| dc.description.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 | nb_NO |
