The facility location problem : modeling and solution methods

Trial lecture held 28 March 2012 before the defence for the PhD in Logistics at Molde University College - Specialized University in Logistic

Minimization of passenger takeoff and landing risk in offshore helicopter transportation: models, approaches and analysis

Offshore petroleum industry uses helicopters to transport the employees to and from installations. Takeoff and landing represent a substantial part of the flight risks for passengers. In this paper, we propose and analyze approaches to create a safe flight schedule to perform pickup of employees by several independent flights. Two scenarios are considered. Under the non-split scenario, exactly one visit is allowed to each installation. Under the split scenario, the pickup demand of an installation can be split between several flights. Interesting links between our problem and other problems of combinatorial optimization, e.g., parallel machine scheduling and bin-packing are established. We provide worst-case analysis of the performance of some of our algorithms and report the results of computational experiments conducted on randomly generated instances based on the real sets of installations in the oil fields on the Norwegian continental shelf. This paper is the first attempt to handle takeoff and landing risk in a flight schedule that consists of several flights and lays ground for the study on more advanced and practically relevant models

Simulated Annealing for the 0/1 Multidimensional Knapsack Problem

In this paper a simulated annealing (SA) algorithm is presented for the 0/1 multidimensional knapsack problem. Problem-specific knowledge is incorporated in the algorithm description and evaluation of parameters, in order to look into the performance of finite-time implementation of SA. Computational results show that SA performs much better than a genetic algorithm in term of solution time, whilst requiring only a modest loss of solution quality

Research Status and Prospect of Nb - Containing Nitride Hard Film

The preparation technology and microstructure properties like crystal structure, hardness, adhesion and wear resistance of the existing TiNbN hard films were analyzed and overviewed in this paper. Besides, the mechanical properties such as hardness, wear resistance and thermal shock resistance of TiAlNbN hard films were summarized. Finally, the development of TiAlNbN hard film was prospected

Preparation of Layered Polyethylene Oxide/rGO Composite: Flexible Lateral Heat Spreaders

In this paper, high thermal conductive polyethylene oxide (PEO)/reduced graphene oxide (rGO) composite is prepared via large-scale green reduction. Flexible layered PEO/GO composites are pre-prepared in aqueous solution. It is demonstrated that PEO chains can form hydrogen bonds with GO. Being driven by hydrogen bonds, GO/PEO composites show homogeneous and lateral highly oriented structures, resulting in excellent mechanical properties. The pre-prepared composite films are large scale soaked into ascorbic acid solution. GO nanosheets in the matrix of the composites can be reduced by ascorbic acid. The results indicate that PEO chains can repair the damage of the films caused by the reduction process. Therefore, the films can maintain their original configuration and still keep excellent flexibility. By comparison, pristine GO films are totally destroyed when the same reduction is experienced. Due to the presence of PEO, the lateral highly oriented structure of the composite will not be damaged. After reduction, the thermal conductivity of the composite reaches to 12.03 W m−1 K−1 along the rGO nanosheet oriented direction

Estuarine groundwater level response to and recovery from extreme precipitation events: Typhoon Lekima in the Yellow River Delta

Extreme precipitation events (EPEs) are projected to become more frequent and intense due to global warming. Understanding how coastal groundwater levels respond to and recover from these severe events is important for estuarine ecosystems to adapt to global change. Numerical model and non-EPE scenario simulation were used to examine groundwater level recovery time (RT) after Super Typhoon Lekima, which triggered EPEs that resulted in groundwater rise and widespread flooding in the Yellow River Delta (YRD). The three-day rainfall during Lekima totaled 290.9 mm, equivalent to 50 % of the annual rainfall for 2019 (581.5 mm), leading to a general rise in groundwater levels. Groundwater recovery to EPE can be divided into two types: inland and coastal. The RT of groundwater levels in monitoring wells in inland areas ranged from 12 to 89 days, with an average of 56.2 days, and there was spatial variation. However, groundwater levels in monitoring wells close to the coast may not recover. Differences in recovery are reflected in the land-sea gradient, with RT gradually increasing from inland highlands to coastal depressions and lowlands. The results showed that inland aquifers were more resistant to EPEs, while coastal aquifers were less resistant. In addition, EPE can cause groundwater flooding, and areas at lower altitude and close to the sea are more sensitive to flooding. Estuarine groundwater and the ecological processes on which it depends are profoundly affected by the direct and legacy effects of EPEs, including salt contamination, widespread flooding, crop damage, and reduced biodiversity. The study of this event provides case support for the response of estuarine groundwater to EPEs, while highlighting the importance of continuous monitoring

Fabrication of high thermal conductive shape-stabilized polyethylene glycol/silica phase change composite by two-step sol gel method

Tetraethyl silicate (TEOS) and polyethylene glycol (PEG) were used to prepare inorganic-organic polyethylene glycol (PEG)/silica (SiO2) nanocomposites by a controllable sol-gel method. Two kinds of counter ions were used to help gelatinize step by step. The presence of metal ions accelerates hydrolysis of SiO2 sol and the presence of ammonium ions allows the system to gel rapidly. PEG forms a shell-and-core structure with SiO2 and this composite shows excellent thermal stability. Moreover, system contains respectively Calcium ions (Ca2+), magnesium ions (Mg2+), aluminum ions (Al3+) were studied. It is very interesting that PEG with different metal ions formed different strength of the chemical force between PEG and metal ions change due to presence of different metal ions. As the force increased, the thermal conductivity increased from 0.25 W m(-1) k(-1) to 0.41 W m(-1) k(-1) and the phase change latent heat of the composites increased from 46.99 to 91.05 J/g, without affecting the phase transition temperature