No abstract

No abstract

The paper provides a critical appraisal of the events surrounding the unexpected settlement of large-diameter monopile foundations for offshore wind energy converters, and will detail experimentation that is to be undertaken to determine the extent of the wear failure mode in such structures. The potential for wear deterioration at the grout-steel interface has been caused by the lack of axial load transfer of the grout and steel and so relative movement between the grout and steel is occurring. Hence, a better understanding of this phenomenon is of key importance to determine the long term integrity of the structure. The remedial solution currently favoured within industry consists of the installation of elastomeric bearings, which however may further exasperate the wear due to its lower stiffness than steel contact currently occurring, also because of the presence of water in the grouted connection. The aim of the testing is therefore to identify the risk to offshore foundation integrity due to grout wear under conditions typically observed during life-time operation of offshore wind turbines. The main factors influencing the wear rate of grout will be identified, and the possibility to define indicators for the integrity of the grout that could be used for offshore inspection will be also considered.

Insufficient axial capacity of large-diameter plain-pipe grouted connections has recently been observed in offshore wind turbine substructures across Europe. Aimed at understanding the implications of this phenomenon, a campaign of structural condition monitoring was undertaken. The measurements showed significant axial displacements occurring between the transition piece and the monopile, which in turn resulted in a considerable amount of wear. Given the existing lack of technical data on the implications that this relative movement has on the wear of grouted connections, a methodology was developed to quantify the likely risk to the foundation integrity of the wear failure mode. The proposed approach consists of a numerical model which applies the wear rate derived from previous experimental testing to the conditions experienced by typical offshore grouted connections, as indicated by the wind turbine generators' supervisory control and data acquisition systems. The output of this model showed that, for a representative sample of the wind farm substructures analysed as a case study, the accumulated lifetime wear would be minimal in the majority of the grouted connection, i.e. less than 0.4 mm over 75% of the connection, but a much greater loss in thickness, of the order of 4 mm, was predicted at the very top and bottom of the connection. This assessment is based on the assumptions that no significant changes occur in the surrounding environmental conditions and that the degradation in the grouted connection does not significantly affect the dynamic response of the foundation structure over its life span. Importantly, these assumptions may affect the model's predictions in terms of cumulated wear over time, not in terms of identifying the individual connections to be prioritised when performing remedial work, which is indeed the main intended use of the model.

No abstract