Multimodal Freight Network Simulator

3.4 Multimodal Freight Network Simulator

The problem addressed by the Multimodal Freight Network Simulator can be stated as follows: given a multimodal network with known service supply attributes and time- dependent O-D demands for multiple commodity classes for the network of interest for each mode, the network simulation model determines the resulting flow of shipments on the road, rail, and sea network for the various time intervals of interest, and the associated service levels and network performance experienced by the shipments.

3.4.1 Freight simulator

The simulation platform is shown in Figure 3-2. The network simulator per se consists of two main components: link moving and node/mode transfer, which process, respectively, flow propagation along links, and through nodes/transfer points. A third component, demand generation and loading, prepares the shipments to be loaded and actually loads them onto the network. The three components are described in the following subsections.

3.4.2 Demand generation, consolidation and loading

Demand is generated during each demand generation interval in economic regions, represented in the network model as nodes, corresponding to centroids of their respective economic regions. Shipments are loaded onto links specified as “generation links”. The inputs to the simulator are time-dependent multiple product O-D shipment flows with corresponding mode shares and path splits, which can be obtained from the intermodal

European Commission UMD and UOB Sixth Framework Programme

Organization Code:

shortest paths stored after the load-up period, or any paths externally specified in the data or freight assignment component.

At the time of generation, each shipment is assigned a product type: either container unit or bulk unit, based on the specified fraction of each product type. Due to the capacity of different conveyances, the number of shipments whose product type is non-bulk varies by conveyance. This gives rise to a consolidation policy, which is used to load several shipments in one conveyance (number of shipments subject to the capacity of that conveyance) at generation links, intermodal transfer terminals, and ports, where shipments could be loaded in conveyances.

The consolidation policy for non-bulk units requires that:

1. all shipments be of the same product type;

2. all shipments have the same next (intermediate or final) destination, which can be an intermodal transfer terminal, port, or destination zone centroid;

3. all shipments have the same mode between the current position and the next destination;

4. all shipments have the same path node sequence between the current position and the next destination;

5. the probability distribution of the number of shipments in one conveyance is based on the product type and location.

After consolidation, conveyances are generated, and their paths are based on those of the shipments they carry. Then, conveyances are loaded into the network based on the actual

Organization Code:

UMD and UOB

European Commission

Classification:

Unclassidied

Sixth Framework Programme

departure time of the loaded shipments. Note that it is possible to load with exogenously determined characteristics directly, instead of generating them based on the given fractions.

European Commission UMD and UOB Sixth Framework Programme

Organization Code:

OD flow; Path split; Mode share.

Demand loading:

Shipments generation; Shipments consolidation; Conveyances loading.

Link moving:

Trucks moving; Shuttle trains moving; Trains moving;

Ferries moving.

t=t+1

Node/mode transfer:

Truck transfers at road intersections; Train transfers at intermediate stations ; Mode transfers at intermodal transfer

terminals, classification yards, and ports .

No Have all shipments reached

their respective destinations?

Or,

Is simulation time at end of

planning horizon?

Yes Stop

Figure 3-2. Multimodal freight network simulator.

Organization Code:

UMD and UOB

European Commission

Classification:

Unclassidied

Sixth Framework Programme

3.4.3 Link moving

During this procedure, the movement of conveyances on the links are simulated according to the speeds of the respective modes. Trucks are moved on the links according to the prevailing speeds. Shuttle trains are moved according to a preset constant speed. Trains and ferries are moved along their respective links according to the given timetables. Delays incurred by shipments on rail links due to meets and overtakes are assumed to be reflected in the given train timetables. This assumption is reasonable for international intermodal freight transportation, where the majority of delays occur at the terminals rather than on the links. For example, in 1996, on average, only 14% of the time taken for a shipment to go from its shipper to consignee was spent on a moving train, and the remainder was at classification yards (Patty, 2001).

3.4.4 Node transfer

Terminal Processes

Node (or terminal) processes, such as sorting in classification yards, loading and unloading in intermodal terminals and ports, contribute a significant portion of total delays. In the present platform, these processes are simulated to estimate the delays that are eventually used in the intermodal shortest path calculations.

Classification yards are used to sort and group railcars, as well as dismantle and make-up trains. Trains arriving at yards are inspected by a yard crew and then are queued on the receiving tracks until they are classified or sorted onto the marshaling or classification track where similarly-bound traffic is combined. The classification operation is performed by

European Commission UMD and UOB Sixth Framework Programme

Organization Code:

pushing the train over the hump in a hump yard or by the switching engines in a flat yard. After classification, the sorted railcars (blocks) wait for dispatch on an appropriate outbound train. The schedule of the outbound train determines the start of the assembly operation for the blocks assigned to that train. After the cut-off time for a departing train occurs, the outbound train is assembled or marshaled on the departure track. Trains then depart to the next yard or to the nearest intermodal terminal or port if shipments are to be transferred to other modes.

Transfers of shipments among rail, road and sea modes are carried out at ports and intermodal terminals. Ports have access to different transport modes: deep sea vessels, barges, trains and trucks. Transshipment processes (loading and unloading) at a port, elaborated in Vis and Koster (2003), generally consist of a three-piece operation: ship to quay with gantry crane, quay to stack with MAFI trailer and reach-stacker and stack to rail wagon/truck with reach-stacker. Direct transfer of shipments is possible if an appropriate outbound vehicle is available during the unloading time, in which case shipments need not be stacked in storage areas.

Intermodal terminals transfer containers and trailers between trains and trucks. These terminals have no direct link to the water mode. Facilities and operations of intermodal terminals are similar to those of ports. Shipments are unloaded from trains onto ramps and then are stacked or transferred directly if a designated truck is available. Shipments that are transferred from trucks to railcars are transported to the nearest classification yard by a local shuttle train.

Organization Code:

UMD and UOB

European Commission

Classification:

Unclassidied

Sixth Framework Programme

Bulk queueing model

As discussed above, real world operations at all kinds of terminals are complicated and differ from one terminal to another. To capture the main characteristics of each terminal, while maintaining generality to be able to apply to all terminals, a bulk queueing model is developed to represent terminal transfer processes and evaluate terminal delay. We associate each terminal with a queueing server with known service (time) distributions. A generalized bulk queueing model is used to model terminal transfer processes as in Fig. 3-3. This model is similar to the one presented by Simao and Powell (Simäo and Powell, 1992a; Simäo and Powell, 1992b). There are two kinds of queueing elements in this study: railcars for classification yards, and shipments for intermodal terminal and ports.

Bulk arrival

Bulk departure

Bulk arrival 1

Departure queue 1

Bulk departure

Bulk departure Bulk arrival m

Arrival queue

Server

Departure queue n

Bulk departure

Figure 3-3. Generalized bulk queueing model.

The bulk queuing model consists of two kinds of queues in a queueing network: arrival queues and departure queues.

European Commission UMD and UOB Sixth Framework Programme

Organization Code:

(1) Arrival queue ( x G / G x / ∑ 1 ):

Since trains, ferries or trucks carry several shipments as they arrive at the terminals, the arrival of elements at the terminals is assumed to follow a bulk-arrival process ( x G ).

Elements (e.g. railcars with specified shipment(s) and destination) queue on the inbound links and are assumed to be served by a single super server. Type of service, service time and cost for the elements depends on the terminal type.

For classification yards, bulk service process ( x G ) is assumed as railcars belonging to

a train are processed at a time. Service times at classification yards reflect the time required for inspection, classification and assembly of the railcars into trains as shown in Figure 3-4. The bulk service process is sensitive to the facilities available at the yards including number of switch engines, number of tracks. Service times can also vary with product type reflecting the operating conditions.

Departure yard Inbound inspection

Receiving yard

Classification yard

Classification &

Scheduled delay

Assembly operation

Outbound inspection

Trains from other yards

Trains queue for

Trains to other yards

service

Shuttle trains from terminals Shuttle trains to terminals

Figure 3-4. Processes at a classification yard.

For ports and intermodal terminals, shipments are assumed to be served individually. The service times reflect unloading, loading and transport time within the terminal as shown in Figure 3-5. The service times are sensitive to the facilities that are available in the

Organization Code:

UMD and UOB

European Commission

Classification:

Unclassidied

Sixth Framework Programme

terminals, e.g. number of cranes, and the loading and unloading rates for ferries, trains, and trucks.

Elements (railcars or shipments) in the arrival queue are processed to estimate the earliest possible departure time (EPDT) for each element.

⎧⎪ i ∑ S for classification yards

AT + W + i i

EPDT = i ⎨ x (3) ⎪⎩ AT + W + S i i i

for terminals and ports

where,

i = element; x

= bulk size; EPDT i = Earliest Possible Departure Time for element i (same for all elements in same bulk); AT i = Arrival Time for element i (same for all elements in same bulk);

W i = waiting time for element i (same for all elements in a same bulk) in arrival queue; and,

S i = service time for element i (stochastic) on process.

(2) Departure queue ( y G GD / 1 ):

At the scheduled departure time of trains, ferries or trucks, processed elements on inbound queues are assigned to corresponding outbound queues and sorted based on destination, EPDT, and priority of the elements, respectively, to generate departure queue for the particular outbound link. The capacity of the outbound vehicle determines the number of elements that depart (bulk-departure, y GD ) from the departure queue at the scheduled time.

The model also considers delays experienced by elements waiting for scheduled connections at classification yards or storage areas in terminals, referred to scheduled delay.

European Commission UMD and UOB Sixth Framework Programme

Organization Code:

The schedule delay of an element i is calculated as follows: SD i = ADT i - EPDT i (4) where,

SD i = Schedule Delay for element i; ADT i = Actual Departure Time for element i based on bulk departure time (e.g. timetable);

G x = general bulk arrival process; GD y = general dependent service process based on bulk departure time (e.g.

timetable); x

= arrival bulk size; and, y

= departure bulk size.

Unloading time

Scheduled delay

Loading time

Truck

Storage Area

unloading Truck loading

Indirect transfer Transport time within

terminal

Direct Transfer

Train unloading Train loading

Ferry unloading

Ferry loading

Figure 3-5. Processes at a port.