Session: 04-01-05 Flexible Pipes and Umbilicals V

Paper Number: 80962

80962 - Numerical Evaluation of the Thermal Performance of Modern Offshore Wind Farm Cables Installed in J-Tubes 

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. Current rating analytical calculations are to-date performed based on several old publications, such as [1] and [2]. These references suggest the use of semi-empirical formulae initially derived either for vertical annulus or even for vertical plates, while their applicability may be limited, as suggested by [2] for Rayleigh (Ra) number that must be lower than 2.3·10⁶. For the large, export cables and J-Tube deigns used today Ra number may be orders of magnitudes higher than 2.3·10⁶, thus rendering the use of those formulae questionable.

Other publications, such as [3], suggest 3-D heat transfer analysis to account for the longitudinal cooling mainly through the cable conductors and take advantage of the increased current rating, while suggesting a height limit for which 2-D analysis may be considered sufficient. However, the authors of [3] use the semi-empirical formulation suggested by [1] and [2] to represent the convective heat transfer mechanism inside the J-Tube.

More recent publications, such as [4], implement Computational Fluid Dynamics (CFD) analysis to represent the convection inside the J-Tube air section. Indeed, CFD results seem to be in good agreement with those published by [3]. However, Ra numbers in [4] are well below the upper limit of 2.3·10⁶ suggested by [2]. Additionally, the cable is treated in a quite simplified manner, i.e., ignoring any domains where convective heat transfer takes place, such as the profile (extruded) fillers used to give a more circular shape to the cable.

This paper evaluates the applicability of the semi-empirical formulae suggested in [1] and [2] 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. CFD coupled with heat transfer analysis is employed for this purpose, while simpler heat transfer models applying the semi-empirical formulae suggested by [1] and [2], 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. Interesting conclusions with regard to the applicability of the analytical methods suggested by [1] and [2] are derived, while guidance are given about how the fillers should be treated from the thermal point of view.

References

[1]    R. A. Hartlein, W. Z. Black, “Ampacity of Electric Power Cables in Vertical Protective Risers”, IEEE in Power Eng. Review, Vol. PER-3, no. 6, pp. 43-43, 1983.

[2]    George J. Anders, “Rating of Cables on Riser Poles, in Trays, in Tunnels and Shafts - A Review”, IEEE Trans. Power Deliv., vol. 11, no. 1, pp. 3–11, 1996.

[3]    R. D. Chippendale, J. A. Pilgrim, K. F. Goddard, P. Cangy, “Analytical Thermal Rating Method for Cables Installed in J-Tubes”, IEEE Trans. Power Deliv., vol. 32, no. 4, pp. 1721–1729, 2017.

[4]    L. You et. al, “Thermal Rating of Offshore Wind Farm Cables Installed in Ventilated J-Tubes,” Energies, vol. 11, no. 3, 2018

Presenting Author: Panagiotis Delizisis Hellenic Cables

Authors:

Panagiotis Delizisis Hellenic Cables
Charilaos Kokkinos FEAC Engineering
Dimitrios Chatzipetros Hellenic Cables
Dionisis Pettas FEAC Engineering
Andreas I. Chrysochos Hellenic Cables
Konstantinos Loukas FEAC Engineering