Structural Dynamic Behavior of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 606

The two processes of load prediction and structural design are highly interactive: loads
and deflections must be known before designers and stress analysts can perform structural
sizing, which in turn influences the loads through changes in stiffness and mass. Structural
design identifies “hot spots” (local areas of high stress) that would benefit most from
dynamic load alleviation. Convergence of this cycle leads to a turbine structure that is
neither under-designed (which may result in structural failure), nor over-designed (which
will lead to excessive weight and cost).
This chapter introduces some of the physical principles and basic analytical tools
needed for the structural dynamic analysis of a horizontal-axis or vertical-axis wind turbine
(HAWT or VAWT). This is done through discussions of two subjects that are fundamental
building blocks of our understanding of the structural-dynamic behavior of wind turbines:
-- single-degree-of-freedom dynamic load model of a HAWT blade, following the
development of the FLAP computer code at the National Renewable Energy
Laboratory [Thresher et al. 1986, Wright et al. 1988]
-- theoretical and experimental analysis of the vibration modes of a VAWT
system, following methods developed at the Sandia National Laboratories
[Carne et al. 1982]
These discussions are written for engineers familiar with structural mechanics, but no
specific knowledge of wind turbine rotors is required. Other technical disciplines with
which the wind turbine structural dynamicist should become familiar are aerodynamics
(required to determine wind forces and aeroelastic effects), rotor dynamics (with its own
special set of accelerations and responses), statistics (required in dealing with wind
turbulence and fatigue life prediction), mathematical modeling , and testing.
Role of Structural Dynamics in the Overall System Design
The design cycle in a typical wind turbine project is usually divided into concept
definition and detail design stages. During the concept definition period trade studies are
conducted to determine the overall configuration. The structural dynamics engineer is
concerned most with the relative load levels and the operational and technological risks
associated with the various design alternatives. Initially, only the feasibility of each concept
is judged, with little time spent on refinements. For example, aeroelastic stability might be
addressed only for unconventional concepts where experience is lacking.
In the detail design stage the structural dynamicist must furnish the design loads and
determine design and operational requirements that insure acceptable dynamic behavior.
These requirements might include drive train damping and spring rates, balance tolerances,
stiffness ranges for bearings, maximum operating wind speed, allowable yaw misalignment,
yaw rate limits, etc. , all based on the results of analysis using various mathematical models.
The key objectives that the design team seeks to achieve by relying on the guidance
and skill of the structural dynamicist include the following:
-- to select a configuration and design approach which alleviate dynamic loads
-- to minimize the sensitivity of dynamic responses to inevitable differences between the
“as-designed” and “as-built” physical properties of the structure
-- to place natural frequencies favorably with respect to turbine operating speeds
-- to insure aeroelastic stability, without which safe operation is impossible
Accomplishment of the first of these objectives must be based on the team's collective
understanding of the dynamic behavior of various wind turbine configurations. It is guided