Modelling alternatives for non-critical components in dynamic offshore power cables are systematically investigated with the aim to clarify the extent to which these components in finite element modelling (FEM) should be considered depending on the purpose of the analysis. Level of simplification, choice of element type and determination of contact stiffness are among the key parameters when modelling such components. Mesh sensitivity on sheath layers, filled bodies and tensile armor wires is carried as well. Their effects on physical properties of the cross section, stick and slip characteristics of helical components are investigated. Axial stress in the tensile armor wires is particularly studied. Software program UFLEX is used for the investigation.

INTRODUCTION

Offshore power cable in this paper refers to the power transmission lines in offshore oil and gas and offshore wind industry. It represents a critical component for power transmission between either the floating unit, such as FPSO or floating wind turbine on ocean surface to subsea facilities, such as wellhead or subsea power grid, or the onshore power supply plant to offshore power distribution system. The power cable is also often used to transmit the power over a long distance on the seabed. A typical power cable consists of a power core, strengthening components such as tensile armor wires, insulation elements and protection sheath. The power core usually consists of three conductor bundles and each bundle may be formed by another three conductor units. The conductor unit consists of several layers of helically stranded copper wires, manufacturing tapes, insulation layers, screening tapes and protection sheath, as illustrated in Fig. 1.

There are two types of power cables according to the characteristics of external loads acted on the cables during the service period. The power cable is called static if variation of the external loads is negligible. The structural design criteria of the static power cable mainly focus on strength of the cross section, such as minimum bending radius, maximum breaking tension and maximum allowable elongation, etc. The power cable is called dynamic if the external loads vary along with time and are characterized by certain statistical parameters. Design of the dynamic power cable will not only consider the factors for the static power cable, but also considering special capabilities related to the variational loads, i.e., fatigue performance of the load bearing components, including the tensile armor wire, copper conductor wires and protection tube for optic fiber components.

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