Analysis of Helical Wound Structures Including Effect of Slip Curvature

Off-shore exploration, including both oil and gas, and floating wind, relies on flexible structure as an enabling technology. Flexible pipes are used to transport fluids in offshore oil and gas production systems, while an umbilical is a multipurpose product for system operations that consists of a group of functional components, such as electric cables, optical fibre cables, hoses, and tubes, laid up or bundled together or in combination with each other, that generally provides hydraulics, fluid injection, power, and/or communication services. Subsea power cables are an essential part of offshore wind projects, where they transport the values produced, electrical power, to shore, via onshore power grids, to the consumers. Power cables are also multi-layered structures consisting of helix-shaped components in a hierarchical build-up. Flexible pipes, umbilicals and cables introduce multilayer unbonded structures with helical wound components to achieve high flexibility in bending and bending stress release. This to increase ultimate capacities and fatigue life of the product. The flexibility in bending due to the helix components comes at a cost; calculating mechanical properties and component stresses is not straight forward and may be costly. Safe design of such structures relies on appropriate simulation tools. Especially, the cross-section design is dependent on appropriate models for local deformations, contact with friction and stress and strain evaluations. During the years, DNV has developed a special purpose tool for such structures, named Helica. Helica perform cross section analysis that supports a consistent evaluation of stress and strain time serie. The mechanical response in the cable component due to cross section forces is based on a semi-analytical formulation. The establishment of long-term stress or strain distributions will require global dynamic time domain analysis followed by cross-sectional analysis for a large number of irregular sea states. The main challenge is therefore to establish an overall manageable computational scheme to establish stress or strain estimates with sufficient confidence. A computational efficient cross-sectional analysis strategy is hence a vital part of the overall fatigue analysis approach. The semi-analytical model capabilities are extended to explore the effect of slip on local curvatures of helical components and the possible influence on ultimate capacities and fatigue life, especially for large diameter components. The updates include both the effect of interlayer slip, and intralayer slip as is seen for three-core components with circumferential contact. The latter is typical for High Voltage Alternating Current (HVAC) cables. The mechanical load and response model relies on a careful separation into axis-symmetric and pure bending behaviour. The axis-symmetric response establishes contact forces between cross section components. The bending model takes due account of stick-slip behaviour. The slip progression and local curvature change is derived for interlayer and intralayer contact. The slip progression of the helical core components in the three-core configuration is complex and different from stick-slip progression between different radial layers. The study evaluates the updated curvatures through a simplified finite element analysis of the bending and slip behaviour both for interlayer and intralayer contact. The influence of the updated curvatures on ultimate capacity and fatigue life is discussed based on changes in stress levels.