Thermal modelling and analysis for offshore submarine high-voltage direct current cable crossings

With an increasing number of offshore high-voltage direct current wind farm projects planned, submarine cable crossings become inevitable. It is important to accurately rate these circuits, because if high temperatures occur at crossing points, the cable may age prematurely. At present, the IEC60287-3-3 crossing rating method is inapplicable to such crossings due to a failure of the `Image´ theory, and it is challenging to carry out other analytical studies due to the difficulties in modelling the subsea thermal environment. As a solution, this study presents a new finite element analysis approach to study the thermal performance of submarine crossings under various continuous rating combinations and rock protection layer conditions, to provide guidelines for crossing operation. With focus on the free thermal convection mechanism within the rock layer, the significance of this mechanism is highlighted in a quantitative manner. It shows that when the free convection is preserved, the upper circuit can operate safely with its stand-alone rating regardless of the crossing; while the lower circuit may require a de-rating if its stand-alone rating is thermally limited.