Experimental investigation of evolution of bending stiffness in dynamic submarine cables and the contribution of polyethylene layers

• A full-scale experimental study revealed a logarithmic reduction in bending stiffness of dynamic submarine cables under combined cyclic loading and current-induced heating. • The study confirmed the existence of a long-term minimum stiffness plateau. • Dynamic Mechanical Analysis (DMA) characterized the temperature- and time-dependent viscoelastic behavior of polyethylene (PE) layers used in dynamic cables. • The combined full-scale and material testing results demonstrated that changes in PE mechanical properties influence the long-term stiffness behavior of the cable. • The work highlights the need for offshore dynamic cable design to account for evolving stiffness characteristics over time, temperature, and loading cycles to ensure reliable performance. This study examines the long-term evolution of bending stiffness in dynamic submarine power cables subjected to extensive cyclic loading. The primary objective is to identify the evolution of bending stiffness over time, with particular attention to the behavior beyond the initial few load cycles. Furthermore, the study aims to examine whether the polyethylene layers may partially account for the mechanisms underlying these observed trends. To achieve this, a novel experimental protocol was developed, involving 100,000 loading cycles of a submarine dynamic cable under realistic operational conditions, including current-induced heating (full-scale test). This protocol was designed to assess the combined effects of temperature and material properties on the degradation of bending stiffness. Moreover, material testing on polyethylene layers was conducted using Dynamic Mechanical Analysis to characterize their time-dependent mechanical behavior and compare it with the results of the full-scale test. The experimental results reveal -for the first time for a dynamic power cable- a logarithmic decrease in bending stiffness as a function of loading cycles, eventually stabilizing at a plateau. In addition, the findings from the material characterization of polyethylene revealed a significant reduction of complex modulus as a function of temperature increase, and a slight reduction in the modulus as a function of the number of loading cycles, which can explain partially the phenomenon. However, the material testing results cannot fully explain the patterns observed at full-scale, suggesting that they play a role in the evolution of the cable’s bending stiffness. Incorporating the findings from this experiment into design methodologies and assessment frameworks is essential to improve the reliability and operational safety of dynamic submarine power cables.