• Energy Research

AbstractRisk of hurricane damage is an important factor in the development of the offshore wind energy industry in the United States. Hurricane loads on an offshore wind turbine (OWT), namely wind and wave loads, not only exert large structural demands, but also have temporally changing characteristics, especially with respect to their directions. Waves are less susceptible to rapid changes, whereas wind can change its properties over shorter time scales. Misalignment of local winds and ocean waves occurs regularly during a hurricane. The strength capacity of non‐axisymmetric structures such as jackets is sensitive to loading direction and misalignment relative to structural orientation. As an example, this work examines the effect of these issues on the extreme loads and structural response of a non‐operational OWT during hurricane conditions. The considered OWT is a 5 MW turbine, supported by a jacket structure and located off the Massachusetts coast. A set of 1000 synthetic hurricane events, selected from a catalog simulating 100,000 years of hurricane activity, is used to represent hurricane conditions, and the corresponding wind speeds, wave heights and directions are estimated using empirical, parametric models for each hurricane. The impact of wind and wave directions and structural orientation are quantified through a series of nonlinear static analyses under various assumptions for combining the directions of wind and wave and structural orientation for the considered example structure. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract This paper introduces a framework for the assessment of damage of offshore wind turbines (OWTs) supported by jackets under extreme environmental loadings. Performance levels/damage states, ranging from operational/undamaged to near collapse/severely damaged, are defined based on static pushover analyses. An example performance assessment is presented for an OWT supported by a jacket based on environmental conditions for a site off Massachusetts along U.S. Atlantic coast. The environmental conditions are characterized based on two methods for estimating wind and wave conditions, one on extrapolation of NOAA buoy measurements and one on a stochastic hurricane catalog, and two models for extreme wave height, one on the crest height and one on the zero-up-crossing height. Using probabilistic models for demands and capacities, two curves of fragility, one estimating the initiation of yielding and the other estimating the onset of collapse, are developed to distinguish between the three damage states. The curves are applied to four combinations of two environmental hazard models and two extreme wave height models, and significant differences are found in the probability of damage among the four combinations of models. The findings have potential implications for the evaluation of the overall risk profile and associated performance for offshore wind farms.

AbstractWind pressures acting on a cubic building with openings exposed to stationary tornadolike vortices were studied experimentally. The effects of opening ratio, single central opening azimuth,...

Abstract On the basis of the measured atmospheric pressure distribution rule fitted by on-the-spot near-ground observation data, the typhoon's height-dependent pressure field was developed with the aid of gas state and hydrostatic balance equations. Probabilistic correlation among mesoscale typhoon field parameters was taken into account. A reduced calculation pattern was proposed by carrying out the scale analysis of three dimensional Navier-Stokes equations to solve the wind velocity field in the typhoon boundary layer. A novel typhoon velocity field model suitable for the gradient layer and boundary layer was then established considering the multi-field parameters correlation and terrain effects. The influence of height-dependent eddy viscosity, which was also closely related to the pressure field and terrain type, on the wind speed profiles under the typhoon boundary layer was considered and discussed. An improved iterative loop algorithm introducing the spatial distribution of eddy viscosity at the low-level boundary layer along the vertical and radial directions was utilized. Furtherly, case studies of the specific typhoon wind field was re-illustrated and compared with the observed wind speed profiles averaged from upper-level dropsondes data and low-level profiles measured by meteorological towers. Three typical regions of vertical wind speed profiles were summarized. Wind speed time series observed by meteorological stations and sea surface wind field snapshots of several typhoons obtained from Hurricane Research Division of National Oceanic and Atmospheric Administration American (H*Wind) were also compared. In general, some specific wind environment characteristics such as non-exponential wind profiles in typhoon boundary layers can be re-illustrated with satisfactory precision by these improved algorithms.

Abstract As offshore wind development is in its infancy along the U.S. Atlantic Coast challenges arise due to the effects of strong storms such as hurricanes. Breaking waves on offshore structures induced by hurricanes are of particular concern to offshore structures due to high magnitude impulse loads caused by wave slamming. Prediction of breaking wave hazards is important in offshore design for load cases using long mean return periods of environmental conditions. A breaking wave hazard estimation model (BWHEM) is introduced that provides a means for assessing breaking hazard at long mean return periods over a large domain along the U.S. Atlantic Coast. The BWHEM combines commonly used breaking criteria with the Inverse First Order Method of producing environmental contours and is applied in a numerical study using a catalog of stochastic hurricanes. The result of the study shows that breaking wave hazard estimation is highly sensitive to the breaking criteria chosen. Criteria including wave steepness and seafloor slope were found to predict breaking conditions at shorter return periods than criteria with only wave height and water depth taken into consideration. Breaking hazard was found to be most important for locations closer to the coast, where breaking was predicted to occur at lower mean return periods than locations further offshore.

Abstract This paper presents a probabilistic debris trajectory model adapted from current 6-degree-of-freedom (6-DoF) deterministic models, in which the aleatoric (inherent) uncertainty is explicitly considered in the proposed probabilistic model. While the inherent randomness in the debris flight trajectory is irreducible due to the wind turbulence, variation in wind direction, gustiness of the wind event, and so forth, the proposed probabilistic model seeks to address these uncertainties through Monte Carlo simulations with the appropriate statistical distributions applied to the governing equations of motion of the debris. Calibration of the probabilistic debris trajectory model is performed through an analytical and visual comparison of the simulated data to wind tunnel test data. Reasonable agreement is observed between the simulated and the wind tunnel test debris landing locations, thus confirming the applicability of the probabilistic wind-borne debris model. The proposed probabilistic model provides an effective method for predicting the variation of debris trajectories in a three-dimensional (3D) space, which is imperative when performing regional building envelope impact risk assessments in which a large amount of debris sources and targets must be considered in the simulation.

Abstract A barrier to the development of the offshore wind resource along the U.S. Atlantic coast is a lack of quantitative measures of the risk to offshore wind turbines (OWTs) from hurricanes. The research presented in this paper quantifies the risk of failure of OWTs to hurricane-induced wind and waves by developing and implementing a risk assessment framework that is adapted from a well-established framework in performance-based earthquake engineering. Both frameworks involve the convolution of hazard intensity measures (IMs) with engineering demand parameters (EDPs) and damage measures (DMs) to estimate probabilities of damage or failure. The adapted framework in this study is implemented and applied to a hypothetical scenario wherein portions of nine existing Wind Farm Areas (WFAs), spanning the U.S. Atlantic coast, are populated with ∼7000 5 MW OWTs supported by monopiles. The IMs of wind and wave are calculated with a catalog representing 100,000 years of simulated hurricane activity for the Atlantic basin, the EDPs are calculated with 24 1-h time history simulations, and a fragility function for DM is estimated by combining variability observed in over one hundred flexural tests of hollow circular tubes found in the literature. The results of the study are that, for hurricane-induced wind and wave, the mean lifetime (i.e., 20-year) probability of structural failure of the tower or monopile of OWTs installed within the nine WFAs along the U.S. Atlantic coast ranges between 7.3 × 10−10 and 3.4 × 10−4 for a functional yaw control system and between 1.5 × 10−7 and 1.6 × 10−3 for a non-functional yaw control system.