Thermal resistivity dry-out curve for cohesive soils based on parametric testing

Client challenge

Vysus Group was commissioned to assess Thermal Resistivity (TR) data collected for the inter-array cables (IAC) of Inch Cape Offshore Wind Farm in the North Sea, due to the presence of clustered TR results above the safe threshold for cable emplacement. The review followed ASTM D5334-14 guidelines to evaluate the testing and accuracy of all results.

The accuracy of the historical data was confirmed, however the extension of the clusters of higher TR could not be mapped through seismic data. As a result, the focus shifted to understanding the relationship between TR and Water Content (%WC) within the burial depth across the encountered soil units, including sands, soft clays and over-consolidated clays.

This behaviour was investigated through the development of dry-out curves, providing insight into how changes in %WC influence soil thermal performance.

How we helped

• Problem Definition: Clusters of elevated TR values posed a potential constraint on cable thermal performance. Mitigation options included either developing a robust understanding of the soil’s thermal behaviour or reducing the depth of cable lowering to avoid clay layers. In practice, reducing burial depth was not always feasible due to lateral soil variability and safety concerns related to seabed disturbance.
• Experience-Led Testing Strategy: Where high-TR soils could not be avoided, Vysus designed a targeted parametric testing programme. This involved laboratory testing of reconstituted soil samples at constant dry densities and varying proportions of in-situ %WC (e.g. 25%, 50%). The controlled approach enabled consistent assessment of thermal behaviour across different soil types.
• Clearer Path to Mitigation: The testing programme allowed the development of dry-out curves and identification of the critical %WC for each unit, below which thermal resistivity increases sharply and non-linearly. These curves provided a transparent framework for benchmarking and validating design TR values derived independently using statistical approaches.

Successful Results

• Increased Client Confidence: It was concluded that the magnitude of the critical %WC were unlikely to be reached in offshore saturated soils, therefore providing confidence in the recommended TR values that were subsequently applied across different sections of the IAC routes.
• Clear Understanding of Study Limits: While parametric testing improves understanding of thermal behaviour in offshore soils, predicting the rate of water loss due to cable heating - and its subsequent replenishment - was beyond the scope of this study. Further research was recommended to explore these processes under in-situ offshore conditions.