Dans cette thèse, nous nous intéressons aux problèmes d'ordonnancement simultané de tâches et de périodes d'indisponibilité. Dans un premier temps, nous réalisons une revue de littérature sur la prise en compte des indisponibilités dans les problèmes d'ordonnancement.Ensuite, nous définissons un modèle général qui englobe des modèles existants de la littérature pour des ateliers à une machine et à machines parallèles. Une approche globale de résolution basée sur les problèmes d'affectation linéaire a été développée. Cette approche permet de résoudre le modèle général comme un simple problème d'affectation. Un grand nombre de critères d'optimisation et de modèles de maintenance peuvent être traités en utilisant cette approche, fournissant ainsi l'accès à tous les modèles qui ont souvent été étudiés séparément dans la littérature. Les résultats élaborés avec cette approche ont permis de résoudre des problèmes d'ordonnancement non traités avant et aussi de généraliser et améliorer des résultats antérieurs.Nous proposons, en dernier lieu, une étude d'un problème flow shop à deux machines en présence d'une période d'indisponibilité sur la deuxième machine. Une étude de complexité est menée sur le problème. Ensuite, nous définissons des propriétés d'optimalité. En se basant sur ces propriétés, trois méthodes de résolution exacte sont proposées; une méthode énumérative, un programme linéaire et une méthode basée sur l'approche de séparation et évaluation B&B. Une analyse expérimentale est présentée afin d'évaluer les performances de ces méthodes. / In classical scheduling problems, machines are assumed to be continuously available. However, in a real manufacturing system, machine may become unavailable during the scheduling period due to preventive maintenance. In this dissertation, we are interested in the problems of jointly scheduling jobs and unavailability periods.We start our study by introducing a general framework for scheduling problems and we present a review of the scheduling problems with unavailability periods.Then, we consider a general model for scheduling jobs on single-machine and unrelated parallel-machines with maintenance interventions. A unified approach is presented to solve this model as an assignment problem. A large number of performance criteria and maintenance models can be treated in this way, thus providing access to models that have often been studied separately in the published literature.Finally, we focus on the problem of a two-machine flow-shop makespan scheduling with the deteriorating maintenance period on the second machine. Then, we establish some conditions of the optimal schedule. In order to solve the problem, we proposed different exact methods: enumerative method, mixed-integer programming (MIP) model and a branch & bound algorithm. Numerical experiments are reported for all the proposed methods.

Compromised welfare and wellbeing of elephants (Loxodonta africana and Elephas maximus) in captive facilities are significant and global problems. The period between birth and two years old is crucial for calf survival and social and environmental learning. Behaviour and developmental processes among captive elephant calves in these first years were compared with those seen in wild calves. Wild elephants calves develop within a complex, varied social context and provide one reference for normal patterns of development. Such comparisons enable insights into welfare at captive facilities. Eleven captive elephant calves born at three UK facilities were studied from birth to 18 months (AsianN=6; AfricanN=5). Older calves (AsianN=2; AfricanN=2) were also sampled up to 3.5 years; making a total of 15 calves studied from 2009 to 2014. Due to the small sample size, the 11 younger calves were also discussed as individual case studies. By 2017, only two of these case study calves were both alive and not orphaned. Three additional calves (AsianN=1; AfricanN=2) died on their day of birth and were not sampled. This small sample highlights the ongoing lack of self-sustaining populations of captive elephants. This thesis collated systematic behavioural observations on captive calves across 373 days (483.5hrs). Calf maintenance activities (feeding, resting, moving), associations with mother and others, interactions and calf play were compared with behavioural observations of wild AsianN=101 (74hrs, Uda Walawe, Sri Lanka) and wild AfricanN=130 (252hrs, Amboseli, Kenya) calves from ~birth to five yrs. Mothers’ (captive: AsianN=4; AfricanN=4; wild: AsianN=90; AfricanN=105) activities were also recorded to explore synchrony with calves. Captive calves raised by their mothers had similar activity budgets to those seen in the wild. Expected age-related declines in suckling were found in captivity. However, captive calves were more independent than wild calves for their age in distance from mother and spent significantly more time in play. A Decision Tree for whether to breed elephants in captivity was developed; benefits that a calf potentially brings to companions, e.g. multi-generational matrilineal groups, enabling social bonding and reducing abnormal behaviours, were considered against space required for families to grow and divide naturally over time, as well as ensuring that captive-bred males are socially sustained. It was recommended that facilities invest in future enclosure/housing designs which permit: free-access to other elephants; 24hr trickle feeding; juvenile males allowed to stay with their maternal group for longer, encouraging learning opportunities and further retaining age-structure/composition. Conversely, facilities unwilling to house a male or provide appropriate group size/composition are recommended to cease breeding.