Derivation of Equivalent Soil Thermal Resistivity for EHVDC Cable Design in Layered Offshore Seabed Conditions

Accurate assessment of soil thermal resistivity is important for the reliable thermal design of buried EHVDC cable systems, especially along offshore routes where seabed conditions are variable and only measured at discrete locations. This paper develops a methodology to derive an equivalent soil thermal resistivity from layered offshore soil profiles for use in standard ampacity calculations and the tendering process. The method combines geotechnical interpretation, statistical characterisation of soil types, and a conformal-mapping-based thermal solver. The methodology is validated against finite-element simulations, showing good agreement, and is further evaluated through sensitivity studies on far-field soil conditions and spatial variability around the cable. The results indicate that far-field soil assumptions and small lateral variations have only a limited effect on the equivalent thermal resistivity, while greater uncertainty arises when high-resistivity layers, layer continuity, or missing measurements affect the soil conditions near the cable depth. The proposed framework provides transmission system operators with a transparent and reproducible basis for interpreting soil thermal data, supporting robust cable design, tender comparability, and offshore route assessment.