Planning logistics operations in the oil industry

In this paper we apply stochastic programming modelling and solution techniques to planning problems for a consortium of oil companies. A multiperiod supply, transformation and distribution scheduling problem—the Depot and Refinery Optimization Problem (DROP)—is formulated for strategic or tactical level planning of the consortium's activities. This deterministic model is used as a basis for implementing a stochastic programming formulation with uncertainty in the product demands and spot supply costs (DROPS), whose solution process utilizes the deterministic equivalent linear programming problem. We employ our STOCHGEN general purpose stochastic problem generator to ‘recreate’ the decision (scenario) tree for the unfolding future as this deterministic equivalent. To project random demands for oil products at different spatial locations into the future and to generate random fluctuations in their future prices/costs a stochastic input data simulator is developed and calibrated to historical industry data. The models are written in the modelling language XPRESS-MP and solved by the XPRESS suite of linear programming solvers. From the viewpoint of implementation of large-scale stochastic programming models this study involves decisions in both space and time and careful revision of the original deterministic formulation. The first part of the paper treats the specification, generation and solution of the deterministic DROP model. The stochastic version of the model (DROPS) and its implementation are studied in detail in the second part and a number of related research questions and implications discussed.     

Search for a spin glass phase in Mn0.22Zn0.78F2

No spin glass behaviour has been found in Mn_0.22Zn_0.78F₂ which is close to the antiferromagnetic critical concentration. Bulk magnetic measurements show no cusp above 1.7 K, no remanent moment after cooling in a field, and no difference between field cooled and zero field cooled susceptibilities. The infinite time susceptibility derived from diffuse neutron scattering measurements is also in good agreement with the bulk measurement, indicating no frozen component of the bulk susceptibility. The temperature dependence of the susceptibility is explained with a simple paramagnetic model.     

Substructure search systems for large chemical data bases

The substructure search systems, S⁴, DARC, HTSS and MACCS are briefly reviewed. The performance of the different systems are compared and evaluated in terms of accuracy and search time. The results show that the accuracy of all four systems is excellent and that S⁴ has by far the best search-time performance. The successful implementation of S^r4 for use with CD-ROMs is also demonstrated.     

Forecasting and the Social Sciences

Journal of the Operational Research Society -     

Surfactant-free "emulsions" generated by freeze-thaw

The stability ofsurfactantless dispersions of surface chemically pure alkanes was studied in the presence and absence of dissolved gas. It was found that simply freezing and thawing a sample of oil and water results in a dispersion. A mechanism based on fingering of the insoluble oil into the aqueous phase, due to local surface tension gradients, followed by separation and nucleation into droplets, is proposed to account for this observation.