Predicting long term performance of offshore wind turbines using cyclic simple shear apparatus

Abstract Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics. Cyclic behaviour (in terms of change of shear modulus change and accumulation of strain) of a typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long term performance prediction of OWT foundations.

[1]  Subhamoy Bhattacharya,et al.  Similitude relationships for physical modelling of monopile-supported offshore wind turbines , 2011 .

[2]  Subhamoy Bhattacharya,et al.  Dynamics of offshore wind turbines supported on two foundations , 2013 .

[3]  Subhamoy Bhattacharya,et al.  Dynamic soil–structure interaction of monopile supported wind turbines in cohesive soil , 2013 .

[4]  Subhamoy Bhattacharya,et al.  An innovative cyclic loading device to study long term performance of offshore wind turbines , 2016 .

[5]  D. Muir Wood,et al.  Observed dynamic soil–structure interaction in scale testing of offshore wind turbine foundations , 2013 .

[6]  Christophe Gaudin,et al.  Centrifuge study on the cyclic performance of caissons in sand , 2014 .

[7]  Christian LeBlanc Bakmar.  Design of Offshore Wind Turbine Support Structures: Selected topics in the field of geotechnical engineering , 2009 .

[8]  Subhamoy Bhattacharya,et al.  Dynamics of offshore wind turbines on two types of foundations , 2013 .

[9]  H B Seed,et al.  VOLUME CHANGES IN SANDS DURING CYCLIC LOADING , 1971 .

[10]  S. John Hogan,et al.  Simplified critical mudline bending moment spectra of offshore wind turbine support structures , 2015 .

[12]  Sumanta Haldar,et al.  Dynamic analysis of offshore wind turbine in clay considering soil–monopile–tower interaction , 2014 .

[13]  Martin Achmus,et al.  Behavior of monopile foundations under cyclic lateral load , 2009 .

[14]  Subhamoy Bhattacharya,et al.  Challenges in Design of Foundations for Offshore Wind Turbines , 2014 .

[15]  S. Bhattacharya.  Winkler Springs ( p - y curves) for pile design from stress-strain of soils: FE assessment of scaling coefficients using the Mobilized Strength Design concept , 2013 .

[16]  Subhamoy Bhattacharya,et al.  Soil–monopile interactions for offshore wind turbines , 2016 .

[17]  Lizhong Wang,et al.  Model Tests on the Long-Term Dynamic Performance of Offshore Wind Turbines Founded on Monopiles in Sand , 2015 .

[18]  Sebastian Thöns,et al.  Resonance phenomenon in a wind turbine system under operational conditions , 2014 .

[19]  Werner Rücker,et al.  On the quasi-static granular convective flow and sand densification around pile foundations under cyclic lateral loading , 2012 .

[20]  Subhamoy Bhattacharya,et al.  Experimental validation of soil–structure interaction of offshore wind turbines , 2011 .

[21]  John H G Macdonald,et al.  Closed form solution of Eigen frequency of monopile supported offshore wind turbines in deeper waters incorporating stiffness of substructure and SSI , 2016 .

[22]  Sumanta Haldar,et al.  Design of monopile supported offshore wind turbine in clay considering dynamic soil–structure-interaction , 2015 .