Ajit Srividya

A comprehensive framework for evaluation of piping reliability due to erosion–corrosion for risk-informed inservice inspection

Abstract Risk-Informed In-Service Inspection (RI-ISI) aims at prioritizing the components for inspection within the permissible risk level thereby avoiding unnecessary inspections. The two main factors that go into the prioritization of components are failure frequency and the consequence of the failure of these components. The study has been focused on piping component as presented in this paper. Failure frequency of piping is highly influenced by the degradation mechanism acting on it and these frequencies are modified as and when maintenance/ISI activities are taken up. In order to incorporate the effects of degradation mechanism and maintenance activities, a Markov model has been suggested as an efficient method for realistic analysis. Emphasis has been given to the erosion–corrosion mechanism, which is dominant in Pressurized Heavy Water Reactors. The paper highlights an analytical model for estimating the corrosion rates and also for finding the failure probability of piping, which can be further used in RI-ISI.

A systems approach to integrated E-maintenance of large engineering plants

Large engineering plants (LEP) such as power plants, process plants, ship-borne machinery and aircraft have certain unique features that a combination of both time and condition based maintenance of the plants is considered necessary. It has, however, been observed that applied research leading to a systematic development of such a maintenance strategy needs further study. Such a strategy should not only harness the advantages of both time and condition based maintenance but also adopt a wholesome ‘Systems’ approach so that the realisation of the overall objectives of maintenance is maximised. e-Maintenance has in the last decade emerged as a coherent and effective amalgamation of information and communication technologies in the arena of maintenance playing a crucial role in maintenance decision making. The focal area of application of e-maintenance has been that of condition based maintenance—e-monitoring, e-diagnosis and e-prognosis. It may be observed that as far as LEPs are concerned, the capability of e-maintenance can be enhanced to add value to the entire maintenance strategy for the plant. In this paper a maintenance strategy for LEPs involving an integration of both time and condition based maintenance which also harnesses e-maintenance features is suggested. Frameworks are presented for maintenance decision making, integrated TBPM and CBPM, e-maintenance of LEPs and a systems approach to maintenance of LEPs.

Machinery Condition Monitoring System Selection A Multi-Objective Decision Approach Using GA

In this paper, a framework for Condition Based Maintenance (CBM) of large engineering systems has been suggested to address the need to optimize the characteristics of both the condition monitoring system and the plant that is being maintained at a macro level. CBM for large engineering systems has been modeled using Markov process. The issues which are critical to CBM, namely, predictability, detectability as well as implications of availability and cost have been considered in the framework to provide effective decision support. The application of the framework has been demonstrated using a numerical example. The trade offs between the Pareto optimal solutions of various objectives have been studied and the effects of variation of the maintenance objective function values with detectability and predictability have been brought out.

ANP-based software reliability prediction using PoCs and subsequent employment of orthogonal defect classification measurements for risk mitigation during prototype studies

In a process centric software development, early reliability prediction plays a vital role in ensuring the project success. Inherent mitigation of risks can be obtained by relying on the conceptualization of Rational Unified Process (RUP). By implanting RUP methodologies into the project build, a continuum of benefits can be reaped intrinsically. Quality of Service (QoS) can be directly improved during the prototyping. Instead of having to strictly depend on prototype implementation for the necessary information that is vital to improvising upon the design functionalities, we have earlier developed frameworks that enable the bypassing of the prototyping modules. Now, a procedure to identify the potential critical phases of a software development process using Analytic Network Process (ANP), before the implementation of prototype is put forward. The results are compared with those obtained on realizing the prototype. While analyzing the information from the prototype realization, measures to strengthen the effectiveness of the whole set-up with the intent of risk mitigation are explained. This involves the reliance on Orthogonal Defect Classification (ODC) for a better reliability prediction.

Optimal time scheduling for carrying out minor maintenance on a steam turbine

Almost all the major equipment of ships or industry today undergo regular maintenance on its components and sub-components. In most of the cases it is observed that there are generally two kinds of maintenance in terms of times to repair and its effect on the availability of the main equipment; minor maintenance and a major maintenance or overhaul. The minor maintenance are generally carried at a component or a sub-component level, either at fixed intervals or based on the component condition. The minor maintenance however, requires that the equipment is brought down from operation for a shorter duration and put back in operation immediately after completion of the minor maintenance. The major maintenance on the other hand is carried out preferably in a workshop, where the equipment is completely stripped down to undertake maintenance which otherwise would not have been possible in its original location (for e.g. inside a ship). The time required for carrying out major maintenance is therefore substantial when compared to the minor maintenance period. The paper puts forward a method for scheduling minor maintenance of a major equipment—a steam turbine of a ship, within the time frame of the major maintenance interval based upon the deterioration of the components. The wear processes of the important components of the turbine are assumed to be time variant gamma processes and allow for continuous monitoring of the wear. Since turbines are a complex mix of components, sub-components and other ancillary systems, most of which are easily replaceable in miniscule of time periods, the paper considers only the most important wearing components which play a direct part in deterioration of performance of the turbine. All other random failures, which based on the past experience are only a very few in numbers and have very little impact on the availability of the turbine, have been ignored. MLE and a Gibbs sampling method has been used to estimate the parameters of distribution of gamma wear processes for the components of turbine.

Use of petrinets for solution of a stern gland optimal inspection interval problem

Petrinets are a useful tool for graphical and mathematical modelling of complex systems as it greatly simplifies the study of the dynamic behaviour of such systems. It is therefore not surprising that it finds its utility in a vast variety of applications. However, its use in analysis of mechanical systems is limited. The limitation in its applicability to mechanical system stems from the fact that most of the mechanical system components, in a race towards failure, follow a non-exponential failure time distribution where the ‘age memory’ of the component plays a significant role. The paper discusses a method to circumvent the above limitations by use of a ‘method of stages’ that can be used to model the wear process as well as exploit the memory less property of various exponentially distributed transition stages and simplify retention of the age memory of the non-exponential process that it models. Application of petrinets for evaluation of various stage probabilities on a sample problem of stern gland of a ship is discussed and results compared with the analytical approach.

Approximation of MTTF calculation of a non-stationary gamma wear process

Gamma distribution is known as the most suitable distribution to model the monotonically increasing wear or deterioration. Examples of wear or deterioration can be found in many of the mechanical components, such as in wear of the bearings, corrosion or erosion wear of pump casings and impellers etc. The component wearing gradually can be considered to have failed when it reaches a pre-specified, optimal wear limit. Calculation of MTTF (mean time to failure or to reach a wear or deterioration limit) is however cumbersome and time consuming as it has to be integrated numerically. The author presents an approximation formula to calculate the MTTF for a gamma wear process with temporal variability which can be easily applied to most of the mechanical components undergoing wear. Error in calculation of MTTF using the theoretical and the approximation formula have been displayed in a graphical form as a ratio of the theoretical MTTF.

Improvement of QoS in Process Centric Software Development Using ANP

To find out the critical phase in the software environment that may be responsible for more defects than other phases involves the selection of a suitable decision making method for realizing the goal. Among Analytical Hierarchy Process (AHP), Fuzzy AHP and Analytical Network Process (ANP), the latter is regarded as a viable alternative, which yields a holistic framework in the selection of the most critical phases in the presence of interdependencies. A practical case study concerning the software development for a financial application which involves the utilization of a tool driven approach like Rational Unified Process (RUP) is made use of to demonstrate the effectiveness of ANP.