Assessment of grouted connection in monopile wind turbine foundations using combined non-destructive techniques

Many large-scale offshore wind farm projects use monopile foundation to obtain a cost effective design. These foundations contain pile-sleeve connections that basically consist of two concentric steel pipes cast together by means of high strength grout. The harsh offshore conditions due to combined action of wind and wave excitations strongly influence the structural integrity as they can lead to potential detachment of the grout. Moreover, at several windfarms, during the construction phase it is noticed that the transition piece (sleeve) is slipping downwards relative to the monopile. This slippage is attributed to a weakening of the adhesion between the grout and the steel causing possible further damage to the grout during extreme operational conditions requiring mitigation to avoid further degradation. In order to eliminate the ambiguities and answer the question as to what extent the damage in the grout is significant for the structural integrity of the monopile, a thorough investigation of grout samples taken from the site is performed. For this purpose, four offshore wind turbines were selected for core sampling. All the samples were subjected to Ultrasonic Pulse Velocity (UPV) Non-Destructive Testing (NDT) and compression testing while a set of samples was also subjected to X-ray analysis. This paper presents the results of the combined use of the X-ray and UPV techniques as well as correlations with compressive strength outputs focusing on the parameters that seem promising for in-situ application.