Estimation of Seismic Load Demand for a Wind Turbine in the Time Domain: Preprint

Abstract

UL 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON a. REPORT

Seismic analysis of offshore wind turbines on bottom-fixed support structures

“…In particular, considering that the results of load case LC3 do not seem affected by the rotor position, a 0 • parked state is assumed. For a given load case, two simulations are carried out for each earthquake, in agreement with the studies by Prowell et al [9,10] on land-based HAWTs. The two simulations differ as the two horizontal components of the earthquake are rotated 90 • , in order to reduce bias from the orientation of the earthquake components relative to the wind direction [9,10].…”

“…Wind and wave environmental states, water depth and soil profile are set in agreement with similar theoretical studies on offshore HAWTs [18,19]. Consistently with the approach followed for land-based HAWTs [9][10][11], the seismic response is investigated for a set of real earthquake records taken from existing databases [20,21], with different frequency content and intensity. Fully coupled nonlinear time-domain simulations are carried out on full system models including nonlinear soil stiffness, using the BLADED software package [22], certified by Germanischer Lloyd for design of wind turbines.…”

“…Figure 4a shows the 5% damped acceleration response spectrum, computed as the maximum square root of the sum of the squares (SRSS) of the acceleration responses under the two horizontal components [10]. It can be seen that many frequencies of Tripod and Jacket, reported in table 1, fall within the range of significant spectral accelerations (for reference, figure 4a includes only the periods of the first and second FA support structure modes, for fixed FM: Following the approach by Prowell et al [9,10], who have extensively studied the seismic response of a land-based NREL 5 MW HAWT, seismic effects are investigated in different scenarios, considering three load cases: LC1 = Earthquake loads and operational wind-wave loads, for a wind speed at hub height V hub = 11.4 m s −1 , a wave period T p = 9.5 s and a significant wave height H s = 5.0 m. LC2 = Earthquake loads and emergency stop loads, for a wind speed at hub height V hub = 11.4 m s −1 , a wave period T p = 9.5 s and a significant height H s = 5.0 m. It is assumed that the emergency stop is activated as the nacelle acceleration exceeds 1 m s −2 . This value is much higher than the nacelle accelerations due to the considered environmental state and is in agreement with the emergency stop nacelle acceleration used for land-based HAWTs [9,10].…”

“…For both Tripod and Jacket, modal damping ratios are set equal to 10 −2 for support structure modes, and 4.775 × 10 −3 for the blades modes [13]. The simulation length is 800 s, with the earthquake ground motion starting 400 s into the simulation, to ensure that the earthquake occurs as the structural response has already attained a steady state [9,10].…”

Fragility analysis of steel and concrete wind turbine towers

“…As a lower-bound, a nacelle ac celeration of Im/s^ was investigated. Prowell et al (2010) suggested this value as the point a t which an emergency shut-down could be induced in the wind turbine system in order to avoid severe damage to sensitive equipment. It was found th a t each record in the suite of ground motions induced this minimum acceleration in all 6 towers.…”