During and following the two world wars, many European countries dumped "shelf-expired" munitions at sea, being the easiest means of disposal. Only half of the millions of sea mines, each containing 350kg of high explosive, that were laid during the wars, were ever recovered. No longer floating, those that remain are rolling around on the seabed.
Rolling alongside these explosives are bombs dropped by aircraft attacking ships or that were jettisoned when they came under fire, plus shells fired by ships and land-based batteries, and torpedoes that missed their targets. The seabed is littered with dangerous unexploded munitions
This of course presents a real threat for wind farm and cable-corridor companies drilling into the seabed to lay foundations for their equipment.
World War l sea mines are discovered in the North Sea around once a decade, but those from the later conflict turn up at least once a year, according to 6 Alpha Associates, a UK-based consultancy that carries out risk assessments for offshore wind-farm developers before they install equipment on a seabed. Bombs, usually in 50kg or 250kg varieties, are also frequently discovered.
Initial risk assessment
In 6 Alpha's ten years of operations, it has completed work on 38 renewable-energy projects, eight of which have been in German waters. It is currently working with transmission system operator Tennet on Borkum Riffgrund 1, a 277MW offshore project in the German North Sea.
All 6 Alpha's work starts in the same way, with a desktop threat and risk assessment. The company uses historical data to work out if there are potential hazards on the client's prospective site. An assessment is then made on what the client intends to do on the seabed and whether this could lead to either an encounter or detonation of pieces of unexploded ordnance.
"In the worst case, you've got 350kg of high explosive in a sea mine, which is specifically designed to sink a vessel in anything up to 50 metres of water," says Simon Cooke, 6 Alpha's managing director. "It could kill or seriously injure people, without a doubt."
Another consequence for wind developers of encountering unexploded ordnance is that they can end up in dispute with their principal contractors, causing costly delays.
"If something was down there, it could blow up and someone could be killed, which all sounds highly dramatic," says Cooke. "But it's much more likely you're going to be tripped up by a switched-on principal contractor either not accepting that you've done nothing on your development, or that what you've done to ameliorate the hazards is insufficient."
Once the desk-based research is completed, and only if any potential threats have been assessed, a geophysical survey of the area is carried out by contractors using a vessel equipped with a survey spread, with a crew of up to 15 people.
This uses sonar detection to identify if any items on the seabed could be unexploded ordnance and involves the use of sophisticated metal detectors, called magnetometers, or several of these strapped together to form a gradiometer array.
The survey data is then sent to shore for another team of specialists, usually geophysicists and bomb-disposal experts, to analyse the findings, although this can be done out at sea on the largest survey vessels.
"You sieve the data to make sure the junk is filtered out," said Cook. "You sort the wheat from the chaff - the wheat being unexploded ordnance, the chaff being seabed junk. That takes a separate set of skills and you might need half a dozen people to do that."
If nothing untoward is found once the data has been captured and interpreted, 6 Alpha's work can end there and it can sign off the necessary safety certification for work to go ahead.
If a potential threat is discovered, another vessel carrying divers and a remotely operated vehicle (ROV) equipped with sensors goes in for a closer look. The principal here is "eyes on, hands off", in order to verify whether a suspicious object on the sea bed is in fact unexploded ordnance.
If confirmed, the next step is to send in yet another specialist team to blow up the object in situ, which is nearly always safer than trying to move it to another location before carrying out a controlled explosion.
Once the decision has been made to blow up a piece of unexploded ordnance, it is time to call in another set of experts, who know exactly where to place the plastic explosive charges.
"When you come to blow it up, this needs subject-matter expertise," says Cooke. "You need properly qualified bomb-disposal officers who are trained to do this sort of thing."
All the people contracted by 6 Alpha to do this work have a military background, either ex-navy clearance divers trained in naval weapons or ex-army bomb-disposal officers who are dive-trained.
Cooke himself was a major in the Corps of Royal Engineers and an advanced-level bomb disposal officer as well as a qualified explosives engineer.
But blowing up 350kg of high-explosive sea mine carries with it a responsibility to the people working near the seat of the explosion and anyone else who happens to be in the area. Extensive permissions and licences must be obtained before this can go ahead. This huge logistical exercise alone can take up to 12 weeks.
"You can't just go blowing things up willy nilly," says Cooke. "You need to issue notice to mariners and airmen to keep them out of the way. There's also no point blowing something up if it's right next door to a wind turbine foundation or cable. If you blow the client's cable up, obviously they're not going to be very happy."
In cases where unexploded ordnance cannot be destroyed in situ because they are too close to existing wind farm foundations they must be carefully moved using ROVs and underwater lift-bags that will not upset the antique mechanism of a 100-year-old bomb. "You do this in a controlled fashion because it is dangerous, and you must assume that it could detonate if moved," says Cooke.
Whether blowing bombs up in situ or moving them, 6 Alpha establishes a safety trace, which is an exclusion zone of perhaps hundreds of metres around the seat of the explosion. The safety trace also applies to sea life, including dolphins, whales and seals.
Devices called seal-scarers emit signals to ward off marine mammals and observers operate at the edge of the safety zone to make sure that no animals swim into the danger zone. An underwater "bubble curtain" can be used to absorb some of the shockwave of the explosion.
For the largest operations, work can involve as many as 40 specialist contractors, depending on what the assessment reveals. Completing the work means that Alpha 6 can provide a certificate stating that the risk of installing any feature of the project has been reduced to as low as is reasonably practicable.
Cooke indicates that the top-end of what it provides, from risk assessment through to employing specialist teams to seek and destroy unexploded ordnance, can cost the client up to £10 million (EUR 12 million).
But without this level of risk assessment, the potential cost of delay to a project that has been started, and with some equipment already installed, far exceeds this sum, with the vessels that install the foundations or cabling for wind farms costing around £200,000 a day to hire.
"In five days, you've burnt £1 million, so you do not want delay," says Cooke. "Our certificate de-risks the project, technically from an unexploded ordnance point of view, but also from a schedule perspective."
Cooke cites the bitter row currently raging between transmission system operator Tennet and developer EWE over which company had responsibility for risk assessment at the Riffgat offshore project as "the worst case I have heard of".
Work started on the export cable route after only a preliminary investigation into the subsoil. No thorough risk assessment was undertaken. Tennet claims the subsequent munitions clearance and compensation for delays has cost EUR 100 million. "Going about it that way is dangerous, and highly expensive," says Cooke.