NPL tests new methods to dispose of unexploded ordnance on seabed

The study compared newly developed methods for ordnance disposal with existing techniques and its findings have published in Marine Pollution Bulletin.

Unexploded ordnance from World Wars One and Two, plus military exercises and tests, poses a major environmental and safety hazard to fishing vessels, maritime traffic and offshore developments such as windfarms. The current removal method - high order detonation - presents a significant risk of injury or death to marine mammals and other fauna. It also causes seabed craters.

In the first study, NPL scientists and researchers from Loughborough University, Aarhus University in Denmark and staff from Aberdeen-based environmental consultancy Hartley-Anderson, conducted a series of experiments, including a field trial in the Baltic Sea, to compare the sound produced by high order detonations with that produced by so-called deflagration.

Developed by Alford Technologies, a Chippenham-based ordnance disposal company, deflagration involves placing a small shaped explosive charge on an unexploded bomb or shell to penetrate its casing and then insert a hot gas such as plasma. This burns away the explosive inside, but at a slow rate so that it doesn’t detonate. The bomb or shell is disarmed, leaving a residue of unburned inert explosive which is then cleared away.

The results showed that the low order deflagration method produces greatly reduced radiated acoustic noise compared to current disposal methods.

Peak sound pressure level and sound exposure level underwater was over 20dB lower than that created by existing methods, which NPL said represents a hundred times less energy released.

These results are from unexploded ordnance up to 460kg TNT equivalent in size and researchers predict that reductions could be significantly higher for larger charge sizes.

The Baltic Sea trial is the latest research conducted by the team in recent years to ensure the safer removal of unexploded ordnance on the seabed, and which is funded by the UK government’s Offshore Energy Strategic Environmental Assessment (SEA) programme. 

A previous study was conducted in the North Sea which analysed the acoustic measurements made during the disposal of 54 items of unexploded ordnance during the pre-construction phase of two offshore windfarms.

The NPL-led team have also developed a protocol for in-situ measurements of underwater sound generated during the disposal of unexploded ordnance. The protocol is said to provide best practice to ensure accurate and effective monitoring of the underwater sound from UXO clearance.

The NPL-led team is set to conduct further controlled tests on other potential new low order UXO disposal techniques, and will also test additional barrier mitigation methods such as the use of bubble curtains.

In a statement, Sei-Him Cheong, senior scientist at NPL, said: “This project is instrumental in addressing the UK offshore wind target. I am thrilled to be a part of this work and it is remarkable to witness every milestone we achieved. The benefits of the numerous collaborations and hard work is bringing us closer to a clean and more sustainable future.”