Statistical Analysis of Structural Plate Mechanical Properties

The purpose of this research is to survey the mechanical properties of A572 and A588 plates produced in North America. The study focuses on three aspects: chemical properties, tensile properties, and toughness properties. Results from this study can be of benefit to specification-writing bodies and other users interested in the variability of mechanical properties of A572 and A588 plates. The results can also help update present databases on plate properties that do not include modern production techniques and new mills and producers.--Introduction

Model Uncertainties for Soil-Structure Interaction in Offshore Wind Turbine Monopile Foundations

Monopiles are the most common type of foundation used for bottom-fixed offshore wind turbines. This investigation concerns the influence of uncertainty related to soil–structure interaction models used to represent monopile–soil systems. The system response is studied for a severe sea state. Three wave-load cases are considered: (i) irregular waves assuming linearity; (ii) highly nonlinear waves that are merged into the irregular wave train; (iii) slamming loads that are included for the nonlinear waves. The extreme response and Fourier amplitude spectra for external moments and mudline bending moments are compared for these load cases where a simpler static pile-cap stiffness and a lumped-parameter model (LPM) are both considered. The fundamental frequency response of the system is well represented by the static pile-cap stiffness model; however, the influence of higher modes (i.e., the second and third modes with frequencies of about 1 Hz and 2 Hz, respectively) is significantly overestimated with the static model compared to the LPM. In the analyzed case, the differences in the higher modes are especially pronounced when slamming loads are not present

The Influence of Foundation Modeling Assumptions on Long-term Load Prediction for Offshore Wind Turbines,”

ABSTRACT In evaluating ultimate limit states for design, time-domain aeroelastic response simulations are typically carried out to establish extreme loads on offshore wind turbines. Accurate load prediction depends on proper modeling of the wind turbulence and the wave stochastic processes as well as of the turbine, the support structure, and the foundation. One method for modeling the support structure is to rigidly connect it to the seabed; such a foundation model is appropriate only when the sea floor is firm (as is the case for rock). To obtain realistic turbine response dynamics for softer soils, it is important that a flexible foundation is modeled. While a single discrete spring for coupled lateral/rotational motion or several distributed springs along the length of the monopile may be employed, a tractable alternative is to employ a fictitious fixed-based pile modeled as an "equivalent" cantilever beam, where the length of this fictitious pile is determined using conventional pile lateral load analysis in combination with knowledge of the soil profile. The objective of this study is to investigate the influence of modeling flexible pile foundations on offshore wind turbine loads such as the fore-aft tower bending moment at the mudline. We employ a utility-scale 5MW offshore wind turbine model with a 90-meter hub height in simulations; the turbine is assumed to be sited in 20 meters of water. For a critical wind-wave combination known to control long-term design loads, we study time histories, power spectra, response statistics, and probability distributions of extreme loads for fixed-base and flexible foundation models with the intention of assessing the importance of foundation model selection. Load distributions are found to be sensitive to foundation modeling assumptions. Extrapolation to rare return periods may be expected to lead to differences in derived nominal loads needed in ultimate limit state design; this justifies the use of flexible foundation models in simulation studies. INTRODUCTION Nominal loads for the design of wind turbines in ultimate limit states are generally established from time-domain aeroelastic response simulations. The accuracy of these derived loads depends on the number of simulations and on how realistically the models used to represent the turbine, support structure, and foundation describe the true structural response. One potential shortcoming in modeling foundations relates to their flexibility. A single pile (often referred to as a monopile) is the most common type of foundation used today for offshore wind turbines; the support structure connects to such a pile foundation that extends some depth below the mudline. One way a monopile foundation could be modeled is by means of a rigid connection at the mudline. This model ignores the soil profile and the associated soil-pile stiffness and, as such, would not account for the pile's expected lateral/rocking movement. Such simplifying assumptions could only adequately simulate the behavior of a monopile founded in rock. Many offshore wind turbines, however, are founded on softer soils where the monopile experiences at least some movement at and below the mudline. It is therefore worth assessing the accuracy of the use of a fixed-base model versus a flexible foundation model. In the present study, we carry out fixed-based model simulations and study turbine loads (specifically, the fore-aft tower bending moment). These are compared with loads derived using a flexible foundation model. This latter model utilizes stiffness properties derived from the soil profile at the location of the turbine by means of a conventional pile foundation analysis and appropriate p-y lateral load-deflection relationships. The flexible foundation model involves derivation of an "apparent fixity length" representing a distance below the mudline where an equivalent cantilever yields the same lateral movement and rotation as the monopile experiences in the pile analysis with the true soil properties. The mass per unit length of the equivalent cantilever is adjusted to match the sub-soil mass o

Luminosity for laser-electron colliders

High intensity laser facilities are expanding their scope from laser and particle-acceleration test beds to user facilities and nuclear physics experiments. A basic goal is to confirm long-standing predictions of strong-field quantum electrodynamics, but with the advent of high-repetition rate laser experiments producing GeV-scale electrons and photons, novel searches for new high-energy particle physics also become possible. The common figure of merit for these facilities is the invariant $\chi\simeq 2\gamma_e|\vec E_{\rm laser}|/E_c$ describing the electric field strength in the electron rest frame relative to the ``critical'' field strength of quantum electrodynamics where the vacuum decays into electron-positron pairs. However, simply achieving large $\chi$ is insufficient; discovery or validation requires statistics to distinguish physics from fluctuations. The number of events delivered by the facility is therefore equally important. In high-energy physics, luminosity quantifies the rate at which colliders provide events and data. We adapt the definition of luminosity to high-intensity laser-electron collisions to quantify and thus optimize the rate at which laser facilities can deliver strong-field QED and potentially new physics events. Modeling the pulsed laser field and electron bunch, we find that luminosity is maximized for laser focal spot size equal or slightly larger than the diameter of the dense core of the electron bunch. Several examples show that luminosity can be maximized for parameters different from those maximizing the peak value of $\chi$ in the collision. The definition of luminosity for electron-laser collisions is straightforwardly extended to photon-laser collisions and lepton beam-beam collisions

Operations and maintenance for multipurpose offshore platforms using statistical weather window analysis

With increasing offshore-related commerce, the choice of appropriate operations and maintenance activities must take into consideration safety, costs and performance targets. Stochastic weather conditions at each site of interest presents uncertain situations. We present an optimized decision making procedure that seeks to maximize monetary benefits while minimizing safety risks. Our proposed approach outlines and illustrates application of such a policy by incorporating traditional weather window analysis using a Markov Decision Process approach. In particular, the approach is applied in case study involving the operation of a multipurpose platform at an offshore Scotland site

Long-term global response analysis of a vertical axis wind turbine supported on a semi-submersible floating platform: Comparison between operating and non-operating wind turbine load cases

This study continues [1] the examination of the long-term global response of a floating vertical axis wind turbine (VAWT) situated off the Portuguese coast in the Atlantic Ocean. The VAWT, which consists of a 5-MW 3-bladed H-type rotor developed as part of the EU-FP7 H2OCEAN project, is assumed to be mounted on the OC4 semi-submersible floating platform. Adding a non-operational load case (wind speed ~35m/s), the sea states identified are used to carry out coupled dynamics simulations using the FloVAWT design tool, for which an improved wave elevation and relative force/moment time signals approach is adopted, as well as also taking into account the drag generated by the wind turbine tower. Short-term turbine load and platform motion statistics are established for individual sea states that are analysed. The long-term reliability yields estimates of 50-year loads and platform motions that takes into consideration response statistics from the simulations as well as the metocean (wind-wave) data and distributions. The results show that it is not necessarily the load cases associated with the highest wind speed that lead to the highest displacements and tensions in the mooring lines; differences among the highest values in some load cases are too similar to establish which one should be considered in a conceptual/preliminary design phase. Also, the aerodynamic drag model needs to be further developed to improve its accuracy. These results and insights contribute to the development of improved floating VAWT design guidelines for reducing the cost of offshore wind energy generation