Blast from the past

Imagine an area 93km by 93km and then fill this area with 3000 wind turbines. Some of the turbine foundations will have only one hole penetrating in the seabed, some will have 3 holes penetrating and some will even have 4 holes. This is what can happen in an area the size of the Dogger Bank OWF wind farm using 5MW turbines to generate the agreed proposed 9GW. Each hole made in the seabed has a chance of hitting a mine or bomb that is still potentially lethal even though it may have lain there in the seabed since 1914.

Approximately 600,000 pieces of explosive ordnance (mines and bombs) are estimated to have been laid in European waters by the British during the 2 World Wars in the last century, of which 500,000 in the North Sea and English Channel areas. These were laid there to form a barrier for shipping between the coast of the UK and Continental Europe. The highest concentration was found on the East Coast, Dover Strait and the South Eastern part of the North Sea in particular.

No one can say with any accuracy where these mines are located. Many of them were laid by aerial mine laying operations, and in the intervening years currents and tidal streams have moved vast amounts of seabed, including what is lying in the seabed..

Shipping lanes have been cleared since 1945 and the risk to normal shipping is calculated in decimal points of a percentage. But fishing vessels dragging trawl nets over the seabed are still occasionally finding mines in their nets, and these trawl nets only disturb the seabed to a depth of about 50cm. There is no way to calculate how many pieces of ordnance have not been cleared or what lies still even deeper in the seabed.

In the oil and gas industry government guidelines have been accepted and followed for thirty years or more. Drilling an oil or gas well is in effect the same as thumping a long piece of pipe into the seabed for a turbine foundation. But never before has the seabed been perforated or stamped on in such a close proximity as is the case with wind farm projects, and this activity will even grow with the Round 3 planned wind farms. The big difference is that oil and gas wells are not drilled every 500 to 750m. Even more risk is apparent with jack-up vessels that are doing the construction work at the wind farms.

Detection

OK, that is the problem … what is the solution, for solution there must be if we are to build the number of wind farms safely.

There are military records, however inaccurate, historic charts and all the information that can be gleaned from the hydrographie surveys of all the North Sea countries available. This data is studied and mapped before any vessel puts to sea with the specialist equipment to search for the ordnance. Then, and only then, is need for a specialist survey determined by a making a risk management plan.

There are companies who not only carry out geophysical surveys but also are able to survey for these explosive items. Fortunately most of the mines and bombs were made with ferrous metal casings and are therefore detectable using magnetometers.

Their instruments track a path at either side of the sensor with widths of 5m, for example, depending on the degree of sensitivity being used. A string of 4 sensors, for example, spaced 5m apart on a towed bar is made so that the vessel can produce a scanned path up and down the target area. The sensors on each end of the tow will overlap the previous scan producing a ‘printout’ pattern similar to the mown grass on a tennis court. Sometimes it is necessary to make the scan lines in the same direction each time to equalise the effect of currents and other variables.

Towing the sensors at a fixed height above the seabed to ensure that the readout produces results that are equal, is not easy but is very necessary. If the results are to show the finds, or lack of finds, to the same scale then all the parameters of the scan must be the same all the time on each scan run.

Some mines laid during World War II had non-ferrous alloy casings that will not be found by magnetic scans though. Scanning for disturbances in the seabed is the only way to detect these mines. In these circumstances it other types of sensors can be added on to the rig to make real time scans of different elements. Using simultaneous multibeam echo sounder and sidescan sonor with the magnetometers produces a really useful amount of data. This extra information from the multiple scan also makes it easier locating any possible finds later when you have to deal with a find.

Assuming that the scans have been carried out with all the above-mentioned parameters taken into consideration then the resulting data has to be processed. The resulting colour charts in 3 dimensional resolution is reminiscent of the Titanic movie. The vast amounts of information produced are ‘money in the bank’ and should to be saved for any further interpretation at a later date.

As this article was being written there was news from Harwich, currently the busiest offshore wind industry port in the UK, concerning the finding of a V2 rocket missile from WW2. Although only fragments of the missile were retrieved from the mud at low tide, the warhead was missing. This find highlights the constant threat of buried ordnance moving, or becoming exposed, with streaming currents and tides after more than 60 years.

Dick Hill