Considering skills evolutions in multi-skilled workforce allocation with flexible working hours

El-Awady Attia, Philippe Duquenne, Jean-Marc Le-Lann
2014 International Journal of Production Research  
OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 19893 To link to this article: The growing need of responsiveness for manufacturing companies facing market volatility raises a strong demand for flexibility in their organisation. Since the company personnel are increasingly considered as the core of the
more » ... tional structures, a strong and forward-looking management of human resources and skills is crucial to performance in many industries. These organisations must develop strategies for the short, medium and long terms, in order to preserve and develop skills. Responding to this importance, this work presents an original model, looking at the line-up of multi-period project, considering the problem of staff allocation with two degrees of flexibility. The first results from the annualising of working time, and relies on policies of changing schedules, individually as well as collectively. The second degree of flexibility is the versatility of the operators, which induces a dynamic view of their skills and the need to predict changes in individual performance as a result of successive assignments. We are firmly in a context where the expected durations of activities are no longer predefined, but result from the performance of the operators selected for their execution. We present a mathematical model of this problem, which is solved by a genetic algorithm. An illustrative example is presented and analysed, and, the robustness of the solving approach is investigated using a sample of 400 projects with different characteristics. . The case where the two workforce productivities, the 'worker-based' and the 'skill-based', one is homogeneous and the other is heterogeneous, we called it 'mixed performance'. One can find this case in the hierarchical classification of workforce productivities, e.g. senior, standard and junior classification that was adopted by Yoshimura et al. (2006), Valls, Perez, and Quintanilla (2009) and Shahnazari-Shahrezaei, Tavakkoli-Moghaddam, and Kazemipoor (2013). The workforce productivities were considered as heterogeneous in the case where neither the worker-based productivity, nor the skill-based productivity is homogeneous. According to the best of our knowledge, the first description of this heterogeneous characteristic is the work of Nelson (1970) in a dual resource constrained job shop. This heterogeneous nature was then adopted in many applications, e.g. the works of Attia, Edi, and Duquenne (2012) , Kolisch and Heimerl (2012) or Yannibelli and Amandi (2012) . Here also, the heterogeneous nature will be considered in addition to the evolutions of productivity as a function of previous workforce allocation (i.e. the dynamic aspect of productivity). Additionally, the workforce flexibility also relies on a temporal concern that plays a vital role in this subject. This flexibility can be developed along two main axes: the traditional overtime working hours and the working time modulation under annualised hours (AH). AH strategy is the possibility to spread irregularly a number of working hours that is predefined on a specified period (often one year), provided some constraints are respected. According to Corominas, Lusa, and Pastor (2002) , it provides a great flexibility with a reasonable cost, but on the other side it can lead to a worsening of the staff's working conditions. To minimise these negative consequences, this strategy of working time has to be negotiated and may be accompanied by some kind of reward or incentive. Simultaneously, legal constraints or collective agreements must be respected to avoid excessively overburdening workers during long, high-demand periods. Responding to the importance of such flexibility, numerous works were conducted; Hung (1999) proposed different scenarios of weekly manpower planning under AH. Grabot and Letouzey (2000) presented a software prototype to check the feasibility between the required workload and workforce capacities for discrete manufacturing short-term planning and scheduling. Corominas, Lusa, and Pastor (2002) presented a mixed-integer linear programme to the planning of workforce all over the year. Azmat, Hürlimann, and Widmer (2004) adopted the problem of single-shift workforce scheduling under AH with Swiss legal constraints. Corominas and Pastor (2010) proposed a reactive re-planning approach of the short-term workforce allocation. Due to this importance, this paper presents two main contributions. The first one presents a mathematical modelling of the staff allocation problem on industrial activities. The proposed model will be developed following three axes; the first is the policy of working time modulation under AH. The second axis is the flexibility resulting from the heterogeneous versatility: each operator may master one skill or more in addition to his core competence, with acceptable cost and quality standard. The last axis is a dynamic vision of the workers' efficiencies (productivities), which refer to the relationship between efficiency and experience. As a result of flexible work schedules and of the variety of the operators' efficiencies, the durations of the activities are not predetermined, and will depend on the performance of those responsible for executing them. This paper expresses the intention to consider any kind of industrial activities, regardless they belong to a project (a set of unique and original tasks) or a production programme (a manufacturing route of repetitive and well-defined jobs); in the same spirit, we will often refer to human resources as actors: an actor may be an operator in a factory as well as an appointed expert in a design office. The other contribution is the platform software that we propose to solve the problem. Starting from the MSPSP (multi-skill project scheduling problem), which is so far a difficult problem in operations research (Bellenguez-Morineau and Néron 2007), considering the actors' efficiencies greatly increases the complexity of a non-linear model either with homogeneous (Li and Womer 2009) or heterogonous actors' productivity rates (Edi 2007; Valls, Perez, and Quintanilla 2009; Attia, Edi, and Duquenne 2012) . Moreover, taking into account their dynamic productivities and working time flexibility, we referred to a method based on metaheuristics (genetic algorithms) to solve this problem. Our contributions and the model we present are likely to be of concern to a wide range of stakeholders. For example, as far as we know, no existing commercial software in the field of project management and resources planning considers altogether these three axes. Some applications can be raised in workforce allocation in aircraft manufacturing for instance. Such production environments, where each task requires a set of highly skilled and qualified workers, depend mainly on the workforce, for sub-assemblies, final assembly of the whole aircraft, testing, repairing if any and painting. According to Démery-Lebrun (2005) , the workforce flexibility is highly appreciated in such industrial sectors. It is especially true since the concept of learning at workstation was originally presented in aircraft manufacturing by Wright (1936) . Furthermore, with relaxing of some constraints, the model can be adopted in planning and scheduling of manual assembly lines of vehicles, electric transformers, etc. It can be useful too in planning of maintenance works where each task or mission requires a specified teamwork with a specified skill for each actor. Due to the diversity of the likely applications of this work, we used 'programme of activities' to name these applications. * According to the French working laws.
doi:10.1080/00207543.2013.877613 fatcat:t6rmrqdkozb6fd5uvw3m26oq5q