Fatigue Safety Factors for Dynamic Power Cables

Abstract

In contrast to traditional power cables for bottom-founded offshore wind turbines, power cables for floating wind turbines penetrate the water column and are exposed to cyclic loading. Due to the dynamic nature of the cable loading, the cables are prone to fatigue failure. Since the evaluation of the fatigue life of complex structures, like power cables, has to deal with a certain degree of uncertainty, fatigue safety factors are introduced. These factors ensure a desired probability of failure based on the degree of uncertainty present in the modeling of the fatigue life. Limited experience in the evaluation of the fatigue life of dynamic power cables is found in the literature, and no sources have been found attempting to calibrate the prescribed fatigue safety factor. Currently, a fatigue safety factor of 10 is advised. This implies that there is much room for reduction in the uncertainties, meaning that there is a potential for making the cables cheaper and more reliable, which will in turn make floating wind energy more competitive with other (renewable) energy sources.
Due to the lack of available literature, comparisons with umbilicals and flexible risers are made. Different methods for the evaluation of fatigue safety factors for flexible risers are evaluated, and judged on how applicable they are to use on dynamic power cables. It is found that a reliability based approach is most suitable for this purpose.
A case study is carried out for a presently relevant scenario to put the proposed method into practice. It is concluded that for a safety class corresponding to a required maximum probability of failure of 10-3 in the last operational year, a fatigue safety factor of 3.5 is needed. This safety factor is required for the wave-load induced fatigue, as opposed to the seabed induced fatigue, to which the dynamic power cable is less vulnerable nearby the touch-down zone. 
The main parameter driving the uncertainty in the fatigue analysis is the sensitivity of the local stress analysis, which is in line with what has been found for similar studies for the oil & gas industry. In order to bring down the cost of dynamic power cables, and make them more reliable, more accurate local stress analysis models need to be developed, being validated against test data, in order to reduce the standard deviation of the local stress computation.