We consider a class of periodic preventive maintenance (PM) optimization problems, for a single piece of equipment that deteriorates with time or use, and can be repaired upon failure, through corrective maintenance (CM). We develop analytical and simulation-based optimization models that seek an optimal periodic PM policy, which minimizes the sum of the expected total cost of PMs and the risk-averse cost of CMs, over a finite planning horizon. In the simulation-based models, we assume that both types of maintenance actions are imperfect, whereas our analytical models consider imperfect PMs with minimal CMs. The effectiveness of maintenance actions is modeled using age reduction factors. For a repairable unit of equipment, its virtual age, and not its calendar age, determines the associated failure rate. Therefore, two sets of parameters, one describing the effectiveness of maintenance actions, and the other that defines the underlying failure rate of a piece of equipment, are critical to our models. Under a given maintenance policy, the two sets of parameters and a virtual-age-based age-reduction model, completely define the failure process of a piece of equipment. In practice, the true failure rate, and exact quality of the maintenance actions, cannot be determined, and are often estimated from the equipment failure history. We use a Bayesian approach to parameter estimation, under which a random-walk-based Gibbs sampler provides posterior estimates for the parameters of interest. Our posterior estimates for a few datasets from the literature, are consistent with published results. Furthermore, our computational results successfully demonstrate that our Gibbs sampler is arguably the obvious choice over a general rejection sampling-based parameter estimation method, for this class of problems. We present a general simulation-based periodic PM optimization model, which uses the posterior estimates to simulate the number of operational equipment failures, under a given periodic PM policy. Optimal periodic PM policies, under the classical maximum likelihood (ML) and Bayesian estimates are obtained for a few datasets. Limitations of the ML approach are revealed for a dataset from the literature, in which the use of ML estimates of the parameters, in the maintenance optimization model, fails to capture a trivial optimal PM policy. Finally, we introduce a single-stage and a two-stage formulation of the risk-averse periodic PM optimization model, with imperfect PMs and minimal CMs. Such models apply to a class of complex equipment with many parts, operational failures of which are addressed by replacing or repairing a few parts, thereby not affecting the failure rate of the equipment under consideration. For general values of PM age reduction factors, we provide sufficient conditions to establish the convexity of the first and second moments of the number of failures, and the risk-averse expected total maintenance cost, over a finite planning horizon. For increasing Weibull rates and a general class of increasing and convex failure rates, we show that these convexity results are independent of the PM age reduction factors. In general, the optimal periodic PM policy under the single-stage model is no better than the optimal two-stage policy. But if PMs are assumed perfect, then we establish that the single-stage and the two-stage optimization models are equivalent. / text

金融監理制度影響一國商業銀行經營績效的相關議題，一直受到學者與政府當局的重視，為瞭解亞洲地區銀行業在中央銀行與監理單位不同管理下的利潤效率，找出最適的制度設計，本研究根據Huang、Huang與Liu（2014）提出之隨機共同利潤邊界（stochastic meta-profit frontier），採用兩階段估計法，蒐集中國大陸、香港、印度、日本、韓國、馬來西亞、巴基斯坦、菲律賓、新加坡、斯里蘭卡、泰國以及阿拉伯聯合大公國等十二國商業銀行資料，分成開發中和已開發國家兩個群組，將環境變數納入無效率模型中，進行實證分析，比較不同群組的利潤效率差異，發掘影響效率的主要變數與方向，從而獲得重要政策意涵。 根據實證分析結果，中央銀行介入銀行監理程度越高，商業銀行利潤效率越低；金融監理單位整合程度越高，商業銀行利潤效率越高；中央銀行獨立程度越高，商業銀行利潤效率越低；已開發國家群組的平均技術缺口比率與共同邊界技術效率值皆高於開發中國家群組，符合預期。共同利潤效率最高的是日本，最低的是韓國。平均而言，各國若在共同利潤邊界上從事生產，能提升41.9%至75%的利潤。 / The effects of degrees of financial supervision on performance of commercial banks have long been important issues and drawn much attention to academic researchers and government authorities. This study applies the stochastic meta-profit frontier, recently developed by Huang, Huang, and Liu (2014), to estimate and compare profit efficiencies of commercial banks from 12 Asian countries, i.e., Mainland China, Hong Kong, India, Japan, South Korea, Malaysia, Pakistan, Philippines, Singapore, Sri Lanka, Thailand, and United Arab Emirates. We divide the sample countries into two groups, i.e., developing and developed countries. This enables us to further investigate the effects of different supervisory systems, enforced by central banks (CB) and supervisory authorities, on commercial banks’ profit efficiencies, as well as to make a suggestion about the optimal supervision regimes in the area. Note that a set of supervisory indices are considered as environmental variables that explain profit inefficiency. Using the two-stage estimation procedure, the empirical results are summarized as follows. First, it is found that bank’s profit efficiency decreases with the increase in a CB’s supervision sectors. Second, the unification of supervisory authority has positive effect on bank’s profit efficiency. Third, the more independent is the CB, the less profit efficient the commercial bank is. Fourth, banks in the group of developed countries are found to have higher technology gap ratios and meta-profit efficiencies than those in the group of developing countries, as expected. Fifth and finally, Japan and South Korea has the highest and the lowest level of meta-profit efficiency, respectively. Evidence is found that if an average commercial bank were adopting the best technology, it can earn roughly 41.9% to 75% more profits than otherwise.

A compact air separator demonstrator based on centrifugally enhanced distillation has been studied. The full size device is meant to be used on board of a Two Stage To Orbit vehicle launcher. The air separation system must be able to extract oxygen in highly concentrated liquid form (LEA, Liquid Enriched Air) from atmospheric air. The LEA is stored before being used in a subsequent rocket propulsion phase by the second stage of the launcher. Two reference vehicles are defined, one with a subsonic first stage and one with a supersonic first stage. In both cases, oxygen collection is performed during a cruise phase (M 0.7 and M 2.5 respectively). The aim of the project is to demonstrate the feasibility of the air separation system, investigate the separation cycle design, and assess that the separator design selected is suitable for the reference vehicles.<p><p>The project is described from original base ideas to design, construction, extended testing and analysis of experimental results. Preliminary computations for a realistic layout have been performed and the motivations for the choices made during the process are explained. Test rig design, separator design and technical discussion are provided for a subscale pilot unit. Mass transport parameters and flooding limits have been estimated and experimentally measured. Performance has been assessed and shown to be sufficient for the reference Two Stage To Orbit vehicles. The technology developed is found suitable without further optimization, although some volume and mass reduction would be desirable for the supersonic first stage concept. There are many ways of optimisation that can be further investigated. The aim of this program, however, is not to fully optimize the device, but to demonstrate that a device based on a simple, robust, low-risk design is already suitable for the launch vehicles. On top of that analysis, directions for improvements are suggested and their potentials estimated. A complete assessment of those improvements requires further maturation of the technological concept through further testing and practical implementations.<p><p>Directions for future work, general conclusions and a vehicle development roadmap have also been provided.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished