Abstract
Offshore wind turbines (OWTs) are deployed in harsh environments often characterized by highly stochastic loads and resistance properties, thus necessitating the need for structural reliability assessment (SRA) to account for such uncertainties systematically. In this work, the SRA of an OWT jacket-type support structure is conducted, applying two stochastic methods to predict the safety level of the structure considering various design constraints. The first method refers to a commercial finite element analysis (FEA) package (DesignXplorer© from ANSYS) which employs direct simulations and the six sigma analysis function applying Latin hypercube sampling (LHS) to predict the probability of failure. The second method develops a non-intrusive formulation which maps the response of the structure through a finite number of simulations to develop a response surface, and then employs first-order reliability methods (FORM) to evaluate the reliability index and, subsequently, the probability of failure. In this analysis, five design constraints were considered: stress, fatigue, deformation, buckling, and vibration. The two methods were applied to a baseline 10-MW OWT jacket-type support structure to identify critical components. The results revealed that, for the inherent stochastic conditions, the structural components can safely withstand such conditions, as the reliability index values were found acceptable when compared with allowable values from design standards. The reliability assessment results revealed that the fatigue performance is the design-driving criterion for structural components of OWT support structures. While there was good agreement in the safety index values predicted by both methods, a limitation of the direct simulation method is in its requirement for a prohibitively large number of simulations to estimate the very low probabilities of failure in the deformation and buckling constraint cases. This limitation can be overcome through the non-intrusive formulation presented in this work.
Highlights
In line with the quest to battle against the energy crisis and global climate change, wind energy is found to provide a technically and economically feasible solution, making it the most promising renewable energy technology in Europe and beyond (i.e., China, United States of America (USA), etc.) [1]
The loads relevant to offshore wind turbines (OWTs) support structures can be roughly classified into six groups, i.e., (1) aerodynamic loads transferred from the rotor, (2) wind loads on the tower, (3) inertia loads, (4) current loads, (5) wave loads, and (6) hydrostatic loads [24]
Methods start by defining the limit statesnon-intrusive taking account of the various design between a Both commercial package and a purpose-developed, stochastic formulation was constraints
Summary
In line with the quest to battle against the energy crisis and global climate change, wind energy is found to provide a technically and economically feasible solution, making it the most promising renewable energy technology in Europe and beyond (i.e., China, United States of America (USA), etc.) [1]. There is a limitation in the further growth of onshore wind farms as a result of visual impact and limited space available to deploy wind turbines. Most of the existing offshore wind turbines (OWTs) are installed on monopile foundations in shallow waters (depths < 50 m). State-of-the-art research on monopile foundations can be found in References [6,7] Space frame structures such as jackets, widely used in the petroleum industry, offer a lighter and yet stiff alternative to monopiles. Effective design of these structures from a turbine system dynamics perspective is a demanding task. Jacket-type support structures could contribute to further developments in the offshore wind industry, research is still required to support their basic design and analysis [5,8], as well as to take into account the requirement for optimization due to both the serial production required and the nature of these unmanned structures
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