Grouted connections for monopiles – limits for large wind turbines

A transition piece is used on monopile wind turbine foundations for transmitting the large moment from the wind on the tower. A grouted connection is generally adopted to connect the transition piece to the pile that is driven into the seabed. The grout connection has been successfully used in the offshore oil and gas industry on drilling and production platform jackets for the last 40 years. However the major difference for a wind turbine structure is that while the grout connection in jackets is mainly intended for large axial forces, the wind turbine grout connection is predominantly subjected to bending. The ability of the grout connection to transfer large moment is not fully understood due to lack of experiments and size effects. The theory that has been used for designing the grout is reviewed thoroughly here in order to understand the limitations. Shortfalls have been exposed in the prediction of transfer of moments. Industry best practice and current code guidelines require a thorough review on the reliability of grout connections transferring large moments given the problems reported recently for offshore windfarms. Although codal guidelines were revised in 2011, there are still some anomalies in predicting the behaviours of grouted connections. A simple theoretical model is presented here for large moment transfer for straight cylinders with grout connection between them. The study identified that urgent research is needed for analysing the behaviour of such connections that transmit large moments, in order to understand the size effects and fatigue effects.