Deflagration: A method for lowering noise from underwater unexploded ordinance (UXO) clearance operations

Significant amounts of historic unexploded ordnance (UXO) are still located in our seas and waterways. Operations to clear waterways of these munitions have increased significantly in the last few decades due to increasing offshore activities, for example, the expansion offshore wind energy development. Various methodologies for Explosive Ordnance Disposal (EOD) exist, but a historically commonly used approach is to blast-in-place (BiP). This process involves detonation of a modern explosive charge next to the historic charge initiating a simultaneous high-order detonation in both. These underwater explosive events are highly energetic and have the potential to generate high acoustic and vibrational levels in both the water column and the seabed with the associated potential for both physiological and behavioral response impacts on a wide range of aquatic fauna. An alternative EOD process has been investigated where a smaller shaped charge is used to initiate a burning process, known as deflagration, in the UXO’s explosive material. This low-order process has the potential to be significantly quieter and less destructive than equivalent high-order detonation events. This chapter presents results from trials carried out in a flooded quarry in the UK in 2019 comparing the acoustics of detonations and deflagration methodologies on like-for-like munitions. Results are also presented of measurements carried out in genuine historic (WWII) UXO located in the Danish Baltic in 2022. Data from both of these trials demonstrated an around 15–20 dB reduction in the water column acoustic levels and seabed vibration levels comparing low-order with high-order methodologies. Additional factors surrounding the long- and short-term toxicity from in situ and cleared UXO processes and the potential for damage to the seabed are also discussed.