Optimization of the preventive maintenance plan of a series components system

One of the most critical problems in preventive maintenance is the determination of the optimum frequency to perform preventive maintenance in equipment, in order to ensure its availability. In this paper, we propose an algorithm to solve the previous problem for equipment that exhibit linearly increasing hazard rate and constant repair rate. Based on this algorithm, we have developed another one to solve the problem of maintenance management of a series system based on preventive maintenance over the different system components. We assume that all components of the system still exhibit linearly increasing hazard rate and constant repair rate and that preventive maintenance would bring the system to the as good as new condition. We define a cost function for maintenance tasks (preventive and corrective) for the system. The algorithm calculates the interval of time between preventive maintenance actions for each component, minimizing the costs, and in such a way that the total downtime, in a certain period of time, does not exceed a predetermined value.     

A bivariate process model for maintenance and inspection planning

The paper describes decision making about monitoring and maintenance of systems described by a general stochastic process. The system is monitored and preventive and corrective maintenance actions are carried out in response to the observed system state. The decision process is simplified by using an associated process as well as the underlying state as decision variables. The bivariate approach allows a wide class of models to be considered including long term memory within a simple probability structure. Both average cost and life-cycle cost models are used as the basis for decision making. The models generalize age replacement and other simple maintenance strategies. The approach can deal with failures that prevent the system functioning further, failures defined by regulation, or by economic considerations. The unified framework developed allows the inclusion of covariates and imperfect inspection or repair.     

Implications of the surry piping failure for other nuclear and fossil units

This paper describes the key elements of an inspection plan to address utility concerns of flow assisted corrosion due to single-phase flow in carbon steel piping. The key elements of the plan are (a) where to look, (b) how to look, (c) when to look, and (d) how to respond. The plan is designed to provide utilities with the ability to predict piping thickness as a function of plant life for a given component and to assess the costs and benefits of a variety of remedy/repair options. The thrust of the inspection plan is to (a) conduct appropriate analysis and a limited but thorough baseline inspection program, (b) determine the extent of thinning, if any, and repair/replace components as necessary, and (c) perform follow-up inspections to confirm or quantify thinning and take longer term corrective actions (i.e. adjust chemistry, operating parameters, or others) as appropriate.     

Combining discrete-time Markov processes and probabilistic fracture mechanics in RI-ISI risk estimates

Based on the research conducted in the Finnish SAFIR project, which is a national nuclear energy research program, discrete-time Markov processes and probabilistic fracture mechanics (PFM) methods are further developed and applied in this paper. The purpose of this work is to increase the accuracy of risk estimates used in RI-ISI, and to quantitatively evaluate the effects of different inspection strategies on risk. Piping failure probabilities are obtained by using PFM analyses. PFM has the advantage that its results are not affected by existing in-service inspection (ISI) activities at the nuclear power plants (NPPs), unlike failure probabilities assessed from existing failure data. The PFM results for crack growth are used to construct transition matrices used in a discrete-time Markov process. The application of Markov process allows the examination of effects of inspections on the failure probabilities. Finally, the developed method and results are showcased by applying them to a selected piping system in an existing Finnish NPP.     

A probabilistic model for evaluating the remaining life ofevaluating the remaining life of electrical insulation in rotatingmachines

A Markov model is introduced for estimating the remaining life of electrical insulation in large motors and generators. The model incorporates various levels of insulation deterioration and periodic (minor) maintenance. The model permits computation of the average time before insulation failure occurs. A mathematical formulation of the expected transition time from any deterioration state to the failure state (expected remaining life) is given. Data requirements are discussed, and numerical examples are presented. This model would also allow assessment through sensitivity studies of the relative effects of various other parameters in the process which are controllable, e.g. maintenance policy and stress level. Even though it is theoretically possible to apply the Markov models to any particular insulation deterioration mechanism, this approach is best applied to an overall insulation determination problem which combines the effects of all individual processes. The models are fairly flexible. They allow, for example, representation of changing stresses acting on the machine insulation as the utility operating environment changes     

Capital improvement project evaluation: a hydroelectric generation economic perspective

Hydropower is a capital‐intensive energy source with low operations and maintenance (O&M) cost and essentially no fuel costs. Consequently, the levelized cost of energy methodology is a very sensitive component to investment costs and interest rates but less sensitive to economic lifetimes. Because current hydroelectric plants are classified as one of the least costly sources of power generation, there are, however, a wide assortment of site‐specific capital costs and capacity factors. Maintenance activities, predominantly fixed O&M costs, are unjustifiable fundamentals in the capital improvement scheme. The conduct of an explicit aging asset in the hydroelectric generation confines is determined by its previous performance and condition assessment. The asset repair, upgrade, and/or replacement strategies are derived from the documented condition valuations to facilitate specific requirements in the capital improvement program. Moreover, the routine evaluation of the equipment with essential inspections and testing delivers a comprehensive insight of its condition before expensive replacement initiatives are imposed. The application of these evaluation approaches and implements described in this paper are designed to aid the decision‐maker with invaluable information of the equipment condition. Copyright © 2015 John Wiley & Sons, Ltd.     

Maintenance optimization for subsea oil pipelines

In this paper, the classical PF interval model, proposed for reliability centered maintenance, is used to optimize the preventive replacement policy of a subsea oil pipeline. The pipeline is subject to failure modes that may lead to oil release and pollution of the sea. The proposed replacement policy is based on regular condition measurements of the quality of an internal pipeline coating (lining). Holes in the coating will expose the pipe material (steel) to a corrosive environment. It is, therefore, important to identify holes in the coating and take action before the corrosion penetrates the pipe wall. The model proposed in the paper can be used to determine the inspection interval that minimizes the average maintenance cost per time unit. The model formulation is outlined and illustrative examples are solved. The mathematical evaluation of the replacement policy requires numerous assumptions. These assumptions have to be tailored to the specific application. The assumptions and the data provided are included for illustrative purposes to exemplify potential results that may be obtained by using the replacement policy.     

可靠度约束下的不完全预防更换维修模型

针对可修系统因维修更换计划不合理而导致的不能充分利用设备剩余寿命问题,在综合考虑动态维修成本和可靠度的基础上,提出一种可靠度约束下的不完全预防更换维修方法,建立基于混合故障率的非等周期不完全预防维修模型.以可靠度为约束,给出设备更换维修周期的计算公式,采用枚举法给出最优更换阈值;以平均维修成本最小为目标,给出最佳预修次...     

European developments in NDE for steam generators

A high standard of integrity is demanded of the steam generator tubing in nuclear power stations to minimise the cost of any loss of availability due to a need for repairs or plugging. If any deterioration should occur it is desirable to identify it before failure in order to permit preventive maintenance to be considered and, particularly for prototype plant, provide early guidance as to the possible need for changes in the design of later plant.

These requirements have resulted in the continuing development of inspection techniques for application at the fabrication stage and subsequently in-service. For the latter it is desirable that inspections do not extend the length of shutdowns beyond that required for core refuelling and routine maintenance and the associated statutory plant inspections.

The above needs apply to all reactor types but the in-service inspection undertaken is very much affected by the type of plant and the available operating experience with similar units. Thus the acceptable leak rate between sodium and water in the fast reactor steam generator is measured in grammes per hour while kilogrammes per hour would be a more appropriate unit for PWRs. For the latter there is an additional requirement to lower the radiation exposure during inspection.

This paper discusses reported European developments in the field and concentrates on the problems of inspecting the tubing and tube-to-tubeplate joints. Also, although the ultimate integrity of a plant unit is dependent on the quality of the original fabrication with its associated inspection, most attention is given to ISI.

Apart from the continuing development of eddy current techniques for austenitic tubing, the past five years have seen an increasing interest in ultrasonic techniques for detailed tubing examination while digital data handling has been demonstrated in the field. In the fast reactor area the development of improved NDT procedures for both fabrication and ISI of explosive welds is helping to provide the basic technological know-how which is essential if explosive welding is to be considered as a serious design option for large scale application for the tube-to-tubeplate joints in large steam generators.     

Cost optimal reliability based inspection and replacement planning of piping subjected to CO2 corrosion

A methodology for cost optimal reliability based inspection and replacement planning of piping subjected to CO₂ corrosion is described. Both initial (design phase) and in-service planning are dealt with. The methodology is based on the application of methods for structural reliability analysis within the framework of Bayesian decision theory. The planning problem is formulated as an optimization problem where the expected lifetime costs are minimized with a constraint on the minimum acceptable reliability level. The optimization parameters are the number of inspections in the expected lifetime, the inspection times and methods. In the design phase the nominal design wall thickness is also treated as an optimization parameter. The most important benefits gained through the application of the methodology are consistent evaluation of the consequences of different inspection and replacement plans, consistent incorporation and handling of uncertainties, and consistent updating of inspection and replacement plans based on inspection results. The latter is achieved through application of Bayesian statistics for updating in combination with structural reliability methods.     

Modelling wind turbine degradation and maintenance

This paper presents an asset model for offshore wind turbine reliability accounting for the degradation, inspection and maintenance processes. The model was developed based on the Petri net method that effectively captures the stochastic nature of the dynamic processes through the use of appropriate statistical distributions. The versatility of the method allows the details of the degradation and maintenance operations to be incorporated in the model. In particular, there are dependent deterioration processes between wind turbine subsystems, complex maintenance rules and the incorporation of condition monitoring systems for early failure indication to enable replacement prior to failure. The purposes of the model are to predict the future condition of wind turbine components and to investigate the effect of a specified maintenance strategy. The model outputs are statistics indicating the performance of the wind turbine components; these include the probability of being in different condition states, the expected number of maintenance actions and the average number and duration of system downtime under any maintenance strategy. Copyright © 2015 John Wiley & Sons, Ltd.     

Study of ageing of laminates and its effects on wind turbine blade deflection

Some wind turbines have exceeded their nominal design service life and are continuing their operation with periodic inspections and maintenance. In the case of rotor blades, the reliability of the inspection is very limited because of the blade structure that comprises laminates and sandwich structures, which are very difficult to monitor. For this reason, wind farm owners are searching for technologies or approaches that will guarantee a safe operation of their wind turbines after the design life has elapsed. The main objective of this paper was to investigate whether detection of ageing of wind turbine blades using deflection as key parameter is feasible using commercial equipment. The paper is divided in three phases. In phase 1, the effect of ageing on a new UD‐0° glass fibre with high moduli was obtained. Using these results and bibliography data, a theoretical study was performed in phase 2 to determine the magnitude of blade deflection along its lifetime due to material ageing. Finally, in phase 3, in‐field deflection measurements where performed on a wind turbine blade to evaluate the utility and limitations of commercial equipment for the detection of blade ageing. It was concluded that material ageing could result in an increase in blade deflection under self‐weight that can be detected using commercial measurement equipment. These results can be used by wind farm owners in their O&M strategies to monitor blades over time and decide whether they should be repaired or replaced.     

Risk-based decision making method for maintenance policy selection of thermal power plant equipment

This study presents a decision-making method for maintenance policy selection of power plants equipment. The method is based on risk analysis concepts. The method first step consists in identifying critical equipment both for power plant operational performance and availability based on risk concepts. The second step involves the proposal of a potential maintenance policy that could be applied to critical equipment in order to increase its availability. The costs associated with each potential maintenance policy must be estimated, including the maintenance costs and the cost of failure that measures the critical equipment failure consequences for the power plant operation. Once the failure probabilities and the costs of failures are estimated, a decision-making procedure is applied to select the best maintenance policy. The decision criterion is to minimize the equipment cost of failure, considering the costs and likelihood of occurrence of failure scenarios. The method is applied to the analysis of a lubrication oil system used in gas turbines journal bearings. The turbine has more than 150 MW nominal output, installed in an open cycle thermoelectric power plant. A design modification with the installation of a redundant oil pump is proposed for lubricating oil system availability improvement.     

基于Markov决策过程的舰船燃气轮机运行与维护成本建模

针对燃气轮机性能退化特性和维护保养策略分析,考虑机组的随机故障失效和退化失效,运用Mark·ov决策过程方法,提出了基于状态的燃气轮机运行与维护成本模型.通过算例分析,验证了模型的可行性,并比较了三种不同保养策略的可用度和经济性.     

Risk analysis of maintenance ship collisions with offshore wind turbines

A large number of offshore wind farms are planned to be built in remote deep-sea areas over the next five years. Though offshore wind sites are often located away from commercial ship traffic, the increased demand for repair or replacement services leads to high traffic densities of “maintenance ships”. To date, the risk analysis of collision between maintenance ship vessels and offshore wind turbines has received very little attention. In this paper, we propose a methodology to evaluate and prioritise the collision risks associated with various kinds of ships used for carrying out maintenance tasks on different subassemblies of wind turbines in an offshore wind farm. It is also studied how the risks of ship collision with wind turbines are distributed between two main types of maintenance tasks, namely corrective and preventative. The proposed model is tested on an offshore wind turbine with seventeen components requiring five kinds of ships to perform the maintenance tasks. Our results indicate that collision risks are mostly associated with maintenance of few components of the wind turbine and in particular, those undergoing a corrective maintenance (replacement). Finally, several mitigation strategies are introduced to minimise the risk of maintenance ship collisions with offshore wind turbines.     

基于威布尔分布的风机齿轮箱元件最优更换时间

研究了考虑单位时间系统维护费用最低的风电机组齿轮箱重要组成元件最优更换时间的计算问题。文章对风机齿轮箱的原理和结构进行了简单介绍,并将齿轮箱作为不可修系统对其构成元件的故障率分布进行了分析;建立了风机齿轮箱重要组成元件的最优更换时间计算模型,该模型的原理是对更换成本和故障成本进行平衡,使系统单位时间的维护费用最低;最后,采用文中建立的模型对某地实际风机齿轮箱齿轮、中速轴承和高速轴承的最优更换时间进行求解,结果表明,此方法得到的最优更换方案能够极大地降低风电机组齿轮箱维护费用。     

A Risk-Based Performance Analysis of Plate-andFrame Heat Exchangers Subject to Fouling: Economics of Heat Exchanger Cleaning

Fouling is one of the major uncertainties associated with the operation and maintenance of plate-and-frame heat exchangers (PHEs) in the steel and process industries. The decision regarding periodic maintenance (i.e., cleaning) to meet the target performance level is generally based on both thermal and economic behavior of the process. In this paper, we present a cost model that includes the risk level and the scatter parameter of random fouling growth models. Two models (namely, power law and exponential fouling growth) are integrated in the model. The non-dimensional cost function of reduced time is examined by considering dimensionless cost parameters, representing additional cost due to a decrease in effectiveness, pumping power cost, anti-foulant cost, cleaning cost, additional pumping power cost of the standby unit during cleaning, and miscellaneous costs. These dimensionless cost elements are examined for a PHE operating in the steel industry. The results are presented in terms of the risk level and scatter parameter for the underlying fouling models. Furthermore, a simplified closed-form solution is also obtained to study the optimal cycle time, which represents the minimum cost of operation and maintenance of heat exchangers. It is found that the optimum dimensionless cost increased by about 21 and 14% for the power law and exponential fouling models, respectively, as the risk level decreased from 0.5 to 0.01     

Wind farm generation forecast and optimal maintenance schedule model

Wind farms must periodically take their turbines offline in order to perform scheduled maintenance repairs. Given that these interruptions impact energy generation and that under Power Purchase Agreements productions shortfalls must be replaced by energy purchases in the spot market, the optimal time to begin maintenance work in a wind farm is a function of both the expected wind speeds and electricity spot prices. In this article, we develop a model to determine the optimal maintenance schedule of a wind farm based on forecasted wind speeds and energy prices. We analyze a window of opportunity in the most likely period of the year and perform weekly updates of expected wind speeds and energy price forecasts. Wind speeds are forecasted with an ARMAX model, where monthly dummies are used as exogenous variables to capture the seasonality of wind speeds, while spot prices are simulated under a standard dual stochastic programing model. The decision to defer maintenance to a future date is modeled in a probabilistic model and also under the real options approach. We test these models with actual data from a wind farm in the Brazilian Northeast and provide comparisons with current practice in order to determine the benefits of the model. The results suggest that this model may provide advantages over a stopping decision that randomly chooses a week to begin maintenance within the opportunity window and is close to the optimal stopping date considering perfect information on future wind speeds and electricity prices.     

基于FMECA和FTA正向综合分析的风电场设备故障管理

将FMECA和FTA综合方法引入到风电场设备故障管理中,在剖析风电场设备结构、功能的基础上,利用FMECA分析风电场设备的故障模式、故障原因、故障影响及危害性,利用故障树分析方法,对高固有风险的故障进行定性、定量分析,指导工作人员制定风电场设备预防性维修决策。     

Failure Identification of Induction Motors by Sensing Unbalanced Stator Currents

The maintenance of electric drives is essential to their efficient and safe operation in any industry. The lack of an effectively designed and implemented maintenance policy will result in increased downtime, increased capital losses from catastrophic failures, and in some cases, a reduced level of personnel safety. Traditional approaches to maintenance include statistical-replacement policies and preventive-maintenance testing. The relative success or failure of these approaches is dependent on the specific industrial setting. However both have inherent limitations. For example, replacement policies tend to increase costs because motors are often taken out of service while still in good condition. Off-line testing procedures, such as insulation-resistance measurements, have limited value since they can be difficult to interpret and are applicable to the detection of a limited number of failure modes. On-line and continuous monitoring methods offer significant advantages in this regard, since incipient failures can be detected, and appropriate maintenance actions can be scheduled, thereby avoiding the costs and problems associated with unexpected failures. Thus far, research in this area has focused on sensing magnetic flux, stator current, rotor current, shaft flux, ground fault current, vibrations, temperature, and speed fluctuations. The first five methods are based on sensing electrical or magnetic parameters, and the remaining are based on monitoring mechanical quantities. A classical type of current transformer is found very effective in sensing the unbalanced currents in the three-phase power system where highresistance grounding is used, such as in an underground coal mine.