting factor for the design life of flexible risers. Until a few years ago, fatigue design was based on SN data obtained by component testing in air, with the implicit assumption that the environment in a pipe annulus is benign with regard to fatigue of armour wires. Service experience has shown that a pipe annulus may contain species that are aggressive with respect to steel, and could affect fatigue strength significantly. In a consistent design methodology these effects should be taken into account. MARINTEK in co-operation with SINTEF Materials and Chemistry is running a Joint Industry Project (JIP) with the aim of developing a basis for fatigue design of armour wire in which the effects due to the chemical environment in a pipe annulus are accounted for. The project started in 2001 and is now into a Phase III to be completed in 2008. A Phase IV is proposed, to be carried out in 2009–2011. Testing is carried out on tensile armour wire in air, and in aqueous environments and with H₂S and/or CO₂ at various partial pressures. SN curves have been obtained for more than 50 different combinations of material grade, environmental composition and loading parameters. In this paper the methodology of the testing is presented, with some general results. The following aspects of corrosion fatigue are discussed: - Procedures for fatigue testing of armour wire in corrosive environments, test protocol. - The scope for establishing common fatigue design criteria for armour wire, based on strength classes. - The effect of loading frequency in corrosion fatigue, assessment of fatigue strength criteria for long lives. - The fatigue limit in corrosion fatigue.