Unexploded Ordnance (UXO) poses a significant safety and operational challenge in both marine and terrestrial environments. However, the types of UXO encountered- and the methods used to manage them- differ markedly between land-based and offshore projects.
Common Marine UXO Threats
In offshore settings, the scope of UXO detection is typically more limited than on land, primarily due to constraints in detection methods and what is reasonable and practical to identify in marine conditions. As such, marine UXO surveys usually focus on larger items—those that pose the greatest potential threat to subsea developments, such as cable routes and offshore wind farms.
The most frequently encountered UXOs in the marine environment include:
- Air-dropped bombs; 50kg is often the munitions routinely investigated offshore, with other sizes such as 100kg, 250kg and 500kg also seen. Typically, the weight is evenly split between the casing and explosive content
- Torpedoes; typically, the largest physical item (potentially up to 7 meters in length) and containing enough explosive power to sink a ship, represent some of the most high-risk items found offshore
- Sea mines; moored and ground mines, were deliberately placed to target naval and merchant vessels during both World Wars
These types of UXO represent a scale of threat that typically exceeds what is commonly encountered on land, and they demand specialised handling and mitigation strategies.
Terrestrial UXO Context
In contrast, land-based UXO clearance—such as that managed by Richard and the team at RSK Ordnance Management—often deals with a wider variety of smaller munitions:
- Anti-aircraft shells; remnants of defensive fire during air raids
- Training ordnance; including thousands of 2-inch and 3-inch mortars fired by the Home Guard and local defence forces
- Rifle grenades and projectiles; including artillery and tank rounds
- Air-dropped bombs; especially in urban or strategic areas targeted during wartime
Urban centres, especially those with ports or rail terminals, like London, Hull, Plymouth, Glasgow, and Edinburgh, have a high concentration of historical UXO. Some items, like hand-dropped bombs from WWI-era Zeppelin attacks, reflect the varied history of aerial bombardment in the UK.
Why ALARP Matters: Managing UXO Risk
At the core of UXO risk management is the principle of ALARP—”As Low As Reasonably Practicable.” This concept underpins all safety-related evaluations and serves as the industry benchmark for managing explosive risk.
ALARP is not just a technical assessment—it is a moral and legal imperative. Developers, clients, and employers have a duty of care to ensure the safety of personnel and equipment. The goal is not to eliminate all risk (which is often impossible), but to reduce it to a level where further mitigation would be grossly disproportionate to the benefit gained.
In practical terms, failing to achieve ALARP can result in severe consequences:
- Risk to People; most obviously, an uncontrolled explosive detonation poses a serious risk to the health and safety of any individuals in the vicinity
- Risk to Equipment; even ordnance that may not pose a risk to people or equipment on the surface, it may damage or destroy tools and infrastructure on the seabed, including trenchers and cables
- Risk to the Project; any uncontrolled explosion could result in significant delays for investigations, expose liability concerns, and potentially halt development, all with significant financial and reputational costs
Conclusion
Understanding the types of UXO likely to be encountered—and implementing a robust risk management process guided by ALARP—is essential for safe and successful offshore and onshore developments. Whether it’s a seabed mine or a buried artillery shell, UXO risks are real, but they can be effectively mitigated with the right expertise, planning, and safety-first mindset.
Written by Patrick Clark, Hydrofix Director and Richard Holmes Managing Director of RSK Ordnance Management.
Created from a Podcast transcript between Patrick and Richard, “Focusing On Everything UXO”.