Maximum Loads on a One Degree of Freedom Model-scale Offshore Wind Turbine. (September 2016)

• Record Type: Journal Article
• Authors: Suja-Thauvin, Loup; Krokstad, Jørgen Ranum; Frimann-Dahl, Joakim Fürst
• Is Part Of: Energy procedia. Volume 94(2016)
• Page Start: 329
• Page End: 338
• Publication Date: 2016-09
• Subjects: extra-large monopile -- 50-year storm -- low damping; Power resources -- Congresses; Power resources -- Periodicals; Power resources; Conference proceedings; Periodicals; 333.7905
• DOI: 10.1016/j.egypro.2016.09.191
• Languages: English
• ISSNs: 1876-6102

Abstract

Abstract: This paper presents the results of an experiment carried out in a wave flume aiming at reproducing a 50-year wave condition on an extra-large bottom-fixed offshore wind turbine mounted on a monopile. The model is a stiff cylinder mounted on a spring allowing rotation of the system around its base only in the wave propagation direction. Under these conditions, the turbine is assumed to be idling, and the damping ratio of the system is 2.4%. The overturning moment at the base of the cylinder is measured, and it is found that the maximum responses are recorded when long steep breaking or near-breaking waves hit the cylinder and excite the first eigenperiod of the structure. For a selected event involving a breaking wave, the response of the system is compared to numerical simulations using the FNV method. The higher order excitation loads from the FNV are approximated as sinusoid pulse loads, and it is shown that since the duration of these pulses lies close to the eigenperiod of the structure, they suffice to trigger the first mode motion, without the need for a slamming model. A consequence of the low damping is that if the structure has been previously excited at its 1 st mode (linearly or by higher order phenomena such as springing), the structure can already have a motion that adds up to the transient response to the pulse loads. The findings of this study also challenge some of the load models currently used by the industry to estimate the response of offshore wind turbines during extreme events.