Integrated End-of-Warranty Assessment of the Borssele 220 kV Offshore Export Cables

North Sea to the Dutch mainland via four 220 kV XLPE submarine cable circuits. Approaching the end of its five-year defect notification period, a comprehensive end-of-warranty (EoW) assessment was conducted to confirm ongoing asset integrity and benchmark system performance against the original cable system design specifications. This EoW phase is also the final opportunity to attribute latent defects to the EPC contractor.

The EoW assessment adopted a holistic methodology, combining four years of operational data, such as distributed temperature sensing (DTS), advanced electrical integrity diagnostic (TDR), depth-of-burial surveys, and subsea structural inspection data, including cable protection systems (CPS). This multi-disciplinary approach provided early-life and end-of-warranty insight into the thermal and mechanical performance of the cable system and established a baseline for subsequent integrity management. Unlike traditional EoW practices that often rely on separate, time-limited tests, such as HV tests, partial discharge (PD), and sheath integrity checks, this integrated approach promotes a more predictive and preventive assessment philosophy.

Thermal analysis is employed to assess how the cable responds to operational loads over time.

The aim is to confirm that temperatures along the cable stay within safe limits. DTS enables continuous thermal profiling across multiple seasons, detecting gradual burial changes and localized thermal anomalies potentially linked to seabed mobility. Occasionally, short-term thermal events may occur due to temporary overloading or environmental fluctuations, and these are analyzed to ensure they do not compromise the cable’s integrity. This supports early intervention and enhances thermal management.

Electrical integrity is assessed using high-resolution time-domain reflectometry (TDR), which enables the detection and localization of electrical discontinuities such as impedance mismatches, joint degradation, and insulation faults along the cable.

Subsea inspections focus on evaluating the physical condition of the cable and its protection systems, identifying issues such as excessive bending or unsupported cable-free spans. Burial depth is measured to ensure the cable is sufficiently protected beneath the seabed. The approach involves comprehensive route depth-of-burial surveys compared to as-built profiles and visual inspections with ROVs. This aligns with advancing industry practices toward complete route verification at EoW.

Where risks are identified, whether thermal, mechanical, or burial-related, targeted mitigations are implemented based on correlated multi-domain data. This includes recalibrating thermal models, refining alarm thresholds, and adjusting operational parameters to reflect actual site conditions. The approach goes beyond static compliance checks by integrating real-time analytics and risk-based logic into the assessment process. This ensures that deviations are not only detected but also understood in context, enabling timely and justified interventions. It also provides evidence of asset condition at the end of the defect notification period and establishes a route-referenced baseline for continued integrity management beyond warranty.

The end-of-warranty (EoW) assessment of the Borssele export cable system, independently confirmed by a third party, verified that the cable system remained well within its design parameters during the EoW period, with all identified deviations either resolved or actively managed. The integrated EoW verification approach enabled a comprehensive, data-driven evaluation and established a framework for continued integrity management, targeted followup monitoring, and future differential assessment of the export cable system.