Dynamics of offshore wind turbines supported on two foundations

Abstract

Offshore wind turbines are currently considered as a reliable source of renewable energy in the UK. These structures, owing to their slender nature, are dynamically sensitive at low frequencies, the first modal frequency of the system (less than 1 Hz) being very close to that of the excitation frequencies. The majority of operational offshore wind turbines situated in UK waters are founded on monopiles in water depths up to 30 m. For future development rounds where water depths are up to 70 m, alternative foundation arrangements are needed. To date there have been no long-term observations of the performance of these relatively novel structures. Monitoring of a limited number of offshore wind turbines has indicated a departure of the system dynamics from the design requirements. This paper summarises the results from a series of 1:100 scale tests of a V120 Vestas turbine supported on two types of foundation: monopiles and tetrapod suction caissons. The test bed used consisted of kaolin clay and sand. Up to 1 . 25 million loading cycles were applied to the scaled model, and the dynamic properties of the system were monitored. The results provide an insight into the long-term performance. Some interesting dynamic soil-structure interaction issues are identified and discussed.

Observed dynamic soil–structure interaction in scale testing of offshore wind turbine foundations

“…Small scale 1-g tests reported by Lombardi (2010), Bhattacharya et al (2012), , Cox et al (2011) showed that the natural frequency of a wind turbine system changes with cycles of loading. The main reason for this change is the alteration of the foundation stiffness due to strain-hardening or strainsoftening behaviour of the soil supporting the foundation owing to the loading cycles.…”

“…The testing apparatus and the methodology for monopile (Figure 4a) and tetrapod foundation (Figure 4b) can be found in Bhattacharya et al (2012). For the tetrapod structure, the model caissons were 7.4cm in diameter, 5.5cm deep and spaced at 40cm apart in two directions.…”

An innovative cyclic loading device to study long term performance of offshore wind turbines

“…Three months after the installation the natural frequency dropped from its initial value of 1.28-1.32Hz to 1.13-1.15Hz. Scaled model tests carried out by Bhattacharya et al (2012Bhattacharya et al ( , 2013aBhattacharya et al ( , 2013b, Yu et al (2014), Guo et al (2015), Cox et al (2014) indicated that natural frequency may change owing to dynamic soil structure interaction.…”

Serviceability of suction caisson founded offshore structures

“…To date a significant amount of research has been conducted into the serviceability of offshore foundations. The main focus of research has been into monopile foundations (LeBlanc et al, 2010a;Bhattacharya et al, 2012). More recently research has considered suction caissons focusing on scaled testing, modelling the monotonic caisson response (Feld, 2001;Houlsby et al, 2005;Villalobos, 2006), and the behaviour of a caisson subjected to a cyclic load (Byrne, 2000;Villalobos, 2006;Zhu et al, 2013).…”