A Tabu search approach for milk collection in western Norway using trucks and trailers

This paper considers a real world problem for a Norwegian dairy company collecting raw milk from farmers. The problem can be classified as a special type of the Truck and Trailer Routing Problem which is a variant of the traditional Vehicle Routing Problem. 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. Most Norwegian farms are small and inaccessible for vehicles with trailers, so the routes that are served with a trailer have to be constructed so that the vehicles carry the trailer to a parking place and leave it there. The truck will then drive a subtour to the farmers and collect milk before it returns 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. The milk can be stored up to three days at the farms, and in this paper we will compare different frequencies of collection, different sizes of the vehicles and the benefit of using trailers compared to driving with single trucks. We will use the tabu search metaheuristic to construct the routes for the different strategies of milk collection.