Depth of cover Algorithm Validation in the Baltic Sea using time-lapse geophysical Surveys

Offshore power cables are typically buried in the seabed for their protection against human activities. Post-installation, the depth of cover (DoC) can vary significantly leading to potential exposure or over-burial of the cable. Where exposure occurs, the risk of external damage and cable fatigue in cases of free hanging cables significantly increases, whilst over-burial can reduce heat dissipation from the cable, resulting in a potential failure of the cable by overheating. Monitoring the spatial and temporal variation in DoC is therefore vital for its protection and long-term operational performance. Traditional approaches to monitoring are time-lapse (typically every 1-3 years) bathymetric surveys; this is expensive and is frequently performed at a much lower frequency than the potential rate of seabed change. A faster, cheaper, and sufficiently accurate alternative is based on the link between depth of cover and thermal dissipation; it provides a way of estimating changes through the combination of distributed temperature sensing (DTS) and physics-based thermal simulation. Beginning with the cable as built information including cross-section, material, nominal depth of burial and seabed parameters, together with the DTS data and the load data, a real time thermal rating (RTTR) based finite element modelling simulation has been developed to calculate conductor temperature Tc. A second computational step calculates the temperature distribution in the cable and the seabed, for a range of DoC’s, as a function of load, time, and distance. Such a concept was validated in real field conditions in an operating windfarm in the Baltic.