Partner: Fugro

In the last century, the tides of conflict swirled around Europe as the world’s largest powers fought for dominance over air, land and sea. The North Sea was a critical theatre that could have determined the future direction of the planet in two World Wars. Conflict raged across these waters between the UK, Continental Europe, and Scandinavia.

Since the guns fell silent on VE Day in May 1945, Europe enjoyed 76 years of peace. The North Sea was transformed into one of the economic hubs of the world. A number of major shipping lanes converge in its cold waters, bringing goods to Europe’s northern coast. 

As well as one of the centres of world trade, the North Sea also contains some of the most promising real estate for current the green energy revolution, as offshore wind farms multiply to take advantage of the stormy conditions. There is just one problem: unexploded ordnance (UXO).

Analysis suggests that there might be as much as 1.3 million tonnes of munitions lying just below the seabed. This makes running cables or foundation work at sea dangerous. So any locations where this Unexploded Ordnance, or UXO is a risk, requires careful site investigation before it can be declared safe.

But some areas are easier to investigate than others, and with explosives, how careful is careful enough?

Searching the seabed

In this article we will learn about some of the innovations being made in the search for unexploded ordnance. In this kind of work, human intervention is increasingly unacceptable, but there are still those hard-to-reach places where a remotely operated vehicle just cannot operate. The surf zone, for example. That is, close to shore; an active tidal region with a lot of churn on both the sea bed and the surface.

Fortunately, this is one of those areas where technology advance is very real, and we have some new kit to talk about. And it has been used for the first time to safely advance a project that will give Europe a clean energy boost and make the future safer for all of us.

But before we get to the new equipment, first we should take a step back and understand a bit more about an UXO search and how it would normally be executed. Vincent van Santen is Fugro’s business development manager for UXO and has been working in the offshore survey industry for over 20 years.

“[UXO searches] are becoming more and more relevant,” he says. “We typically think of World War 1 or World War 2, but people do not realise that NATO countries have dumped a lot of ammunition [in the seas] way into the 1990s.

“UXO risk is a real risk in the world. Not only because of the explosive, but also because of environmental issues. And if we can get rid of that, I think we will make a better world.”

Now, with the seas becoming more and more crowded with growing trade and energy generation, it is increasingly difficult to stay in regions that are known to be perfectly safe. And UXO surveys are becoming more serious than ever before.

“So every time that a certain area has been identified, that is going to have some sort of construction work, whether it’s an offshore wind generator or a high voltage power cable, we need to do an analysis whether that is there is an UXO risk,” says van Santen.

Starting a study

The work always begins with a desk study, looking at the geodata to determine how the seabed looks now, and how it looked 50 years ago. Combining historical open-source data and restricted data, as well as the specialist knowledge of the experts in the field, locations are assigned various risk levels.

Then comes what is called an offshore UXO survey.

“In an offshore environment we will use a typically a survey vessel that tows certain survey equipment behind it and we will have survey sensors on the vessel as well. We use acoustics like a multi beam sensor to have a 3D model of the bathymetry of the of the seabed we have a side scan sonar and the sub bottom profiles are there different acoustic sensors with different frequencies.”

This is to detect objects on the surface of the seabed or objects buried to a depth of a couple of metres.

“And in addition to that, we can also use magnetometers, or if we use multiple magnetometers, we call it a gradiometer, and what we in effect do is we measure the differences in relation to the natural magnetic field of the Earth… the earth is one big magnet, it has its own magnetic field. And if there are any metal objects, you can detect these disturbances. So, you have little spikes in your data.”

This is cutting edge survey equipment, then the specialists will process the data and come up with what is known as a ‘master target list’. A list of locations, each a potential resting place of unexploded ordnance. Then comes the next phase, the UXO Identification Phase.

“We will do a more detailed survey or specific targets. And we use again magnetometers, we will use electromagnetic equipment. So that’s where you generate electromagnetic fields. And this is similar to [a hobbyist’s] metal detector.

“So you detect disturbances with those sensors and also acoustics. So there’s all different types of survey sensors. Again, if it’s an offshore environment, we will use an ROV that’s remote operated vehicle, that’s like an subsea robot, we send to specific targets with the sensors, and we do a detailed sensor on the target to get a better understanding of what it is. And following those detailed surveys on specific targets, then it’s only a small percentage that is that we can really identify, that’s a real potential UXO.”

Depending on the shape and the data signature that they get. Any targets that are left, have to be cleared. In the modern world this means a dredge pump on an ROV starts digging away at the seabed, while operators on a nearby vessel look on through cameras. Then if it is a bomb or shell, there is a decision to be made. Perhaps the asset owner wants to relocate its asset, which could be expensive. If they decide otherwise, the object needs to be addressed.

Depending on the national legislation, the project may be required to call in the coast guard or the marines for safe disposal. Technology has advanced to the point that sometimes the UXO can be extracted from the location, but often this means a controlled explosion. However, this can disrupt and kill marine life up to a radius of 30km, so a bubble curtain is created around the bomb containing pressurised air. This disrupts the pressure wave from the explosion and protects the local fauna.

“So even though we do this low order detonation, you make sure that if there isn’t any shock wave coming for you, that is you break down the shock wave, it won’t pass through this bubble curtain,” says van Santen.

The end result of all of this work is to give the owner of the project ALARP sign-off, ‘as low as reasonably possible’ so that it can begin work on the project in question. This is sign-off from an independent UXO expert to say an area has been surveyed to such an extent that it is has passed a threshold to allow construction work to begin.

Shallow water work

Most of the survey work in shallow marine environments is really challenging. ROVs struggle to moor themselves in the surf zone, there is heavy turnover of sediment on the seabed, and even barges struggle.

The standard working platform in these surf zones is called a jack-up. A jack-up is any non-propelled or self-propelled vessel that is fitted with legs and a jacking system that allows it to elevate its hull above the sea surface. This creates a stationary working platform from which to go after unexploded ordnance.

“So it jacks itself up from the surface of the water while the legs stand on the on the seabed. But this is a stationary situation. So to move a jack up, you need to go it needs to go down again in a floating position. And then it can relocate either by a tugboat brought to another location or sometimes it’s even self-propelled. And then it can move to the next location, then the cycle starts again and starts pushing those legs down and injects itself up to a stationary position,” says van Santen.

It is incredibly slow-moving if there are a lot of nearby targets. And in a shallow surf zone those struggling ROVs might tempt a project manager to opt for putting divers at risk. For Fugro and its engineers, the situation cried out for something better.

A lot of offshore work in the modern world is around wind farms. The interesting thing about these offshore wind farms, from an UXO perspective, is they need to be connected to land by subsea High Voltage Director Current Interconnector cables. For more information on these, check out episode #157: Interconnectors: The Green Link to Ireland.

Connecting to these farms means traversing this traditionally difficult surf zone. And one project is a critical onshoring hub for this offshore generation. It is being constructed by Tennet, to ‘onshore’ power from several neighbouring windfarms. The work was critical, and so it called for two new pieces of equipment.

Ido Dillisse, Project Manager for Fugro says, “So you have an offshore wind farm, which is a few miles out of the coast, and it gets connected to the national grid via some serious high voltage cables. And in order to instal these cables, the area needs to be needs to be cleared. And it was specifically the surf sound at the landing site of these cables, that was proven to be an impossibility to, to have that to have that cleared.”

Dillisse is referring to the conventional approach to working with such high stakes on such a critical project.

“And hence the solution of the Wave Walker came in and came into play.”

The Wave Walker is the first piece of equipment we will look at. It is a replacement for those jack up barges. Where as they stand stationary, on four legs and move slowly and with difficulty.

“The wave Walker has got eight legs, and which can reposition themselves alongside the deck. So what you would have as you would have four legs standing on the seabed, and the other four legs, repositioning themselves lower than on the seabed. Pull up the previous ones. And in that case, you can make a walking motion.”

It is basically a spider-like sea platform walking along the seabed from UXO target to UXO target, eliminating the need for tugboats within the project site.

“So we have a base for our work. We [also] have the scope of work which shows the locations of the of the targets. The locations are provided to the to the barge master and we ask him: “Please, could you relocate the wave Walker to that in that position”, so wave Walker starts walking, we arrive on site. There are some positioning checks that that are going to happen, systems are switched on excavator is switched on. And then the tool goes into the into the water over that location and try to find at first the magnetic anomaly that was first observed. If we can’t find it, the dredging operation so start, so we try to dig the hole a little deeper. And when it’s deep enough, we do another survey sweep. And we continue like that. Until we until we find the targets.”

Although it eliminates the need for tugboats within the target site… we do mean the target site, the Wave Walker won’t win any marathons. It can achieve about 8m per hour at maximum. But in operational terms within a target area, this is a generational improvement.

This platform gets the project team to the best location, but there was another tool developed in recent years that allows safe UXO intervention.

Martin Valk, Solution Owner – UXO at Fugro says, “There was definitely a technology gap. We worked a lot offshore doing UXO identification offshore basically with ROV’s and those are fit for purpose to excavate targets in a low current environment up to one and a half, two metres of water depth. But we realised as soon as we moved closer to the beach, or closer, closer into an estuary where every cable lands at some point, ROV’s are really struggling in those areas, struggling with the current, but also often struggling with a larger depth of burial.”

ROVs can typically penetrate to 1.5 or 2m below the seabed, but in these surf zones objects can be buried in excess of 5m due to the highly mobile sediment. It is very dynamic and often the initial impact of the explosive buries it.

“And we set ourselves a target to solve that problem. And from that, we started the initial discussions to develop Sea Auk.”

Basically, the Sea Auk is a tool that is mounted on the boom of an excavator, which itself sits on the Wave Walker. It is a very large dredge head, about 4m long, to deal with remove large amounts of sediment.

“Then we integrated quite smartly on the right positions and Acoustic Camera, which allows to give visibility in moments when there is no visibility… by sight basically. So it’s an acoustic camera. We integrated an electromagnetic system. So, that tool is used to reconfirm the position of the of the potential UXO.”

Then because an electromagnetic system is limited to detecting ferrous objects to 2m deep, they also incorporated magnetometers to look more deeply. The tool also has water injection capabilities to stir up stiffer materials and help the dredging process.

Then, if the object isn’t UXO, for example a piece of discarded metal, it is safe to remove. Valk says that it can easily be grabbed and removed from the cable corridor.

This replaces the use of ROVs in environments inappropriate for them. The currents and the waves would result in low workability, low productivity. Divers were always an option, but there is the health and safety risk to consider. If divers can be avoided, it is always better to do so. Hence the Sea Auk. And innovation is underway to improve the Sea Auk’s efficiency at the lower end of its depth range, to overlap with ‘shore’ work.

It’s fascinating to see an industry adapt in real-time to a game-changing technology, and how different teams decide how best to optimise it. In the end, the team achieved the best result for any UXO survey, no UXO whatsoever. They however did find steel beams several centuries old, originally cast in Sweden and presumably left on the seabed due to a past shipwreck.

“The client was notified,” says Dillisse, “and when we got back the archaeologists were already informed so they were on the quayside waiting for us to offload that material. They took a few pictures and yeah, the next day we heard that they confirmed it was about 250–300-year-old steel. And that’s a few 100 years old. And a few museums already showed an interest in displaying some of these items.”

At the end of a industrial project looking to avoid the hazards we have inherited from the past it is fantastic and somehow fitting to instead be left with a link to our industrial history