This story features in Unearthed: Geoscience for good

Undersea unexploded ordnance (UXO) are a risk to windfarm construction in the UK’s North Sea, with a historical twist that makes detecting them more complex than it should be. Seequent’s Strategic Account Executive Matt Grove explains how geophysics helps this branch of the clean energy sector build more confidently and efficiently.

What we thought were tightly defined areas of risk have turned out to be entire corridors of risk, with ordnance ranging from high explosive to chemical.

Windfarms are an important source of clean energy, and around the UK, the North Sea is an ideal place to put them. Lots of wind so lots of energy, and not an eyesore for anyone. But they face a problem in that these huge turbines need to be built in locations where there’s a risk of unexploded munitions right below them. What we try to do is mitigate that risk.

Why should that be the case? Well post World War ll there was a real urgency to dispose of unused ordnance. The plan was to drop them far out into the North Sea, well away from shipping lanes or infrastructure. However, the commercial vehicles commissioned to dump them in these carefully selected and marked off polygons on the map, were paid by the load. As soon as they left port and were out of sight, many just ditched the munitions over the side and came back to load up again.

Consequently, there is unexploded ordnance peppered all over the place, and often in areas closer to shore where windfarms like to be.

Add to that the complication of the North Sea being a very mobile environment. Sand waves are moving and migrating on a daily basis, shifting these munitions with them. One day they might have a dune over them, the next they’re lying exposed on the seabed.

Never underestimate what sits on the seafloor

What we thought were tightly defined areas of risk have turned out to be entire corridors of risk, with ordnance ranging from high explosive to chemical. It’s never a good idea to underestimate what might be on the seabed. For example, there was once a practice flight with a live nuclear bomb on board that ran into trouble and had to ditch the bomb in the sea. That’s still not been found to this day….

Therefore surveys of these areas not only face a number of challenges but have a limited lifespan as the seafloor conditions change. It’s impossible to have any confidence in the old maps and charts, so it’s often simpler and cheaper to survey the entire area of a windfarm and deem it safe than make a guess of where you should look for UXO.

Ten years ago, surveying such a wide area might have been impossible. You would have been towing a single magnetometer at maybe 30 or 50 metre line spacing, and ending up with barely any data to work with. Now it’s possible to mount 10 magnetometers on a frame and be towing four or five frames at a time, and arrive at very dense data with few gaps in your coverage. It’s known as gradiometry.

It’s true that there are other industries operating in the North Sea that have traditionally exhibited less concern about the potential presence of UXO, and in fairness, incidents have been rare. But that attitude is changing. The windfarm sector is something of a pioneer in this area. I think that’s because it’s an industry trying to do ‘the right thing’ about energy and demonstrate where we should be going in the future, so they’ve decided to set a precedent on this.

 

 

Matt Grove 
Strategic Account Executive, Seequent