Impact of seabed properties on the ampacity and reliability of submarine HV cables

• Main Author: Emeana, Chinedu John
• Published: University of Southampton 2016
• Subjects: 621.31
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720148

The expansion of offshore renewable energy infrastructure and the need for trans-continental shelf power transmission require the use of submarine High Voltage (HV) cables. These cables have maximum operating surface temperatures of up to 90°C and are typically buried 1-2 m beneath the seabed, within the wide range of substrates found on the continental shelf. However, the heat flow pattern and potential effects on the sedimentary environments around such anomalously high heat sources in the near surface sediments are poorly understood. Temperature measurements from a 2D laboratory experiment representing a buried submarine HV cable are presented, and the thermal regimes generated within a range of typical unconsolidated shelf sediments - coarse silt, fine sand and very coarse sand are identified. Several experiments were carried out in a large (2 x 2.5 m) tank filled with water-saturated artificial (ballotini - spherical glass beads) and natural sediments (fine marine sand) with a buried heat source and 120 thermocouples to measure the time-dependent 2D temperature distributions. The observed steady state heat flow regimes and normalised radial temperature distributions were compared with outputs from corresponding Finite Element Method (FEM) simulations. The results show that the mechanisms of heat transfer and thus temperature fields generated from submarine HV cables buried within a range of sediments are highly variable. Coarse silts with ~10-13 m2 permeability, are shown to be purely conductive with 10-60 °C radial temperature distribution within 40 cm from a 60°C above ambient source. Fine sands with ~10-11 m2 permeability, demonstrate a transition from conductive to convective heat transfer at c. 20°C above ambient with 10-55 °C asymmetric temperature rise up to 1 m above a 55°C above ambient heat source. Very coarse sands with ~10-9 m2 permeability, exhibit dominantly convective heat transfer even at very low (c. 7°C) operating heat source temperatures and with 10-18 °C significant asymmetric temperature rise of the surrounding sediments over 1 m above an 18 °C heat source. The computed controlling thermal properties demonstrate a distinct variation of thermal diffusivity and conductivity within different sediment types; sandy (fine sands) sediments are about twice more effective at diffusing heat than muddy (coarse silts) sediments. The occurrence of convection heat transfer within high permeability sediments is an important insight that are currently neglected in the existing IEC 60287 standard for current ratings estimation. Significant convection supports more efficient heat transfer leading to reduced cable temperature, increased current ratings and ampacity, decreased degradation rates of cable insulation and thus increased life span and decrease manufacturing costs of submarine cables. Also the varying sediment thermal conductivity around submarine HV cables further implies that cables buried around sandy sediments will uptake heat more rapidly than in muddy sediments, which are also not considered in the existing IEC 60287 standards. In addition, these findings are important for the surrounding near surface environments experiencing such high temperatures and may have significant implications for chemical and physical processes operating at the grain and sub-grain scale as well as biological activity at both micro-faunal and macro-faunal levels.