Assessment of grouted samples from monopile wind turbine foundations using combined non-destructive techniques

The vast majority of offshore wind farms uses wind turbines on monopile foundations for cost effective designs. These foundations are complex structures consisting of steel and a high strength cementitious grout that fills the annulus between the two concentric steel pipes known as monopile and transition piece. The grouted connection is potentially prone to structural failures when subjected to harsh offshore conditions due to combinations of extreme wind and wave excitations. Already grouting failures related to slippage of the transition piece relative to the monopile due to weakening of the adhesion between the grout and the steel have been observed at several windfarms during the construction phase. Therefore, a thorough investigation of the grouted connection is of utmost importance. In this study, a large population of cementitious cores were sampled from as many as four offshore wind turbines. The samples were subjected to Ultrasonic Pulse velocity (UPV) non-destructive testing (NDT) and compression testing while a smaller set of samples was subjected to X-ray analysis as well. This paper presents the results of the combined use of the UPV and X-ray techniques as well as correlations with compressive strength results and critically discusses the possibility of using the techniques for in situ application. To the authors' knowledge it is the first time that these techniques are applied for assessment of the grouted connection of offshore wind turbines on monopile foundations.