Aero-elastic loads on a 10 MW turbine exposed to extreme events selected from a year-long Large-Eddy Simulation over the North Sea

This article describes a study in which the loads are assessed as calculated on a 10 MW wind turbine in response to extreme wind events on the North Sea. The turbine on which the loads are calculated is the 10 MW Reference Wind Turbine as designed in the EU project AVATAR (Sieros, et al., January 2015). The study is carried out within the Dutch national project DOWA in a cooperation between ECN part of TNO and Whiffle. The extreme wind events have been selected from a year-long simulation with the operational LES code GRASP from Whiffle (Gilbert, et al., 2019). GRASP is an atmospheric LES model nested in a global weather model which allows the detailed modelling of meteorological phenomena on a spatial and temporal grid resolution which is fine enough for aero-elastic load calculations. The resulting extreme wind events are then fed as wind input to the aero-elastic solver PHATAS from WMC (now LM) as used by ECN part of TNO (Lindenburg, 2005). PHATAS is coupled to the AeroModule which is a code with two aerodynamic models, a Blade Element Momentum (BEM) method and a Free Vortex Wake (FVW) method AWSM (Boorsma, Grasso, & Holierhoek, 2012). The calculated loads as response to these extreme wind events are compared with the loads from a reference design load spectrum which is available from the AVATAR project (Stettner, et al., April 2015) This reference design load spectrum is calculated with a conventional procedure along the IEC standards. By comparing the loads in response to the extreme events with those from the conventional design load spectrum, the importance of extreme wind events can be assessed for practical (load) purposes. The structure of the present article is as follows In section 3 a short description is given of the turbine on which the load calculations are performed. It also describes the location where the turbine is located. Section 4 describes the load modelling of extreme wind events. It explains the GRASP model and the selected extreme events with the validation using measurements. It also describes the rotor modelling from PHATAS and AeroModule and the interface between PHATAS and GRASP. Section 5 describes the calculation of the reference design load spectrum. In section 6 the comparison between the loads from the extreme events and those from the reference spectrum is given together with an evaluation of results. Special attention will be given to the analysis of results at an extreme Low Level Jet, since these events are often believed to have significant impact on turbine loading, see e.g. (Duncan, November 2018) Finally, in section 7 the conclusions and recommendations are given.