UNEXPLODED ORDNANCE (UXO): Unexploded ordnance does not go away

The threat of unexploded ordnance (UXO) in the marine environment never completely goes away. The types of munitions that might be encountered in the North Sea today consist of High Explosive (HE) bombs and sea mines, which are there not only as a result of military activities associated with both World Wars, but also extensive sea dumping of expired munitions.

During the course of both World Wars, hundreds of thousands of mines were laid by Allied and Axis forces, in both a defensive and offensive manner throughout the North Sea, from the East Coast of Britain to the German Bight. The precise numbers of mines laid throughout both wars is not known, as many mine laying operations by both sides were either inaccurately recorded or not recorded at all.

These mines, laid by vessels during WWI and by both vessels and aircraft during WWII, were variously designed to sit on the seabed or float either on or just below the surface of the water. Today, wartime mines have lost any buoyancy they may have once had and are all to be found on the seabed’s surface or buried just beneath it.

As a result, some mines may migrate due to tidal and other weather and seabed mobility factors. Records kept by the Dutch authorities during WWI show that almost 6,500 mines drifted into neutral coastal areas of the Southern Netherlands during the four years of war.

Unfortunately, wartime records concerning mine laying and mine clearance are not always particularly accurate. While it is clear that mine sweeping operations were undertaken post 1918, the precise extent and success rate is not known. Similar clearance operations were conducted during and post-WWII, during which an estimated 30% to 70% of all laid mines were made safe. Clearly, a significant number of mines were not accounted for, which may pose an offshore threat to this day.

In addition to the sea mine contamination, UXO risks are generated by a variety of other offshore sources. For example various vessel and aircraft wrecks sunk during the course of WWII contained UXO, such as bombs and torpedoes. Merchant and war fighting vessels also transported munitions. While such wreck areas tend to be well charted, they can break up over the years, and their deadly cargos may spill from them, contaminating the seafloor.

Dumped munitions are also a significant threat, as millions of tonnes of unexploded munitions, including chemical varieties, have been deposited in the World’s oceans since 1945. Some dumps are well charted, but others were never accurately recorded in the first instance.

Furthermore, those employed to dump such munitions offshore commonly dumped their cargo on the way to and from the formal dump sites, which tended to spread the contamination.

Such activities have traditionally caused a major problem for fishermen who frequently trawl the seabed and often accidentally pick up UXO in their nets, which, when trapped and brought on-board, can pose serious risks to the lives of the crew as well as compromising the integrity and safety of the vessels themselves. More often than not, such munitions are dumped back overboard, thus spreading and perpetuating the hazard.

Now, offshore wind developers must also come to terms with this threat. Despite the obvious safety issues associated with encountering with UXO in the marine environment, several wind farms have been safely constructed and there are plans to increase wind-generating capacity by a factor of three in the next few years.

Those wind farms that have been the subject of UXO risks have generally mitigated those risks by employing UXO consultants to scope the nature of the UXO and the risks posed, and then planning to mitigate those risks by rerouting cables or relocating wind turbine foundations. In cases where this is not possible, the clearance of verified UXO has been necessary in order to afford installation safety.

Once the turbine foundations and cables have been installed, the issue of UXO might ordinarily be expected to disappear. However, because those suspicious anomalies that were modelled as UXO during geophysical surveys have been avoided during installation, some UXO might remain in place during the operations and maintenance (O&M) period, which can of course last for decades. And those munitions might also migrate with time.

Empirical evidence has shown that wartime UXO in some circumstances, has migrated hundreds of meters, which is certainly sufficient to require UXO hazards to be considered and mitigated during the O&M period. Clearly static and migrating UXO could pose a significant risk to vessels and their crews during O&M.

Mitigating risks

Offshore WIND spoke to Simon Cooke, Managing Director of offshore UXO risk management consultants, 6 Alpha Associates. We asked him about the extent of the UXO risk and how it might be solved in order to ensure safety, not only during installation, but also during the O&M period.

Mr Cooke explained there are a number of key questions that must be answered concerning UXO risk during both installation and O&M, which include:

“What is the likelihood of encountering UXO at the project site? What sort of activities might be undertaken in close proximity of such UXO hazards and will they have sufficient energy to initiate UXO if they are encountered, e.g. as a result of piling, cable installation or vessels jacking up?
What are the seabed soil conditions and what is its capacity to both transfer and ameliorate such energy with respect to distance from prospective UXO?
What safety distances should be put in place to ensure vessel and crew safety?
What is the likelihood of UXO migration and how might that affect safety during installation and O&M activities?”

He intimated that answers to those questions determine not only the initial safety strategy for dealing with UXO installation risk, but critically how safety might continue to be addressed throughout the project life cycle.

He went on to say, “At 6 Alpha we tend to reduce risks for our developer clients in accordance with the legal principle of As Low As Reasonably Practicable, also known as the ALARP principle, rather than trying to reduce the UXO risk to zero, which is both impractical and can be extremely expensive.”

He described the company’s practice of not only producing “a specific desk top Threat and Risk assessment, but also (and often in parallel), Munitions Migration Assessments”, which take into account all of the above factors, affecting both the installation and the O&M phases of an offshore wind farm. The company has developed a Marine Risk Management Framework for UXO, which has five distinct phases including historical research through to UXO clearance at ALARP level.

A detailed high-resolution geophysical survey is typically required to identify and avoid UXO. And safety buffers should be designed to ensure that UXO is avoided by a margin that will deliver vessel and crew safety.

Mr Cooke continued, “A high resolution geophysical UXO survey might result in 80% to 90% of the total area of the prospective wind farm not being surveyed; however every geotechnical investigative position and all foundations and cable runs ought to be surveyed in order to afford vessel safety.”

Assessing munitions migration

Whilst the UXO risks associated with such may have been reduced ALARP at time of installation by a combination of avoidance and investigation, a couple of years later UXO may have migrated, changing the location of risk and prospectively elevating it. In an example given by Mr Cooke, he said, “A small mass UXO (which could still be dangerous and present a significant hazard) might remain static or drift between, say, 2m to 8m per annum.”

He continued, “We have developed a Munitions Migration Assessment model which predicts the rate of prospective drift and enables us to enhance the longevity of safety sign-off certification. This has saved our clients time and resources by avoiding or delaying follow-on geophysical survey for UXO risk management purposes.” In order to estimate the possible rate and magnitude of UXO migration, there are many factors that must be taken into account. For example bathymetry, seabed medium and morphology can dictate whether or not UXO may have been buried upon its initial deployment and whether or not it remains buried.

Topography and gradient will affect whether UXO could roll down a seabed slope. Other key factors include the water depth, currents and weather patterns. Strong currents and unpredictable weather are more likely to move UXO than still calm waters. The morphology of the munitions also plays a key role. For example, a round mine is far more likely to be mobile, than a long, cylindrical torpedo.

All of these factors can be applied to anticipate the potential movement of an item as well as the possible scouring around the item, giving a predictable rate and direction of migration.

Dick Hill