The presence (and disposal) of UneXploded Ordnance (UXO) from past military conflicts, (e.g. World War I and II) poses a major hazard for, inter alia, offshore windfarm, hydrocarbon pipeline, and cable laying and defence projects, etc. Prior to any construction on the seabed, extensive geophysical surveys are performed, usually in conjunction with Remotely Operated Vehicle (ROV) and commercial-diver surveys to identify a target, assess, and apply appropriate Explosive Ordnance Disposal (EOD) methods. Charges are placed preferentially by an ROV, but also by a commercial diver (if necessary), followed by in-situ detonation, where a donor charge is positioned adjacent to each UXO target, which is then detonated successively.
UXO disposal procedures pose substantial threats to marine fauna, especially marine mammals and their prey (e.g. fish). This is because underwater explosions generate shock waves and bubble-pulse propagation, the energy of which is determined by type and weight of chemical charge, as well as detonation depth. Propagation of sound waves is influenced by local environmental factors such as seabed type, bathymetry, and local oceanographic conditions (Todd et al., 2015). Noise produced is impulsive, high intensity, and sound Source Levels (SLs) which can sometimes exceed 260 decibels (dB) re 1 µPa Root Mean Square (RMS) at 1 m over a wide range of frequencies.
To reduce potential injury to marine fauna, alternative methods of UXO disposal, including low-order detonation or deflagration, are becoming increasingly popular. Explosives disposal through deflagration entails penetration of the UXO shell by means of a shaped charge with insufficient power to trigger detonation; this, in turn, engenders a rapid burning reaction of the explosive material within the UXO target, thus preventing a full explosion. The blast wave generated by underwater deflagration carries lower energy than that produced by high-order detonation of UXO, and has been demonstrated to produce peak Sound Pressure Level (SPL) >20 dB lower than high-order detonation (Robinson et al., 2020).
Injuries to marine mammals and fish from explosions can be severe and fatal. Exposure to high-intensity noise could result in organ damage and rupture of gas-filled cavities, including those in the auditory system. This, in turn, can result in permanent hearing loss, brain damage, or intestinal haemorrhaging. Behavioural alterations and disruptions of communication signals via masking or induction of temporary or permanent hearing loss are also possible.
OSC wrote the industry book on marine mammal mitigation, available on www.amazon.co.uk. We provide Marine Mammal Observers (MMOs), Passive Acoustic Monitoring Operators (PAMOs), and Acoustic Deterrent Devices (ADDs) for EOD operations MMOs and PAMOs implement Joint Nature Conservation Committee (JNCC) mitigation guidelines and adhere to project-specific licencing.
All OSC MMOs and PAMOs are trained, qualified, and experienced, fully insured, and have the full suite of offshore training qualifications and medicals (see Marine Mammal Observers for more information). OSC can also provide project-specific Marine Mammal Mitigation Plans (MMMPs), advice documents, and specialised reporting.
Contact us for further details.
Robinson, S.P., Wang, L., Cheong, S.-., Lepper, P.A., Marubini, F., and Hartley, J.P. (2020): Underwater acoustic characterisation of unexploded ordnance disposal using deflagration. Marine Pollution Bulletin 160, 111646.
Todd, V.L.G., Todd, I.B., Gardiner, J.C., and Morrin, E.C.N. (2015): Marine mammal observer and passive acoustic monitoring handbook. Pelagic Publishing Ltd, Exeter, UK.