Thermo-chemo-mechanical influences of impingement flow on the degradation of organic coatings in the underwater zone of offshore wind turbines

Most offshore wind turbines use monopile structures anchored to seabed. In the underwater zone of these structures, water attacks the surface of the steel monopiles which is protected by organic coatings. The influence of applied stresses due to the water flow on the degradation of organic coatings is the focus of this study. For this purpose, an impingement flow chamber was employed to resemble the situation of monopile structures exposed to the shallow water currents in underwater zone. Electrochemical impedance spectroscopy was utilized to measure the changes of barrier properties of coatings exposed to the impingement flow of a 3.5 wt% NaCl solution with a variety of flow rates. Computational fluid dynamics (CFD) modeling as well as the particle image velocimetry measurements were employed to calculate and validate the magnitude of applied stresses on the surface of the coated samples. Thermal activation theory, thermal elasticity laws, and the Fick’s laws were utilized to investigate and analyze the influence of stresses on coating properties. The comparison among the obtained electrochemical parameters under different flow rates shows the direct influence of stresses on the degradation of the coatings. The utilized methods, in addition to the developed analytical procedure, can potentially predict the behavior of organic coatings applied on offshore wind turbines in underwater zone. It was concluded that the applied stresses do not influence the apparent diffusion coefficient of water in coatings, but a higher flow rate increases the volume fraction of water and coating capacitance significantly.