Abstract
Currently, in the design standards for environmental sampling to assess long-term fatigue damage, the grid-based sampling method is used to scan a rectangular grid of meteorological inputs. However, the required simulation cost increases exponentially with the number of environmental parameters, and considerable time and effort are required to characterise the statistical uncertainty of offshore wind turbine (OWT) systems. In this study, a K-type jacket substructure of an OWT was modelled numerically. Time rather than frequency-domain analyses were conducted because of the high nonlinearity of the OWT system. The Monte Carlo (MC) sampling method is well known for its theoretical convergence, which is independent of dimensionality. Conventional grid-based and MC sampling methods were applied for sampling simulation conditions from the probability distributions of four environmental variables. Approximately 10,000 simulations were conducted to compare the computational efficiencies of the two sampling methods, and the statistical uncertainty of the distribution of fatigue damage was assessed. The uncertainty due to the stochastic processes of the wave and wind loads presented considerable influence on the hot-spot stress of welded tubular joints of the jacket-type substructure. This implies that more simulations for each representative short-term environmental condition are required to derive the characteristic fatigue damage. The characteristic fatigue damage results revealed that the MC sampling method yielded the same error level for Grids 1 and 2 (2443 iterations required for both) after 1437 and 516 iterations for K- and KK-joint cases, respectively. This result indicated that the MC method has the potential for a high convergence rate.
Highlights
Pollution due to energy production is harmful for our living environment; an alternative energy source is increasingly required
The Monte Carlo (MC) method presents significantly reduced computational cost at high dimensions because the theoretical convergence rate is independent of dimensions
The accumulated fatigue damage of the tubular joints of the jacket substructure of an Offshore wind turbine (OWT) was mainly due to the wind loads
Summary
Pollution due to energy production is harmful for our living environment; an alternative energy source is increasingly required. Offshore wind turbine (OWT) technology offers an opportunity in this field [1]. According to the Global Wind Energy Council (GWEC), the global wind turbine installation capacity was 539.1 GW by the end of 2017 and is estimated to reach 840.9 GW in 2022 [2]. Jacket-type substructures are commonly used for the OWT systems for water depths of 20 to 50 m. The tubular joint connections in jacket substructures are subject to approximately 109 load cycles during their designed lifetime of 20 years [3] and are the critical locations that may undergo fatigue failure. Time-domain rather than frequency-domain analyses for OWTs are typically used to handle
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