Mechanical characterization of lead alloys for subsea high voltage power cable applications

Abstract

Subsea power cables are an important component in electrical grids, connecting both offshore facilities to the mainland and interconnecting countries through sea stretches, for lengths up to hundreds of kilometres. Such cables ideally consist of three components: the conductor, copper or aluminium, the insulation, mass impregnated oil-paper or crosslinked polyethylene and the armouring, to withstand axial and radial loads. Cables for the use with voltages in excess of 52 kV require a sheathing layer of a stable, watertight material, to prevent electrical failure of the powerline due to water penetration through the polymer layers. The materials universally adopted for this purpose are lead alloys, due to their great chemical stability, ductility and ability to be continuously extruded for great lengths. The low melting point of lead, together with low self-diffusion activation energy and vacancy formation enthalpy make so that these alloys are affected by time dependent deformation, or creep, even at room temperature. It would certainly be of great advantage for the industry and the environment to gain a deeper understanding of the damage mechanics of these alloys, in particular the interaction between creep and fatigue phenomena, in order to acquire the necessary tools for a conscious estimate of their structural integrity, in contrast to what has been done traditionally. That is relying on experience and previous designs rather than scientific knowledge, thus leading to potentially overconservative designs naturally affected by excessive use of a potentially hazardous material with a consequent economic and environmental negative impact. The purpose of this thesis is to investigate the mechanical properties of a lead alloy commercially used for producing submarine power cable sheathing with a goal focused approach based on the special necessities of this industry. The work performed, here summarized in the form of the papers appended, consists in the study of the tensile properties at different strain rate with the influence of microstructure, the fatigue performance of full-scale cables and small scale specimens cut out from the component together with in situ observations of the monotonic and cyclic deformation behaviour, focused on the qualitative observation of the mechanisms causing deformation and damage. Due to the high ductility of the material tested, numerous practical challenges have arisen and the digital image correlation has been a fundamental tool for computing the strain experienced by the material. The results have been used for the calibration of material models able to account for the time dependent deformation behaviour and used for the full scale modelling of the component with the goal of connecting some of the design parameters with the stress state of the sheathing and, consequentially, with the full scale fatigue performance.