Fatigue of Copper Conductors for Dynamic Subsea Power Cables

Abstract

Dynamic subsea power cables are used for distribution of electric power to subsea units for oil and gas production and for power distribution from offshore wind mills. As the cables are suspended from the sea level to sea floor, a dynamic analysis is normally required to ensure that the cables are able to withstand the dynamic tension and bending caused by waves and vessel movements. The fatigue property of a conductor is determined by the mechanical properties of the individual wires and stress concentration caused by wire interaction. Previous publications on steel wires and ropes have brought light on the effect of R-ratio and wire interaction such as fretting and inter wire slippage. For copper conductors the complexity of fatigue is multiplied by the fact that the individual wires have a nonlinear stress-strain behavior well below the defined yield stress limit and poor creep properties below maximum cable operation temperatures. Moreover, the interaction between the wires is determined not only by the contact between copper surfaces but also by the water blocking compound in between the wires normally required for subsea cables. A test method, simulating the fatigue mechanism in a dynamic power cable, including the effect from friction, fretting, creep properties of copper and high tension at deep waters is presented together with test results for a representative conductor.