27,394 research outputs found
A Closed-Form Technique for the Reliability and Risk Assessment of Wind Turbine Systems
This paper proposes a closed-form method to evaluate wind turbine system
reliability and associated failure consequences. Monte Carlo simulation, a widely used
approach for system reliability assessment, usually requires large numbers of computational
experiments, while existing analytical methods are limited to simple system event
configurations with a focus on average values of reliability metrics. By analyzing a wind
turbine system and its components in a combinatorial yet computationally efficient form,
the proposed approach provides an entire probability distribution of system failure that
contains all possible configurations of component failure and survival events. The
approach is also capable of handling unique component attributes such as downtime and
repair cost needed for risk estimations, and enables sensitivity analysis for quantifying the
criticality of individual components to wind turbine system reliability. Applications of the
technique are illustrated by assessing the reliability of a 12-subassembly turbine system. In
addition, component downtimes and repair costs of components are embedded in the
formulation to compute expected annual wind turbine unavailability and repair cost
probabilities, and component importance metrics useful for maintenance planning and
research prioritization. Furthermore, this paper introduces a recursive solution to closed-form
method and applies this to a 45-component turbine system. The proposed approach proves
to be computationally efficient and yields vital reliability information that could be readily
used by wind farm stakeholders for decision making and risk management
Multi criteria risk analysis of a subsea BOP system
The Subsea blowout preventer (BOP) which is latched to a subsea wellhead is
one of several barriers in the well to prevent kicks and blowouts and it is the
most important and critical equipment, as it becomes the last line of protection
against blowout. The BOP system used in Subsea drilling operations is
considered a Safety – Critical System, with a high severity consequence
following its failure. Following past offshore blowout incidents such as the most
recent Macondo in the Gulf of Mexico, there have been investigations, research,
and improvements sought for improved understanding of the BOP system and
its operation. This informs the need for a systematic re-evaluation of the Subsea
BOP system to understand its associated risk and reliability and identify critical
areas/aspects/components.
Different risk analysis techniques were surveyed and the Failure modes effect
and criticality analysis (FMECA) selected to be used to drive the study in this
thesis. This is due to it being a simple proven cost effective process that can
add value to the understanding of the behaviours and properties of a system,
component, software, function or other. The output of the FMECA can be used
to inform or support other key engineering tasks such as redesigning, enhanced
qualification and testing activity or maintenance for greater inherent reliability
and reduced risk potential. This thesis underscores the application of the
FMECA technique to critique associated risk of the Subsea BOP system.
System Functional diagrams was developed with boundaries defined, a FMECA
were carried out and an initial select list of critical component failure modes
identified. The limitations surrounding the confidence of the FMECA failure
modes ranking outcome based on Risk priority number (RPN) is presented and
potential variations in risk interpretation are discussed.
The main contribution in this thesis is an innovative framework utilising
Multicriteria decision making (MCDA) analysis techniques with consideration of
fuzzy interval data is applied to the Subsea BOP system critical failure modes
from the FMECA analysis. It utilised nine criticality assessment criteria deduced
from expert consultation to obtain a more reliable ranking of failure modes. The MCDA techniques applied includes the technique for order of Preference for
similarity to the Ideal Solution (TOPSIS), Fuzzy TOPSIS, TOPSIS with interval
data, and Preference Ranking Organization Method for Enrichment of
Evaluations (PROMETHEE). The outcome of the Multi-criteria analysis of the
BOP system clearly shows failures of the Wellhead connector, LMRP hydraulic
connector and Control system related failure as the Top 3 most critical failure
with respect to a well control. The critical failure mode and components
outcome from the analysis in this thesis is validated using failure data from
industry database and a sensitivity analysis carried out. The importance of
maintenance, testing and redundancy to the BOP system criticality was
established by the sensitivity analysis. The potential for MCDA to be used for
more specific analysis of criteria for a technology was demonstrated.
Improper maintenance, inspection, testing (functional and pressure) are critical
to the BOP system performance and sustenance of a high reliability level.
Material selection and performance of components (seals, flanges, packers,
bolts, mechanical body housings) relative to use environment and operational
conditions is fundamental to avoiding failure mechanisms occurrence. Also
worthy of notice is the contribution of personnel and organisations (by way of
procedures to robustness and verification structure to ensure standard expected
practices/rules are followed) to failures as seen in the root cause discussion.
OEMs, operators and drilling contractors to periodically review operation
scenarios relative to BOP system product design through the use of a Failure
reporting analysis and corrective action system. This can improve design of
monitoring systems, informs requirement for re-qualification of technology
and/or next generation designs. Operations personnel are to correctly log in
failures in these systems, and responsible Authority to ensure root cause
analysis is done to uncover underlying issue initiating and driving failures
Structural Vulnerability Analysis of Electric Power Distribution Grids
Power grid outages cause huge economical and societal costs. Disruptions in
the power distribution grid are responsible for a significant fraction of
electric power unavailability to customers. The impact of extreme weather
conditions, continuously increasing demand, and the over-ageing of assets in
the grid, deteriorates the safety of electric power delivery in the near
future. It is this dependence on electric power that necessitates further
research in the power distribution grid security assessment. Thus measures to
analyze the robustness characteristics and to identify vulnerabilities as they
exist in the grid are of utmost importance. This research investigates exactly
those concepts- the vulnerability and robustness of power distribution grids
from a topological point of view, and proposes a metric to quantify them with
respect to assets in a distribution grid. Real-world data is used to
demonstrate the applicability of the proposed metric as a tool to assess the
criticality of assets in a distribution grid
Assessing the critical material constraints on low carbon infrastructure transitions
We present an assessment method to analyze whether the disruption in supply of a group of materials endangers the transition to low-carbon infrastructure. We define criticality as the combination of the potential for supply disruption and the exposure of the system of interest to that disruption. Low-carbon energy depends on multiple technologies comprised of a multitude of materials of varying criticality. Our methodology allows us to assess the simultaneous potential for supply disruption of a range of materials. Generating a specific target level of low-carbon energy implies a dynamic roll-out of technology at a specific scale. Our approach is correspondingly dynamic, and monitors the change in criticality during the transition towards a low-carbon energy goal. It is thus not limited to the quantification of criticality of a particular material at a particular point in time. We apply our method to criticality in the proposed UK energy transition as a demonstration, with a focus on neodymium use in electric vehicles. Although we anticipate that the supply disruption of neodymium will decrease, our results show the criticality of low carbon energy generation increases, as a result of increasing exposure to neodymium-reliant technologies. We present a number of potential responses to reduce the criticality through a reduction in supply disruption potential of the exposure of the UK to that disruption
PointMap: A real-time memory-based learning system with on-line and post-training pruning
Also published in the International Journal of Hybrid Intelligent Systems, Volume 1, January, 2004A memory-based learning system called PointMap is a simple and computationally efficient extension of Condensed Nearest Neighbor that allows the user to limit the number of exemplars stored during incremental learning. PointMap evaluates the information value of coding nodes during training, and uses this index to prune uninformative nodes either on-line or after training. These pruning methods allow the user to control both a priori code size and sensitivity to detail in the training data, as well as to determine the code size necessary for accurate performance on a given data set. Coding and pruning computations are local in space, with only the nearest coded neighbor available for comparison with the input; and in time, with only the current input available during coding. Pruning helps solve common problems of traditional memory-based learning systems: large memory requirements, their accompanying slow on-line computations, and sensitivity to noise. PointMap copes with the curse of dimensionality by considering multiple nearest neighbors during testing without increasing the complexity of the training process or the stored code. The performance of PointMap is compared to that of a group of sixteen nearest-neighbor systems on benchmark problems.This research was supported by grants from the Air Force Office of Scientific Research (AFOSR F49620-98-l-0108, F49620-0l-l-0397, and F49620-0l-l-0423)
and the Office of Naval Research (ONR N00014-0l-l-0624)
Universal Organization of Resting Brain Activity at the Thermodynamic Critical Point
Thermodynamic criticality describes emergent phenomena in a wide variety of
complex systems. In the mammalian brain, the complex dynamics that
spontaneously emerge from neuronal interactions have been characterized as
neuronal avalanches, a form of critical branching dynamics. Here, we show that
neuronal avalanches also reflect that the brain dynamics are organized close to
a thermodynamic critical point. We recorded spontaneous cortical activity in
monkeys and humans at rest using high-density intracranial microelectrode
arrays and magnetoencephalography, respectively. By numerically changing a
control parameter equivalent to thermodynamic temperature, we observed typical
critical behavior in cortical activities near the actual physiological
condition, including the phase transition of an order parameter, as well as the
divergence of susceptibility and specific heat. Finite-size scaling of these
quantities allowed us to derive robust critical exponents highly consistent
across monkey and humans that uncover a distinct, yet universal organization of
brain dynamics
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