Integrated planning of asset-use and dry-docking for a fleet of maritime assets

In maritime industry, moving assets (e.g., naval ships, dredgers, pilot vessels) are subject to obligatory inspections based on calendar time. These inspections consist of exhaustive operations that need the assets to be towed into specialized facilities referred to as dry-docks. In addition, there are maintenance operations needed as a result of usage-related deterioration of the assets, also requiring the assets to be dry-docked. In practice, a common approach for a fleet of assets is to synchronize these inspection and maintenance operations to avoid unnecessary dry-dockings. However, when and how these operations, some of which are calendar-based and some of which are usage-based, should be synchronized, and whether synchronizing them is always optimal remain as important questions. Since how an asset is used influences when it requires maintenance, answering these questions requires solving an integrated planning problem that combines the planning of asset-use and the planning of dry-docking. Operational constraints such as the locations of assets, limited dry-docking capacity, and the requirement to meet the demand for asset-use in each location make the problem even more challenging. This real-life problem is formulated as a mixed integer linear programming model which minimizes the total discounted cost for a finite time horizon and ensures the full satisfaction of the demand in every time period. The resulting optimal policy is compared with a sequential planning approach to quantify the economic benefit of integrated planning for asset-use and dry-docking. Additionally, two alternative planning approaches are presented for large problem instances. Results of the numerical analysis show that integrated planning can save up to 28.5% of the total cost.