Willy Herroelen

Project scheduling under uncertainty: Survey and research potentials

The vast majority of the research efforts in project scheduling assume complete information about the scheduling problem to be solved and a static deterministic environment within which the pre-computed baseline schedule will be executed. However, in the real world, project activities are subject to considerable uncertainty, which is gradually resolved during project execution. In this survey we review the fundamental approaches for scheduling under uncertainty: reactive scheduling, stochastic project scheduling, fuzzy project scheduling, robust (proactive) scheduling and sensitivity analysis. We discuss the potentials of these approaches for scheduling under uncertainty projects with deterministic network evolution structure.

Resource-constrained project scheduling: a survey of recent developments

Resource-constrained project scheduling involves the scheduling of project activities subject to precedence and resource constraints in order to meet the objective(s) in the best possible way. The area covers a wide variety of problem types. The objective of this paper is to provide a survey of what we believe are the important recent developments in the area. Our main focus will be on the recent progress made in and the encouraging computational experience gained with the use of optimal solution procedures for the basic resource-constrained project scheduling problem (RCPSP) and important extensions. We illustrate how the branching rules, dominance and bounding arguments of a new depth- first branch-and-bound procedure can be extended to a rich variety of related problems: the generalized resource-constrained project scheduling problem, the resource-constrained project scheduling problem with generalized precedence relations, the preemptive resource-constrained project scheduling problem, the resource availability cost problem, and the resource-constrained project scheduling problem with various time/resource(cost) trade-offs and discounted cash flows.

Project network models with discounted cash flows. A guided tour through recent developments

Abstract The vast majority of the project scheduling methodologies presented in the literature have been developed with the objective of minimizing the project duration subject to precedence and other constraints. In doing so, the financial aspects of project management are largely ignored. Recent efforts have taken into account discounted cash flows and have focused on the maximization of the net present value ( npv ) of the project as the more appropriate objective. In this paper we offer a guided tour through the important recent developments in the expanding field of research on deterministic and stochastic project network models with discounted cash flows. Subsequent to a close examination of the rationale behind the npv objective, we offer a taxonomy of the problems studied in the literature and critically review the major contributions. Proper attention is given to npv maximization models for the unconstrained scheduling problem with known cash flows, optimal and suboptimal scheduling procedures with various types of resource constraints, and the problem of determining both the timing and amount of payments.

Proactive heuristic procedures for robust project scheduling: An experimental analysis

Solution-robust project scheduling is a growing research field aiming at constructing proactive schedules to cope with multiple disruptions during project execution. When stochastic activity durations are considered, including time buffers between activities is a proven method to improve the stability of a baseline schedule. This paper introduces multiple algorithms to include time buffers in a given schedule while a predefined project due date remains respected. Multiple efficient heuristic and meta-heuristic procedures are proposed to allocate buffers throughout the schedule. An extensive simulation-based analysis of the performance of all algorithms is given. The impact of the activity duration variance structure on the performance is discussed in detail.

Proactive and reactive strategies for resource-constrained project scheduling with uncertain resource availabilities

AbstractnResearch concerning project planning under uncertainty has primarily focused on the stochastic resource-constrained project scheduling problem (stochastic RCPSP), an extension of the basic RCPSP, in which the assumption of deterministic activity durations is dropped. In this paper, we introduce axa0new variant of the RCPSP, for which the uncertainty is modeled by means of resource availabilities that are subject to unforeseen breakdowns. Our objective is to build axa0robust schedule that meets the project deadline and minimizes the schedule instability cost, defined as the expected weighted sum of the absolute deviations between the planned and the actually realized activity starting times during project execution. We describe how stochastic resource breakdowns can be modeled, which reaction is recommended, when axa0resource infeasibility occurs due to axa0breakdown, and how one can protect the initial schedule from the adverse effects of potential breakdowns. An extensive computational experiment is used to show the relative performance of the proposed proactive and reactive strategies. It is shown that protection of the baseline schedule, coupled with intelligent schedule recovery, yields significant performance gains over the use of deterministic scheduling approaches in axa0stochastic setting.n

The construction of stable project baseline schedules

The vast majority of project scheduling efforts assume complete information about the scheduling problem to be solved and a static deterministic environment within which the pre-computed baseline schedule will be executed. In reality, however, project activities are subject to considerable uncertainty, which generally leads to numerous schedule disruptions. It is of interest to develop pre-schedules that can absorb disruptions in activity durations without affecting the planning of other activities, such that co-ordination of resources and material procurement for each of the activities can be performed as smoothly as possible. The objective of this paper is to develop and evaluate various approaches for constructing a stable preschedule, which is unlikely to undergo major changes when it needs to be repaired as a reaction to minor activity duration disruptions.

A Classification Scheme for Project Scheduling

The basic concern of scheduling is commonly described as the allocation of limited resources to tasks over time (Lawler et al. 1993, Pinedo 1995). The resources and tasks may take many forms. In project scheduling the tasks refer to the activities belonging to one or more projects. The execution of project activities may require the use of different types of resources (money, crews, equipment, …). The scheduling objectives may also take many forms (minimizing project duration, minimizing project costs, maximizing project revenues, optimizing due date performance,…). The result is a wide and steadily growing variety of problem types which motivates the introduction of a systematic notation that can serve as a basis for a classification scheme.

The scheduling of activities to maximize the net present value of projects

Abstract We review and critique the approaches offered for the resolution of the problem of maximizing the net present value of a project through the manipulation of the times of realization of its key events. We offer an elementary approach that maintains the essential simplicity of the problem.

Critical Chain Project Scheduling: Do Not Oversimplify

Critical chain scheduling/Buffer management (CCS/BM)—the direct application of the theory of constraints (TOC) to project management—has received much attention in project management literature. There still is controversy over the merits and pitfalls of the CCS/BM methodology. This paper focuses on the fundamental elements of CCS/BM logic and pinpoints some intricacies that are not commonly referred to in the available literature. The authors’ analysis is based on a critical review of the relevant sources and experimentation with both commercial CCS/BM software and an internally developed CCS/BM-based tool.

Stability and resource allocation in project planning

The majority of resource-constrained project scheduling efforts assume perfect information about the scheduling problem to be solved and a static deterministic environment within which the precomputed baseline schedule is executed. In reality, project activities are subject to considerable uncertainty, which generally leads to numerous schedule disruptions. In this paper, we present a resource allocation model that protects a given baseline schedule against activity duration variability. A branch-and-bound algorithm is developed that solves the proposed resource allocation problem. We report on computational results obtained on a set of benchmark problems.