Reliability of mooring lines and shared anchors of floating offshore wind turbines
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Copyright 2017 by the International Society of Offshore and Polar Engineers (ISOPE). The trend of offshore installations to move to deeper water further offshore has motivated exploration into the installation of farms utilizing Floating Offshore Wind Turbines (FOWTs). To address the high cost of FOWT anchors and their installations, multiline anchor configurations are proposed in which multiple turbines share a common anchor. Multiline anchors reduce the total number of anchors installed at the expense of the anchors requiring multidirectional load carrying capacity. The multiline anchor also introduces a system of interconnectedness throughout the FOWT wind farm network that may lead to potential cascading failure if components (mooring lines or anchors) of the network begin to fail. This research assesses the increased likelihood of component failures for the multiline FOWT network, and compares the likelihoods to the single line anchor case. Component failure probabilities are assessed through the use of a risk framework in which distributions of capacities and demands are sampled. The framework is then applied to a numerical example of a semisubmersible floating platform assessed at the design environmental conditions to assess intended levels of reliability of the numerical design. The probability of failure of mooring lines is shown to increase from 0.039%-0.085% for the single line case to 0.06%-0.42% for the multiline case, while the probability of failure of anchors is shown to increase from 0.0004%-0.0030% for the single line case to 0.002%-0.008% for the multiline case. The framework allows for further refinement of mooring line and anchor capacity and demand probability distributions, including multidirectional wind and waves as well as the effect of variability of soil conditions throughout the wind farm.
