Exerimental and numerical investigations on grouted connections for monopile offshore wind turbines

Tziavos, Nikolaos (2019). Exerimental and numerical investigations on grouted connections for monopile offshore wind turbines. University of Birmingham. Ph.D.

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Abstract

The future growth of the offshore wind (OW) energy sector greatly depends on the development of reliable substructures which will enable the utilisation of generators with larger capacity at greater water depths. Current substructures are solely relying on grouted connections (GC) to withstand the harsh offshore environmental conditions. The structural performance of the connections is vital, however during early days their design was mainly based on experience drawn from oil and gas (O&G) structures, limited experimental data and simplified analytical models. This led to unexpected settlements of monopile GCs owing to insufficient assumptions, lack of dedicated tests and absence of monitoring tools. Within this context the current research project aims to offer an improved understanding and a comprehensive insight on the structural behaviour of GCs with shear keys. Moreover, real-time monitoring of the joint’s condition has been carried out using acoustic emission (AE) monitoring.
On modern offshore GCs the loading regime is better described by reversed loading conditions. As such, in order to assess the influence of load reversal, a new set of experiments was conducted on tubular GCs. It was revealed that for the designed configuration the ultimate bending capacity of the tested GC was not reduced by the preceding reversed bending loads and the specimens survived the cyclic protocol. However, crushing at the tip of the shear keys was reported following the visual inspections owing to the large shear key ratio. The experimental campaign was complemented by detailed numerical studies employing finite element analysis (FEA). Several numerical models of varying geometries were validated with very good agreement against the author’s tests and experiments from the literature. For stockier cross-sections a cohesive-based modelling approach was suggested to model the grout confinement with accuracy. Furthermore, principal geometrical features related to shear key height and spacing were also evaluated and the robustness of the numerical model was proven for varying configurations. It is suggested that the presented numerical scheme and FE models could accurately supplement the design of GCs.
Ultimately, monitoring of the grout integrity during destructive testing by means of AE was benchmarked. The results are presented in the form of a pilot case-study which establishes the use of AE as an excellent alternative for assessing the condition of the grout. Damage patterns were traced by means of b-value analysis which was shown to be an excellent tool for crack detection. A parametric analysis revealed several key performance indicators (KPIs), which can be used for detection, assessment and evaluation of the GC condition.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):