ResearchIntegrated remanufacturing, maintenance and spares policies towards life extension of a multi-component system
Introduction
Due to market competitiveness, customer social responsibility drive and compliance to reduce adverse environmental effects (e.g., lower carbon footprint, minimize waste generation), organizations are increasingly considering integrating sustainable remanufacturing strategies within their maintenance processes, to achieve lower environmental impact in asset maintenance management. This is equally critical towards the United Nations sustainable development goals (SDG) number 12 - Responsible Consumption and Production [1]. Here, extending the lifespan of a deteriorating and ageing equipment should meticulously be accounted for when designing maintenance strategies, to substantially reduce waste generation.
The extension of the lifespan of deteriorating and often ageing assets, especially at their End of Life (EOL), presents a critical challenge which a good deal of maintenance-intensive plants are facing. As an example, the lifespan of several plants, including nuclear reactors in the United States, the United Kingdom and France, have been extended by twenty years [2]. Besides, new large capacity refineries have not been constructed in the US over the past 40 years [3]. When equipment approaches the designed EOL, and the decision on the end of its service life is not taken, two critical challenges are introduced.
Firstly, managers of ageing technical systems face challenges of developing decisions to mitigate expected increased age-related equipment failures, while considering both reductions in maintenance and operational cost and waste generation.
The second challenge entails the technical aspects like spare part obsolescence, which may adversely affect the operation of a manufacturing facility or plant [4]. Unexpected obsolescence refers to sudden changes, which means the spare part is no longer available, which may include logistical obsolescence where the part is no longer produced, or functional obsolescence, where the item does no longer comply with current operational requirements [5]. Hence, plants may require an alternative spare-parts management strategy to address this potential challenge.
Therefore, to address these two challenges, there is a need to formulate and evaluate an integrated maintenance approach while considering equipment life extension and spare-part management strategies of an ageing plant. Maintenance and life extension strategies considering the end-of-life phase involves the shift from a linear economy to a Circular Economy. Linear economy represents the traditional production of take-make-use-dispose, while circular economy considers principles of reducing, reuse and recycle to slow, close or narrow the resource loops [6]. Typical examples of equipment life extension strategies include repair, reuse, remanufacturing and reconditioning of a component [7]. Integration of maintenance and remanufacturing provides the solution to these two challenges. Remanufacturing offers immense benefits like extending the life of an asset by facilitating reuse, restoring a used part to as good as new condition, the reduced cost compared to new products, reduced repair time and part supply lead time [8,9].
Despite the envisaged benefits of remanufacturing, several authors (e.g., [9]) have argued that remanufacturing strategies have marginalized the active role of end-users and the maintenance function, albeit decisions related to remanufacturing a component ultimately are in the hands of the end-user. By contrast, research in the remanufacturing domain has been devoted to addressing the perspectives of the manufacturer (OEM) and remanufacturer (OEM/R), like optimization of the logistics and remanufacturing process and production, and marketing of “remanufacturable” products [9]. Here remanufacturers (OEM/R) include OEM and third-party agents involved in remanufacturing. For example, the OEM/R evaluates a returned component from the end-user and ultimately decides how best to remanufacture the part, basing their decision on a remanufacturability index. The index is derived as a product of indices representing environmental, cost and technical aspects, of which these metrics focus on the remanufacturing process, for instance, cleaning, assembly and disassembly [10]. However, notwithstanding remanufacturing being a complicated process, disregarding the maintenance function and the end-user role in contributing to the remanufacturing decision may lead to suboptimal decision support. This is because of the following reasons.
The maintenance strategies selected and employed by the end-user significantly affect the core characteristics (quality, demand, and condition). For instance, the end-user application of interventions like major repair may significantly reduce the remanufacturability of a component compared to less intense interventions like an adjustment. Moreover, suboptimal maintenance activities may lead to an increased failure rate and affect the availability of cores [11]. For example, tightening bolts below the torque settings may catalyze vibrations and wear, ultimately cause the failure of a component, often rendering it non-remanufacturable. Hence, the critical role of maintenance strategies developed and implemented by the end-user should be viewed from the lens of the remanufacturing. Currently, this is not the case as the maintenance role and how it affects the core status is not considered by the OEM/R. This view can be seen by the stakeholders as; first, an enabling system to sustain the equipment throughout its life cycle. Secondly, as a vital tool to keep the renewal potential or remanufacturability of the equipment.
Looking at the roles of the end-user, they are responsible for assessing when a component is obsolescent, and if so, to send it to the OEM/R for remanufacturing or exchange with a remanufactured part from a common spares pool. The introduction of the remanufactured exchange parts implies that industrial plants must maintain multiple types of spares in their inventory provisioning strategy. The inventory strategies exhibit unique cost, supply chain and inventory management characteristics which the end-user should apply. For example, the used part returned to OEM/R in exchange of a remanufactured part, should be remanufacturable to avoid incurring the surcharge expense. Therefore, for optimal and timely remanufactured exchange parts replenishment, the OEM/R would require parts or cores returned for remanufacturing to retain high remanufacturability. This exposes a cardinal challenge where the end-user needs to accurately predict and classify a part or core as remanufacturable to evade risks like protracted downtime and additional surcharge expenses charged by the remanufacturer.
Several authors have considered incorporating maintenance and life extension strategies [11], [12], [13]. Most of the studies are devoted to exploring the OEM/R perspectives, for instance, planning and optimizing of maintenance and remanufacturing process during the Preventive Maintenance (PM) policy. Previous studies do not provide direct evidence of considering the end-user activities in the remanufacturing decision process. Another common assumption made in the literature (e.g., [14]), is the optimization of life extension strategies like remanufacturing (role of the OEM/R) as “stand-alone” while disregarding maintenance policies (end-users role). This assumption disregards alternative end-user maintenance interventions employed under corrective maintenance (CM), like minimal repair and direct reusing a part that ultimately affects the core condition of the part and influences the remanufacturing decision. The OEM/R utilizes the remanufacturability index developed and assigned during research and design of a part for remanufacturing decision. The use of this index by the OEM/R for remanufacturing decision may cause a misunderstanding by the end-user because it is not empirically identifiable and apprehensible to the end-user. Hence, from the end-user perspective, component remanufacturability assessment and decision process seem to be perceived as a ‘black box’. Additionally, cross-cutting issues like alternative options to new spares and costs, generate potential conflict between the stakeholders due to their divergent preferences. For instance, the use of new parts instead of remanufactured exchange parts may primarily be inferred by OEM/R, due to benefits like accurate reliability and higher margins. However, the end-user may prefer remanufactured exchange parts due to their reduced cost compared to purchasing new parts. These, among other conflicting roles and preferences between the end-user and OEM/R, introduces an asymmetric relationship.
The interactive and asymmetric roles played by both the end-user and the OEM or OEM/R while deciding the recovery strategy (referring to both maintenance and life extension strategies) to employ, remain under-explored in literature. Moreover, a systematic understanding of how the maintenance function and the end-user contributes to the remanufacturing decision making is still lacking. Owing to these aspects being under-explored, a clear view of the end user's impact and involvement in the decision derivation is not permitted. Hence, maintenance and equipment life extension strategies like remanufacturing should be integrated into a singular framework, therefore.
- (i)
to derive robust decision support while considering the interactions between these strategies, and
- (ii)
to incorporate the asymmetric relationship between the end-user and OEM/R in the decision process.
This paper, therefore, develops a holistic framework integrating maintenance, remanufacturing, and multiple spare strategies for life extension of an ageing multi-component system. Multiple spares strategy is employed to mitigate ageing effect challenges like spare part obsolescence and the increased failure rate of the system. Moreover, the framework considers the asymmetric relationship while defining the roles and interactions of the stakeholders; in this case, the end-user and OEM or OEM/R towards optimizing recovery cost.
The remaining part of the paper proceeds as follows: In Section 2, a brief review of relevant literature is presented. In Section 3, the study and assumptions are delved in, while Section 4, advances the case study simulation model formulation. Section 5 involves the presentation of the results and their discussion, while lastly, Section 6 presents the conclusions and suggestions for future research.
Section snippets
Review of relevant literature
As alluded in Section 1, four vital areas of concern in this research are reviewed in this section. The domains examined include the various product life extension strategies, maintenance policies, applicable spares inventory policies, and lastly the strategy employment decision making.
Notations
Number of recovery actions; = {1,2,3,4,6,7} Number of components modelled; = {1,2,3,4} Severity levels; = {1,2,3,4} Remanufacturability levels; = {1,2,3,4,5} Inventory identifiers Process identifiers Remanufactured spare parts Remanufacturing process New parts Minimal Reuse process Empty Cell Minimal Repair process Decision variables Percentage residual life-reuse PM interval (hrs.) Percentage residual life-repair Cleaning interval (hrs.) Number of times Recovery actions Number of
Case study system description and assumptions
In this paper, a use-case simulation-based approach of a turbocharger system in a thermal power plant is advanced to demonstrate the methodology developed. The turbine rotor assembly was established as critical following a criticality analysis of components in a turbocharger by [38]. Therefore, the components modelled include the turbine rotor shaft, turbine blades and compressor wheel. We include the bearings which suspend the rotor assembly inside the turbocharger, therefore, part and parcel
Results and discussion
The case study of the turbocharger is simulated in Arena 15.10, where the simulation time of 105,120 hours ( 12 years) is used, depicting the second half of the system lifespan. The complete lifespan of the system is 24 years. A warmup period of 10000 hours is implemented. The model is run with the current system conditions and variables to derive the base scenario results (Section 5.1). To establish the significant modelling parameters impacting the performance measure, a design of experiment
Managerial implications
The most prominent finding to emerge from the analysis is that incorporating the use of remanufactured exchange spare parts as an additional spare provisioning policy within both CM and PM, and remanufacturing within PM, has shown significant impact in recovery cost reduction. Interestingly, it could be hypothesized that the integration of remanufacturing, maintenance and multiple spare parts strategies elaborately, has significant effects on the system, in terms of recovery cost savings. These
Conclusion
This study set out to develop an integrated model, subsuming joint recovery (maintenance and remanufacturing) and spare policies for the ageing multi-component system approaching their end of life, from the end-user context. The study modelled the role and asymmetric relationship of the end-user and the OEM, whose decision logic is based on technical and economic criteria. The paper incorporates the end-user's role in remanufacturing by linking a component's empirical observation with its
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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