Dry Bulk Ports (Top 25 List Calculation)

Background

The Fixing American’s Surface Transportation (FAST) Act requires the PPFSP to report on the top 25 ports as measured by dry bulk cargo tonnage. Dry bulk cargo tonnage refers to the weight of solid, dry cargo, commonly shipped in vessels designed for such cargo (dry bulk vessels). Examples of dry bulk commodities include coal, ores, and grain. The data are included in the PPFSP Annual Reports and interactive Port Profiles.

Concept

Dry bulk is not a standard categorization for tonnage statistics. Therefore, the Bureau of Transportation Statistics (BTS) used the methodology developed with the U.S. Army Corps of Engineers (USACE) and Maritime Administration (MARAD). This methodology was approved by the PPFSP Working Group to identify the top 25 dry bulk ports.

Methods

Dry bulk cargo was defined as any cargo carried in dry bulk vessels. The 13 International Classification of Ships by Type (ICST) codes listed in the table below were identified as dry bulk cargo vessels for this purpose. The following list was submitted to the PPFSP Working Group, which did not request adjustments:

SOURCE: An International Classification of Ships by Type (June 1994), available at https://op.europa.eu/en/web/eu-vocabularies/dataset/-/resource?uri=http://publications.europa.eu/resource/dataset/icst-comas of February 2020.

BTS calculated the most recent year's tonnage for every commodity handled at U.S. ports on those vessel types, using the cargo database compiled and maintained by the USACE Waterborne Commerce Statistics Center (WCSC). The results were used to identify the top 25 dry bulk ports.

Data Sources

The most recent year's cargo tonnage data were obtained from the USACE WCSC, using a special tabulation and same process as previous years.

Uses and Limitations

This methodology has inherent limitations. Using vessel classes to identify dry bulk cargo is likely imperfect due to the overlap in commodities shipped on various vessel types. For instance, ICST vessel code 222 “Bulk/container carrier” likely includes vessels that could have carried cargo in containers as well as in bulk. This approach could lead to small differences in tonnage estimates between similarly ranked ports.

Container Ports (Top 25 List Calculation)

Background

The Fixing American’s Surface Transportation (FAST) Act requires the PPFSP to report data on containerized cargo throughput and capacity for the top 25 ports as measured in twenty-foot equivalent units (TEU) of container cargo. A TEU is a measure of container cargo volume and the capacity of container ships, with each unit nominally equal to one 20-foot container.

Concept

The top 25 container ports were selected based on TEU throughput data published by the U.S. Army Corps of Engineers (USACE) Waterborne Commerce Statistics Center (WCSC). Container throughput for this purpose was calculated by totaling:

1. Loaded containers inbound from a domestic origin
2. Loaded containers outbound to a domestic destination
3. Loaded containers inbound from a foreign origin
4. Loaded containers outbound to a foreign destination

Empty containers were omitted from the measurement of TEU throughput for identifying the top 25 container ports the WCSC container dataset.

Data Sources

The top 25 container ports were selected using TEU throughput data published by USACE WCSC. USACE compiles these data differently for domestic and foreign cargo. Domestic freight flows are reported directly from manifest data (under Federal law, vessel-operating companies must report domestic waterborne commercial vessel movements directly to USACE). Foreign waterborne cargo data are derived from several other sources, including U.S. Customs and Border Protection (CBP), the U.S. Census Bureau, and the Port Import Export Reporting Service (PIERS).

While there exist other sources of data on TEU throughput, BTS chose to select the top 25 container ports using data from the WCSC for several reasons. The WCSC container dataset is the publicly available, updated annually, and reported in a consistent manner for all ports across the Nation, including single-terminal ports, multi-terminal ports, state-operated port authorities, multi-state entities, and river ports with terminals spread over a wide area. The dataset therefore meets the recommendations articulated by Section 6018 of the FAST Act and the PPFSP Working Group that this data be timely, nationally consistent, and readily accessible to the public.

Methods

The top 25 container ports were identified based on USACE WCSC container throughput statistics.

Uses and Limitations

The use of USACE data that excludes empty containers shipped to or from other locations to identify the top 25 container ports results in a list that may differ from one based on total TEU handled. Despite this fact, the availability of a nationally consistent source in the USACE data makes it the best option for selecting the top 25 container ports for the PPFSP.

Vessel Calls

Background

A vessel call is a single visit to a terminal or port by a waterborne vessel. The annual number of vessel calls provides insight into port throughput.

Concept

The vessel call metric indicates the total number of cargo vessel calls that each port handled in the most recent year for which data are available. The vessel call metric aligns with the throughput statistics that categorize cargo into three types (twenty-foot equivalent units (TEU), overall tonnage, dry bulk tonnage). Dry bulk vessels and other freight vessel categories have been further split between barges and other vessels. Vessel call counts reflect each barge call, so groups of connected barges can result in high vessel counts and overshadow non-barge vessel calls. Separating barges from other vessels therefore allows for a better understanding of trends in non-barge vessels.

Methods

Vessel calls are reported as a total number for the most recent year for which data are available and the change from the previous year. Vessel calls are also shown by percentage of total, as divided into five categories of vessels based on the ICST codes listed in the tables below. This method excludes ferries, cruise, and other passenger vessels. The five categories include:

1. Container: Vessels identified as carrying containers. A container vessel is usually a cellular container ship loaded and unloaded using shoreside container cranes. Some ports handle containers on general cargo vessels, roll-on/roll-off (Ro/Ro) vessels and/or barges. These vessel calls are not included in the container vessel counts unless they are specifically classified as container vessels, as it is not feasible to identify which of the other vessel calls carry containers (see ICST Code 338, Ro/Ro Container).
2. Dry bulk: Non-barge vessels identified as carrying dry bulk cargo. The method for selecting dry bulk vessel types was developed to quantify dry bulk port cargo volumes and to select the top 25 dry bulk ports. BTS selected 13 types of vessels to include in dry bulk cargo tonnage and dry bulk vessel calls. Six of the 13 types are self-propelled or otherwise classified as non-barge vessels and are included in this list.
3. Dry bulk barge: The remaining seven vessel types that were identified as carrying dry bulk cargo, and as barges.
4. Other freight: All other vessels that predominantly handle freight, are not identified as container or dry bulk vessels, and are not barges. These include crude oil tankers, liquefied natural gas (LNG) tankers, chemical tankers, general cargo vessels, and vehicle or Ro/Ro carriers. The combination of “Other freight vessel” calls and “Other freight barge” calls equals overall cargo tonnage minus container and dry bulk cargo tonnage.
5. Other freight barge: Barge vessels that were identified as carrying non-containerized, non-dry bulk freight cargo.

Vessels that either do not or rarely carry cargo but play a role in the movement of cargo at ports, such as tugs and push boats, were not included in the vessel call analysis. The large number of support vessel movements can hide trends in the other five categories. The following tables show the assignment of ICST codes used to identify vessels in each of the five vessel categories:

Container Vessels

Dry Bulk Vessels (Non-Barge)

Dry Bulk Barge Vessels

Other Freight Vessels

Other Freight Barge

Data Sources

Vessel calls were tabulated by USACE WCSC each year.

Uses and Limitations

The interactive Port Profiles included display total cargo vessel calls in the most recent year for which data are available.

Food & Farm Product Index

Background

The food and farm product index is a quarterly index of food and farm product tonnage, including commodities such as fish, grains, oilseeds, vegetables, and dairy products.

Methods

The index is calculated by dividing the current month’s tonnage by the average of the previous 4 quarters. This step was taken to smooth out short-term cyclical fluctuations and highlight the long-term trends.

Data Sources

The U.S. Army Corps of Engineer, Waterborne Commerce Statistics Center, Waterborne Commerce of the United States is the data source.[1] The index is based upon the tonnage of major commodity group 60 Food and Food Products. See the table for a complete list of commodity groups or the Waterborne Commerce of the United States publication cited below for additional information.

Food & Farm Products

[1] U.S. Army Corps of Engineer, Waterborne Commerce Statistics Center, Waterborne Commerce of the United States, available at https://www.iwr.usace.army.mil/About/Technical-Centers/WCSC-Waterborne-Commerce-Statistics-Center/ as of February 2020.

Port Vicinity Maps

Background

The Fixing American’s Surface Transportation (FAST) Act requires the PPFSP to include the top 25 ports as measured by total tonnage, container / twenty-foot equivalent (TEU) volume, and dry bulk tonnage. Many ports rank in the top 25 in more than one category. Each interactive Port Profile includes a map of the “Port Vicinity.”

Concept

As the exact boundaries of each terminal and port facility are complex and are often unavailable, “Port Vicinity” maps are useful to portray the general location of port facilities within the region while still providing insight into the area in which port-related activity is focused. These maps reflecting the approximate port facility location, along with local relevant infrastructure such as roads, rail lines, and limiting bridges.

Methods

Each “Port Vicinity” map may include the following elements (where applicable) overlaid on a base map:

• Authorized and minimal depth of the approach channel
• Class 1 and relevant short line railroads
• Interstate highways, national highways, and principal roads
• Limiting bridges
• State and/or national borders

The “Port Vicinity” was derived from BTS-generated port terminal polygons, the Port Statistical Area published by the U.S. Army Corps of Engineers' Navigation Data Center, and land surveys and maps published by individual port authorities on their websites. Google satellite imagery was used to verify facility locations at many ports. The shaded vicinity zone was expanded to include the area between the port and terminal boundaries and the water’s edge to create a contiguous area. The port vicinity determined the extent and scale of each map.

The source data for the base map included water body names and state and national borders. Bridge points were added manually based on port vicinity and bridge locations. Roadway locations were imported from the Global Roads Open Access Data Set (gROADSv1) published by NASA and the Center for International Earth Science Information Network (CIESIN) and modified to fit the base map coordinate system. Railroad locations were imported from BTS National Transportation Atlas Database and were similarly modified to fit the base map coordinate system. The railroad data set was filtered to show active main network, and major and minor industrial rail lines, and distinguishes between Class 1 and non-Class 1 railroads. Class 1 railroads include:

• BNSF
• Canadian National
• Canadian Pacific
• CSX
• Kansas City Southern
• Norfolk Southern
• Union Pacific

Data Sources

• Port Vicinity: derived from BTS-generated port and terminal boundaries using 2016 AIS data, plus land surveys and maps published by individual port authorities on their websites and Google satellite imagery, as of 2018.
• Limiting bridges: were compiled by U.S. Coast Guard and verified using National Oceanic and Atmospheric Administration (NOAA) charts, as of November 2017.
• Roads: gROADSv1 layer was downloaded from the Center for International Earth Science Information Network/Columbia University, and Information Technology Outreach Services/University of Georgia, Global Roads Open Access Data Set, Version 1, as of December 2018.
• Rails: was downloaded from the U.S. Department of Transportation, Bureau of Transportation Statistics, National Transportation Atlas Database, Open Access Data Set, as of December 2018.
• Base Map: The World Light Gray Base, Web Mercator Auxiliary Sphere (WKID 102100) coordinate system as of December 2018, was imported from Esri, Garmin, HERE, MapmyIndia, INCREMENT P, © OpenStreetMap contributors, and the GIS community.

Uses and Limitations

The “Port Vicinity” maps illustrate the approximate location of port areas and surrounding facilities, but may not provide exact facility locations. The maps should not be used for detailed analysis. For all ports, the area was drawn broadly to ensure that no facilities were missed. In some cases, the AIS data differed from port websites and satellite imagery. In these cases, the map included AIS data as part of the strategy to ensure inclusion, and the port vicinity may differ significantly from other port facility maps. The scale of each map differs based on the extent of the port vicinity. Some ports are compact, with all facilities located close together. Other ports are much larger, such as the Ports of Cincinnati-Northern Kentucky, which extends for over 200 miles along a river. While the mapping process was consistent across ports and tailored to port vicinity extent, the base maps for individual ports vary in level of detail shown. The limiting bridges included on some port vicinity maps were selected based on air draft, and illustrate the potential limits on vessel size. Some bridges were left off maps for improved clarity. Other ports may be limited by bridges that do appear on port vicinity map.

Port Terminal Polygons

Background

Port terminal polygons are digital geospatial boundaries for cargo-handling maritime terminal facilities. The polygons were defined using AIS vessel position data in a geographic information system (GIS) overlaid on publicly available satellite maps, and were used to calculate key performance metrics.  The scope of the project was limited to container, liquid bulk, and roll-on/roll-off (Ro/Ro) terminals at the Nation’s top container, dry bulk, and tonnage ports, as identified from U.S. Army Corps of Engineers (USACE) Waterborne Commerce Statistics Center (WCSC) data. All the polygons originally created for the PPFSP were drawn in ArcGIS by the BTS Office of Spatial Analysis and Visualization. BTS and USACE has periodically validated and updated the port terminal polygons to ensure accurate capture of vessel operations at the Nation’s top ports for the PPFSP Annual Report and interactive Port Profiles.

The purpose of this effort was to:

1. utilize cross-verifying data sources to create a GIS polygon layer reflecting the location, time, and ship details of cargo vessels at berth; and
2. identify and demarcate, with a repeatable and nationally consistent methodology, the land and waterside extent of port facilities where cargo is stored or transferred to other transport modes.

No nationally consistent methodology, framework, or best practices previously existed for drawing port terminal polygons. In broad terms, the framework for including port terminal polygons in the PPFSP was established in three steps. First, the team analyzed AIS data showing where vessel activity occurred over a year in the context of ancillary data such as terminal maps, satellite imagery, and land use shapefiles. Second, the team created polygon shapefiles around the terminals where both cargo handling facilities and cargo activity exist. Third, the team shared the shapefiles with U.S. Army Engineer Research and Development Center (ERDC), to be used in conjunction with the ERDC’s Automatic Identification System Analysis Package (AISAP). The results were then used to generate performance measures used in the PPFSP.  

While a shared protocol was followed to create the polygons, differences in port and channel layouts required flexible interpretation of the guidelines. The steps presented should then be considered best, but very general, practices. Ultimately, AIS data determine the shape and size of any boundary, and each polygon exists to provide specific information about a given port terminal or facility. 

Querying and Filtering the Initial Data

AIS data associated with each port was acquired via the NAIS and provided to the team by USACE in the form of comma-separated values (CSV) files. The files were parsed into monthly files using a query that retained only AIS data from vessels with a navigation status of moored and a speed of zero. Filtering data by these parameters ensures that the dataset is limited to information from vessels that are truly stationary and therefore reasonably able to load or unload cargo.

Using ArcMap, the monthly AIS files were joined to a CSV file containing information provided by USACE for individual cargo vessels by their MMSI numbers. This process provided additional detail for each AIS data point including vessel name, the confirmed MMSI number, ship type, and ship subtype.

All data points for non-cargo vessels were removed from the dataset. Those data points left unmatched by MMSI number were removed from consideration to limit the influence of human error on the analytical process.

Drawing the Polygons     

In ArcMap, the cleaned files were converted to points, and overlaid onto satellite imagery basemaps. Additional data pertaining to port facilities, including coordinates for thousands of docks in the United States and their structural and usage attributes (commodity processed, owner, operator, and name), were obtained through the USACE Master Docks Plus database (which contains 40,000 port-and-waterway facilities and other navigation points of interest), converted to points, and added to the ArcMap document. Each port contains multiple docks and each eventual port terminal polygon contains at least one dock. 

Clusters of AIS data points within a reasonable distance of dock facilities were identified. For the purposes of this project, AIS clusters are defined as 10 or more transmissions from the same vessel within close proximity. Although many clusters contain 100+ pings, the lower limit of 10 ensures that a vessel is stopped at a given terminal.

The boundaries of port terminal polygons were then drawn in ArcGIS around individual docks or terminals that contained at least one AIS cluster. Where Master Docks Plus points existed, polygons were linked in the underlying database according to the listed owner or operator, and were presumed to be handling similar cargo types. Where no Master Docks Plus point existed, USACE was notified and secondary sources were used to identify ownership. If this process was fruitless, the polygon was drawn without associated ownership information. This process provides the PPFSP the opportunity to complete analysis at both the port and terminal level.

As stated above, a shared protocol was followed, but differences in port and channel layouts require flexible interpretation of the guidelines. The distance that port terminal polygons extend into the channel from physical dock structures, visible through satellite imagery, is a prime example of this flexibility. The different standards for water-side polygon boundaries for three general terminal types are described below and show in the figure below:

1. Container and Ro/Ro terminals: Polygons may extend 400 feet from the edge of the waterside boundary of the dock. This measurement can be reduced if the dock shares a boundary with an adjacent terminal or if the channel is particularly narrow. This ensures no erroneous capture of vessels moored in the channel. Most terminals used for Ro-Ro were previous used for container records, a few additional polygons were added to ensure complete coverage.
2. Oil and chemical terminals: Polygons may extend 150 to 200 feet from the waterside boundary as these terminals are often located in narrower channel areas. Oil and chemical docks may also be floating berths located further from the visible waterside dock boundary and connected via piping to a processing area. In such cases, the polygon must be shaped appropriately to capture any AIS data present. As noted above, BTS and ERDC has periodically validated and updated the port terminal polygons to ensure accurate capture of vessel operations, especially for tankers.
3. Miscellaneous cargo terminals: Polygons may extend 150-250 feet from the waterside boundary. These docks may be equipped to handle one of several cargo types including break-bulk, dry bulk, and Ro/Ro cargoes. As with the oil and chemical facilities, these facilities are often found in narrower channels or inner parts of the port.

Vessel Type Length Ranges

The landside portions of the port terminal polygons were drawn to cover each terminal’s loading areas as well as storage and processing facilities. Landside boundaries were drawn to follow internal or access roads. Boundaries stop at any major roads, railways, or developed areas that are far away from the water and not part of another port terminal. 

In many cases, port terminal polygons were drawn adjacent to each other. In this situation, it is appropriate to align the waterside borders if AIS data extends into water at a similar distance, and align the landside borders if the boundaries follow the same human-made features. In many ports, terminals that process the same or similar commodities are located adjacent. The process of aligning boundaries and the nature of AIS data may result in having AIS clusters that span two or more polygons. This should not affect the annual results presented in any of the performance measures.

Container Terminal Minimum Project Depth

Background

Channel depth limits the sailing draft (the vertical distance between the waterline and keel) of vessels that can call at a port. Container terminals may be faced with greater channel depth requirements as the size of containerships increases. Multiple terminals within the same port may have different channel depths as different Federal navigation projects cover the route between open water and each terminal.

Concept

U.S. Army Corps of Engineers (USACE) constructs and maintains Federal navigation projects to an authorized depth as specified in congressional legislation. Not all channels are constructed or maintained to their exact authorized dimensions. Maintained depths may be less than authorized due to several factors. In some cases, limited annual budget allocations may have precluded maintaining the entire navigation project to full authorized dimensions; this is particularly true when the initial deepening results in significantly higher-than-expected sediment loads accumulating in the channel. In other cases, the difference is temporary, pending completion of ongoing channel deepening activities, which can require several years depending on the scope of the required dredging. As channel depths vary over the course of the year, any reporting of actual depths risks being out of date shortly after publication. The interactive Port Profiles for the top 25 container ports therefore detail the minimum Federal navigation project depth encountered on the path between open water and each terminal.

Methods

BTS worked with USACE to develop protocols that treated all terminals equally. The primary sources for determining the minimum Federal navigation project depth for each terminal were the project dimensions tabulated from USACE hydrographic surveys that are processed and uploaded by USACE Districts and available via the eHydro data portal.[1]. The path between terminal and open water was plotted for each container terminal, and the Federal navigation projects along that path were identified. The maintained Mean Lower Low Water (MLLW) depth for each Federal navigation project along a given path was recorded, and the minimum depth encountered along each path was assigned as the minimum project depth for the terminal. Turning basins were not included unless a path used them on the way to a terminal. Several terminals are located at ports with natural deep-water channels, and project information was not available for the path that connected those terminals to open water. In those cases, the profiles report N/A for the minimum project depth. A representative of USACE subsequently confirmed the paths and depths.

Data Sources

The primary source for the project dimensions were hydrographic surveys conducted by USACE and disseminated via the eHydro data portal. These charts include soundings (in feet) of depth throughout a project and the maintained MLLW depth (in feet) of mapped Federal navigation projects. USACE performs ongoing channel depth surveys, with new readings incorporated into National Oceanic and Atmospheric Administration (NOAA) [2] and USACE charts and documents. Surveys were selected based on the most recent calendar year to align with other data points included in the interactive Port Profiles.

Uses and Limitations

The ongoing changes in channel depths due to tidal impact or localized shoaling from sediment accumulation mean that the reported minimum project depths are not substitutes for actual controlling depths, and instead provide insight into the size of vessels the terminals can handle. Further, ongoing or recently approved channel deepening projects may not be captured in the charts available. As described above, several terminals reported N/A to reflect issues collecting Federal navigation project depths.

[1] U.S. Army Corps of Engineers, Hydrographic Surveys, available at https://www.mvr.usace.army.mil/Missions/Navigation/Hydrographic-Surveys/ as of February 2020.

[2] U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Office of Coast Survey, Nautical Charts, available at https://www.charts.noaa.gov/InteractiveCatalog/nrnc.shtml as of February 2020.

Container, Tanker, & Roll-on/Roll-Off (Ro/Ro) Vessel Dwell Times

Background

Vessel dwell time refers to the continuous time a vessel spends within a defined geographic area. When applied to terminal berth areas within coastal ports, vessel dwell time can be used as a proxy metric for time spent in port securing the vessel, discharging or loading cargo, and other activities. In the context of the PPFSP, vessel dwell times at coastal port terminals provide insight into the relationship between vessel size, cargo volumes, and throughput.

To support this vessel dwell time analysis, researchers with the U.S. Army Engineer Research and Development Center (ERDC) processed and analyzed archival vessel position reports from calendar year 2016 through the most recent year available. The vessel position data were acquired via the U.S. Coast Guard’s (USCG) Nationwide Automatic Identification System (NAIS). The geofenced watch areas used to define the port terminal areas were provided to the ERDC team by the BTS Office of Spatial Analysis and Visualization.

Concept

The dwell time analysis uses geofenced watch areas and archival Automatic Identification System (AIS) vessel position reports to generate a wealth of useful information. The large number of observations allows for calculation of meaningful summary statistics for each port area.  These include mean dwell times and distributions.

Methods

Vessel Calls

Vessel calls within a defined port area are determined by identifying each instance of a unique vessel entering, stopping in, and exiting the watch zones. Short-duration events are filtered out from consideration. Examples of short-duration events include brief harbor tug visits or vessel transits that skirt the outer portions of the watch zone.

International Classification of Ships by Type (ICST) Codes

The broadcast AIS messages from each vessel include a general ship type field selected by the operator, and vessel type labels such as Cargo, Tanker, Tug, Tow, and Passenger Notably, there are no separate AIS broadcast classifications for container ships, vehicle carriers, or for the various sub-types of tanker vessels. To overcome this limitation, another field in the broadcast AIS message, the International Maritime Organization (IMO) ship identification number, is used to match the vessel identities and types in a separate inventory of vessels compiled by the ERDC team from the U.S. Customs Vessels Entrances and Clearances inventory. This inventory is available via the USACE Navigation Data Center.[1] This data set includes the ICST codes that provide classifications for container vessels, chemical tankers, and LNG carriers, among others of interest to the PPFSP. Vessels broadcasting via AIS with an invalid or blank IMO field were manually matched using the vessel name included in the AIS message. The ICST matching process allows for the AIS-derived vessel call and dwell time measures for each port area to be categorized across 42 possible vessel classifications.

Dwell Time Statistics

Dwell times are estimated as the difference between the date-time stamps of the earliest and last sequential AIS position reports from a stationary vessel within a geofenced area. Summary statistics are calculated based on the available dwell time observations at each port for all the most recent full calendar year.

Data Sources

The AIS data standard is set by the International Telecommunication Union (ITU). The 27 message types embedded within the AIS broadcast include vessel name, maritime mobile service identity (MMSI) number (a unique nine-digit code used internationally to identify an AIS unit), AIS ship type, discrete time-stamped position (latitude/longitude) records, and vessel operating parameters such as speed, course over ground, heading, and rate of turn.

To support the PPFSP, archival AIS position reports from the USCG’s NAIS were accessed by the ERDC team via a suite of web services provided to USACE through a standing inter-agency security agreement. The ERDC-developed Automatic Identification System Analysis Package (AISAP) was used to query, compile, spatially filter using the geofenced areas, and interpret and analyze AIS data to derive records of vessel calls.  The AIS data was collected within the NAIS archive beginning in 2016 through the most recent year available for the ports featured in this report. To keep data transfer and computational times manageable, the vessel position histories were sampled at five-minute intervals.

Uses and Limitations

Not all vessels found within the AIS record could be matched via the IMO number, as discussed previously. In most cases, the unmatched vessels were harbor tugs and other U.S.-flagged vessels that do not have IMO numbers. Since IMO registry is mandatory for all cargo ships of at least 300 gross tons, nearly all commercial vessels of interest to the PPFSP can be matched via this process, with only a few inconsistencies typically identified at each port location for each calendar year.

AIS messages are broadcast over very high frequency (VHF)-band range and can be subject to disruptions from terrain or weather events. Likewise, equipment problems with the onboard AIS transceiver units or the shore-based NAIS receiving towers can occasionally cause interruptions in coverage for individual vessels or within particular areas.  During interruptions, some vessel call events may be missed completely, or interruptions may cut the associated dwell time observations if they happen to span the period when the vessel entered or exited the geofenced watch area. Signal disruptions or software issues may also sometimes lead to inaccurate geo-coordinates for some position reports, causing vessels docked at terminals to appear to briefly leave the geofenced terminal area. Absent any quality control steps, these issues with position accuracy may be incorrectly interpreted as multiple calls by the same vessel, when in fact only one has occurred. The following table summarizes three filter thresholds used to post-process the vessel call event counts and dwell time observations derived from the geofenced port terminal boundaries and the NAIS archival data. 

Vessel Call and Dwell Time Post-Processing Filters

One key assumption for the dwell time evaluation is that vessels of all types require a minimum of two hours inside the terminal boundary to constitute a valid observation wherein cargo could be discharged or loaded. This threshold filters out cases wherein vessels are merely passing through the terminal area or maneuvering for transit elsewhere within the port area. Multiple, consecutive dwell time events by the same vessel are combined into a single entry if less than two hours elapse between them. The duration of this new, combined entry is based on the respective start and end times of the first and last observed dwell time events, and thus spans any coverage gaps. The last entry in the table above refers to instances where there was more than a one-hour gap prior to the first observation of a vessel position report within the geofenced terminal area, or more than a one-hour gap after the last observation within the area. These cases are counted as valid vessel call events. For most port areas, between one and three percent of vessel call observations are screened out from the summary statistics calculation due to these gaps in the coverage record.

Container and Ro/Ro Vessel Dwell Times

Because it takes between one and three hours to secure a container or Ro/Ro vessel on arrive and prepare it for departure, calls of less than four hours were deemed too brief for significant container cargo transfer. Calls of over 120 hours were considered anomalies, possibly due to delays unrelated to cargo handling, and were not included in further calculations. Most Ro-Ro vessels broadcast the vessel type of "Cargo" in their AIS message, thus further disambiguation using the ICST vessel classification was necessary. The ICST vessel types used to identify Ro-Ro vessels as distinct from cellular containerships were: Other Ro-Ro Cargo, Ro-Ro Container, and Ro-Ro Cargo Ship. Only Ro/Ro-Cargo Ship entries were used in the dwell time analysis, and those were further verified as being vehicle carriers, as opposed to some type of combination vessel, using fleet listings compiled from commercial lists and open source Internet databases. To assign vessel TEU capacities, vessel names were compared with fleet listings compiled from commercial lists and open source Internet databases. Vessels were limited to geared or cellular container ships; barges or Ro/Ro vessels handling containers would not have comparable dwell times. The analysis of container and Ro/Ro vessel dwell times included additional filtering steps, as shown in the following table:

Container and Ro/Ro Vessel Dwell Time Post-Processing Filters

Tanker Vessel Dwell Times

The analysis of tanker vessel dwell times also included additional filtering steps, as shown in the table below. Because it takes between one and three hours to secure a tanker vessel on arrival and prepare it for departure, calls of less than four hours were deemed too brief for significant liquid bulk cargo transfer. Calls of over 120 hours were considered anomalies, possibly due to delays unrelated to cargo handling or vessels remaining idle, and were not considered.

Tanker Vessel Dwell Time Post-Processing Filters

As shown in the table below, the AIS tanker vessel data included eleven possible ICST vessel types. For the PPFSP analysis is was not necessary to distinguish vessel type on this fine level of detail. The associated simplified types were used instead to make the data analysis more manageable and the graphic displays clearer.

ICST and Simplified Tanker Vessel Types

Several metrics are used for tanker vessel size. The most widely available metric is Gross Register Tonnage (GRT), the total internal volume of a vessel expressed in “register tons” equivalent to 100 cubic feet each. GRT is thus a measure of volume rather than of weight. To assign vessel GRT capacities, vessel names and MMSI numbers were compared with fleet listings compiled from commercial lists and open source Internet databases. Vessels were limited to types shown in the table above.

[1] U.S. Army Engineer Institute for Water Resources, Navigation Data Center, available at http://www.navigationdatacenter.us/ as of February 2020.