This paper presents improved dynamic modelling of subsea power cables using new models for the determination of non-linear cable mechanical properties. The modelling has been developed for cables typically used in offshore wind and for interconnectors, as well as dynamic power umbilicals. The results provide a better simulation of the dynamic response and allow better integration of local and global modelling for determination of stress and fatigue in offshore power cables.
Cable response due to bending is modelled by including non-linear adhesion induced stresses due to a yielding bond between armour wire and neighbouring layers, which captures the effects of temperature and strain rate and provides better representation than purely friction-based modelling especially at low tension. Local armour bending stiffness is included by using average wire strain energy after slip along the strained helical path to determine the armour layer bending stiffness contribution. Mathematical modelling for mechanical properties is verified by sample testing and FEA, to provide a robust method for predicting cable response.
Although dynamic subsea power cables are essentially non-bonded structures there is a certain amount of adhesion within the structure. Previous work has focused on friction-tension based modelling of armour wire-core interaction, that is not appropriate as critical slip curvatures at low tension are understated and full-slip stress distributions do not account for work done against friction during further bending. The principal result of this new approach is the improved determination of lifetime stresses for critical components within the cable structure. Non-linear bend stiffness modelling produces characteristic moment-curvature relations including hysteresis on reversal of bending. These curves provide an improved representation of the onset of slip in the armour wires and allow for the influence of temperature and strain rate on the cable bending stiffness to be included. The bend-stiffness model has been validated against test data both of complete bundles and individual components.
The overall result is a methodology that typically results in increased fatigue life and can reduce the requirement for ancillary products such as ballast/buoyancy and bend stiffeners/restrictors. Additionally, the non-linear, hysteretic response of a cable significantly reduces certain phenomena that are often associated with numerical modelling of cables using a linear bend stiffness. Specifically, a cable catenary attached to a vessel and modelled with a linear bend stiffness will often experience ‘compression waves’ when the vessel is moving in response to wave loading. Use of a non-linear, hysteretic bend stiffness minimises the compression wave phenomenon, giving a much more realistic response and often greatly improving operability windows for offshore operations.