1. Department of Safety, Economics and Planning, Faculty of Science and Technology, University of Stavanger, P.O. Box 8600 , Stavanger 4036, Norway
2. Norwegian Defence Research Establishment (FFI), P.O. Box 25 , Kjeller 2027, Norway
3. Department of Mathematics, University of Oslo, P.O. Box 1053 Blindern , Oslo 0316, Norway