Extreme mooring tensions due to snap loads on a floating offshore wind turbine system

Many floating offshore wind turbine (FOWT) concepts are light displacement platforms moored in shallow water and exposed to significant storms with high winds. Shock loads (also called snap loads) on the mooring lines can occur when an FOWT experiences large wave- and wind-induced motions. A typical snap event is characterized by a temporary slackness in the line followed by a sharp spike in tension whose magnitude can considerably exceed typical values of local tension maxima. In this study, we investigate seven experimental tests of a moored FOWT under survival storm conditions (i.e., a 100-year storm). For all these cases, the significant wave height and peak wave period are held constant at 10.5 m and 14.3 s respectively, while the wind speed is varied from 0 to 30.5 m/s. The wind conditions range from a steady wind to turbulent wind characterized by a chosen spectrum. Several snap events are found to result in the windward mooring line. The duration of these events range from 7.5 to 10.5 s, and the maximum tension values recorded are 37-68% higher than the corresponding cyclic non-snap tension maxima. The dynamic tension in the measured time histories are separated into snap-induced events and those that are not associated with a snap event based on criteria specified in marine operation practices; probability distributions of these separate events are analyzed. The exceedance probability curve of the dynamic tension in the higher ranges contributed to by snap-induced tension shows different characteristics compared to the lower tension range values that are related to the cyclic dynamic tension. There appears to be a clear demarcation point for this change in the probability curve characteristics. We propose a composite Weibull probability distribution for the mooring line dynamic tension that incorporates the effects of snap events. The model is composed of two Weibull distributions with different characteristics on either side of a transition tension value, and whose parameters are estimated from test data. The transition tension is related to the maximum cyclic dynamic tension in an extreme storm event, as specified by recommended practices. The proposed distribution model provides a good fit to the measured tension data, particularly in the extreme range. A design extreme value is developed for systems where snap loads are generally non-existent or are associated with very low probabilities of occurrence. When the shock load incidence probability is higher, the developed composite Weibull distribution model could offer a good starting point for the prediction of extreme dynamic tensions of a FOWT mooring system. (C) 2017 Elsevier Ltd. All rights reserved.