It's on us. Share your news here.

DredgeWire Exclusive: The Tappan Zee Bridge Project Shows a Way Forward for Baltimore’s Francis Scott Key Bridge

The U.S. Navy released this CODA EchoScope image merged with LiDAR point cloud data, of the twisted Francis Scott Key bridge wreckage. illustrating the difficulty of removal. Courtesy of U.S. Navy’s Naval Sea System Command; Supervisor of Salvage and Diving

Posted on April 10, 2024

This Sonar And Positioning Based Method Can Reduce And Remove Bridge Debris Methodically And Economically

By Judith Powers

Once the visible structure of the downed Francis Scott Key bridge has been removed, the ever more complicated job of removing the submerged debris will begin. Sonar and positioning technology will be needed to identify, locate, reduce, and rig the heavy metal sections, and robust cranes must lift the pieces from the bottom to adjacent barges for transport to a yard in nearby Sparrows Point.
These maneuvers will be most economical with accurate machine positioning and guidance, augmented with visualization by sonar, each positioned by the Global Navigation Satellite system (GNSS).

With the mass of metal constantly settling and shifting, frequent multibeam surveys of each work area will be necessary, the resultant 3D point clouds obtained being vital to the work of machine operators and divers alike, the work carried out with attention to hazards present in the wreckage.

Fortunately, a method for safe deconstruction and removal of underwater debris exists and has been proven over a number of years , most visibly on the East Coast on the deconstruction of the Tappan Zee Bridge on the Hudson River. The method is a GNSS based system which provides real-time machine positioning and guidance for on-water cranes/ excavators/ and other machines aided and augmented by sonar verification.


When the replacement bridge – the Gov. Mario M. Cuomo Memorial Bridge – was up and running, the above water structure of the old bridge was removed and the underwater portion knocked into the water. A large 1,916-ton crane called the Left Coast Lifter (later renamed I Lift New York) arrived from its previous job in San Francisco, and in the spring of 2015, Tappan Zee Constructors, LLC (TZC) contacted Lou Nash, whose company Measutronics Corporation, had installed a positioning and guidance system on the Lifter during the summer of 2009.This was Nash’s introduction to TZC where he and Measutronics went on to workwith them in formulating a time saving and economical deconstruction debris removal process. Measutronics played a supporting role in the project through May of 2020.As Trimble, Inc.’s Advanced Marine Partner, Measutronics had first provided machine positioning and guidance, augmented with near real time 3D sonar imageryin support of underwater construction with Weeks Marinein 2003. The Tappan Zee Bridge deconstruction project provided the scope and duration to improve on hardware and software integration, in providing real time machine positioning, guidance and underwater imaging.

The system uses multibeam sonar returns of the bottom that create three-dimensional point clouds, each point having an X,Y,Z location creating a crisp image of the bottom and debris, showing dimensions, angles, depth, and aspect of all bottom objects. At the sonar operator’s console, a real-time, accurate view of the bottom and debris is displayed on a computer screen. This image is in turn displayed in near real time on any number of machine operator’s positioning and guidance display screens.

“This is a small community,” said Nash. “We’ve worked in one capacity or another with most all of the contractors already on site in Baltimore. As best I know, there are at least five different models of scanning sonars on site, representing three of the major manufacturers, and we’ve interfaced with each sonar in some fashion over the years. There are only a few choices for each sensor and sonar, and we have the drivers for most of them in our software already, along with some “one-offs” that we’ve created internally” he said.


Nothing is stationary when working on the water. Machine positioning and guidance requires a number of sensors, used in combination, to position a machine, thus enabling guidance of its bucket or other attachments. For example on clamshell cranes, rotational encoders on the cable drum are able to measure the cable paid out while also accounting for the reduction in diameter of the cable laysafter multiple rotations of the drum. A similar encoder on the bucket closing line shows the bucket open and close and can indicate if the bucket isn’t fully closed. Similarly, dual axis inclinometers on the machine cab, boom, stick, and bucket capture all the motion of barge mounted long stick excavators. The guidance software supports any number of excavator attachments: buckets, grapples, pincers, hydraulic hammers, and more.

Calibration of the crane movements and tool actions is applied geometry. Strategically located encoders, such as on the cable drums, crane pivot point, crane tip and underwater tool send constant measurements to the software. Similar sensors are installed as needed on any tool or attachment required for the task at hand.

The system allows the operators to see what and where the submerged debris lies,thus facilitating its removal. The ongoing multibeam scans provided to machine operators ensure that nothing is missed.

On the Tappan Zee project, machine operators were assigned an area in which they reduced and removed materials using buckets, pincers, shears, hammers, grapples, jackhammers and more.

The project included up to 10 barge-mounted machines at times, with accompanying debris barges, tugs and other support vessels configured to break, cut, and remove debris. Concrete was demolished using a jackhammer guided with centimeter level accuracy; metal was cut in sizes conducive to retrieval. At that point, clamshell cranes were used to remove the debris.

When a machine completed a task, such as cutting a length of metal or breaking a caisson, its operator contacted the survey boat to create a new scan of that area. This provided an “as working” view of the job, an up-to-the-minute view of the machine’s progress.

The Trimble Marine Construction (TMC) guidance software was input with the target elevation (“existing mud line”), and showed all features above final grade in green, changing to yellow as it approached grade – another feature that helped operators know what is left to pick up. Once an area was deconstructed, and the debris removed, operators called for a new 3D scan. Having progressed on the 3D point cloud generated surface from green, through yellow and on to red(like an inverted traffic light) the operator called for a barge move to the next assigned work area, confident that nothing was missed or left behind.


Nash recently described to DredgeWire how the system could be valuable in removing Francis Scott Key (FSK) Bridge debris.

“We can guide each vessel to the debris and show its location and to the machine operator who can then use the guidance system, showing orientation of the bucket or other attachment relative to the targeted object, for efficient removal.” he said.

Nash added, “Because of the chaotic nature of the debris, it’s important to know exactly what is under each barge and vessel as it approaches the debris. With the sonar scan integrated into the machine guidance software, tug and barge operators can maneuver in safety, without hitting or dropping spuds on an underwater structure, or fouling anchor wires in debris.”

The detailed view of the bottom, including dimensions and shifts in the mass through frequent scans, also greatly reduces the necessity of sending divers into a hazardous situation.
Nash stressed that every project is different and that he and his team can assess what is needed according to the nature of the application requirements . He sees the Key BridgeFSK project as well within tried and tested technology to facilitate deconstruction and removal safely and economically.

According to a 2022 case study of the Tappan Zee project in Marine Construction magazine, the system that Nash and Measutronics created, provided the world’s first application of real-time machine guidance for on-water cranes/ excavators/ jack hammers, with sonar verification, eliminated “working in the blind” and enabled safer, faster, more accurate demolition. It accelerated demolition and debris removal by giving a “bird’s eye view,” showed elevation, location, depth, bucket position and real-time 3D view of debris with precise accuracy. It improved revenue capture with documentation of “as-building” capabilities. Multibeam sonar and image capture document benchmarks were met.

It's on us. Share your news here.
Submit Your News Today

Join Our
Click to Subscribe