Browsing by Author "Arwade, Sanjay R."

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    Convergence of Extreme Loads for Offshore Wind Turbine Support Structures
    Stewart, Gordon; Lackner, Matthew; Arwade, Sanjay R.; Myers, Andrew T.; Hallowell, Spencer (Virginia Tech, 2015-06-11)
    Extreme loads of wind turbines are historically difficult to predict through simulation due to uncertainty in input conditions as well as in the simulation models. In addition, many long time series must be simulated for the statistics of the peak loads to become stationary. Offshore wind turbines require even more simulation due to the addition of stochastic wave loading. Floating offshore wind turbines, the subject of this paper, experience free-body motion as a result of wind and wave loading, and the phasing of wind turbulence, turbine motion, and large waves can be very influential in determining extreme loading. The International Electrotechnical Commission's 61400-3 standard covers loads analysis of offshore wind turbines, including only cursory references toward floating offshore wind turbines. This IEC design standard requires six 1-hour simulations to estimate extreme loads, which is not long enough for convergence of the statistics of peak loads for offshore turbines, especially floating turbines, which have higher and more variable loads due to platform motion. In this paper, 50-year wind and wave conditions are synthesized from data from the National Oceanographic and Atmospheric Administration's (NOAA's) floating data buoys for a suite of ocean sites suitable for floating or fixed bottom offshore wind turbines. The simulation software used in this paper is FAST, developed by the National Renewable Energy Laboratory, which is a coupled aero-hydro-servo-elastic wind turbine design tool. The current version of FAST which is used in this research includes second-order hydrodynamic effects, which may be important sources of loading in extreme conditions for certain floating platforms. TurbSim is used to create full-field turbulent wind files using the mean wind speed determined from the buoy data, while the hydrodynamics module within FAST handles the wave height time series using a JONSWAP spectrum. The OC3 spar buoy and the OC4 semi-submersible floating platforms are used as examples of realistic platform designs and a monopile is used as the fixed bottom example. A large number of 1-hour simulations are run to determine the convergence characteristics of each platform at each ocean site. These results are discussed and recommendations for future revisions of the design standard are made. Future work concerning various methods that will reduce the simulation cost of determining the converged extreme load will also be discussed.
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    Impact of Hurricane Wind/Wave Misalignment on the Analyses of Fixed-Bottom Jacket Type Offshore Wind Turbine
    Wei, Kai; Arwade, Sanjay R.; Myers, Andrew T.; Valamanesh, Vahid; Pang, Weichiang (Virginia Tech, 2015-06)
    A high risk of hurricane is threatening the development of offshore wind energy in the east coast of the United States. Hurricane loads on an offshore wind turbine, namely wind and waves, not only exert large demands but also have rapidly changing characteristics, especially wind and wave directions. Waves are, in general, inert to rapid changes, whereas wind can change its properties within very short time scales. Misalignment of local winds and propagating ocean waves occurs regularly in a hurricane environment. It is a common practice to design monopile support structures for offshore wind turbines (OWTs) under extreme conditions by the highest wind/wave loads when they are assumed to come from the same direction. However, this co-directional wind/wave assumption can be hazardous for non-axisymmetric fixed bottom support structures for deeper water such as jackets due to their sensitive capacity to loading directions. The goal of this work is to examine the impact of wind/wave misalignment on the extreme loads and structural response under hurricanes. We select a fixed-bottom jacket type offshore wind turbine located in a water depth of 50m as the example structure. The hurricane induced wind and wave loads on the structure system are calculated from a reduced set of 1000 simulated full-track hurricane events, selected from a database of 200,000 years of simulated hurricanes, to represent the hazard of Nantucket, Massachusetts. The meteorological ocean (met-ocean) conditions and wind/wave directions for each hurricane are identified from the track data by physical models. The wind direction, wave direction at different time and location and the orientation of structure are included to capture the misalignment impact on the structural analyses over a wide range of possible engineering designs and conditions. It will let us clearly understand the impact of wind/wave misalignment on the analyses of a jacket-type support structure. Summary : This work, (1) predicts environmental conditions and directions from a reduced set of synthetic hurricane catalogue based on the analytical models; (2) calculates wind and wave forces on a jacket-type offshore wind turbine example (3) studies impact of wind/wave misalignment on the structural analyses over a wide range of possible engineering designs and conditions
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    The Role of Damping in Offshore Wind Turbine Dynamics
    Fontana, Casey; Carswell, Wystan; Arwade, Sanjay R.; Degroot, Don (Virginia Tech, 2015-06-08)
    The stochastic environmental loading present in potential offshore wind farm locations leads to high costs for the turbine's support structure. As a result, it is important to determine the contribution each damping source on these types of loading. The least is known about foundation damping, therefore it is conservatively neglected in structural design. A parameter study will be presented to show how foundation damping affects structural demands over a variety of wind, wave, and operating conditions. Given that fatigue analysis is important for ensuring the design life of OWTs, the effective contribution of damping on fatigue damage accumulation will also be estimated. Examining the sensitivity of structural loading to damping may allow foundation damping to be advantageously incorporated into design guidelines, potentially leading to a more efficient OWT design and reduction in the large cost of the support structure.