Modal Dynamics of Large Wind Turbines With Different Support Structures

Abstract

This paper presents modal dynamics of floating-platformsupported and monopile-supported offshore turbines, which are gaining attention for their ability to capture the immense wind resources available over coastal waters. Minimal dynamic loads and the absence of instability are imperative to the success of these turbines. Modal dynamics determine both loads and instabilities to a large extent, and therefore must always be analyzed. Also, to model the turbine, several aeroelastic computer codes require modes of the major components, e.g., the rotor blades and the rotor-nacelle support structure. To compute such modes, we used a recently developed finiteelement code called BModes. The code provides coupled modes either for the rotating blades or for the support structure. A coupled mode implies presence of coupled flexural, axial, and torsion motions in a natural mode of vibration. In this paper, we use BModes to provide modes only for flexible towers, which carry head mass (rotor-nacelle subassembly modeled as a rigid body) and are mounted atop either a floating platform or a soilsupported monopile. The code accounts for the effects of hydrodynamic inertia, hydrostatic restoring, and mooring lines stiffness on the floating platform. It also accounts for the distributed hydrodynamic mass on the submerged part of the tower and for the elastic foundation surrounding the monopile. Results are obtained for three turbine configurations: land-based turbine, floating-platform-supported turbine, and monopilesupported turbine. Three foundation models are used for the monopile configuration. Results show that the hydrodynamic and elastic-foundation effects strongly influence the turbine modal dynamics.

Hydraulic-electric hybrid wind turbines: Tower mass saving and energy storage capacity

“…The baseline tower condition (at zero tower mass savings) represents a design whose natural first mode frequency (f 1 ) is 0.32 Hz (shown by solid black line). This frequency has been checked against the rotor (1P) and blade (3P) rotational frequencies by NREL engineers, in order to avoid any resonance [10,11]. This frequency is 1.4 times larger than the rated rotor rotation frequency f R of 0.23 Hz (shown by dashed black line), which is calculated based on rotor diameter D rotor , tip speed ratio l ¼ 7.5, and average wind speed v ¼ 12 m/s.…”

“…Wind turbine towers are structurally designed so that they are not easily resonated by blade rotation; therefore the natural frequencies are important characteristics of tower design and safety. To predict the tower's natural frequencies, NREL employed finite-element codes called BModes and ADAMS [11] to determine modes of this baseline wind turbine tower assuming a fixed connection at the base. Two cases were considered: 1) without head mass, and 2) with head mass (including rotor and all nacelle contents).…”

Vibration control in wind turbines to achieve desired system-level performance under single and multiple hazard loadings

“…q i is the generalized displacement, and Q i is the generalized loading corresponding to the i th degree of freedom (DOF). The full‐scale reference 5‐MW National Renewable Energy Laboratory wind turbine was used in this study that the tower and blades characteristics are presented in Tables and …”

Full-scale modal wind turbine tests: comparing shaker excitation with wind excitation

“…The optimal design of a turbine is dependent on reducing vibrations, which can be done by accurately identifying turbine's natural frequencies and mode shapes [2]. By determining the modes of the wind turbine, it can be ensured that the turbine's operational conditions preclude resonant frequencies, thereby minimizing dynamic loads and lengthening the life of the turbine [2] [3]. In order to develop accurate structural models, extensive experimental testing must be done to refine and validate computer simulations.…”

“…Richard Osgood (1) , Gunjit Bir (1) , Heena Mutha (1) , Bart Peeters (2) , Marcin Luczak (2) , Gert Sablon (2) (1) INTRODUCTION During operation, wind turbines, like many engineering structures, are subject to dynamic loads, because the aerodynamic and gravitational loads vary with time as the turbine rotates [1]. The optimal design of a turbine is dependent on reducing vibrations, which can be done by accurately identifying turbine's natural frequencies and mode shapes [2].…”