Bridge structures that must withstand being rammed into by a big ship — eerily similar to the MV Dali – were required for the Delaware Memorial Bridge almost five years ago.
Eight round, concrete, rock and sand cylinders almost 80 ft in diameter, known as “dolphins,” were sunk 45 ft into the Delaware River bed south of Philadelphia to protect the towers of the twin-deck bridge from a 156,000-ton ship traveling 7 knots.
The weight and speed of the MV Dali as it crashed into the Francis Scott Key Bridge would be 156,000 tons if it was about half full and it was traveling 7.6 knots before the collision.
The similarity may have you thinking that the 80 ft diameter dolphins around the Delaware Memorial Bridge were designed for the worst-case scenario by considering a ship as big as the MV Dali. They were not. MV Dali did not even crack the ten biggest container ship list in 2019, when the biggest ship at the time was the OOCL Hong Kong, a 100 m longer than the MV Dali and able to carry twice the load.
Most reports of the MV Dali incident judge either miss seeing the four dolphins installed around the Francis Scott Key Bridge or judge them inadequate. From an arial photo, we estimate them to be 28-ft in diameter. They probably would not have taken a direct hit, but that is a moot point since the path of the ship, as determined by AIS (automatic identification system) data read by MarineTraffic.com, shows that the MV Dali avoided the dolphins. The dolphins guarding the south tower had assumed ships would travel parallel to the shipping channel. Instead, the Dali veered off the shipping channel and came in on an angle, sailing past the dolphin and into the bridge.
Lessons applied to Delaware Memorial Bridge protection
In 1980, the Sunshine Skyway Bridge in Tampa Bay collapsed after being struck by an empty bulk carrier weighing less than 20,000 tons that may have been sailing at 9.5 knots, according to the NTSB accident report. The collapse cost 35 lives, including 23 people in a Greyhound bus that fell 150 ft into the water.
Modjeski and Masters, a bridge engineering firm headquartered in Mechanicsburg, Pennsylvania, was involved in the investigation and applied what it learned to create the “Criteria for Design of Bridge Piers Against Ship Collision in Louisiana Waterways.” This served as a reference for bridge protection until it was replaced by the AASHTO Guide Specifications for Vessel Collision Design in 1990. Years later, in a June 2019 press release, Modjeski and Masters announced that it was awarded the contract to design the bridge protection system for the Delaware Memorial Bridge.
A new worst-case emerged
While the size and number of dolphins around the towers of the Delaware Memorial Bridge may withstand a direct hit by an MV Dali-sized ship traveling parallel to the shipping channel, would they protect the bridge tower against:
- Ships much larger than the MV Dali or traveling faster?
- A ship veering off the shipping channel, as MV Dali did?
The MV Dali veered off the shipping channel and struck the Francis Scott Key Bridge at roughly a 23° angle to the channel. Bridge authorities who calculate the likelihood of bridge strikes may consider the possibility of a ship veering off course after an unfortunate series of events like MV Dali’s too unlikely to occur again.
But for engineers, this is a new worst case to be seared into our souls and for future generations, as is Galloping Gertie, another bridge failure we never foresaw and now cannot unsee.
The dolphins around the Delaware Memorial Bridge towers are staggered and placed inward towards the eight-lane shipping channel. But an MV Dali-sized ship that veers as little as 31° from perpendicular to the bridge could slip between the dolphins and hit the tower with a glancing blow — which could be more dangerous than a direct hit since the tower’s island is weakest in the direction parallel to the bridge. A dolphin placed midway between the inside dolphins would preclude the possibility, however unlikely, of a ship managing to slip through the dolphins.
Also, we can’t dismiss the possibility of a deliberate attempt to do what MV Dali did accidentally. The U.S. Department of Homeland Security (DHS), tasked to protect the nation’s infrastructure from deliberate attempts to harm it, does not mention ship strikes in its archived August 2007 article Protecting Bridges from Bad Guys. In an April 1 video of the DHS’s activities, its only mention of the Francis Scott Key Bridge collapse is a thank-you to the U.S. Coast Guard for their response in rushing to the scene.
Reference
MV Dali Hitting the Key Bridge - Track and Video Analysis, What is Going on in Shipping? YouTube Channel, Salvatore Mercogliano