631 research outputs found
A coloured Petri net framework for modelling aircraft fleet maintenance
The aircraft fleet maintenance organisation is responsible for keeping aircraft in a safe, efficient operating condition. Through optimising the use of maintenance resources and the implementation of maintenance activities, fleet maintenance management aims to maximise fleet performance by, for example, ensuring there is minimal deviation from the planned operational schedule,that the number of unexpected failures is minimised or that maintenance cost is kept at a minimum. To obtain overall fleet performance, the performance of individual aircraft must first be known. The calculation of aircraft performance requires an accurate model of the fleet operation and maintenance processes. This paper aims to introduce a framework that can be used to build aircraft fleet maintenance models. A variety of CPN (coloured Petri nets) models are established to represent fleet maintenance activities and maintenance management, as well as the factors that have a significant impact on fleet maintenance including fleet operation, aircraft failure logic and component failure processes. Such CPN models provide an ideal structured framework for Monte Carlo simulation analysis, within which calculations can be performed in order to determine numerous fleet reliability and maintenance performance measures
Supporting group maintenance through prognostics-enhanced dynamic dependability prediction
Condition-based maintenance strategies adapt maintenance planning through the integration of online condition monitoring of assets. The accuracy and cost-effectiveness of these strategies can be improved by integrating prognostics predictions and grouping maintenance actions respectively. In complex industrial systems, however, effective condition-based maintenance is intricate. Such systems are comprised of repairable assets which can fail in different ways, with various effects, and typically governed by dynamics which include time-dependent and conditional events. In this context, system reliability prediction is complex and effective maintenance planning is virtually impossible prior to system deployment and hard even in the case of condition-based maintenance. Addressing these issues, this paper presents an online system maintenance method that takes into account the system dynamics. The method employs an online predictive diagnosis algorithm to distinguish between critical and non-critical assets. A prognostics-updated method for predicting the system health is then employed to yield well-informed, more accurate, condition-based suggestions for the maintenance of critical assets and for the group-based reactive repair of non-critical assets. The cost-effectiveness of the approach is discussed in a case study from the power industry
A Repairable System Supported by Two Spare Units and Serviced by Two Types of Repairers
We study a one-unit repairable system, supported by two identical spare units on cold standby, and serviced by two types of repairers. The model applies, for instance, to ANSI (American National Standard Institute) centrifugal pumps in a chemical plant, and hydraulic systems in aviation industry. The failed unit undergoes repair either by an in-house repairer within a random or deterministic patience time, or else by a visiting expert repairer. The expert repairs one or all failed units before leaving, and does so faster but at a higher cost rate than the regular repairer. Four models arise depending on the number of repairs done by the expert and the nature of the patience time. We compare these models based on the limiting availability
, and the limiting profit per unit time
, using semi-Markov processes, when all distributions are exponential. As anticipated, to maximize , the expert should repair all failed units. To maximize a suitably chosen deterministic patience time is better than a random patience time. Furthermore, given all cost parameters, we determine the optimum number of repairs the expert should complete, and the optimum patience time given to the regular repairer in order to maximize
Some stochastic problems in reliability and inventory
An attempt is made in this thesis to study some stochastic models of both reliability and
inventory systems with reference to the following aspects:
(i) the confidence limits with the introduction of common-cause failures.
(ii) the Erlangian repair time distributions.
(iii) the product interactions and demand interactions.
(iv) the products are perishable.
This thesis contains six chapters.
Chaper 1 is introductory in nature and gives a review of the literature and the techniques
used in the analysis of reliability systems.
Chapter 2 is a study of component common-cause failure systems. Such failures may
greatly reduce the reliability indices. Two models of such systems (series and parallel)
have been studied in this chapter. The expressions such as, reliability, availability and
expected number of repairs have been obtained. The confidence limits for the steady
state availability of these two systems have also been obtained. A numerical example
illustrates the results.
A 100 (1 - a) % confidence limit for the steady state availability of a two unit hot and
warm standby system has been studied, when the failure of an online unit is constant and
the repair time of a failed unit is Erlangian.
The general introduction of various inventory systems and the techniques used in the
analysis of such systems have been explained in chapter 4.
Chapter 5 provides two models of two component continuous review inventory systems.
Here we assume that demand occurs according to a poisson process and that a demand
can be satisfied only if both the components are available in inventory. Back-orders
are not permitted. The two components are bought from outside suppliers and are
replenished according to (s, S) policy. In model 1 we assume that the lead-time of
the components follow an exponential distribution. By identifying the inventory level
as a Markov process, a system of difference-differential equations at any time and the
steady-state for the state of inventory level are obtained. Tn model 2 we assume that the
lead-time distribution of one product is arbitrary and the other is exponential. Identifying
the underlying process as a semi-regenerative process we find the stationary distribution
of the inventory level. For both these models, we find out the performance measures such
as the mean stationary rate of the number of lost demands, the demands satisfied and the
reorders made. Numerical examples for the two models are also considered.
Chaper 6 is devoted to the study of a two perishable product inventory model in which
the products are substitutable. The perishable rates of product 1 and product 2 are two
different constants. Demand for product 1 and product 2 follow two independent Poisson
processes. For replenishment of product 1 (s, S) ordering policy is followed and the
associated lead-time is arbitrary. Replenishment of product 2 is instantaneous. A demand
for product 1 which occurs during its stock-out period can be substituted by product 2 with
some probability. Expressions are derived for the stationary distribution of the inventor}'
level by identifying the underlying stochastic process as a semi-regenerative process. An
expression for the expected profit rate is obtained. A numerical illustration is provided
and an optimal reordering level maximising the profit rate is also studied.
To sum up, this thesis is an effort to improve the state the of art of (i) complex reliability
systems and their estimation study (ii) muitiproduct inventory systems. The salient
features of the thesis are:
(i) Analysis of a two-component reliability system with common-cause failures.
(ii) Estimation study of a complex system in which the repair time for both hot standby
and warm standby systems are assumed to be Eriangian.
(iii) A multi-product continuous review inventory system with product interaction, with a
(s, S) policy.
(iv) Introduction of the concept of substitutability for products.
(v) Derivation of expressions for various statistical measures.
(vi) Effective use of the regeneration point technique in deriving various measures for both
reliability and inventory systems.
(vii) Illustration of the various results by extensive numerical work.
(vii) Consideration of relevant optimization problems.Mathematical SciencesPhD (Statistics
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A review of asset management literature on multi-asset systems
This article gives an overview of the literature on asset management for multi-unit systems with an emphasis on two multi-asset categories: fleet (a system of homogeneous assets) and portfolio (a system of heterogeneous assets). As asset systems become more complicated, researchers have employed different terms to refer to their specific problems. With an
objective to facilitate readers in searching conducive studies to their interests, this paper establishes a novel classification scheme for multi-unit systems in accordance with essential features such as diversity of assets and intervention options. Moreover, discerning differences in characteristics between cross-component and cross-asset interactions, we select three types of potential multi-component dependencies (performance, stochastic, and resource) and extend their notions to be applicable to multi-asset systems. The investigation into these dependencies enables the identification of problems that could exist in real industrial settings
but are yet to be determined in academia. Ultimately, we delve into modelling approaches adopted by previous researchers. This comprehensive information allows us to offer the insights into the current trends in multi-asset maintenance. We expect that the output of this review paper will not only stress research gaps on multi-asset systems, but more importantly
help systematise future studies on this aspect
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