Abstract
The fatigue assessment of mooring lines for floating offshore wind turbines represents a challenging issue not only for the reliable design of the stationkeeping system but also for the economic impact on the installation and maintenance costs over the entire lifetime of the offshore wind farm. After a brief review about the state-of-art, the nonlinear time-domain hydrodynamic model of floating offshore wind turbines moored by chain cables is discussed. Subsequently, the assessment of the fatigue damage in the mooring lines is outlined, focusing on the combined-spectrum approach. The relevant fatigue parameters, due to the low- and wave-frequency components of the stress process, are estimated by two different methods. The former is based on the time-domain analysis of the filtered stress process time history. The latter, instead, is based on the spectral analysis of the stress process by two advanced methods, namely the Welch and Thomson ones. Subsequently, a benchmark study is performed, assuming as reference floating offshore wind turbine the OC4-DeepCWind semisubmersible platform, equipped with the 5 MW NREL wind turbine. The cumulative fatigue damage is determined for eight load conditions, including both power production and parked wind turbine situations. A comparative analysis between time-domain and spectral analysis methods is also performed. Current results clearly show that the endorsement of advanced spectral analysis methods can be helpful to improve the reliability of the fatigue life assessment of mooring lines.
Highlights
The reliability of long-term mooring systems for floating offshore wind turbines (FOWTs), deployed on intermediate and deep-water depth, is a key factor to move the offshore wind energy sector towards the commercialization phase, provided that in the last two decades several accidents, mainly due to mooring failures, occurred throughout the world [1]
Past research activities focused on a variety of topics involved in the fatigue analysis of mooring systems, among which were the following: the simultaneous presence of low- and wave-frequency components in the mooring line stress process [6], the occurrence of bimodal non-Gaussian random processes [7], the application of spectral analysis and rainflow counting methods for the fatigue analysis of mooring lines [8]
The paper focused on the assessment of the cumulative fatigue damage in the mooring lines of an FOWT, deployed on intermediate water depth, by time-domain and advanced spectral analysis methods
Summary
The reliability of long-term mooring systems for floating offshore wind turbines (FOWTs), deployed on intermediate and deep-water depth, is a key factor to move the offshore wind energy sector towards the commercialization phase, provided that in the last two decades several accidents, mainly due to mooring failures, occurred throughout the world [1]. Following the increasing demand for energy from renewable sources and the rapid development of the offshore wind technology, attention was paid to the design and analysis of stationkeeping systems for FOWTs deployed on intermediate and deep-water depths. Most of past research activities in this field focused on the design and optimization of mooring systems, with the main aim of reducing the installation costs of wind farms and making the offshore wind energy sector more competitive on the international market, in terms of the levelized cost of energy. Brommundt et al [9]
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