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Baseefa Ltd

Managing the offshore UXO threat

15 September 2014

Two world wars and years of munitions dumping have left behind a substantial risk legacy on the European seabed. For developers in Europe seeking to build offshore energy infrastructure projects, negotiating the obstacle of thousands of tonnes of unexploded ordnance (UXO) has posed a considerable challenge. Simon Cooke of 6 Alpha Associates describes the threat to energy projects in global waters and how this risk can be diffused. 

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Today’s UXO threat in European waters is entirely man-made. It is the result of war fighting and other activities over the last century, including mine laying, naval battles and bombing during the two world wars and dumping of expired munitions by a number of European nations continuing until the 1980s.

The scale of the risk is substantial but difficult to quantify. We know, for example, that up to 30% of the sea mines laid in the period between 1914 and 1945 remain unaccounted for, whilst the nature of post-war dumping operations means that it is hard to gain an accurate figure for the number of tonnes of expired munitions currently sitting on the seabed, although best estimates suggest that this amounts to hundreds of thousands of tonnes.

A cocktail of explosive weaponry is on the ocean floor in the North Sea and English Channel, ranging from sea mines, which have since lost buoyancy, to bombs, shells and torpedoes that missed their intended targets, and, most worryingly perhaps, chemical weapons and mustard gas of the sort disposed of in German waters after WW1I.  Some of those munitions are buried sub-surface and others move with tides and current.

The result, in more stark terms, is that, while offshore workers in the North Sea discover a WW1 sea mine only once a decade, WWII mines are found on an annual basis and iron bombs of varying size and provenance turn up regularly. UXO therefore presents a tangible and calculable risk to any intrusive activity taking place on the seabed, ranging from dredging to the construction of offshore energy infrastructure, including drilling, cable laying and driving of monopile foundations for offshore wind turbines.

The latter is of particular concern to UXO risk management practitioners. While offshore oil and gas platforms are typically towed into place and located in deep water far from shore, away from the highest risk areas, offshore wind developments are currently restricted to sites nearer the coast where UXO is generally more prevalent.

The threat to offshore wind developments

Offshore wind projects are beginning to proliferate – The European Wind Energy Association (EWEA) estimates that 418 offshore turbines came online in 2013 in Europe, a third more than in 2012. If this trend continues in 2014, more than 550 turbines might be brought online. It is therefore of increasing importance that developers are aware of the threat posed by UXO in the marine environment. Any high impact disturbance to the seabed is capable of causing unintended detonation of surface or buried ordnance, an event that, at the very least, will cause substantial damage, delay and associated costs and, in the very worst case scenario, may result in worker injury or loss of life.

Subject to the size of the high explosive charge and the depth of water (amongst other factors), the underwater detonation of UXO will often generate a series of shock waves, which will be sufficiently powerful to damage and possibly sink vessels at some distance from the seat of the explosion. Clearly any UXO that might inadvertently be brought back to the vessel (e.g. if it is trapped or entangled in equipment) might cause much more damage if it initiates in close proximity to the hull and/or personnel on board.

One of 6 Alpha’s greatest challenges lies in educating developers, financiers, project managers and construction professionals not only that UXO is a tangible, widespread threat that will, more often than not, be present on their site but also that it might severely impair project delivery. Clearly Developers and their Principal Contractors are legally responsible for the safe management of all risks to sub-contractors and installers.

Readers of HazardEx will no doubt be familiar with The Construction Design & Management Regulations and Corporate Manslaughter and Homicide Act, both of 2007. Should the worst happen, stakeholders may be liable for unlimited fines and even imprisonment (if negligence is proven and people have been critically injured as a result), alongside inevitable and irreparable reputational damage. 

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Even in cases where detonation does not occur and UXO is unexpectedly discovered on site during the construction phase of an offshore wind farm or marine cabling project, the developer is at significant risk of project delay, which is always much more expensive to manage once the project is underway.

6 Alpha estimates that it is approximately ten times more expensive to ameliorate UXO risk once the project is underway, as compared with managing the risk proactively before the installation phase begins.

On offshore wind sites, for example, the cost of hiring a large jack-up vessel for turbine installation can be as much as £200k/day, meaning that long periods of inactivity will begin to quickly erode contingency sums.

Recent project setbacks have served to illustrate this point. A dispute over UXO ensued between German grid generator TenneT and the developer of the Riffgat Offshore Windfarm, concerning the responsibility for UXO along the export cable route. TenneT estimated that it had spent €100m because the amount of UXO along the route significantly exceeded initial estimates. Some €37m of the total was spent on clearing munitions and €43m on compensation caused by delays.

More recently at RWE’s Gwynt-y-Môr offshore wind site, work was initially suspended when three WWII bombs were discovered. Following the imposition of exclusion zones around the bombs, permission was sought from the authorities for their detonation. Once permission had been granted, a process that can take weeks, vessels were again excluded from the site as the bombs were detonated in a series of controlled explosions.

While the probability of inadvertent UXO discovery and detonation might be considered low, the risk of delays engendered by UXO discovery and consequential contractual dispute is all too apparent and such a risk might be mitigated easily and early in the project-planning phase.

Perhaps the most important lesson to learn, however, is that providing developers adopt a proactive and preventative approach to marine UXO risk management, the costs of dealing with the threat need not run into the millions. The best UXO specialists will develop strategies that rely on thorough assessment and analysis of project sites before work begins and will coordinate the safe removal of marine UXO whenever and wherever it is discovered.  In this way costly delays and disputes can be avoided during the time and resource critical construction phases.

Marine UXO management

6 Alpha’s approach to marine UXO management can be divided into seven clear phases, namely:

•  Preliminary Threat Assessment

•  Detailed Threat and Risk Assessment

•  Risk Management Strategy

•  Risk Mitigation Design and Specification

Offshore wind projects are often close to the coast where UXO is more prevalent
Offshore wind projects are often close to the coast where UXO is more prevalent

•  Contracting

•  Risk Management during the O&M period

•  Decommissioning.

The first four stages are significant as the company seeks not only to warrant that a site is safe in terms of risks having been reduced ALARP but also to ensure that a cost-effective strategy is put in place. In short, while some UXO removal contractors will seek to employ an expensive 100% clearance approach to a site, 6 Alpha determines what kind of action is appropriate – often it makes far greater economical sense to simply avoid geophysical survey anomalies that model as UXO (but may in fact be scrap), rather than attempt complex and often unnecessary investigative procedures.

The first stage of effective UXO risk management takes place at desktop level. 6 Alpha carries out preliminary desktop threat assessments using an extensive database of all known and mapped UXO sites in European waters. This allows the company to quickly answer the question “Could there be UXO on this client’s site?” If the answer is a definitive “no”, then no further UXO risk management action is considered necessary. But if foundations and/or cables are likely to be installed where there is a UXO threat, stage 2 will need to be undertaken.

This takes the form of a detailed and transparent threat and risk assessment, in which the company employs high-resolution Bomb Search risk mapping to examine the area in question to confirm what kind of UXO may be present on site, where and why it might be there.

The output of the threat and risk assessment informs the next phase of the process; the design of a risk management strategy tailored to reduce risks to As Low As Reasonably Practicable (ALARP) as the law requires (rather than towards zero, which is not only unnecessary but also prohibitively expensive).

The risk management strategy constitutes sharing, transferring, mitigating and tolerating UXO risks. Risk mitigation is a key element of the strategy and forms the fourth stage of the process.

Stage four involves the detailed design and specification of UXO risk mitigation measures which often includes geophysical survey specifications for the detection of UXO in the threat spectrum; prove out trials; data processing; anomaly grading and selection and investigation/verification criteria.

The fifth stage of work sees the engagement and supervision of geophysical survey contractors, ROVs and divers in order to identify, avoid and verify and if necessary remove those anomalies that model as UXO and might pose a risk to installation operations.

On the majority of marine sites, on-site survey is conducted aboard a manned vessel equipped with both side-scan sonar and a gradiometer array, which functions like an advanced metal detector. Combined, these techniques deliver an overview of how potential UXO is distributed and if it is on the surface and/or how deep it may be buried under the sand on the seabed.

The majority of objects detected during this process will be harmless items of seabed debris and a team of geophysicists and munitions experts are employed to analyse the data and determine which might pose an explosive threat. In order to verify the accuracy of this procedure and calibrate survey equipment, 6 Alpha deploys ‘surrogate’ items, designed to resemble UXO, upon the seabed prior to survey in order to verify the survey spread and to determine data processing criteria.

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Once this data has been gathered and analysed, the company will determine whether action is required to further investigate and neutralise any potential threat. Should a hazard be discovered that will directly affect a cable route or the site of a turbine foundation, for example, we will advise on the most suitable strategy. If it is possible to avoid a threat by making subtle changes to project plans, it will often save time and resources.

If this kind of adjustment is not feasible, 6 Alpha will coordinate the work of specialist bomb disposal contractors – all with military training – to render safe the UXO. Firstly a diver or an unmanned ROV will be sent in to inspect the object and determine whether it poses an explosive hazard. If so, munitions, (which are often unstable due to the degradation of trigger mechanisms), will usually be remote detonated in situ, unless a direct threat is perceived to existing infrastructure in the vicinity, in which case the UXO might be remotely floated (with specialist equipment), with great care and towed to a safe location far from the site for disposal.

Most detonations require the establishment of an exclusion zone of not less than 1km, alongside further measures to prevent shockwave damage to surrounding infrastructure and marine mammals.

The critical action at the end of this phase of work is a sign-off certificate which warrants that the UXO risk to investigative installation and/or installation operations has been reduced ALARP.

Subsequent stages of work need to consider UXO risks separately along with O&M and decommissioning.

While the threat of UXO to marine infrastructure projects has been well-publicised of late, the young offshore wind sector is still coming to terms with the nature of the UXO risk posed to its projects. Likewise, the marine construction industry as a whole currently lacks centralised formal guidelines for the management of this risk.

In a reflection of this, 6 Alpha has recently been commissioned, alongside Royal Haskoning DHV, to co-author the upcoming CIRIA guidance for the Assessment and Management of Unexploded Ordnance (UXO) Risk in the Marine Environment.

This work should go some way, not only to raise awareness of the marine UXO threat, but also to create best practice for the industry-wide management of this type of risk. In the meantime, offshore wind developers, marine cabling firms and all of those involved in marine construction should take note of the potential obstacle that these unexploded munitions pose to the safe and timely completion of their projects and learn how to manage it in a proactive, strategic and appropriate manner.

About the author:

Simon Cooke has been Managing Director of 6 Alpha Associates Ltd for the past 10 years. Prior to that he was Technical Director at an international business concerned with UXO remediation and land mine clearance, and was in the British Army’s  Corps of Royal Engineers for 17 years.


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