Submarine, export cables behave, to some point, as long, flexible cylindrical bodies. Their mechanical performance is crucial during laying and operating processes, which depends to a large extent on their stiffness. Although theoretical methods, used to estimate cable stiffness, are currently available, it is difficult to account for the various physical mechanisms involved, such as internal friction, residual torsion and ‘relaxation’ effects. These mechanisms are expected to affect cable stiffness and should be included some way. To represent more realistically cable stiffness, full-scale tests are performed in this paper. The deviation between theoretical and experimental values appears to be significant in certain cases: hence, non-realistic values for cable stiffness would occur if the stiffness estimation relied only on the theoretical methods. Interesting results, affording an in more depth insight and allowing for a better understanding of the cable mechanical performance, are presented in this paper.

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.

For an installed submarine power cable, fatigue is one of the main causes of failure. For static power cables there are cases such as free spans, spans between touch-down point (TDP) and J/I-Tube bell mouth and repair/sea joint procedures that may lead to failure due to fatigue as a result of Vortex-Induced Vibrations (VIV). Similar failure modes are applicable to dynamic cables since they are exposed to current flows for their expected operational life of more than 25 years. Although much research has been done in the field of oil and gas for VIV phenomena on flexible pipe lines, there is no extended and in-depth corresponding literature on submarine power cables. The scope of the present work is to investigate and demonstrate the effect of parameters such as Strouhal number, reduced bandwidth and added mass coefficient on the predicted VIV behavior of submarine power cables, both static and dynamic. The standard industry software SHEAR7 is used in order to assess the effects of each of these parameters. Finally, a comparative study was performed with the results of a scaled down experiment, which took place in the basin of Maritime Research Institute of Netherlands (MARIN) within the scope of the Horizon 2020 (H2020) FLOTANT project and studied the response of a towed dynamic power cable.