Analytical Thermal Rating Method for Cables Installed in J-Tubes

One possible thermal pinch point along the route of a wind farm export circuit is a J tube, commonly used to provide mechanical protection to cable sections between the sea floor and the offshore platform. Current ratings for such cable sections are not covered by the scope of IEC 60287, while the existing publications covering such systems have limitations. This paper presents an updated 2-D analytical method and a 3-D extension for the rating of J tubes with short air section lengths. Continuous rating comparisons have been made against a 3-D finite-element model which shows a 4.5% variation in rating from the 2-D analytical model for air section lengths greater than 10 m, rising to 13% for short air section lengths. With the addition of longitudinal heat transfer within the new 3-D analytical approach, this variation decreases by 2.5%. Both methods proposed can be solved readily using conventional spreadsheet tools and are broadly compliant with the IEC 60287 methodology.