Preposition moves are a subset of housekeeping moves. They possess a higher priority based on the move time horizon.
XPS conducts preposition moves in preparation for scheduled events such as:
Vessel loads
Rail loads
Truck visits
As these events enter the XPS preposition-planning horizon, XPS considers the locations of the appropriate containers and evaluates if the containers can be moved to positions closer to the location where they will be needed. For example, XPS attempts to reposition vessel-load containers closer to the waterside buffer as the vessel load approaches.
In order for XPS to plan a preposition move, the container must meet the following criteria:
The container is not on hold.
The container has a move time set.
When a container is planned to depart by vessel and its scheduled move time is set, XPS re-evaluates its position 90 minutes prior to vessel loading. This is based on the following EC Parameters:
PHMLOD - Determines the amount of time before the vessel load move that the container can be considered for imminent pre-handling
PHTIPI - Determines the imminent prehandle runtime interval.
When a truck arrives at the gate to pick up the container, XPS attempts to reposition the container to reduce the chassis load time (based on the EC prehandle parameter PHMROD).
The score for the container position must be improved by more than the threshold value set by the EC prehandle parameter PHTLIT. PHTSIT is not considered.
The ASC must be idle for the number of seconds set in OPTPID to consider scheduling preposition moves.
For details on other EC prehandle parameters that affect when and how XPS processes prehandles, see Setting up block optimization (on page 1).
In the preposition process, XPS does the following:
Determines if a container's proximity to the transfer zone can be improved to provide shorter transit distance for its final movement to the TZ.
Calculates the new position for a container using decking allocations.
Plans the preposition move to a background preposition work queue. If the background work queue does not exist, XPS automatically creates it.
If the container is a reefer, XPS also sets the desired connection state to Disconnected, which causes the container to show on the Reefer Checker's disconnection list.
If XPS cannot find a position that is significantly better, it does not plan the move.