With the expansion of offshore wind farms and oil platforms, industrials are concerned about J-tubes and the thermal limiting point imposed to submarine cables installed inside. Nowadays, current rating of such cables is not covered by the scope of IEC 60287 [1, 2] and a de-rating coefficient of 0.88 is generally used to avoid overheating of cables, defined by ERA in 1988 . Furthermore, load cycles and weather conditions create unsteady thermal behaviour inside the offshore cable.
Currently only a few works deal with this situation by using time consuming finite elements calculation, in steady state and transient [4, 5, 6]. Other works uses simple models based on 1D energy balance, reliable only in steady state, which return conductors temperature exclusively [7, 8, 9].
In this paper, we introduce a model based on Lumped Element Method, which is convenient to simulate an energy cable in different environment, especially here in a J-tube. This method relies on electrical-thermal analogy by representing heat flux through thermal conductances, allowing us to have access to 2D temperature field in the cable and J-tube using thermal nodes. In a first step, we validate this model by comparison with previous work in steady state, for a 132Kv SL-type XLPE insulated cable . Then, we extend this work to transient by adding thermal capacitances to each node, and we study the cable thermal behaviour depending on weather conditions (sun, wind, ambient temperature). In addition, based on IEC 60853 standards , we study the effect of load cycles on the offshore cable, which are very present in offshore windfarm due to the fluctuation in the energy production (wind variability). A comparison is made with thermal measurements through a thermocouple in the cable installed in a J-tube on an offshore platform.
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