A Tabu Search Approach for Milk Collection in Western Norway Using Trucks and Trailers

Milk collection is a problem which is well known in rural areas all around the world. This paper considers a real world problem for a Norwegian dairy company collecting raw milk from farmers. Chao (2002) and Scheuerer (2006) describe the Truck and Trailer Routing Problem (TTRP) as a variant of the traditional Vehicle Routing Problem (VRP). In this problem customers are divided into vehicle customers who can be reached with a complete vehicle with a trailer, and truck customers who only can be reached by a single truck. Most Norwegian farms are however small and inaccessible for a large truck with a trailer, and thus our problem can be classified as a special type of the TTRP where all customers are defined as truck-customers. The company uses a fleet of heterogeneous trucks with tanks for the milk, and a truck can either drive the route by itself or carry a trailer with an additional tank. The routes are then designed in a way that allows the trailer to be parked at a parking place while the truck is visiting the farms and collecting the milk. The possible parking places are prearranged mostly at large parking lots or petrol stations. When the tank on the truck is full, the truck will return to the parked trailer. It can then fill the milk over from the truck tank to the trailer tank and start on a new subtour from the same spot, or it can drive the trailer to a new parking place, fill the milk over and start a subtour from there. Figure 1 shows a graph which represents a solution to the milk collection problem with the constraints mentioned. The circle marks the depot, while the squares are the parking places which are used as roots for the subtours that visits the suppliers marked with triangles. This type of solutions, with the trailer serving as a mobile depot for the truck, was first described by Vahrenkamp (1989) for the milk collection in Western Germany.