Abstract
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 0.4mm over 75% of the connection, but a much greater loss in thickness, of the order of 4mm, 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.
Highlights
The concept of grouted connections has been extensively used in the oil and gas industry, and more recently in the offshore wind energy sector [1], as they offer an efficient solution to join the piles driven into the seabed with the top-side substructure, while accommodating significant installation tolerances [2]
Aimed at better understanding and quantifying the long-term implications of the grout wear failure in large-diameter plain-pipe grouted connections, a numerical model has been developed to predict the accumulation of wear in the grouted connections for the actual load conditions experienced over a given period
The proposed model has been derived and calibrated based on limited site Structural condition monitoring (SCM) and supervisory control and data acquisition (SCADA) data of two operational WTG substructures afflicted by wear of the grouted connections, along with experimentally-derived wear rates
Summary
The concept of grouted connections has been extensively used in the oil and gas industry, and more recently in the offshore wind energy sector [1], as they offer an efficient solution to join the piles driven into the seabed with the top-side substructure, while accommodating significant installation tolerances [2]. The available data has been transformed into relative displacements and compressive stresses within the grouted connections through relationships derived from SCM deployed on two WTG substructures and transfer functions based on structural analysis of the substructure Fed with this information, the proposed model provides an indication of the distribution of wear around the circumference and depth of the grouted connection, which will help to determine if further remediation work is going to be required within the remaining operational life of the WTG. This paper will briefly describe the development and calibration of the proposed numerical model and the experimentation used to derive wear rates It will present the results for a representative case study, showing the distribution of wear around the depth and circumference of the grouted connection, along with the variability of wear across a typical wind farm
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