Efficient Shallow Underwater UXO Retrieval
As a result of past military training and weapons testing activities, residual unexploded ordnance (UXO) is present at sites designated for Base Realignment and Closure (BRAC), at Formerly Used Defense Sites (FUDS), on currently-active training ranges, and on private lands and marine resource and recreational areas adjacent to current and former ranges. Many of the sites associated with military practice and test ranges contain significant marine areas.
The National Defense Center for Environmental Excellence (NDCEE) has released a report reviewing and summarizing the current state-of-the-art in modern UXO remediation technologies. This report focuses on remotely operated and automated retrieval technologies with the intent of emphasizing safety and reducing UXO recovery costs. The only technologies cited for underwater applications involve either remotely operated underwater vehicles (intended for operation at significant depths) or surf zone/beachcomber systems for shoreline applications. None of the cited approaches assumes either that digital geophysical UXO surveys have been conducted or that retrieval of specific targets with known coordinates is an objective. On shore UXO target recoveries (in benign environments) typically cost approximately $200 per dug target using commercially available technologies. Recovery of the same targets in shallow water offshore costs 5-8 times more. Currently underwater UXO remediation requires hands-on, UXO qualified diver intervention.
The currently used approach for underwater UXO retrieval requires a team of divers to manually locate and remove each individual target. The process begins with the dive team re-acquiring the target position from a boat using a hand held global positioning system (GPS). The target location is then marked using a weight and buoy or a rigid pole with a flag. An underwater metal detector is then used by a diver to reacquire the magnetic anomaly and refine the buoy placement.
Once the target is located, the diver begins the investigation and recovery process. Either using his hands or hand tools, he uncovers the item. Targets buried more than approximately 1.5 ft typically cannot be successfully recovered using this approach regardless of whether the bottom sediments are sand, shell, silt/mud, or clay. Divers typically use only small military-style entrenching hand tools for digging. Excavation sidewalls routinely collapse into the excavation if it is deeper than about 1 ft. Water visibility typically drops to zero once the bottom surface is disturbed. For shallower buried objects, after the target is uncovered, the diver identifies the target visually if possible, or by feel if visibility is limited. The diver, in conjunction with the UXO supervisor then determines if the item can be safely moved or whether it must be blown in place. In typical UXO marine environments, it is often impossible (or impractical) to investigate or recover more than half of the magnetic anomalies following modern digital UXO geophysical surveys.
Objectives of the Demonstration
The objective of this project was to design, build, and demonstrate a semi-automated system to provide an efficient, relatively economical, and safe approach for use in recovering single UXO targets in shallow water (less than 15 ft). In this project, the approach has been addressed by combining technologies based upon commercial off-the-shelf (COTS) components to create an integrated system that can semi-autonomously uncover UXO buried in marine sediments, visualize the uncovered target (using TV and/or imaging Sonar), and remotely recover the target to the surface using an electromagnet or a mechanical grapple. Currently underwater UXO recovery operations typically involve explosive ordnance disposal (EOD) or commercial UXO divers precisely locating the positions of a metallic object with a metal detector, then uncovering the targets using hand tools. The identity and fuzing of the target is determined either by sight or by feel. Small targets can be brought to the surface by the diver, while larger targets require lift bags or winches to break them free of the sediment and raise them to the surface.
For this demonstration, it was the project team’s intention to use the new system to investigate and recover UXO targets from the Currituck Sound adjacent to a former test range, the Former Duck Naval Target Range, North Carolina. The demonstrators have previously surveyed the offshore area involved in this demonstration and at the time of the original survey recovered 100 underwater targets.
The support vessel (anchoring systems, winches and spuds, and deployment and retrieval systems) operations were successful. The shroud deployed well, the vacuum dredge and water jet were very efficient in excavating within the shroud in a variety of sediment types. The electromagnet array retrieval system operated flawlessly, meeting all project goals. The TV camera visualization system (along with the water clarification system) uniformly failed to operate adequately to identify unknown ordnance and/or to determine its fuzing. This was exacerbated by water visibility of less than 3 in however, the TV optics design and lighting system were also determined to be of an inappropriate design for this task. The dual-frequency identification sonar (DIDSON) imaging sonar, which might have accomplished the visual recognition tasks also failed because the plastic optics in the system had dried out and degraded from several months of non-use. There were insufficient funds available to rebuild the optical system of the sonar.
The major obstacle to future implementation of this system is with underwater visualization. This proved to be a more difficult task than originally anticipated. The water filtration system improved the water clarity enough to allow the target to be visualized from a few inches. This did not provide an extensive enough view of the target to allow for identification. For this system to be successfully operated in the field, the visualization must be improved. This should be accomplished with additional filtration, additional lighting, improved camera design and/or a rebuilt sonar imaging system.
Overall, the system was straightforward to operate. Positioning the vessel was easily accomplished using winch controlled anchors. The mechanical winches were effective at raising and lowering the stabilizing spuds. The 4 function hydraulic crane allowed the shroud to be positioned efficiently over the marked target. The dredge was easy to manually control using the dock lines. Completing automation of the dredge/water jet system would improve its operation. A hydraulic controlled system would improve the positioning of the suction head and water jet, and allow for more efficient dredging. The electromagnet was able to lift the inert items used in the shakedown tests and it is unknown how effective it would be at lifting larger partially buried items.
All components used to construct this system were either COTS or easily manufactured items. No special skills or training is needed to operate the technology. A UXO technician/diver is required at all times during operation if ordnance items are potentially going to be encountered