Detecting Damage in Grouted-joints of Wind Turbine Support Structures —Application to a Large-scale Experiment

Abstract

SHM is a growing field for new and ageing infrastructure around the globe. Not least, since revenues for industry, operators and insurances are becoming increasingly obvious. Offshore wind turbines (OWT) represent an ideal application case – Being expensive, critical infrastructure with a remote, sometimes inaccessible, location, SHM can help to increase availability and reduce costs for operation and maintenance. Furthermore, with a tall slender structure and a large head mass, WT have the tendency to an extensive dynamic behavior, facilitating vibration based SHM. In the field of SHM for OWT and WT, diverse research has been carried out and some industrial applications e.g. for ice detection on rotor blades exist. In the offshore wind industry, grouted connections between piles and support structure have been adapted from the oil and gas industry. Nevertheless, the dynamic loading differs significantly form the one at large offshore platforms in the oil and gas industry and a deep understanding for damage initiation, progress and consequences is not yet fully achieved and under investigation [1]. Nevertheless, most research deals with axial loading, typical for Jacket structures, and the favored solution for OTW being monopoles with a loading dominated by bending. The presented large scale tests in the test center for support structures in Hannover Germany focus on damage in such connections. After investigating the influence of scour, marine growth and loosened bolts in the preceding project UnderwaterINSPECT [2], the current project QS-M Grout focusses on damages at grouted connections. For this purpose, two grouted connections in transition pieces (TP) are tested and damaged in the current project. Both, in a sand pit with a pile as footing and in a span where damage relevant cyclic loads are applied to the TP. The paper focusses on two main points: One is the influence and decrease of the TP stiffness along with cyclic loading, the other is the damage detection through vibration based SHM procedures. In the second case different condition Parameters will be compared to each other to evaluate the SHM-performance. The two TP were initially placed into the sand pit, where a reference load was applied by an electrodynamic shaker, mounted on top of the structure.