Long-term Response Analysis of Wind Turbines with an Emphasis on Fault and Shutdown Conditions

Abstract

Wind turbines are exposed to a variety of load conditions during their 20-year service life. After installation, every turbine experiences operational, shutdown, and parked conditions, and any of these conditions might cause critical loads. Although wind turbines are unmanned, it is always important to improve their reliability to reduce costs and increase the competitiveness of wind-generated power. To achieve this goal, a better understanding of the design loads is required. Turbines should be designed to ensure a sufficiently low probability of ultimate and fatigue failures, and time-domain numerical simulations should be performed to predict the responses of wind turbines. International design standards provide a number of design load cases, but those related to fault conditions are not well defined. Although wind turbines differ in type and location of deployment, it is likely that fault cases drive the design for certain turbines. A wind turbine experiences operational, parked, and shutdown load conditions, and unfavorable responses may occur if a severe fault occurs during operation and is not handled properly. This scenario could apply to bottom-fixed as well as floating wind turbines, which are proposed for extraction of wind power in deep water (>200 meters). The design problem for floating wind turbines is complex and challenging, and the relevant design standards are still under development. A particular need exists for insights into the response characteristics of floating wind turbines under various load conditions, including fault conditions. A wind turbine consists of many subsystems and components. For traditional wind turbines with gear transmissions, the gearbox is among the most expensive components, and gearbox failure rates are often higher than expected. One possible cause is a lack of understanding of the dynamic loads in wind turbine gearboxes. Components such as bearings are designed by the manufacturers using in-house design codes, and these designs may be inappropriate if the loads are not properly assessed. Therefore, it is necessary to develop methods to better understand the effects of the dynamic load conditions on the gearbox components. This thesis addresses the response analysis of 5 megawatt (MW) spar-type floating wind turbines with reference to a land-based wind turbine. Specific procedures were implemented in integrated dynamic analysis tools. Parked conditions that consider pitch mechanism faults were first studied on a spar-type wind turbine. An aero-hydro-servo-elastic tool (HAWC2) was used to model the phenomena and analyze the effects of faults on the aerodynamic and hydrodynamic loadings, and the responses under extreme environmental conditions were compared. Second, the dynamic response of wind turbines during transient events was investigated. Using the external dynamic link libraries in the HAWC2 code, scenarios were simulated that involve pitch mechanism faults, grid loss events, and shutdowns. Interesting response phenomena were reported for the land-based and floating wind turbine. The focus was on the extreme responses that occurred during shutdown. Furthermore, a comparative study of the National Renewable Energy Laboratory (NREL) 5 MW land-based and OC3 Hywind turbines was conducted, with consideration of shutdown procedures, using various blade pitch rates and grid statuses. A nonlinear computational tool (SIMO-RIFLEX-AeroDyn) was used to examine the extreme responses and fatigue damage of structural components, including themooring lines. Additionally, a long-term fatigue analysis of the planetary bearings in a 750 kW land-based drivetrain was performed. Multilevel integrated analyses were performed using the HAWC2 aeroservo- elastic code, the SIMPACK multibody dynamics code, the Calyx three-dimensional finite element code, and the lifetime prediction models for rolling contact fatigue. This thesis work can be classified using two main themes: • Application of aero-hydro-servo-elastic analysis tools to increase understanding of the global dynamic behaviors of both land-based and floating wind turbines • Application of analytical methods and multiple simulation tools to understand the dynamic behaviors of the drivetrain and to improve its design