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Offshore wind energy deployment in the US is expected to increase in the years to come, with proposed wind farm sites located in regions with high-risk for tropical cyclones. Yet, the wind turbine design criteria outlined by the International Electrotechnical Commission for extreme events may not account for the severe wind conditions in tropical cyclones, even the weaker storms that are likely to reach mid-Atlantic wind resource areas. To evaluate if current design standards capture the extreme conditions of these storms, we perform idealized large-eddy simulations of five tropical cyclones (two category-1, two category-2, and one category-3 storms) using the Weather Research and Forecasting model. Wind conditions near the eyewall of category-1, category-2 and category-3 storms can exceed current design standards for offshore wind turbines. Hub-height winds can exceed design criteria for Class I and Class T turbines for 50-year recurrence periods. Moreover, wind speed shear across the turbine rotor layer is larger than assumed in design specifications. Vertical variations in wind direction across the turbine rotor layer are also large for tropical cyclones of all intensity levels, suggesting design standards should include veer, which can amplify loads in wind turbines.

Offshore wind is a promising renewable energy source for many coastal populations, predicted to supply as much as 10% of global electricity by 2050. However, the technology has followed a cautious development path, using an onshore-style of turbine and tower, held steadily with an oil-industry type of platform. This approach has avoided major risks but is responsible for high costs, supply chain and infrastructure challenges, and relatively slow innovation due to the difficulties of offshore repair. It may now be appropriate to rethink that approach in search of lower cost and reduced infrastructure requirements. This paper presents ongoing work on a light floating turbine concept that yaws like a ship at anchor, has a shallow draft, widely spaced buoyancy, and high center of gravity (CG), and holds the rotor both upwind and downwind. Rather than seeking to minimize wave disturbances, the design aims to stiffly follow water elevation by using shallow floats designed to prevent heave amplification. Regular and irregular motions of the system are studied for various mass distributions to identify deviations from the intended wave following motion.

This paper explains the importance of education in the dying process and outlines a full curriculum to create a society more prepared for difficult end-of-life situations. This course is structured to be taught as a high school elective course in which the principles and approaches to death and dying are discussed in depth. For a high school student, death can be approached from diverse perspectives. Within these perspectives, there are several vital topics to understand in order to ensure a student’s proper education on death. The complexity of death is represented by the titles of the nine chapters of the course. Chapter One, titled “Definition of Death, Consciousness, and Persistent Vegetative State,” highlights the development of death throughout history, the general consensus about death today, and the morality of different levels of treatment for dying patients. Chapter Two, “End Stage Medical Conditions,” outlines the most common end-of-life health conditions along with their symptoms and possible treatments. In Chapter Three, titled “Bad News and Communication,” the course introduces patient-physician communication, the current shortcomings of medical education in this vital area, and the possible protocols for effectively conveying both truth and support to a patient. Chapter Four, labelled “Do Not Resuscitate Orders, Living Wills/Advance Directives, and Durable Powers of Attorney for Health Care,” presents the appropriate options available to prepare for a patient’s inability to make informed decisions autonomously. Chapter Five, “Ordinary vs. Extraordinary Means: The Issue of Tube Feedings,” discusses the distinction between different types of end-of-life treatment and when certain treatments can be morally excused. Chapter Six, titled “Palliative Care and Hospice: A Paradigm for End-of-Life Care,” gives effective options for terminally ill patients that ensure care is never stopped and a patient's dignity is never lost. Chapter Seven, “Pain Management,” highlights the difficulties and recommendations for establishing proper pain management. Chapter Eight, entitled “Medical Futility,” presents the debate surrounding patient autonomy with physician’s beneficence and nonmaleficence, forgoing and requesting treatments, and the values associated with this issue. Finally Chapter Nine, titled “Spirituality and End of Life Care,” emphasizes the inevitable linkage between healthcare, death, and spirituality and the benefits of proper support for a patient. This paper also highlights the ethical implications surrounding the death and dying process. The importance of this subject lies within the death-denying reality that society is today in which death is avoided and human dignity is increasingly sacrificed as a result. Discussions and education among society’s youngest adults can improve general knowledge about the dying process and lead to overall better preparation for handling this inevitable stage of humanity.

Hurricanes induce multiple hazards including high wind speeds, large waves, and storm surge. The characteristics of these hazards have an important effect on the design of offshore structures and determine the suitability of simplifying assumptions for modeling, such as whether the maxima can be reasonably modeled as occurring simultaneously during the hurricane and whether the N-year values of the hazards can be modeled separately and combined. Such assumptions are common in practice. Assessing the implications of these assumptions to better inform hazard modeling along the U.S. Atlantic coast is the central objective of this paper. To this end, the characteristics of wind speeds, waves, and storm surge during hurricanes are evaluated for ten areas where development of offshore wind is proposed. The assumptions are evaluated for a model of an offshore wind turbine using time histories of hazards during intense hurricanes and using so-called environmental contours constructed through a novel implementation of the Inverse First Order Reliability Method. The results of this study show that these assumptions are reasonably accurate, especially for wind and wave in shallow waters and for wind and surge in deep waters. Hazard ratios are proposed as a suitable representation of the characteristics of hazard maxima.

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.

A shared multiline anchor system may reduce material and installation costs for floating offshore wind farms, but the concept also expands the physical space over which the analysis of the system must be considered. The multiline anchor force is a combination of forces that originate from different floating offshore wind turbine (FOWT) platforms, and this interconnectivity mandates investigation of spatial coherence of waves over the area of the connected turbines. The goal of this work is to determine if anchor force characteristics are sensitive to spatial coherence of the wave field, or if an assumption of independent wave fields at different FOWT locations provides sufficiently accurate anchor load characterizations. Spatially coherent irregular wave fields are generated and applied to FOWT systems in 3-line and 6-line anchor systems, revealing negligible sensitivity of multiline anchor force to wave spatial coherence. These results are continued with a parametric study showing how wave characteristics affect correlation lengths of spatially coherent waves. A review of installed offshore wind turbine spacings is presented as a context for these results. In each section, regular waves are first used to examine the dynamics in a simple context, then irregular waves are used to more realistically simulate ocean conditions.

Metocean conditions during hurricanes are defined by multiple parameters (e.g., significant wave height and surge height) that vary in time with significant auto- and cross-correlation. In many cases, the nature of the variation of these characteristics in time is important to design and assess the risk to offshore structures, but a persistent problem is that measurements are sparse and time history simulations using metocean models are computationally onerous. Surrogate modeling is an appealing approach to ease the computational burden of metocean modeling, however, modeling the time-dependency of metocean conditions using surrogate models is challenging because the conditions at one time instant are dependent on not only the conditions at that instant but also on the conditions at previous time instances. In this paper, time-dependent surrogate modeling of significant wave height, peak wave period, peak wave direction, and storm surge is explored using a database of metocean conditions at an offshore site. Three types of surrogate models, including Kriging, Multilayer Perceptron (MLP), and Recurrent Neural Network with Gated Recurrent Unit (RNN-GRU), are evaluated, with two different time-dependent structures considered for the Kriging model and two training set sizes for the MLP model, resulting in a total of five models evaluated in this paper. The performance of the models is compared in terms of accuracy and sensitivity towards hyperparameters, and the MLP and RNN-GRU models are demonstrated to have extraordinary prediction performance in this context.

Offshore wind turbines are poised to become a vital part of the global energy landscape — particularly the floating types which give access to a much greater wind power resource. The design possibilities for floating turbines are so different from onshore and offshore fixed-bottom turbines, that a cost-reducing re-imagining may be justified. Apart from the expense of an offshore transmission cable and substation, the costs of hardware for offshore bottom-fixed wind turbines (CAPEX) are roughly twice as much as for onshore. Much of the extra cost can be attributed to the mass of the above- and underwater support and expensive installation. In this paper, we reconsider two aspects of present-day offshore turbines: (a) maintaining a land-turbine architecture (a slender tower with the rotor cantilevered from a yawing, equipment-filled nacelle); (b) seeking to minimize wave-induced motion and loads of the above-water plant. Our aim is an offshore floating design that is potentially less expensive than offshore fixed-bottom units. We outline the preliminary structural analyses that underlie a design focused on weight, cost reduction, and ease of manufacturing. This includes lattice towers, tubular hub and axle, and needle-roller bearings. The biggest concerns about the proposed lightweight system involve motions and forces induced by waves. Shallow-draft floats will follow the waves, leading to greater rotor translation and precession; and lattice towers may be subject to impact loads, ice buildup, and fouling. We present analysis of some of these motions and forces, along with resulting estimates for required structural weight as a preliminary investigation into the feasibility of this lightweight concept.

Environmental contours are used to simplify the process of design response analysis. A wide variety of contour methods exist; however, there have been a very limited number of comparisons of these methods to date. This paper is the output of an open benchmarking exercise, in which contributors developed contours based on their preferred methods and submitted them for a blind comparison study. The exercise had two components—one, focusing on the robustness of contour methods across different offshore sites and, the other, focusing on characterizing sampling uncertainty. Nine teams of researchers contributed to the benchmark. The analysis of the submitted contours highlighted significant differences between contours derived via different methods. For example, the highest wave height value along a contour varied by as much as a factor of two between some submissions while the number of metocean data points or observations that fell outside a contour deviated by an order of magnitude between the contributions (even for contours with a return period shorter than the duration of the record). These differences arose from both different joint distribution models and different contour construction methods, however, variability from joint distribution models appeared to be higher than variability from contour construction methods.

Surrogate modeling of the variability of metocean conditions in space and in time during hurricanes is a crucial task for risk analysis on offshore structures such as offshore wind turbines, which are deployed over a large area. This task is challenging because of the complex nature of the meteorology-metocean interaction in addition to the time-dependence and high-dimensionality of the output. In this paper, spatio-temporal characteristics of surrogate models, such as Deep Neural Networks, are analyzed based on an offshore multi-hazard database created by the authors. The focus of this paper is two-fold: first, the effectiveness of dimension reduction techniques for representing high-dimensional output distributed in space is investigated and, second, an overall approach to estimate spatio-temporal characteristics of hurricane hazards using Deep Neural Networks is presented. The popular dimension reduction technique, Principal Component Analysis, is shown to perform similarly compared to a simpler dimension reduction approach and to not perform as well as a surrogate model implemented without dimension reduction. Discussions are provided to explain why the performance of Principal Component Analysis is only mediocre in this implementation and why dimension reduction might not be necessary.

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.

This paper reports the findings from the 2016 Wind Energy Research Workshop held in Lowell, MA. The workshop examined the state-of-the-art in wind energy research within the following three core topic areas: (A) Wind Turbine Design and Manufacturing including: blades, towers/foundations and nacelle, (B) Wind Farm Development including: offshore installations/siting, flow characterization and loads/waves/wind characterization, and (C) Wind Farm Operations including: controls, power production, wind farms, sensing, diagnostics, testing, structural health monitoring, reliability, energy storage, the grid and power transmission. Research challenges and future directions were discussed and are reported for each sub-topic area.

This research assesses the reliability of floating offshore windfarms utilizing two different anchor configurations: a conventional single-line system in which each anchor is loaded by a single mooring line and a multiline system in which each anchor is loaded by three mooring lines. While there are advantages to adopting a multiline system for floating offshore wind farms, the interconnectedness of this concept introduces disadvantages, such as reduction of system reliability and the potential for cascading failures among multiple structures. The reduction in system reliability is investigated here by running Monte-Carlo simulations in which mooring line and anchor demands and capacities are sampled from probability distributions. Demand distributions are generated through dynamic simulations with environmental conditions corresponding to the 500-year storm. Failure of mooring lines or anchors are initiated when their capacity is exceeded by their demand. The results of this research show that the reliability of the multiline system degrades significantly when progressive failures are taken into consideration. This research also shows that design considerations, such as the sizing of mooring lines and anchors and designing for single-line or multiline loads, significantly influence the system reliability of a floating offshore wind farm.

The objective of this paper is to advance the design of slender steel tubes by developing a practical approach for utilizing high-resolution measurements of geometric imperfections to estimate the buckling location and strength of such tubes in bending. This approach includes a novel measure of imperfection severity that is designed to be insensitive to noise. The ability of this measure to predict buckling behavior of slender tubes in bending is assessed through comparison with eight large-scale tests, and, for this set of data, the predictions are promising. This study is intended to be a starting point in the development of a simple, but accurate, design method to quantify the impact of imperfections on the buckling behavior of slender tubes.

Environmental contours are useful for designing and assessing risk to offshore structures. The Inverse First-Order Reliability Method (IFORM) and the Rosenblatt transformation are widely used to calculate environmental contours but have several challenges. In this approach, the conditional distribution is often expressed in a parametric form with parameters defined as functions of the independent variable. The distribution should be accurate for both extreme and frequently occurring values, for both the dependent and independent variables, and this is a challenging requirement for complex datasets. A new IFORM-Rosenblatt framework is proposed here to overcome this challenge. The framework includes a method to approximate a distribution with a complex form, including parametric, empirical, and mixed parametric/empirical forms, using a reduced set of parameters that are designed to prioritize accuracy for the quantiles of the distribution that contribute to the contour and a set of metrics to evaluate the goodness-of-fit of the resulting contour. A hindcast dataset is used as an example to calculate the 50-year contour using the proposed framework and a traditional approach. The proposed framework is demonstrated to generate accurate contours, although it requires more parameters and is more complex than conventional approaches.

While the offshore wind industry has shown a steady trend towards floating turbines, costs of these systems remain high. A multiline anchor concept may significantly reduce the high cost of floating wind, in which floating turbines share anchors. This work investigates the potential cost benefit of implementing a multiline anchor system relative to the conventional single-line anchor system over a range of spatial parameters. The OC4 DeepCwind semisubmersible platform is used to design catenary mooring systems for different water depths and turbine spacings. In all cases, the maximum anchor force in the 3-line anchor system is less than or equal to that of the single-line anchor system. Cost models for the mooring lines, anchors, installation and geotechnical site investigation are presented and discussed. In a 100-turbine farm, the multiline anchor system results in a 9–19% reduction in stationkeeping costs, with high and low estimates for the cost models additionally included. Larger reductions in the combined line and anchor cost result from mooring system configurations with smaller ratios of water depth to turbine spacing. Due to perimeter effects in the multiline configuration, larger cost reductions can be achieved for larger farm sizes.

The vast spatial scale of offshore structures causes wave loading to be correlated amongst nearby structural members. Certain engineering activities including health monitoring, maintenance, and preliminary design of offshore structures requires the prediction of wave forces on said structural members. The high cost and low availability of environmental wave measurements requires the reconstruction of wave kinematics and force profiles to accurately capture the forcing history on offshore structures. A method for predicting wave forces on a cylinder from nearby wave elevation measurements is proposed. The formulation utilizes the Fast Fourier Transform to calculate wave kinematics propagation in the frequency domain and applies the kinematics to the Morison equation for calculation of cylinder forces. The prediction equations are applied to three types of waves: regular periodic waves, random irregular waves, and solitary breaking waves, and the error in both elevation prediction and force prediction when compared to measured values is calculated. The force prediction equations were shown to perform best for small wave heights, with errors as low as 5% in the force predictions for small regular and irregular waves. The error in force prediction increases nonlinearly with the increase in wave height due to the deficiencies of the linear dispersion relationship used in the formulation.

This paper describes the analysis of a wind turbine and support structure subject to simulated hurricane wind fields. The hurricane wind fields, which result from a large eddy simulation of a hurricane, exhibit features such as very high gust factors (>1.7), rapid direction changes (30∘ in 30 s), and substantial veer. Wind fields including these features have not previously been used in an analysis of a wind turbine, and their effect on structural loads may be an important driver of enhanced design considerations. With a focus on blade root loads and tower base loads, the simulations show that these features of hurricane wind fields can lead to loads that are substantially in excess of those that would be predicted if wind fields with equally high mean wind speeds but without the associated direction change and veer were used in the analysis. This result, if further verified for a range of hurricane and tropical storm simulations, should provide an impetus for revisiting design standards.

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.

Offshore wind turbine (OWT) monopile foundations are subjected to cyclic loading from wind, waves, and operational loads from rotating blades. Lateral monopile capacity can be significantly affected by cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency degradation and accumulated permanent rotation at the mudline, respectively. This paper introduced novel combinations of existing p-y curve design methods and compared the impact of short-term cyclic loading on monopiles in soft, medium, and stiff clay. The results of this paper indicate that short-term cyclic loading from extreme storm conditions are unlikely to significantly affect natural frequency and permanent accumulated rotation for OWT monopiles in stiff clays, but monopiles in soft clay may experience significant degradation. Further consideration is required for medium clays, as load magnitude played a strong role in both natural frequency and permanent rotation estimation.

The most common wind tower structure, a tapered tubular steel monopole, is currently limited to heights of ~ 80 m due to transportation constraints which arise because tower sections are manufactured at centralized plants and transported to site for assembly. The need to transport the sections imposes a limit on their size, whereby maximum tower diameters are dictated by bridge clearances rather than by structural efficiency. New manufacturing innovations, based on automated spiral welding, may enable on-site production of wind towers, thereby precluding transportation limits and permitting the manufacture of taller towers, which can harvest the steadier, stronger winds at higher elevations. Taller towers, however, are expected to have cross-sections with slenderness that is uncommon in structural engineering (i.e., diameter-to-thickness ratios up to ~ 500) and much larger than those of conventionally manufactured towers (i.e., diameter-to-thickness ratios up to ~ 300). Tubular structures with highly slender cross-sections are imperfection-sensitive, and the welding process is known to influence imperfections. To account for this sensitivity, slender tubes are usually designed based on empirical knockdown factors, however there are few experiments of tubes in flexure with slenderness as high as what is expected for spirally welded wind towers, and there are no experiments on tubes within this slenderness range and manufactured with spiral welding. This paper reviews the state-of-the-art for designing spirally welded tubes as wind towers and identifies deficiencies. Relevant experimental and analytical research is summarized and research needs to efficiently design tapered spirally welded steel tubes as wind towers are identified.

The evaluation of natural catastrophe risk to structures often includes consideration of uncertainty in predictions of some measure of the intensity of the hazard caused by the catastrophe. For example, in the well-established method of probabilistic seismic hazard analysis, uncertainty in the intensity measure for the ground motion is considered through so-called ground motion prediction equations, which predict ground motion intensity and uncertainty as a function of earthquake characteristics. An analogous method for evaluating hurricane risk to offshore structures, referred to herein as probabilistic offshore hurricane hazard analysis, has not been studied extensively, and analogous equations do not exist to predict offshore hurricane wind and wave intensity and uncertainty as a function of hurricane characteristics. Such equations, termed here as wind and wave prediction equations (WWPEs), are developed in this paper by comparing wind and wave estimates from parametric models with corresponding measurements during historical hurricanes from 22 offshore buoys maintained as part of the National Data Buoy Center and located near the US Atlantic and Gulf of Mexico coasts. The considered buoys include observations from 27 historical hurricanes spanning from 1999 to 2012. The 27 hurricanes are characterized by their eye position, translation speed, central pressure, radius to maximum winds, maximum wind speed, Holland B parameter and direction. Most of these parameters are provided for historical hurricanes by the National Hurricane Center’s H*Wind program. The exception is the Holland B parameter, which is calculated using a best-fit procedure based on H*Wind’s surface wind reanalyses. The formulation of the WWPEs is based on two parametric models: the Holland model to estimate hurricane winds and Young’s model to estimate hurricane-induced waves. Model predictions are made for the 27 considered historical hurricanes, and bias and uncertainty of these predictions are characterized by comparing predictions with measurements from buoys. The significance of including uncertainty in the WWPEs is evaluated by applying the WWPEs to a 100,000-year stochastic catalog of synthetic hurricanes at three locations near the US Atlantic coast. The limitations of this approach and remaining work are also discussed.

In comparison with monopile support structures commonly used for offshore wind turbines (OWTs) in shallow water, fixed bottom support structures for deeper water such as jackets tend to lack axisymmetry and have different capacities when loaded in different directions. Design of such a structure may therefore benefit from consideration of the directional characteristics of loading. This paper focuses on a rational and efficient approach to assess the structural safety of jacket type OWTs under directionally dependent extreme environmental loads. The Incremental Wind–Wave Analysis (IWWA) framework is reviewed and used to assess capacity of jacket-type OWTs under directional environmental wind–wave conditions. The approach uses static pushover analysis of OWT jackets subject to combined wind and wave load patterns corresponding to increasing mean return periods (MRPs). The wind and wave conditions are calculated independently and assumed to occur simultaneously. The loading direction and jacket orientation are both included in the analysis. To illustrate the approach, metocean conditions at different sites along the U.S. Atlantic coast are obtained from the historical database of the National Oceanic and Atmospheric Administration (NOAA) Data Buoy Center. Two models are introduced for estimating the occurrence probability of wave direction, one of which directly uses the frequency of wave directions and the other uses a Gaussian kernel to represent the range of wave directions represented by given directional spectra. For each combination of wind–wave direction and structural orientation, an IWWA analysis gives the capacity in terms of the MRP conditions leading to formation of a fully developed plastic mechanism in the jacket. Those capacities, convolved with MRP models for loading intensity, yield direction-dependent structural reliabilities, and when those reliabilities are convolved with the probability density function for load direction, a total failure probability is obtained that accounts for wave directionality. Example analyses are conducted for an OWT supported by a four-leg jacket adapted from a design published as part of the European Union UpWind Project. Effects of extreme load directionality, structural orientation, structural geometry and site specification on the ultimate capacity of the jacket are carefully discussed.

The prediction of ultimate and fatigue demands for the design of offshore wind turbines (OWTs) requires accurate simulation of the dynamic response of OWTs subject to time-varying wind and wave loads. The magnitude of damping in an OWT system significantly influences the dynamic response, however, some sources of damping, such as foundation damping, are not explicitly considered in design guidelines and may increase damping significantly compared to commonly assumed values in design. Experimental and analytical studies have estimated the magnitude of foundation damping to be between 0.17% and 1.5% of critical, and this paper investigates how increased damping within this range affects load maxima and fatigue damage for a hypothetical 5MW OWT subjected to a variety of wind, wave, and operational conditions. The paper shows that increased damping effects the greatest percentage reduction of ultimate moment demands and fatigue damage when the OWT rotor is parked and feathered. In such cases, the aerodynamic damping is relatively low, allowing for additional damping from the foundation to account for a relatively larger proportion of the total system damping. Incorporating foundation damping in design guidelines may lead to more efficient structures, which is a crucial factor in overcoming the high cost barrier associated with offshore wind development.

The International Electrotechnical Commission (IEC) design standard (IEC 61400-3) for offshore wind turbines includes a design load case which considers loads on the turbine during extreme conditions, when the wind turbine is not operating and the blades are feathered. The recommendation of this design standard is to simulate 6 one-hour periods, each with wind and wave fields modeled as random processes, and to calculate design loads as the mean of the maximum from the six simulations. Previous studies have investigated this recommendation for fixed bottom offshore wind turbines and raised concerns about the stability of the estimate of the design load calculated using these guidelines. The research presented in this paper calculates statistics of extreme design loads as a function of the number of one-hour simulations considered for three offshore wind turbine support structures: a fixed-bottom turbine supported by a monopile and two floating turbines, a spar buoy and a semi-submersible. The study considers one extreme design load, the moment at the tower base, one set of 50-year metocean conditions representive of the Northeast U.S. Atlantic coast, and five combinations of wind and wave models, including linear and nonlinear representations of irregular waves. The data generated from this study are used to assess the stability of the estimate of the extreme load for various numbers of one-hour simulations for each turbine type. The study shows that, for the considered metocean conditions and models, the monopile exhibits the least stability in the estimate of the extreme load and therefore requires more one-hour simulations to have comparable stability as the two floating support structures. Explanations for this result are provided along with a probabilistic formulation for estimating variability of design loads more efficiently, using fewer one-hour simulations.

The purpose of this work is to develop and validate a frequency domain analysis (FDA) algorithm to predict loads on a monopile-supported offshore wind turbine (OWT). FDA is faster than the time domain simulations that are conventionally used to perform structural analyses of OWTs. An FDA algorithm that could accurately predict loads would enable expedient design and assessment of risk for OWTs. The current work presents an FDA algorithm for predicting variance of wave-driven loads during the non-operational condition with validation from both simulated and measured data.

The objective of this paper is to analyze and characterize high-resolution measurements of geometric imperfections taken from a set of seven slender tapered steel tubes in order to provide insights that can improve methods for predicting buckling behavior. The seven tubes are each ~3400 mm long with diameters between 800 mm and 1100 mm, diameter to thickness ratios between 300 and 350, and taper angles between 0.67° and 0.86°. The tubes are manufactured from steel plates using an innovative spiral welding process. The geometric imperfections of these tubes are characterized with harmonic analysis of the overall imperfection measurements and with regression analysis of the measured shapes of weld depressions. The results show a consistent imperfection signature caused by the manufacturing process including distinct features attributed to both the rolling and welding processes, i.e. anticlastic deformations and weld depressions. Variability in the imperfection measurements is also analyzed and used to generate a probabilistic scheme capable of generating random fields of geometric imperfections that are consistent with the measurements considered here.

The construction of environmental contours using the Rosenblatt transformation requires the parameters defining the conditional distribution to be determined for all values of the conditioned random variable, including extreme values which are extrapolated from data in most cases. In the typical case where the conditional distribution is described by many parameters, the fitting process of these parameters is cumbersome, and the resultant extrapolation is prone to error. A re-parameterization technique is proposed here to approximate such a conditional distribution, which depends on many parameters, with a two-parameter Weibull distribution, so that fewer distribution parameters are involved in the fitting and extrapolation processes. This technique is shown to have a better performance compared to conventional approaches, as demonstrated here for a numerical example involving the construction of a 50-year environmental contour for wind and wave at offshore locations.

Fixed-bottom offshore wind turbines (OWTs) are typically located in shallow to intermediate water depth, where waves are likely to break. Support structure designs for such turbines must account for loads due to breaking waves, but predictions from breaking wave models often disagree with each other and with observed behavior. This variability indicates the need for a better understanding of each model’s strengths and limitations, especially for different ocean conditions. This work evaluates and improves the accuracy of common breaking wave criteria through comparison to Computational Fluid Dynamics (CFD) simulations of breaking waves. The simulated ocean conditions are representative of potential U.S. East Coast offshore wind energy development sites, but the discussion of model accuracy and limitations can be applied to any location with similar ocean conditions. The waves are simulated using CONVERGE, a commercial CFD software that uses a Volume of Fluid (VOF) approach and includes adaptive mesh refinement at the evolving air-water interface. First, the CFD model is validated against experimental data for shoaling and breaking wave surface elevations. Second, 2D simulations of breaking waves are compared to widely-used breaking wave limits (McCowan, Miche, and Goda) for different combinations of wave height, wavelength, water depth, and seafloor slope. Based on these comparisons, the accuracy and limitations of each breaking limit model are discussed. General usage guidelines are then recommended.

In order for our nation to develop its vast offshore wind resource in a manner that respects the ocean environment and its many stakeholders, it is critical that we develop a coordinated, long-term strategic vision for advancing American innovation in this new industry. This vision is particularly relevant to the Commonwealth of Massachusetts, which has, inter alia, set a target of 25% of its electricity needs supplied by renewable sources by 2030. In accordance with this goal the Commonwealth has also mandated that 1600 MW of offshore wind be installed by 2027. This has been followed by opening up of three leases in federal waters off Massachusetts, selected with the help of the Bureau of Ocean Energy Management. The Massachusetts Research Partnership (MRP) in Offshore Wind, launched with seed funds from the Massachusetts Clean Energy Center's Renewable Energy Trust, includes six Massachusetts academic and research institutions - Northeastern University, Tufts University, UMass Amherst, UMass Dartmouth, UMass Lowell and Woods Hole Oceanographic Institution. The goal of the MRP is to develop a data driven multi-disciplinary system level framework for offshore wind research needed to galvanize academia, industry, stakeholders, and policy makers in order to create a resilient, low-risk, productive, and world-leading offshore wind infrastructure and portfolio of wind farms. The MRP was established to prepare a national research agenda that will align the efforts of federal, state, and other entities that share a vision for offshore wind, and suggest how stakeholders can collectively work together to further develop and support an effective national research program. The Wind Energy Center and the Clean Energy Extension program at the University of Massachusetts at Amherst have active education and research programs in the area of offshore wind. With the support of the MRP workshops on Development of a National Offshore Wind Research Agenda were held during 2016 – 17. These workshops mobilized key players in the New England area and beyond and set the stage for further collaboration. Working closely with the National Renewable Energy Laboratory and other institutions, the Partnership for Offshore Wind Energy Research (POWER-US) initiative was recently formed. POWER-US will develop national collaboration that will accelerate the growth of the nascent offshore wind industry in the US. This paper will talk about the initiatives that are currently in progress in the Commonwealth of Massachusetts, including findings from the MRP workshops on Development of a National Offshore Wind Energy Research Agenda, and ongoing research and development initiatives that support development of the supply chain for the offshore wind industry in the Commonwealth and in New England.

The objective of this study is to develop and validate a practical finite-element modeling protocol for predicting the flexural strength and collapse behavior of thin-walled spirally welded...

A monopile supporting an offshore wind turbine (OWT) is currently designed for both strength and stiffness. Regarding strength, the monopile is designed to have sufficient capacity to withstand demands under both 50-year operational conditions, when the rotor is spinning and blades are oriented to optimize power generation, and 50-year extreme conditions, when the rotor is parked and the blades feathered to minimize aerodynamic loads. Regarding stiffness, the monopile is designed to have sufficient stiffness such that the first structural frequency of the OWT falls between the 1P and 3P frequencies (rotation frequency and blade passing frequency for a three-bladed turbine). For six case studies, including three sites along the U.S. Atlantic coast and two mudline conditions (fixed and compliant), this paper delineates the conditions under which stiffness and strength govern the design of the monopile. This distinction has important implications for the overall risk profile of an OWT, as monopiles controlled by stiffness will have more reserve capacity than monopiles controlled by strength. The six case studies are intended to consider a range of water depths, metocean environments and mudline conditions that is representative of conditions suitable for installing OWTs supported by monopiles along the U.S. Atlantic coast. The monopile designs are controlled by stiffness for two of the six cases studies and, for these two cases, a modest (6–8%) reduction in monopile area (and mass) could be achieved if dynamic design requirements were achieved through means other than increasing monopile stiffness. Monopile designs for the remaining four cases are controlled by operational moment demands.

The contribution of foundation damping to offshore wind turbines (OWTs) is not well known, though researchers have back-calculated foundation damping from “rotor-stop” tests after estimating aerodynamic, hydrodynamic, and structural damping with numerical models. Because design guidelines do not currently recommend methods for determining foundation damping, it is typically neglected. This paper investigates the significance of foundation damping on monopile-supported OWTs subjected to extreme storm loading using a linear elastic two-dimensional finite element model. The effect of foundation damping primarily on the first natural frequency of the OWT was considered as OWT behavior is dominated by the first mode under storm loading. A simplified foundation model based on the soil-pile mudline stiffness matrix was used to represent the monopile, hydrodynamic effects were modeled via added hydrodynamic mass, and 1.00% Rayleigh structural damping was assumed. Hysteretic energy loss in the foundation was converted into a viscous, rotational dashpot at the mudline to represent foundation damping. Using the logarithmic decrement method on a finite element free vibration time history, 0.17%-0.28% of critical damping was attributed to foundation damping. Stochastic time history analysis of extreme storm conditions indicated that mudline OWT foundation damping decreases the maximum and standard deviation of mudline moment by 7–9%.

Most offshore wind turbines (OWTs) are designed according to the international standard IEC 61400-3 which requires consideration of several design load cases under 50-year extreme storm conditions during which the wind turbine is not operational (i.e. the rotor is parked and blades are feathered). Each of these load cases depends on combinations of at least three jointly distributed metocean parameters, the mean wind speed, the significant wave height, and the peak spectral period. In practice, these variables are commonly estimated for the 50-year extreme storm using a simple but coarse method, wherein 50-year values of wind speed and wave height are calculated independently and combined with a range of peak spectral period conditioned on the 50-year wave height. The IEC Standard does not provide detailed guidance on how to calculate the appropriate range of peak spectral period. Given the varying correlation of these parameters from site-to-site, this approach is clearly an approximation which is assumed to overestimate structural loads since wind and wave are combined without regard to their correlation. In this paper, we introduce an alternative multivariate method for assessing extreme storm conditions. The method is based on the Nataf model and the Inverse First Order Reliability Method (IFORM) and uses measurements or hindcasts of wind speed, wave height and peak spectral period to estimate an environmental surface which defines combinations of these parameters with a particular recurrence period. The method is illustrated using three sites along the U.S. Atlantic coast near Maine, Delaware and Georgia. Mudline moments are calculated using this new multivariate method for a hypothetical 5 MW OWT supported by a monopile and compared with mudline moments calculated using simpler univariate approaches. The results of the comparison highlight the importance of selecting an appropriate range of the peak spectral period when using the simpler univariate approaches.

Offshore wind turbine (OWT) support structures are subjected to non-proportional environmental wind and wave load patterns with respect to increases in wave height and with respect to wind and wave com- bined loading. Traditional approaches to estimating the ultimate capacity of offshore support structures are not ideally suited to analysis of OWTs. In this paper, the concept of incremental wind-wave (IWWA) analysis of the structural capacity of OWT support structures is proposed. The approach uses static push- over analysis of OWT support structures subject to wind and wave combined load patterns corresponding to increasing mean return period (MRP). The IWWA framework can be applied as a one-parameter approach (IWWA1) in which the MRP for the wind and wave conditions is assumed to be the same or a two-parameter approach (IWWA2) in which the MRPs associated with wind and wave conditions are related to a joint probability density function characterizing the wind and wave conditions at the site. Example calculations for monopile and jacket supported OWTs at Atlantic marine sites are performed under both one parameter and two parameters IWWA framework. The analyses illustrate that: the results of an IWWA analysis are site specific; and structural response can be dominated by either wind or wave conditions depending on structural characteristics and site conditions. Finally, reliability analyses for both examples excluding uncertainties in structural resistance are estimated based on their IWWA results and probabilistic models for site environmental conditions.

A new manufacturing process allows for the production of tapered spirally welded steel tubes. This paper describes a series of eight large-scale tests examining the behavior of such tubes in flexure and investigates the impact of imperfections on the flexural strength of the tube. The tests are performed on tapered circular steel tubes with diameters between 0.7 and 1.1 m and maximum diameter-to-thickness ratios between 200 and 350. Specimen geometries are selected to provide flexural test data at slenderness ratios not commonly tested in the literature and to be representative of tapered tubes applied as wind turbine towers. The geometries of the specimens are measured with laser scanners before and during testing to characterize initial imperfections and the evolution of local buckling. Results are compared to design strengths per Eurocode EN 1993 1-6. All specimens meeting Eurocode manufacturing quality requirements exceed predicted strengths. The location and orientation of the local buckling ...

The bog turtle Glyptemys muhlenbergii is a highly endangered species inhabiting freshwater wetlands of the eastern United States. Habitat degradation is considered one of the chief causes of bog turtle declines. Most of the studies investigating habitat requirements of bog turtles focus on intrawetland influences on bog turtle habitat, but few have explored landscape-level drivers of bog turtle distribution. To better understand causes of bog turtle declines, we combined bog turtle occurrence records with geographic data to contrast 12 landscape variables among wetlands containing extant populations of bog turtles (n = 65) and 500 nearby and 500 distant, randomly located wetlands in southeastern New York State. In contrast to randomly chosen wetlands, bog turtles have persisted in relatively large, open-canopy wetlands (typically >0.42 ha) within large watersheds (typically >13.54 ha), in wetlands with more numerous stream connections, and more often in wetlands positioned atop carbonaceous rock types. Wetlands occurring within carbonaceous surficial geological settings may be more resistant to bog turtle habitat degradation and hence likely to support more persistent populations of bog turtles. Our findings that geomorphic features and intrawetland vegetation are the most useful predictors of bog turtle occurrence and persistence in southeastern New York State are important for generating more detailed hypotheses regarding drivers of bog turtle declines and are also helpful for prioritizing sites for bog turtle conservation and guiding surveys to discover new populations.

A mooring and anchoring concept for floating offshore wind turbines is introduced in which each anchor moors multiple floating platforms. Several possible geometries are identified and it is shown that the number of anchors for a wind farm can be reduced by factors of at least 3. Dynamic simulation of turbine dynamics for one of the candidate geometries and for two directions of wind and wave loading allows estimation of multiline anchor forces the preview the types of loads that a multiline anchor will need to resist. Preliminary findings indicate that the peak demand on the anchor may be reduced by as much as 30% but that anchors used in such a system will need to be able to resist multi-directional loading.Copyright © 2016 by ASME

The stress modified critical strain (SMCS) criterion provides a local index for the initiation of ductile fracture in metals as a function of plastic strain and stress triaxiality. Previous research has confirmed the SMCS criterion to be an accurate index for fracture initiation in mild steels and demonstrated its application to civil/structural engineering. To facilitate practical implementation of the SMCS criterion, two key aspects of its calibration for steel materials are examined. The first pertains to the sensitivity of the measured SMCS material toughness parameter to the size of the test coupon. New results from 23 tests of cylindrically notched tension (CNT) specimens of various sizes and notch geometries indicate that the toughness parameter is relatively insensitive to calibration specimen size. This finding validates the use of miniature bar specimens to calibrate the SMCS model for thin plate steels and in-service structures, where extraction of larger coupons is impossible. The second aspec...

Results from six two-thirds scale tests on moment-resisting steel column base plates are presented. The test specimens incorporate Complete Joint Penetration (CJP) and Partial Joint Penetration (PJP) weld details between the column and the base plate. The test data indicate that both details are resilient to fracture and sustain inelastic column hinging to story drift ratios of 6%–9%, which exceeds the typical acceptance criteria of 4% drift ratio for seismically detailed special moment frames. In five of the six tests, fractures initiated in the Heat Affected Zone (HAZ) at the fusion line between the weld and column flange. In the sixth test, fractures initiated at the inside face of the column flange at the upper edge of the weld access hole of the CJP weld detail. Contrary to the initial expectations, the specimens with the PJP welds exhibited higher displacement ductility than those with CJP welds. This is attributed to the fillet reinforcing that strengthens the PJP welds and enables them to sustain stresses and strains necessary to fully develop yielding in the column flanges. The test data further support the adequacy of the FEMA 350 provisions for determining the required strength of the welds based on the probable moment demand with the material overstrength factor of R y = 1.1 and strain hardening factor of C p r = 1.2 .

Results from 18 large-scale tests of steel bracing members are presented to examine their inelastic buckling and fracture behavior as related to the seismic design of concentrically braced frames. The brace specimens include square hollow structural shapes (HSS), pipe, and wide-flange sections. The effect of various parameters, including width–thickness and slenderness ratios, cross-section shape, loading history, loading rate, and grout fill on the performance of these braces is investigated. Among these parameters, loading history, width–thickness ratio and slenderness ratio are shown to have the largest influence on brace ductility. The test data suggest that for some HSS and pipe specimens, current seismic design provision limits on maximum width–thickness ratios may not provide sufficient ductility for seismic design. Effects of loading rate are found to be insignificant, and the grout fill is shown to provide a modest improvement in cyclic ductility. Measurements of brace stiffness, tensile strength...

The development of renewable energy sources is a critical global need. The Atlantic coast and Gulf of Mexico of the U.S., with large wind resources and proximity to major population centers, are natural places for such development; however, these regions are also at considerable risk from severe hurricanes or tropical cyclones. Current international guidelines for the design of offshore wind turbines (OWTs) do not explicitly consider loading under hurricane conditions, however subsequent editions are anticipated to include language specific to hurricanes. Variability in extreme loads is greater in areas where hurricanes are likely and the design loads and risk profile of offshore structures installed in such areas are expected to be strongly influenced by hurricanes. For many offshore structures, environmental conditions at design recurrence periods and beyond are often estimated through extrapolation of long-term (i.e. multiple decades) wind and wave measurements from buoys, however, for offshore structures located at areas exposed to hurricanes, it is accepted practice to use physics-based models to augment the historical record of Atlantic hurricane activity and generate a stochastic catalog of synthetic hurricanes that provides tens of thousands of realizations for one year of potential hurricane activity. Once a stochastic catalog has been established, appropriate hazard intensity measures (e.g. the one-minute sustained wind speed, the significant wave height, and the peak spectral wave period) can be estimated for each storm at any site using well-known wind and wave parametric models. In this study, we consider several sites along the Atlantic coast and quantify the impact of estimating hazard for design recurrence periods and beyond for three different methods. The first is based on an extrapolation of wind and wave measurements from buoys, and the second and third are based on a stochastic catalog of synthetic hurricanes with wind and wave intensities estimated based on deterministic and probabilistic relationships.Copyright © 2015 by ASME

Approximately 75% of installed offshore wind turbines (OWTs) are supported by monopiles, a foundation whose design is dominated by lateral loading. Monopiles are typically designed using the p-y method which models soil-pile resistance using decoupled, nonlinear elastic Winkler springs. Because cyclic soil behavior is difficult to predict, the cyclic p-y method accounts for cyclic soil-pile interaction using a quasistatic analysis with cyclic p-y curves representing lower-bound soil resistance. This paper compares the Matlock (1970) and Dunnavant & O’Neill (1989) p-y curve methods, and the p-y degradation models from Rajashree & Sundaravadivelu (1996) and Dunnavant & O’Neill (1989) for a 6 m diameter monopile in stiff clay subjected to storm loading. Because the Matlock (1970) cyclic p-y curves are independent of the number of load cycles, the static p-y curves were used in conjunction with the Rajashree & Sundaravadivelu (1996) p-y degradation method in order to take number of cycles into account. All of the p-y methods were developed for small diameter piles, therefore it should be noted that the extrapolation of these methods for large diameter OWT monopiles may not be physically accurate; however, the Matlock (1970) curves are still the curves predominantly recommended in OWT design guidelines. The National Renewable Energy Laboratory wind turbine analysis program FAST was used to produce mudline design loads representative of extreme storm loading. These design loads were used as the load input to cyclic p-y analysis. Deformed pile shapes as a result of the design load are compared for each of the cyclic p-y methods as well as pile head displacement and rotation and degradation of soil-pile resistance with increasing number of cycles.

Successful development of the offshore wind energy resource depends upon the reliability of the structures that support the electricity-generating rotor-nacelle assemblies that form the core of a wind turbine system. The risk posed to offshore wind turbines (OWTs) by extreme environmental events such as Atlantic hurricanes requires a more detailed examination of post-elastic structural performance during events occurring at greater return periods than the design conditions. This paper focuses on the assessment of the fragility of OWTs with jacket-type support structures under extreme environmental loadings. Performance levels ranging from undamaged to near collapse of the jacket supported OWT are defined and are assessed using static pushover analysis of the support structure for given wind and wave conditions. A numerical example is presented based on an environmental hazard model for a site off the coast of the state of Massachusetts along the Atlantic coast of the U.S. Using probabilistic models for the structural demands and capacities, fragility curves are developed for two damage states. It is shown how such fragility curves can be convolved with estimates of the economic consequences associated with the various performance levels to develop a complete risk profile for the support structure.