The supply vessel planning problem is a maritime transportation problem faced by amongst others the energy company Statoil. A set of offshore installations requires supplies from an onshore supply depot on a regular basis, a service performed by a fleet of offshore supply vessels. The problem consists of determining the optimal fleet composition of offshore supply vessels and their corresponding weekly voyages and schedules. We present a voyage-based solution method for the supply vessel planning problem. A computational study shows how the solution method can be used to solve real-life problems. Statoil has implemented a planning tool based on the voyage-based solution method and reports significant savings.