Andrew T. Myers

Probabilistic Formulation of the Cyclic Void Growth Model to Predict Ultralow Cycle Fatigue in Structural Steel

The previously developed cyclic void growth model (CVGM) has been demonstrated to accurately simulate ductile fracture initiation under monotonic and ultralow cycle fatigue loading for a variety of steel materials and geometric configurations. Prediction of ductile fracture initiation involves significant uncertainty, particularly where there is high variability in the material (e.g., welded connections) subjected to irregular cyclic loading. The reliability of the model predictions is improved through a probabilistic formulation based on maximum likelihood parameter estimation. The probabilistic formulation, which incorporates information from both the failure and nonfailure loading cycles, has the following features: (1) the calibration of model parameters provides the maximum likelihood of agreement for a given set of cyclic fracture observations, and (2) fracture predictions are provided in a probabilistic sense by generating a distribution of the expected instant of fracture. The benefit of the approach is twofold. First, it eliminates an inconsistency that is inherent in the deterministic calibration procedure, as proposed in the original development of the CVGM. Second, the degree of certainty of fracture predictions is quantified. In combination, these features significantly enhance the robustness of the framework within which the model is implemented. Although this paper applies this approach in the specific context of the CVGM, the method can be generalized to other models that share similar characteristics.

Variability of breaking wave characteristics and impact loads on offshore wind turbines supported by monopiles

Most existing and planned offshore wind turbines (OWTs) are located in shallow water where the possibility of breaking waves impacting the structure may influence design. Breaking waves and their associated impact loads are challenging to model because the breaking process is a strongly non-linear phenomenon with significant statistical scattering. Given the challenges and uncertainty in modeling breaking waves, there is a need for comparing existing models with simultaneous environmental and structural measurements taken from utility-scale OWTs exposed to breaking waves. Overall, such measurements are lacking; however, one exception is the Offshore Wind Turbines at Exposed Sites project, which recorded sea state conditions and associated structural loads for a 2.0 MW OWT supported by a monopile and located at the Blyth wind farm off the coast of England. Measurements were recorded over a 17 month campaign between 2001 and 2003, a period that included a storm that exposed the instrumented OWT to dozens of breaking waves. This paper uses the measurements from this campaign to categorize and identify breaking waves and quantify the variability of their impact loads. For this particular site and turbine, the distribution of measured mudline moments due to breaking waves has a mean of 8.7MN-m, a coefficient of variation of 26% and a maximum of 14.9MN-m. The accuracy of several breaking wave limits and impact force models is compared with the measurements, and the impact force models are shown to represent the measurements with varying accuracy and to be sensitive to modeling assumptions. Copyright

Sensitivity of the Dynamic Response of Monopile-Supported Offshore Wind Turbines to Structural and Foundation Damping

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.

Wind-wave prediction equations for probabilistic offshore hurricane hazard analysis

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.

Efficient Multiline Anchor Systems for Floating Offshore Wind Turbines

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

The Impact of Peak Spectral Period in the Design of Offshore Wind Turbines for the Extreme Sea State

Most offshore wind turbines (OWTs) are designed according to the international standard International Electrotechnical Commission (IEC) 61400-3 which requires consideration of several design load cases under extreme sea state conditions during which the wind turbine is in survival mode (i.e. the rotor is parked and blades are feathered). Each of these load cases depends on combinations of two random variables, the mean wind speed and the significant wave height, both with a mean return period of 50 years. The response of an offshore wind turbine under wave loading is known to be sensitive to both the significant wave height and a frequency measure of the sea state such as the peak spectral period. The IEC standard states that design calculations for the extreme sea state should be based on values of peak spectral period which result in the highest loads acting on the structure, but does not provide additional guidance. The Standard does provide a deterministic range for the period of the extreme wave conditioned on the significant wave height, and this can be converted to a range of peak spectral period using published empirical relationships. This paper considers an offshore location off the coast of Georgia, where the National Oceanic and Atmospheric Administration (NOAA) buoy 41008 is located, and shows that a deterministic range of peak spectral period converted from the range provided in the IEC Standard may not accurately represent measured data. Moreover, the paper shows that the response of a hypothetical offshore wind turbine, installed at this location and supported by a monopile foundation, is sensitive to variation in the peak spectral period, emphasizing the importance of modeling the turbine for an appropriate and possibly site-specific range of peak spectral periods. A probabilistic approach is proposed to find an appropriate and site-specific range of the peak spectral period for the design of offshore wind turbines under the extreme sea state.

Reliability analysis of monopile offshore wind turbine support structures

We probe the reliability of monopile support stru ct res designed to support industrial scale turbines along the coastal United States using stochastic m odels for the wind and wave loadings, and representations of the uncertainty associated with soil propert ies. The turbine support structure investigated is that promulgated by the National Renewable Energy Laboratory as t pical of a monopile support structure designed for tens of meters of water depth and a characteristic wind/w ave environment. We investigate the structural reliability using structural finite element models develop ed in MATLAB and a commonly used industry tool, FAST, developed and distributed by NREL. Reliability investigations include the effect of spatial c orrelation of soil properties on reliability with respect t o serviceability and the combined effects of loading and soil property uncertainty on structural performance. We also comment on the interaction between the tower/pile desi gn space and the resulting reliability, allowing us to comment on the effect tower geometry may have on reliability . FAST uses reduced order structural models in the pursuit of c omputational efficiency, and we evaluate the efficacy of these models for structural behaviors which may e nter the nonlinear regime. These investigations include the ability of FAST to capture structural model shap es with large curvature gradients, and the effect of mode shape approximation on time-history dynamic analysis .

Large Scale Tests and Micromechanics-Based Models to Characterize Ultra Low Cycle Fatigue in Welded Structural Details

Fracture and fatigue-induced failure in welded structural details is an important limitstate in earthquake resistant design. Despite its significance, fundamental, physics-based models to simulate Ultra Low Cycle Fatigue (ULCF) in base and weld metals are not readily available, and many of the popular approaches predict ULCF in an empirical manner without considering the complex interactions of stress and strain histories responsible for it. While convenient, these empirical methods may not be reliably transferable to untested details or connections. In this paper, newly developed physicsbased models that aim to simulate ULCF at a continuum level (and apply them through finite element analyses) are introduced. Preliminary results from experiments on six column base plate specimens are presented. These tests, part of a NEESR project, seek to validate these physics-based models. From a practical standpoint, these experiments provide important insights into modes and hierarchies of failure of column base plate details, especially fracture originating in the welds and heat affected zone. The parameters considered include variations in cyclic loading histories and in weld details similar to configurations commonly used in engineering practice.

Static Flexural Local Buckling Tests on Large Scale Spirally Welded Tubes for Use as Wind Turbine Towers

Taller, more economical wind turbine support towers are necessary for the future optimization of wind energy generation. Trends in current wind tower designs clearly demonstrate this need. Many large turbine manufacturers have begun production on large towers (taller than 120 m) that employee non-traditional wind turbine tower manufacturing techniques. Keystone Tower System’s (KTS) spiral welding manufacturing process for tapered steel towers is one example of a potential tall-tower manufacturing solution. Spiral welding is common in the pipeline industry where structural demands are influenced by internal pressurization. In contrast, the demand on wind turbine towers is dominated by flexural loading without internal pressurization. The lack of existing studies on the flexural behavior of spirally welded tubes is exacerbated by the high sensitivity to local buckling inherent in wind turbine towers, which have diameter-to-thickness ratios (D/t) that can exceed 300. For such structures, failure is dependent on initial geometric imperfections, including those induced by welding and other manufacturing procedures. Since spirally welded wind turbine towers will have a unique combination of welds, the impact of this particular manufacturing process on the ultimate local buckling strength of the towers must be understood. For this reason, a series of static, flexural tests have been undertaken on large scale spirally welded tubes. The preliminary results of these tests will be presented and discussed.

Comparison of Cyclic P-Y Methods for Offshore Wind Turbine Monopiles Subjected to Extreme Storm Loading

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.Copyright