Front Pages and Table of Contents
Index and Author Bio
Mark W. Atherton
It is a fact the underwater substructure inspection of bridges, docks and dams has taken a back seat to above-water surveys. “Out of sight, out of mind.” Federally mandated inspection of the underwater structural element of all bridges in the United States is every five years, whereas the above-water component of the same structure must be inspected every two years.
The numbers vary slightly depending on the source, but in the United States there are approximately 590,000 road and highway bridges. Of these, the Federal Highway Administration (FHWA) has identified 17,000 of these as “scour critical,” and another 86,000 as “scour susceptible.” Most owners of private bridges (including railway) and loading facilities follow the FHWA guidelines; however, it is left up to these companies to determine what inspection schedule is appropriate.
There is one fact that is not disputed: “Scour is the primary cause of bridge failure in the United States.” And the same problem exists world-wide.
Traditional Methods of Underwater Inspection
With the collapse of the Silver Bridge on the Ohio River on December 15, 1967, the FHWA started the maintenance and inspection program of bridges above and below the waterline. Although above-water inspection techniques and equipment have improved, the underwater inspection of bridges is still being performed using commercial divers to complete a visual and/or tactile inspection, as it has for the past 100-plus years.
Diving inspections are often challenged by high current flows, deep water, lack of visibility (due to sediment load) and debris accumulation around the structure. In addition to limiting the diver from completing a thorough inspection, debris buildup around bridge piers is potentially one of the most dangerous conditions the inspection diver faces. It introduces the possibility of entanglement, and even worse, diver entrapment if the debris moves. In spite of these hazards and conditions, commercial divers and engineering firms with the proper commercial diving certification have done a remarkable job inspecting bridges and other underwater structures.
Irrespective of the professionalism of the diver, one common concern within all inspection programs is inconsistency in the substructure inspection and the associated inspection report. Inconsistencies can and do exist, not only from one contractor to the next, but between individual divers. This leaves the regulating government agency with questionable substructure data, and difficulty in determining fund allocation for immediate or timely repairs.
Augmenting Diver Inspection with Sonar
Diver inspections have been augmented by echo sounders and towed side-scan sonar systems. Side-scan, in most instances, provides excellent visualization of the bottom. However, even in moderate current, this instrument’s data quality can be affected when there are problems maintaining vessel course.
The alignment of the side-scan beam relative to a bridge or dock face is primarily perpendicular. This limits it capability to detect narrow linear targets at right angles to the structure (due to its transverse/along-track resolution). When working in a river or in current conditions, side-scan is typically towed up or downstream, again limiting structural acoustic coverage.
High-frequency side-scan can be used to visualize the vertical component of the substructure, but the towfish must be rolled 90º and fixed to a bracket so the beam scans vertically through the water column. Generating high-quality records with this type of deployment heavily depends on positioning the towfish close to the structure while maintaining vessel speed and heading.
Nonetheless, achieving these views is extremely difficult in all but minimum current or when the survey vessel stability is affected by surface waves. Even the presence of light to moderate current makes it difficult to visualize the upstream and downstream sides of bridge piers using pole-mounted side-scan or scanning sonar (set to side-scan mode of operation).
An echo sounder, which is not restricted by towing requirements of side-scan, is constrained in coverage, however, due to its narrow beam angle. Positioning the vessel closely enough to the structure to obtain the required data set is determined by vessel size, maneuverability and the influence of localized current. When using a small vessel in calm conditions, an echo sounder can effectively collect bathymetric data adjacent to a structure, but is unable to provide visualization data.
Images and Illustrations
Tricks of the Trade and Author’s Notes
Check the company insurance policy. Do not assume the coverage includes equipment deployed underwater. Meet with the insurance provider and tell them exactly how the gear is used. Have them list any constraints in the insurance coverage; get this information in writing otherwise policy interpretation may differ if a claim is made.
Record the serial numbers of all equipment every time the gear leaves the office. Carry a copy of the list and leave one in the project folder. Photocopy every major purchase receipt and file the originals in a secure place. Immediately report to the insurance company if the event equipment is lost, damaged or stolen (and the police for the latter) and supply the serial numbers and proof of purchase for every piece of kit that is claimed against the insurance policy.
Author’s note: Winches and cables are expensive. Doubling up on using one winch/cable for multiple sensors or instruments is an appreciable cost savings. Consider the number and type of conductors, and the voltage/amperage requirements for each system that may be winch-deployed. If there is concern about the voltage drop in the cable, combine conductors to reduce resistance and voltage losses.
Since every instrument has a different connector and wiring arrangement, the trick is to use “pup-whips”—a connector interface that combines the conductors and mates to both the winch cable and the instrument. When a subsea pup-whip is used, another interface cable is needed top-side between the slip-ring and the instrument processor.
Author’s note: When using an unanchored surface vessel, never underestimate the value in training the helmsperson to follow an ROV. Those running search programs are under pressure to get systems loaded, operational, and the project underway. There may be resistance to allocating training time. If the skipper is not familiar with ROV operations, there is risk of losing the vehicle early in the project. Global Positioning Systems and computer charts are commonplace on the bridge of most vessels. However, the ability to use these tools to hold the vessel in station-keeping mode is not a task required of most skippers.
Take the time—as much as needed—to familiarize the ship’s personnel with reading the navigation screen, and precisely understanding how much leeway they have in keeping the vessel over the ROV. This is especially important when using a smaller vehicle as it and the umbilical are easily sucked into the vessel’s propeller or bow thruster.
Start the training in shallow water with a surface buoy anchored to the seabed. Have the helmsperson hold the vessel on station and maintain heading adjacent to the buoy. Continue this exercise until they can hold position without making erratic forward and reverse adjustments.
The next task is to recover the buoy and put a marker on the navigation screen. Instruct the helmsperson to maintain position and heading on the electronic display marker.
Place additional electronic markers on the navigation screen to simulate the path of the vehicle moving on bottom. Have the helm operator move the vessel from its current position to each of the electronic markers in a controlled manner, avoiding any dramatic adjustments to the engine speed or heading.
The search can potentially expand into a 24-hour operation, so all helm operators need training. There is also the chance of fouling the ROV on bottom, requiring the ship to remain on station for an extended duration.
This exercise may seem time-consuming and nonproductive. Temper that thought by remembering the price of replacing an ROV inadvertently dragged into the ship’s propeller or that is torn from its umbilical.