Lifecycle optimization of multi-fuel tugboat fleets for port decarbonization

Tugboats play an essential role in port operations, providing towing, pushing, and escorting services for vessel movements. However, traditional diesel-powered tugboats emit substantial greenhouse gases, contributing significantly to port carbon footprints. In busy ports, where high demand for tugboat services coincides with increasing pressure to decarbonize, these dual challenges render propulsion upgrades and effective service schedules critical for achieving decarbonization goals while maintaining high service quality under green shipping initiatives. Given the long lifespan of tugboat assets, early greening efforts often result in diverse tugboat types and refueling systems. In this paper, we present an integrated decision-making framework that jointly addresses strategic multi-fuel tugboat fleet planning and the operational scheduling of the resulting heterogeneous fleet. Strategically, it evaluates six propulsion options: electricity, LNG, hybrid (diesel-electric), hydrogen, methanol, and ammonia, and compares fleet renewal pathways through newbuild purchases or retrofitting under two policy instruments: government subsidies and carbon taxes. Operationally, it coordinates the scheduling of a multi-fuel tugboat fleet across multi-zone ports, incorporating varying horsepower requirements, time windows, and speed decisions. A mixed-integer linear programming model is formulated to minimize total lifecycle costs, and a two-stage iterative solution approach with enhancement strategies is proposed for practical instances. Extensive numerical experiments based on real-world data of Lianyungang Port show that considering stakeholder willingness and policy incentives enables a fleet with 70% alternative-propulsion tugboats, 86% of which operate with zero emissions. Compared with a conventional fuel-powered fleet, this multi-fuel strategy can reduce carbon emissions by approximately 62%. Sensitivity analyses further reveal that external policy incentives exert a stronger influence on decarbonization than internal willingness: subsidies for retrofits and purchases can cut emissions by an additional 30%, while raising the carbon taxes can achieve nearly 39% further reductions.