Gap height prediction for bolted ring flange connections based on measurements

Wind turbine towers and their flange connections are highly fatigue loaded structures. Previous investigations have shown that initial parallel gaps between L-flange connections, which are resulting from their respective flatness of the contact surfaces, can increase fatigue loads of the connecting fasteners significantly. Flange flatness and/or gap sizes in between flanges must comply with limitations given in relevant standards and therefore must be measured after the manufacturing process and/or during installation. More than 1900 flatness measurements of several offshore wind projects were collected and used to determine initial gaps between flanges analytically. A statistical evaluation allows to determine the distribution of expected gap sizes. The increase of gap height with increasing gap length can be described with linear correlations. Furthermore, the distributions of gap heights are approximated with log-normal distributions. Parameters for generic log-normal distributions are derived considering all performed mating simulations. Following the described methodology, gaps can be determined from any large number of flatness measurements. It is proposed to consider the results for flange design.