Optimizing fleet size

Transportation Optimization supports the use of a transportation asset on multiple routes. Each asset can be assigned a sequence of routes during the planning horizon. For example, a sequence of three routes can be assigned to a single asset. The cost of the sequence will be the asset fixed cost (counted once) plus the sum of each individual route cost.

When you use the Optimize Fleet Size option, the solver looks for a solution that attempts to minimize the total cost and the number of vehicles. The constraints that apply to this problem are the same as those for standard optimization, with the exception of additional constraints specific to the asset, as defined in Constraints for optimize fleet size.

When solving with Optimize Fleet Size, Transportation Optimization performs the following general steps:

• Generate a set of routes for each asset type.
• Attempt to minimize the total cost for the set of routes defined in the first step.
• Assign each route to an asset.

Since minimizing cost is an objective of Optimize Fleet Size, it is advisable to model appropriate costs for your assets. You do this through a combination of the Rate table records and the Fixed Asset Cost on the Transportation Asset records.

When using Optimize Fleet Size, Transportation Optimization seeks to return the lowest cost solution with the least number of assets. You can use this capability to model mode selection, in which different assets can represent different fleets, carriers, or other transportation options.

With Optimize Fleet Size, the asset will return to the home base at the end of each route.

Constraints for optimize fleet size

You control assets using constraints on the Transportation Assets table:

If you want Transportation Optimization to assign multiple routes to an asset, you select the Optimize Fleet Size option in the Transportation Optimization options. Transportation Optimization will then assign an asset ID to each route. It will populate the Assets optimization output table with the details of each asset.

You must also set the Is Round Trip column to “Yes” in the Transportation Asset table.

The Route Summary table includes an Asset ID column that displays the identifier for the asset to which the route belongs. It also displays the Asset Name used to identify the specific type of asset used. The Assets output table provides details about the asset, including the total distance, total cost and asset ID.

Assets output tables

Information about the assets when using Optimize Fleet Size are provided in several optimization output tables:

• Assets – provides details about the assets that have routes assigned to them when you use Optimize Fleet Size in Transportation Optimization.
• Asset Summary – provides information about the number of available assets and the number used in the solution.
• Route Summary – provides information for routes assigned to assets when you use Optimize Fleet Size in Transportation Optimization:
  • Asset ID – The identifier of the asset (from the Assets table) to which the route is assigned.
  • Asset Sequence ID – The sequence number of the route on the asset.
  • Segment Utilization – The segment utilization is calculated as:

  

  • Route Utilization – The route utilization is calculated as:

    (maximum load on the asset) / (asset capacity)

Asset fixed cost

The Fixed Asset Cost on the Transportation Assets table is included in the Transportation Optimization problem. Transportation Optimization will consider the fixed cost of the asset when assigning assets to routes. The value you enter should be the fixed cost incurred over the model horizon. Therefore, if you are modeling a week and you have an annual fixed cost of 80,000, the Fixed Asset Cost should be 1,538.

Optimize fleet size high level algorithm summary

The following provides a high level review of Optimize Fleet Size:

• Route enumeration
  • Generate an initial set of candidate routes for each asset type and pool site.
  • Generate a large amount of promising routes using column generation techniques.
• Initial route selection, asset assignment and initial solution generation
  • Solve a mixed integer problem (MIP) of the route selection problem. This selects the routes and asset types to be used to build the initial solution.
  • Another MIP is solved to assign routes to each specific asset based on the solution from the route selection problem.
  • The solution from the two MIPs is used to create a sequence of routes for each specific asset.
• Solution improvement phase
  • This phase utilizes the improvement phases built for the standard optimization in addition to some modifications and unique improvements specific for the management of the assets.
• Post processing and reporting