Seismic risk analysis of wind turbine support structures - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 714

p

= ∑ 1

( ) = ( )

γ xw ,, q θ h xw ,

[26.2]

k k ki ki

i

normalized explanatory functions that might
be signifi cant in correcting ˆ k ( x , w ), and p

where θ k = [ θ ki ], h ki ( x , w ) =

= the number of unknown

model parameters. The model parameters, σ k ), can be estimated

following a Bayesian approach using fi eld, laboratory, and/or virtual
(simulated) data.

Θ k = ( θ k ,

26.3 Demand models for the support structure of
offshore wind turbines
In this section we develop probabilistic deformation, shear, and moment
demand models for horizontal axis offshore wind turbines rated between
0.5 and 5 megawatts (medium to large wind turbines). The wind turbines
of interest in this chapter are supported by a tubular steel tower, which is
seated on a steel mono-pile foundation at the base and installed in water
depths less than 30 meters. We predict deformation, shear, and moment
demands on the support structures subject to seismic excitation, in addition
to wind, wave, current, and turbine operational loadings.

26.3.1 Deterministic demand model
An ideal deterministic model ˆ k ( x , w ) should be simple and yet accurate,
and commonly accepted in practice. Because of these reasons, Mardfekri
and Gardoni (2012) used FAST (Fatigue, Aerodynamics, Structures, and
Turbulence) to compute deterministic predictions of the deformation, shear
and moment demands on the support structure of wind turbines subject to
wind, wave, current, and turbine operational loadings. FAST is a compre-
hensive aeroelastic simulator capable of predicting both the extreme and
fatigue loads of two- and three-bladed horizontal-axis wind turbines. FAST
employs a combined modal and multibody dynamics formulation. The
structural model consists of nine rigid bodies (earth, support platform, base
plate, nacelle, armature, gears, hub, tail, and structure furling with the rotor)
and fi ve fl exible bodies (tower, three blades, and a drive shaft) that relates
through 24 degrees of freedom (Jonkman and Buhl, 2005). Internally, FAST
uses the computer program AeroDyn (Laino and Hansen, 2002) to compute
the aerodynamic forces on the rotating blades. In FAST, the support plat-
form is modeled as a rigid body with six degrees of freedom that allows
applying external loads other than wind (i.e., wave, current and earthquake)
on the support structure.
For a given value of the mean wind speed and turbulence intensity, a time
history of wind speed is generated internally to FAST by TurbSim and used