Dynamic umbilicals are widely used in the wet tree development of deep-water field. In the application of dynamic umbilicals, fatigue is a key failure mode which may cause severe economic consequences. The fatigue behavior of dynamic umbilical is complicated by the mechanical properties of the material, the cross-section design and the interaction of different components. Published studies have shown that the interaction friction stress of steel wires is critical for fatigue analysis. Coefficient of friction were used as a constant during the fatigue life analysis. However, the friction stress and the coefficient of friction may change during the wear of the steel wire.
This paper presents a new experiment method and device to evaluate the change of the coefficient of friction of steel wire. Different cycles of reciprocating wear were carried out on samples to simulate interaction of the steel wires during the process of fatigue. The samples are pairs of steel wires which were cut on a dynamic umbilical. The interaction stress and friction stress were measured during the test. Micro morphology on the surface of steel wire at the contract zone were measured to investigate the wear of the sample.
The test result shows that the coefficient of friction and stress changes during the test. Worn marks were found on the contact zone of the steel wires. The change of the coefficient of friction may affect the accuracy of fatigue life analysis of dynamic umbilical. Conclusions were also presented on the coefficient of friction test and theoretical calculation method to approach a more accurate fatigue life for dynamic umbilical design.