TY - JOUR

T1 - Bolted ring flanges in offshore-wind support structures

T2 - in-situ validation of load-transfer behaviour

AU - Weijtjens, Wout

AU - Stang, Andre

AU - Devriendt, Christof

AU - Schaumann, Peter

N1 - Funding Information: The current research was conducted in the frame of the O&O Nobelwind, a research project with the support of the agency Flanders Innovation Entrepreneurship. Wout Weijtjens is a Post-doctoral researcher at the Research Foundation Flanders (FWO). At the Institute for Steel Construction, the research work was conducted within the ForWind joint research project "ventus efficiens- Joint research for the efficiency of wind energy converters within the energy supply system". This project is financially supported by the Ministry for Science and Culture in Lower Saxony, Germany. The financial support and cooperation is kindly acknowledged. The current paper is an equal effort of Vrije Universiteit Brussels (VUB) and Leibniz University Hannover (LUH). In which the VUB executed and processed the long measurement campaign, LUH developed the FE-Models and derived the corresponding load transfer functions. The authors also want to show their gratitude to the people of Parkwind and Nobelwind for their support during this project.

PY - 2021/1

Y1 - 2021/1

N2 - Offshore wind turbines are subjected to cyclic loads from wind and waves, ultimately resulting in a fatigue-driven design. These cyclic loads result in cyclic shell forces above the bolted ring flange and ultimately into the cyclic loading of the bolt tension forces. To model the transfer of the shell forces into the bolt, Load Transfer Functions (LTF) are used, of which the tri-linear function according Schmidt/Neuper is the most known approximation but more contemporary approaches exist. As the LTF has a significant impact on the estimated fatigue lifetime of the bolt, it is relevant to investigate the accuracy of the currently used LTF. This paper uses in-situ monitoring data from three offshore wind turbines at the Nobelwind windfarm, each equipped with an instrumented bolt in the monopile to transition piece bolted flange connection and strain-gauges above said flange. Long-term monitoring data is used to derive empirical load transfer coefficients (LTC), i.e. the derivatives of the LTF. In parallel a finite element model (FE model) of the flange connection was developed. The found load transfer coefficients from measurements were compared to the values obtained from the function according to Schmidt/Neuper and the FE model. It is concluded that the observed LTC in the offshore wind farm were favourable compared to the values expected from Schmidt/Neuper. The FE model matched more closely, especially when the MP-flange inclination was included. The inclusion of the MP-flange inclination in the model had a positive effect on the final LTF, which was confirmed by the measurements.

AB - Offshore wind turbines are subjected to cyclic loads from wind and waves, ultimately resulting in a fatigue-driven design. These cyclic loads result in cyclic shell forces above the bolted ring flange and ultimately into the cyclic loading of the bolt tension forces. To model the transfer of the shell forces into the bolt, Load Transfer Functions (LTF) are used, of which the tri-linear function according Schmidt/Neuper is the most known approximation but more contemporary approaches exist. As the LTF has a significant impact on the estimated fatigue lifetime of the bolt, it is relevant to investigate the accuracy of the currently used LTF. This paper uses in-situ monitoring data from three offshore wind turbines at the Nobelwind windfarm, each equipped with an instrumented bolt in the monopile to transition piece bolted flange connection and strain-gauges above said flange. Long-term monitoring data is used to derive empirical load transfer coefficients (LTC), i.e. the derivatives of the LTF. In parallel a finite element model (FE model) of the flange connection was developed. The found load transfer coefficients from measurements were compared to the values obtained from the function according to Schmidt/Neuper and the FE model. It is concluded that the observed LTC in the offshore wind farm were favourable compared to the values expected from Schmidt/Neuper. The FE model matched more closely, especially when the MP-flange inclination was included. The inclusion of the MP-flange inclination in the model had a positive effect on the final LTF, which was confirmed by the measurements.

KW - (Offshore) Wind energy

KW - Bolted flange connection

KW - FE modeling

KW - Load transfer function

KW - Measurements in-situ

UR - http://www.scopus.com/inward/record.url?scp=85092341842&partnerID=8YFLogxK

U2 - 10.1016/j.jcsr.2020.106361

DO - 10.1016/j.jcsr.2020.106361

M3 - Article

AN - SCOPUS:85092341842

VL - 176

JO - Journal of Constructional Steel Research

JF - Journal of Constructional Steel Research

SN - 0143-974X

M1 - 106361

ER -