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Preventing Ship Allisions of Bridges

Bridge Pier Protections

Place Piers on Land

Bridge piers are the posts that hold up bridges. The first line of defense is to position bridge piers on land or in shallow water, if possible, to avoid having ships be able to reach the bridge piers.

Figure 1:  Cable-stayed bridge in Croatia, with piers on land. [Wiki]
Figure 2:  Cable-stayed bridge with piers on land, at Savannah Port, Georgia, USA. Remaining land piers of the removed old bridge are shown. [Army]
Figure 3:  Container cargo ship, cable-stayed bridge, and a remaining land pier from the removed old bridge (right). Savannah Port. [Wiki]

Artificial Islands

If bridge piers are built in water that a large ship could reach, instead of built on land or shallow water, an artificial island can be built around each pier that is in or near the ship channel.

An artificial island protecting a bridge pier needs to be big enough for a ship to run aground on the artificial island before reaching the pier. If the artificial island is too small, the bow overhang of a ship could strike the piers high above the water level before the ship runs aground on the artificial island below.

Figure 4:  Artificial islands protect the shipping channel piers of the Commodore Barry Bridge, Pennsylvania. [Wiki]
Figure 5:  Cable-stayed bridge pier protected with artificial island, Charleston, South Carolina. [RyanPethick]
Figure 6:  Artificial islands protecting piers of the cable-stayed bridge at Brunswick, Georgia, USA. [DevinMorris]
Figure 7:  Brunswick bridge under construction (center), and old bridge (left). Artificial island was extended to the left after the old bridge was removed. [BobWebster]

Apron

If an artificial island has a seawall along its perimeter, the seawall is called an apron.

Figure 8:  Aprons protecting the piers of Rupsha Bridge in Khulna, Bangladesh. [TarifEzaz]

Aprons must be far enough away from the bridge piers to block a ship before the ship’s bow overhang could reach a pier. Aprons that are close to a pier are called fenders and are not able to prevent bow allision.

Faux Pier Islands (Dolphins)

Additional protection is achieved by constructing artificial islands separate from and next to bridge piers. These islands, that are separate from the bridge, are called low pier islands, faux pier islands, or dolphins.

These protection islands are essentially low piers that do not hold up anything but can take a ship strike, blocking ships from reaching the actual bridge piers.

Compared to the height of bridge piers, the faux piers do not extend vertically high above the water. The lower height reduces leverage that could break the faux piers.

The faux piers are called dolphins because they may be in the water along a ship’s bow wake — like dolphins swimming along the bow wake of a ship in the ocean.

These dolphins (faux piers) can be filled with concrete, sand, or other fill.

Figure 9:  View of one side of the shipping channel of the cable-stayed Tampa Bay Bridge in Florida. The bridge has artificial islands and dolphins (faux piers). The dolphins must have large diameters and be plentiful, as shown here. [MBarrison]
Figure 10:  Zoomed-in portion of the preceding photograph. Arrow points to a small recreational boat with people in it, to the right of a dolphin (faux pier). The dolphin is much bigger than the recreational boat, despite being low compared to the bridge pier. [MBarrison]
Figure 11:  Constructing the walls of a steel-sided low pier in Japan, for use in a seawall (similar structural properties as dolphins). [Kashima]
Figure 12:  The steel-sided low pier is in position (left) and filled with sand from a barge (right). [Kashima]