Vessel Traffic
Vessel Traffic Characteristics:
Physical and Operating Characteristics
General knowledge of the operation of vessels and their characteristics is essential for safe bridge design. The types of commercial vessels encountered in navigable waterways may be divided into ships and barge tow
Ships:
Ships are self-propelled vessels using deep draft waterways. Their size can be determined on a DWT basis. DWT means a metric ton (1 ton = 2205 lb = 9.80 kN) of cargo, stores, fuel, passenger, and the ship carrying personnel when fully loaded. There are three main categories of merchants
Ships: bulk carriers, product carriers/tankers, and freight carriers. General information on ship profiles, dimensions, and sizes as a function of ship class and its DWT are given in the references [6,7]. The dimensions given in the references [6,7] are typical values, and because of the large variety of vessels present, they should be treated as general estimates.
Steering of ships in coastal waterways is a difficult process. This involves constant communication between the shipmaster, the helmsman, and the engine room. There is a time delay before the ship begins to respond to an order of change of speed or trajectory and the response of the vessel is relatively slow.
Therefore, the shipmaster must be familiar with the waterway and know in advance about obstacles and navigation, and weather conditions. Often local pilots use ships to navigate a particular section of the coastal waterway. When navigation conditions are difficult, tug boats are used to assist ships in turning.
Ships need speed to control and maintain steering. A minimum vessel speed of at least 5 knots (8 km / h) is usually required
To maintain the steering. Fully loaded vessels are highly maneuverable, and in deep water, they are stable in direction and can take a radius equal to one to two times the length of the vessel.
However, because the under-keel clearance is reduced to less than half the vessel, many vessels become unsteady, meaning that they require constant steering to travel in a straight line. On coastal waterways in the United States, the key clearance of many Lauden ships may be below this threshold, in some cases as small as 5% of the ship’s draft. Ships riding in ballast with shallow draft are less maneuverable than loaded ships and maybe more affected by wind and currents. Historical accident data indicates that most bridge accidents involve empty or parked vessels.
Barge Tows:
Barge tows use the deep draft and shallow draft waterways. Most existing bridges cross shallow-draft waterways, where the fleet consists of barge tows. In the United States, the size of barges is usually defined in terms of shipping capacity in short tons (1 ton = 2000 pounds = 8.90 kN). Inland barges include open and closed hoppers, tank barges and deck barges.
They are rectangular in shape and they can travel in tows as their dimensions are quite standard. The number of barges per bar can vary from one to 20 and their configuration is affected by waterway conditions. In most cases, the boats are pushed by a ferry boat.
Information on barge dimensions and capacity, as well as information on barge tow configurations is included in the references [6,7]. Statistical analysis of barge tow types, configurations and dimensions utilizing barge traffic data from the Ohio River is reported in Ref. [12].
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Barge tows are difficult to control and drive, especially in waterways with high stream speeds and cross currents. Taking a turn on a fast waterway with high current is a serious task. In conducting a bending, due to inertial forces, the tows will experience a sliding effect in the opposite direction of the bending direction, which is often coupled with the current flow. Sometimes, bridge poles and fenders are used for pulling before turning.
The fastest waterway bridges near the turn of the channel were probably hit by barges many times in their lifetime. Generally, there is a high probability of hitting any bridge element that can be reached by barge over the life of the bridge.
Vessel Fleet Characteristics
The vessel data required for the design of the bridge include the types and size distributions of vessels, shipping frequencies, typical vessel speeds, and loading conditions. To determine the size distribution of vessels at the bridge site, detailed information on current and projected future ship traffic is required. Collecting data on vessel fleet characteristics for the waterway is an important and often time-consuming process.
Some of the sources in the United States for collecting vessel traffic data are listed below:
• U.S. Army Corps of Engineers, District Offices
• Port authorities and industries along with the waterways local pilot associations and merchant marine organizations
• U.S. Coast Guard, Marine Safety & Bridge Administration Offices
• U.S. Army Corps of Engineers, “Products and Services Available to the Public,” Water Resources Support Center, Navigation Data Center, Fort Belvoir, Virginia, NDC Report 89-N1, August 1989
• U.S. Army Corps of Engineers, “Waterborne Commerce of the United States (WCUS), Parts
1 thru 5,” Water Resources Support Center (WRSC), Fort Belvoir, Virginia
• U.S. Army Corps of Engineers, “Lock Performance Monitoring (LPM) Reports,” Water Resources Support Center (WRSC), Fort Belvoir, Virginia
• Shipping registers (American Bureau of Shipping Register, New York; and Lloyd’s Register of Shipping, London)
• Bridge tender reports for movable bridges Projections for anticipated vessel traffic during the service life of the bridge should address both changes in the volume of traffic and in the size of vessels. Factors that need to be considered include:
• Changes in regional economics;
• Plans for deepening or widening the navigation channel;
• Planned changes in alternate waterway routes and in navigation patterns;
• Plans for increasing the size and capacity of locks leading to the bridge;
• Port development plans.
Ship traffic projections from the US Department of Transportation, Port Authorities, and the US Army Corps of Engineers are good sources of information for bridge design along with planned channel-deepening plans or lock replacements. As a large number of factors affect ship traffic in the future, it is important to review and update planned traffic over the life of the bridge.
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