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The submarine cables used to interconnect Offshore Wind-Farms (OWFs) with the mainland typically pass through large, metallic risers having a J-shape when approaching the Wind Turbine Generators (WTGs). These so-called “J-Tubes” consist of three main parts: water, air and platform sections. The air-section is typically sealed at both ends for practical reasons: its bottom is bounded by the water surface, while the top end by means of the armor hang-off cap. Hence, the cable may experience adverse thermal conditions within this enclosed section and the latter is likely to be the thermal bottleneck of the entire subsea route. Unfortunately, no Standardized method currently exists to calculate the current rating of cables installed in J-Tubes. This paper evaluates the applicability of the semi-empirical formulae existing in literature given the modern export cable and J-Tube designs presently used, which in many cases can give Ra numbers substantially higher than 2.3 · 106. Computational Fluid Dynamics (CFD) coupled with heat transfer analysis is employed for this purpose, while simpler heat transfer models applying the existing analytical methods are also used for comparison purposes. Furthermore, the cable is represented in a more realistic way and the convective heat transfer in the filler interior is also considered. As demonstrated in the paper, the use of analytical methods gives relatively good results for lower Ra numbers compared to the more realistic numerical models developed. However, the analytical methods may lead to very optimistic conductor temperature values when Ra becomes higher than 2.3 · 106, thus making their use rather questionable for larger J-tubes. |
