Calibration of Long-Term Time-Domain Load Generation for Fatigue Life Assessment of Offshore Wind Turbine

Abstract

This study evaluated, by time-domain simulations, the fatigue life of the jacket support structure of a 3.6 MW wind turbine operating in Fuhai Offshore Wind Farm. The long-term statistical environment was based on a preliminary site survey that served as the basis for a convergence study for an accurate fatigue life evaluation. The wave loads were determined by the Morison equation, executed via the in-house HydroCRest code, and the wind loads on the wind turbine rotor were calculated by an unsteady BEM method. A Finite Element model of the wind turbine was built using Beam elements. However, to reduce the time of computation, the hot spot stress evaluation combined FE-derived Closed-Form expressions of the nominal stresses at the tubular joints and stress concentration factors. Finally, the fatigue damage was assessed using the Rainflow Counting scheme and appropriate SN curves. Based on a preliminary sensitivity study of the fatigue damage prediction, an optimal load setting of 60-min short-term environmental conditions with one-second time steps was selected. After analysis, a sufficient fatigue strength was identified, but further calculations involving more extensive long-term data measurements are required in order to confirm these results. Finally, this study highlighted the sensitivity of the fatigue life to the degree of fluctuation (standard deviation) of the wind loads, as opposed to the mean wind loads, as well as the importance of appropriately orienting the jacket foundations according to prevailing wind and wave conditions.

Fatigue Life Analysis of Offshore Wind Turbine Support Structures in an Offshore Wind Farm

“…The concept of wind turbulence is explained in DNV [9] as "the natural variability of the wind speed about the mean wind speed U 10 in a 10-minute period" for which "the short-term probability distribution for the instantaneous wind speed U can be assumed to be a normal distribution" with standard deviation  U . The short-term ambient wind states in the present study were modeled on the IEC 61400-1 Normal Turbulence Model (NTM) [6], with a reference turbulence intensity of I ref = 0.16, in accordance with the requirements of the MOEA [10] that the wind turbines installed in the Formosa I Offshore Wind Farm must be IEC 61400-1 Class I A compliant. Following the NTM, the standard deviation σ U of the wind speed for a given U 10 is calculated by Equation (1):…”

“…To generate more realistic short-term wind states, we employed our previous [6] approach of calibrating the wind speed fluctuations against real on-site data by applying a regression analysis to the wind speed spectra ( Figure 2), and then reconstructing the signals (Figure 3 The increased turbulence in the downstream wakes was modelled by means of an analytical model proposed by Larsen [12,13], which has been shown to reasonably predict the added turbulence in the so-called far wake [14]:…”

“…Offshore wind turbine support structure finite element model and CF expressions for load parameters at the mudline The details of the Closed-Form expression calibration and validation were presented in Reference [6]. Note, however, that in the present study the wave/current load components were omitted to isolate the effects of the wind loads on the fatigue.…”

“…One of the more obvious challenges faced by Taiwan"s wind farms are the extreme loads exerted on the wind turbine and support structures during typhoons [3,4], which strike the island, on average, four times every year [5]. Additionally, the authors previously showed that fatigue damage due to Taiwan"s prevailing North Easterly trade winds can be just as significant a design problem [6]. This previous study considered the long term environmental loads exerted on the jacket support structure of a single wind turbine during its 20-year design life, and highlighted the sensitivity of the fatigue life to the degree of fluctuation (standard deviation) of the wind loads, as well as the importance of appropriately orienting the jacket foundations according to prevailing wind conditions.…”

“…Section III discusses the short-term ambient environmental conditions, which were stochastically generated in accordance with IEC and DNV guidelines, and the in-farm environmental conditions, for which we employed both analytical and computational models to determine the effects of the upstream wind turbine wakes on the downstream flow field. The numerical models employed to calculate the short-term wind loads on the individual wind farm units and to calculate the flow field throughout the wind farm are respectively described in Sections IV and V, while Section VI briefly presents the hot spot stress evaluation approach which the authors previously showed [6] to produce a good trade-off between computational efficiency and simulation accuracy by combining Finite Element Model (FEM)-derived Closed-Form expressions of the nominal stresses at the tubular joints and stress concentration factors. Finally, the fatigue damage was assessed using the Rainflow Cycle Counting scheme together with appropriate SN curves.…”