考虑事故后频率约束的两阶段经济调度

为了保证事故后电力系统仍能正常运行,基于多阶段决策的思想提出了考虑事故后频率约束的两阶段随机经济调度模型。该模型中同时考虑事故前(第一阶段)和事故后(第二阶段)系统的运行成本,分别称为"当前成本"和"未来成本"。第一阶段的优化目标函数考虑在满足供电可靠性的条件下使常规机组和风电机组的发电成本最小;为了保证系统频率在事故后较短时间内恢复正常,第二阶段考虑了对低频减载的约束,目标函数为常规机组和风电机组的发电成本,风能高估低估的惩罚成本以及减负荷的惩罚成本最小。针对模型的非光滑性,采用光滑化方法对其进行处理,并以IEEE30节点系统进行仿真,仿真结果验证了新模型和算法的有效性。     

计及需求响应的风电储能两阶段调度优化模型及求解算法

为解决风电功率不确定性对系统稳定运行带来的影响,在含风电的系统优化调度问题中引入需求响应和储能系统。首先利用区间法模拟风电场景并构建了基于Kantorovich距离的场景削减策略,然后分别在需求侧和发电侧引入需求响应和储能系统,结合2阶段优化理论,以风电日前预测功率和超短期预测功率作为随机变量及其实现,构建了计及需求响应的风电储能2阶段调度优化模型。为求解该模型,在传统二进制粒子群算法中引入混沌搜索,构建了混沌二进制粒子群算法。最后,以IEEE 36节点10机系统进行算例仿真。结果表明,混沌二进制粒子群算法能够得到全局最优解,适用于风电储能系统2阶段模型求解;利用需求响应和储能系统的协作效应,可以抑制风电功率的不确定性,提高系统风电利用效率,降低系统发电煤耗水平,因此综合效益显著。     

External corrosion to concrete sewers: a case study

The aim of this study was to investigate the deterioration of concrete sewers and identify responsible factors; this investigation was a part of an ongoing asset evaluation for Rennes (France). The sewer studied was a 300 mm spun concrete pipe laid in 1992. Core samples (D 28 mm, L 40 mm) were taken from different areas of a pipe (invert, crown and side wall) and analysed to quantify the condition of the concrete and the extent of damage to the internal and external surface of the pipe. Physical and mechanical properties of the material were characterised. The depassivation of the concrete was measured by testing with a phenolphthalein solution. The study showed that in this particular case the external corrosion was extensive with minimal internal corrosion.

It is proposed that the corrosion was possibly linked to the use of a contaminated backfill or organic acids in the ground water. Recommendations are proposed concerning the direction of future research.     

CFRP confinement of bridge columns damaged by premature concrete deterioration: a case study in Texas

In 1998, the Texas Department of Transportation (TxDOT) identified two concrete distress mechanisms occurring on its bridge over Lake Ivie in the west central part of the state: alkali – silica reaction (ASR) and delayed ettringite formation (DEF), collectively referred to by TxDOT as premature concrete deterioration (PCD). The agency sponsored two research projects to investigate the problem. One focused on determining the remaining capacity of PCD-damaged concrete members with a case study of the Lake Ivie bridge and the other assessed treatments that could be applied to members with PCD damage. Based on findings of the study, TxDOT applied a system that waterproofed the damaged columns and then confined them using resin-bonded carbon fibre reinforced polymer (CFRP) fabric strips. This paper describes the efforts of TxDOT to mitigate PCD damage for its Lake Ivie bridge.     

Corrosion initiation time updating by epistemic uncertainty as an alternative to schedule the first inspection time of pre-stressed concrete vehicular bridge beams

As traffic demands grow constantly and some vehicle bridges deteriorate because of corrosion issues, bridge agencies require non-expensive procedures to support decisions about cost-effective maintenance schedules. In this article, a reliability-based formulation is proposed for the prediction of the optimal first inspection time including both the corrosion deterioration and the epistemic uncertainty on the corrosion initiation time. For the identification of the bridge integrity state, where little or no follow-up has been previously developed, the prediction of a damage state implies a great deal of epistemic uncertainty. The impact of this kind of uncertainty on the corrosion initiation time prediction is appraised in order to include the conservative estimations of such a time, according to the bridge revenues/cost ratio of further and more detailed studies. The time-varying bridge reliability is calculated in terms of the bridge corrosion deterioration, which induces a moment capacity reduction of the bridge beams. Epistemic uncertainty is introduced in the corrosion initiation time, and the optimal first inspection time is obtained as a probability distribution. Consequently, a procedure to calculate the first time for inspection on girder bridges has been proposed, based on updating a known distribution after considering the effect of epistemic uncertainty, using a lognormal distributed factor as ‘evidence’, by means of the Markov chain Monte Carlo technique.     

Functional and residual capital values as criteria for water pipe renewal

Water pipes are considered as tangible assets designed to provide a level of service throughout an expected lifetime. The asset can be characterised by two types of values: capital or accounting value and functional value. The capital value of assets depreciates according to a specific rate per time period. The depreciation is assessed by the annual amortisation of the capital asset, which constitutes a potential self-budgeting for future renewal. The development in the last 20 years of public sector accounting standards boards dedicated to tangible capital assets management has encouraged water and wastewater utilities to consider capital depreciation as a decision-making criterion for asset prioritisation. At the same time, when the capital depreciates the asset function deteriorates, but this deterioration is more difficult to assess. In fact, each asset can be evaluated according to a functional value that indicates the capacity of the asset to deliver the designated service at the required level. This value declines during the service life because of the occurrence of unexpected events (failures, breaks, leaks and degradation). The functional deterioration, also referred to as ‘obsolescence’ or ‘reliability’, is still misunderstood for water pipe assets. This study addresses the following questions: How can the functional value be estimated for water systems both at the pipe and at the network scale? How can the decision-making process for pipe renewal be improved by incorporating specific indicators based on both functional and accounting values? The current research focuses on the definition of an appropriate time-dependent functional value, based on the following hypothesis: the functional value depends on both structural and hydraulic deterioration of the pipe asset, but is partially restored by curative maintenance actions. The calibration of the functional value is mainly based on historical incident data, roughness data and managers' opinions. In order to carry out the decision-making process, the evolution of the functional value is simultaneously analysed with the depreciation of the capital asset value. The implementation of the approach at the pipe and the network level leads to an assessment of innovative criteria for the purpose of assessing possible renewal policies.     

Optimal intervention strategies for multiple objects affected by manifest and latent deterioration processes

In the existing infrastructure management systems, optimal interventions strategies (OISs) are determined for objects that deteriorate gradually (manifest deterioration process, MDPs), under the assumption that with appropriate inspection and intervention strategies the probability of failure of object can be neglected. Objects that deteriorate suddenly (latent deterioration process, LDPs), for example, due to scouring during a flood or earth movements during an earthquake are not considered. The determination of OISs for an object that deteriorates due to both MDPs and LDPs requires the consideration of both. The latter, however, means that the probability of failure of the object must be considered. In this article, a Markov model is presented that can be used to determine OISs for multiple objects of multiple types affected by uncorrelated MDPs and LDPs. The model is an extension of the model proposed by Mayet and Madanat (Incorporation of seismic considerations in bridge management systems. Computer-Aided Civil and Infrastructure Engineering, 17:185–193, 2002). In the model, a set of condition states (CSs) is used to describe the condition of objects of each type, where each set is composed of non-failure CSs and failure CSs. The probabilities of going from each non-failure CS to each failure CS are estimated using normalised fragility curves, and the probabilities of going from each non-failure CS to each non-failure CS are initially estimated using the Markov deterioration prediction model of Kobayashi, Kaito, and Lethanh (A Bayesian estimation method to improve deterioration prediction for infrastructure system with Markov chain model. International Journal of Architecture, Engineering and Construction, 1:1–13, 2012a) and later adjusted taking into consideration the probabilities of entering the failure CSs. The use of the model is demonstrated using a road link comprising one road section and one bridge.     

Characterisation of the behaviour of steel bridge bearings under cyclic load reversal

Seismic hazards pose a significant concern for older steel bridges in the Central and Eastern USA. The majority of these bridges still use steel bearings that are not designed to sustain earthquake loads. To assess older bridge vulnerability, an understanding of the cyclic behaviour of their components, such as steel bolster and rocker bearings, is needed. A theoretical analysis is conducted to understand steel bearing behaviour and their potential limit states, which also serves to validate bearing finite element models. The high fidelity models are used to evaluate the hysteretic behaviour of the bearings. The influence of gravity load on the response is evaluated to consider different bridge configurations, while varying the friction coefficient between steel surfaces allows for consideration of effects from ageing and corrosion. These parameters are shown to influence the response suggesting the need to consider ageing effects in assessing the vulnerability of in situ bridges.     

Robustness of corroded reinforced concrete structures – a structural performance approach

The aim of this work is to provide new contributions in order to define more accurately the structural robustness concept, particularly when applied to corroded reinforced concrete (RC) structures. To fulfil such a task, several robustness indicators are analysed and discussed with special emphasis on structural-performance-based measures. A new robustness definition and a framework are then proposed for its analysis, based on the structural performance lost after damage occurrence. The competence of the proposed methodology is then tested comparing the robustness of two RC foot bridges under corrosion. The damage considered is the longitudinal reinforcement corrosion level, and load carrying capacity is the structural performance evaluated. In order to analyse corrosion effects, a finite element (FE) based on a two-step analysis is adopted. In the first step, a cross-section analysis is performed to capture phenomenons such as expansion of the reinforcement due to the corrosion products accumulation; damage and cracking in the reinforcement surrounding concrete; steel–concrete bond strength degradation; effective reinforcement area reduction. The results obtained are then used to build a 2D structural model, in order to assess the maximum load carrying capacity of the corroded structure. For each foot bridge, robustness is assessed using the proposed methodology.     

Optimal FRP-strengthening strategy for corrosion-affected reinforced concrete columns

Abstract

Deterioration of reinforced concrete (RC) structures is unavoidable after their long time in service, with corrosion being the major mechanism of deterioration. In order to ensure safety of deteriorated structures, an effective rehabilitation plan is essential. Although considerable research on strengthening of RC structures using fibre-reinforced polymers (FRPs) composites has been undertaken, more is on the methods of strengthening and effects of corrosion on strength of RC columns than that on the prediction of optimum strengthening time. This paper presents a methodology for determining the optimal strengthening time and the required number of FRP layers for corrosion-affected RC structures with application to columns. The methodology is based on the time-dependent reliability method and the renewal theory. An example is provided to illustrate the application of the proposed methodology. It is found in this study that an optimum point for the formulated objective function exists, and that outcomes of optimisation problem, i.e. strengthening time and number of required FRP layers, are sensitive to corrosion rate. The significance of the proposed methodology is that it provides guidance for practitioners and asset managers to decide when and how to strengthen deteriorated structural members.