Optimal policies for maintaining a supply service in the Norwegian Sea Kjetil Fagerholt a, *, HaÊkon Lindstad b a Department of Marine Systems Design, Norwegian University of Science and Technology, Otto Nielsens vei 10, 7491 Trondheim, Norway b Norwegian Marine Technology Research Institute, Trondheim, Norway Received 1 March 1999; accepted 1 August 1999 Abstract This paper considers the real problem of determining an ecient policy for a supply operation in the Norwegian Sea, where a number of oshore installations are serviced from an onshore depot by supply vessels. The purpose of the study was to evaluate the eect on the total supply cost of having some or all of the oshore installations closed for service during night, and to determine an optimal routing policy. Six scenarios were developed, varying in the opening hours and the number of weekly services of the installations, as well as an algorithm to ®nd an optimal routing policy (which vessels to operate and the coherent weekly schedules) for each of the scenarios. By also evaluating the qualitative aspects of the solution of each scenario, a routing policy was recommended, involving potential savings of 7 million dollars. The proposed policy has been implemented and the experience so far is good. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Scheduling; Transport; Application; Ships; Supply 1. Introduction After the ®rst discoveries of oil and gas in the North Sea in the sixties, Norway has become a major oil and gas producer and exporter. Several companies are now operating various kinds of oshore installations in the North Sea and the Norwegian Sea. Crucial for an e- cient operation of these installations is that they receive regular supplies of various commodities. Some of the oil and gas companies operate depots along the Norwegian Coast, either on their own or in co- operation with other companies. The task of each of these depots is to supply a given set of oshore instal- lations. In case of rush orders, helicopters may be used in this supply, but most often, the commodities are transported from the depot to the installations by supply vessels. The work presented in this paper was performed on request from Statoil, one of the major oil and gas com- panies operating in the North Sea and the Norwegian Sea. The main task was to evaluate the existing oper- ation of the supply vessel services from a given depot servicing a number of oshore installations. At that time, the night shift operation of the cranes to dis- charge the supply vessels had recently been abolished at two of the installations. As a consequence, these Omega 28 (2000) 269±275 0305-0483/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0305-0483(99)00054-7 www.elsevier.com/locate/orms * Corresponding author. +47-73-595-582; fax: +47-73-595- 575. E-mail address: kjetil.fagerholt@marintek.sintef.no (K. Fagerholt).