Abstract
The global transition towards renewable energy has led to a rapid expansion of offshore wind power, placing increasing demands on the performance and reliability of subsea power cables. As installations move into deeper waters, the need for dynamic cables has increased, while the continuous upscaling of power transmission requires higher operating voltages. This, together with environmental and health concerns associated with the traditional lead sheath, has driven the development of wet-design cables. While the lead free design improves sustainability and dynamic mechanical performance, it also introduces challenges related to degradation of the dielectric insulation system, the most critical being water treeing. Water treeing occurs in cross-linked polyethylene (XLPE) under the combined influence of moisture and electrical stress. Among the different types, vented water trees, originating at the interfaces between the semiconductive (SC) screens and the XLPE insulation, are particularly critical, as they can ultimately lead to premature failure of entire cable sections. Although extensively researched, the mechanisms governing their inception in modern wet-design cables remain insufficiently understood. Addressing this knowledge gap is the primary objective of the present work.
The initial phase of the study investigated the inception sites and surrounding regions of vented water trees in a laboratory-aged high voltage wet-design cable. A comprehensive analysis of the microstructure revealed a consistent correlation between degradation structures inside the SC screens and water tree inception sites. Specifically, voids with diameters ranging from 15 to 200 μm were observed within the SC bulk. The voids were connected to the water tree inception sites by nanostructured tracks exhibiting a porous morphology. Crystalline NaCl was identified within the degradation structures by a combination of chemical and structural analyses. A spatial overlap between Si and O surrounding the pores that constitute the nanostructured tracks was observed. Absorption bands characteristic of Si-O and Si-C bonds were identified through Fourier transform infrared spectroscopy (FTIR).
To assess the spatial distribution of inorganic impurities on a larger scale, a synchrotron wide-angle X-ray scattering (WAXS) technique was developed. This approach confirmed NaCl as the dominant crystalline impurity, spatially confined to the voids, nanostructured tracks, and water trees. Crystalline NaCl was also observed within the SC screen of an unaged section of the same cable, and impurity extraction of the as-received SC compound confirmed the presence of NaCl prior to extrusion. Other crystalline impurities were effectively ruled out with respect to the observed degradation mechanism by the spatial mapping from synchrotron WAXS data.
To further investigate the prerequisites of the observed degradation structures, NaCl particles were embedded in disc-shaped model objects of the SC compound. Voids closely
resembling those observed in the cable formed around the embedded particles after accelerated wet ageing in deionised water. During wet ageing, water vapour diffuses into the SC compound and condenses on the hygroscopic NaCl, forming a concentrated electrolyte. The resulting osmotic pressure is sufficiently high to generate voids of the observed size. Consequently, void formation was attributed to the deliquescence of NaCl and the associated osmotic pressure-driven volumetric expansion of the surrounding polymer.
The absence of nanostructured tracks in the disc-shaped model samples motivated a study of the effect of residual mechanical stresses, which are known to be substantial in extruded cable insulation systems. The mechanical stresses were achieved by rapidly cooling the SC model objects with embedded NaCl immediately after crosslinking. A finite element (FE) model of the discs was developed to simulate the experimentally applied cooling scenarios. SEM analyses revealed the presence of nanostructured tracks emanating from the voids after accelerated wet ageing. These tracks were predominantly located in regions of elevated shear stress, with the principal tensile stresses oriented orthogonally to the direction of track propagation. These findings established residual mechanical stress as the third prerequisite, alongside NaCl and water, for the formation of nanostructured tracks.
A two-step mechanism for the degradation of the SC screens was proposed based on the combined results from cable analyses, model experiments, and FE simulations. In the first step, hygroscopic NaCl particles within the SC compound undergo deliquescence upon exposure to elevated humidity beyond 70 %, forming a concentrated electrolyte. The resulting osmotic pressure drives local volumetric expansion of the polymer, leading to the formation of spherical voids that act as reservoirs for the electrolyte. In the second step, the combined action of residual mechanical stresses, ionic species, and the presence of liquid water promotes localised degradation of the polymer network at the void surface. We propose a mechanism where the presence of siloxane (Si-O-Si) bonds within the SC compound, inferred from spectroscopic analyses, provides a chemically susceptible pathway for this process. Siloxane bonds are prone to stress-assisted hydrolysis in the presence of an electrolyte, resulting in the formation of silanol groups and the development of nanoscale porosity. This process gives rise to the observed nanostructured tracks, which facilitate enhanced electrolyte transport to the SC screen/XLPE interface and ultimately lead to the inception of vented water trees.
This thesis elucidates the root causes of vented water tree inception in state-of-the-art wet-design subsea power cables, by combining a wide range of techniques and model samples. The work identifies NaCl contamination, present in the SC compound prior to extrusion, as the primary driver of degradation. Void formation and nanostructured track development follow as the key interacting processes. Hygroscopic impurities smaller than the size range monitored by the manufacturer were identified as critical for the observed phenomenon, highlighting the need for improved material cleanliness and detection methods. Furthermore, the demonstration that degradation can occur in the absence of an electric field provides new insight into the early stages of cable ageing. The results establish a direct mechanistic link between impurity-driven physicochemical processes in the SC screens and the subsequent inception of vented water trees, offering a scientific basis for improving cable reliability and lifetime through enhanced impurity control, optimisation of material formulations, and management of residual mechanical stresses.