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. Accurate estimates of the aerodynamic loads on the individual towers and on the overall group are essential for the safe and economic design of the quayside's supporting structures and foundations. Wind tunnel tests represent the main way to address the problem, but results may lead to overconservative designs due to inevitable mismatches in the Reynolds number. This crucial issue is dealt with here using an original engineering solution based on concentrated but discontinuous surface roughness, which allows, for the first time in the case of finite-height towers arranged in groups and subjected to an atmospheric boundary layer flow, the successful simulation of the target high Reynolds number regime. 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 single-row groups, since biased flow sometimes occurs in symmetric or nearly-symmetric configurations. Dynamic loads are also inspected, and gust factors in good agreement with Eurocode 1's 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.