Aerodynamic loads on groups of offshore wind turbine towers stored on quaysides during the pre-assembly phase

Abstract

Offshore wind turbine towers are pre-assembled and temporarily held in close proximity to each other in group arrangements on port quaysides during which time they are highly sensitive to wind action. Accurate estimates of the aerodynamic loads on the individual towers and on the overall group are therefore essential for the safe and economic design of the quayside’s supporting structures and foundations. Given the many possible group configurations, the key role played by wind direction and the limited literature on this topic, wind tunnel tests represent the main way to address this issue. The problem is that such tests tend to lead to overconservative designs due to inevitable mismatches in the Reynolds number, which is typically subcritical in experiments and transcritical at full scale. This crucial issue is dealt with here using an original engineering solution based on concentrated but discontinuous surface roughness, which allows the satisfactory simulation of the mean loads occurring at high Reynolds number. This case study assumes slender wind turbine towers with a height of 115 m, and for the sake of generality, the investigations focus principally on a cylindrical shape rather than the more complex real-world geometry. The constant diameter of the towers is determined based on the theoretical equivalence of the mean overturning moment. The rationality of this procedure is verified a posteriori using a set of measurements on the real-shape towers. The experiments show a regular behavior of the maximum mean base shear force and moment for towers arranged in double-row groups, while the results are more complicated for the heavily loaded single-row groups; indeed, despite the simulated transcritical regime and the turbulent wind profile, biased flow sometimes occurs in symmetric or nearly-symmetric configurations. Dynamic loads are also inspected, and gust factors in good agreement with Eurocode 1 prescriptions are found. Several parametric studies are carried out, the most extensive of which is devoted to assessing the role of tower height. A complicated non-monotonic pattern of the load coefficients with the tower height is encountered; therefore, the use of simple correction coefficients for practical design purposes must be handled with care. Moreover, the analysis reveals end-effect factors non-negligibly higher than those suggested by Eurocode 1 and ESDU 80025.