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Offshore wind power promises to be the way of the future for Europe's burgeoning electricity needs. Locating unexploded ordnance on the seabed is an important first step.
By Graham Chandler
According to the European Wind Energy Association, 308 new offshore wind turbines generating 883 MW in eight new wind farms were connected to European national grids in 2010—a 51% increase over 2009. It was a new record that brought the region’s offshore power to 1,136 connected turbines; totaling 2,946 MW in 45 wind farms throughout nine countries.
It is to continue. The EWEA forecasts 1,000 to 1,500 MW of new offshore wind capacity will be fully grid connected in Europe for 2011. Ten farms totaling 3,000 MW are under construction this year. When completed, Europe’s installed offshore capacity will increase to 6,200 MW. Moreover, 19,000 MW have been approved. There’s great potential to go even further: “Europe’s offshore wind potential is enormous and able to power Europe seven times over,” states the EWEA.
The rush has spawned a boom in UXO seabed surveys. They are critically important: according to a story in the August 2010 issue of Offshore Wind Engineering, experts estimate that ten percent of military ordnance deployed during the Second World War failed to detonate. Still-live naval mines and air-delivered iron bombs present the greatest danger. This seabed contamination includes residue from defensive barrage minefields not fully cleared at war’s end, torpedoes, land to sea artillery, dumped munitions and shipwrecks. In the English Channel alone, UXO dangers lurk along the coastline as far north as the Humber, Tyne, and Tay estuaries—all key wind farm sites.
The task of undertaking seabed surveys—critical before any sort of construction can get under way—falls to specialty firms like Gardline Marine Sciences of Norfolk, UK. UXO surveys aren’t a new concept in Europe’s seas; the oil and gas industry has been drilling offshore for several decades and government guidelines for these are already in place.
But they don’t necessarily apply to wind farm sites. “There are no guidance notes on survey standards for wind farms like there are for oil and gas (the international Association of Oil and Gas Producers’ guidelines) but these have been adapted to offer what is required, as they offer best practice,” says Roger Birchall, Geophysical Manager at Gardline Geosurvey. “There has been the need for the contractors to better understand the requirement of an engineer having to design 92 pile foundations to fit into an area roughly 12x8 km as opposed to three piles in a 1 km square as has been traditional for oil and gas clients. So intelligent survey design is required.” Birchall adds that updated guidelines are in the wind however. “RenewableUK have recognized that standard guidelines are an issue,” he says; and the OSIG (The Society of Underwater Technology’s, Offshore Site Investigation and Geotechnics Committee) guidelines are under active review.
For a new survey, first steps are to research what UXO might be expected—this ‘desktop’ study is typically done by a third party.
“They usually subcontract to specialist companies,” explains Birchall. “Typically the consultancies will have access to their own records and will look at military records from all protagonists, historic maps, UKHO data base of wrecks. Some of the specialist UXO companies have jealously guarded charts mapping British minefields laid in the wars. For instance I have seen these but never been able to take copies, unfortunately we don’t have the German maps for where they laid mines off our coast, but it is interesting that they did not lay theirs on top of ours but along side or close to ours, so they had an idea where ours were.” Other sources of information can be academic institutions and historical literature relating to actions around proposed wind farm sites.
Richard Hamilton is a geophysicist with Southampton-based EMU Limited, a marine consultancy which just wrapped up its survey in the outer Thames estuary some 20 km from the Kent and Essex coasts. “In addition to all available archives being consulted, any pre-existing survey data from the area is reviewed during this stage,” he says. “This data is used to put together a risk register and risk management plan, which can then be used to determine the most suitable survey techniques.” On a recent job he says the desk based study identified that different types of UXO could be present on different parts of the site. “50 kg bombs were a threat item within 5 km of the shore, 250 kg bombs were a threat item across the site and MK19 sea mines were threat items in the south west of the site.”
Predicted locations of potential threats don’t necessarily dictate which areas to concentrate a survey, either. “The threat picture can be very similar over an entire area,” explains Arne Röd Lauridsen, geologist and geophysicist who manages offshore UXO surveys at DONG Energy. He says UXO items don’t always stay where they first sank beneath the waves, either. “Ordnance can travel around in the area due to currents over the seabed,” he says. “Some are exposed, some buried; some have become exposed, then travelled, then buried in another place.”
“The whole point of these desk top studies is to offer a quantifiable risk of the existence of UXO,” summarizes Gardline’s Birchall. “Once this has been done and the need for a UXO survey is determined, [we] are then contracted to carry out the survey.”
The next step is planning the sensing equipment to be used and selection of a grid pattern. “EMU currently uses Geometrics G-882 marine magnetometers for UXO survey work,” says Hamilton. “They have been chosen for their reliability, accuracy and rapid update rates.” Typical UXO surveys also employ sidescan sonar (SSS) and multibeam bathymetry to identify objects on the seabed and a seismic pinger to identify buried objects.”
Roger Birchall says Gardline also generally chooses the Geometrics 882 magnetometers. “For SSS we use Edgetech 4200s, 410khz which gives us an effective range of 75m, however we are also looking at some of the high resolution sonars such as the Edgetech 4125s.” Gardline often uses SSS and magnetometers together because it offers a cost effective solution to survey as opposed to dual pass surveys. “Sidescan can be used to clearly identify objects on the seabed,” he says. “The large magnetometer anomaly could well be an item of debris lost overboard and not UXO. Also, many German sea mines were made of alloys making them undetectable by magnetometers. Magnetometers are useful as they can identify anomalies that are buried, and in the rapidly shifting sands of the southern North Sea this is imperative.”
Magnetometer track lines are generally just parallel lines; because of the vastness of the areas, adding cross-lines can make a study prohibitively expensive. Normally spreads are 5m but can vary from 2.5m to 7m. “It depends on the threat picture,” says Lauridsen. “And also the expected background noise. The signal received from the UXO should be larger than the background noise. On one project we chose a spacing of 2.5m and that was quite an intense campaign.” He adds that where the UXO are expected to be exposed on the seabed the SSS often delivers sufficient information, which significantly reduces survey costs because it allows vessel trackline spacing to be increased.
Staying on pattern is a constant challenge, surveyors find. Offshore wind farm locations are purposely chosen for their constant high winds in order to maximize electrical output time—which doesn’t make accurate survey data easy to acquire. Added to that is the need to maintain a constant magnetometer altitude of 4m off the seabed. “The contractors usually are willing to go as close as three to four metres above the seabed so that the altitude of the magnetometer is very important,” says Lauridsen.
Hamilton would agree: “The biggest problems were expected to be keeping the magnetometers within 5m of the seabed and no more than a 7m horizontal gap in the data across the required survey site,” he says. “We spent some time researching ways to maintain the altitude to varying degrees of success, and really had to rely on our experienced skippers to keep the sensors over the survey lines.”
Roger Birchall says these line spacings are indeed tight, and a challenge which has prompted some looks at new techniques. “A line spacing of 5m is an order of magnitude above our usual site survey work,” he says. “We recently completed a trial towing four magnetometers with a spacing of 5m between each. The idea is that the outside magnetometer is run over the last line from the previous path, reducing the possibility of gaps due to currents and tides on opposing lines, and a grid is typically run with the emphasis on one direction with due consideration to the prevailing current and/or seabed conditions.” He says sand waves offer a particular problem when the requirement is to fly at 4m above the seabed in an area with sand waves to over 7m above the seabed, “common in the southern North Sea.”
EMU developed a similar tow arrangement a couple of years ago and has now completed over 35,000 km of survey with it. “For the Round 2 wind farm sites we developed a towing system allowing four magnetometer sensors to be towed within 5m of the seabed and accurately positioned by USBL from our 24m survey vessel RV Discovery,” says Hamilton.
Choice of ship varies. “The vessel required is normally determined by the site rather than the survey kit which we can usually adapt to fit any vessel,” says Hamilton. “On recent jobs we have operated from a 7m vessel with only a 0.5m draft to survey the intertidal section on a cable route.”
So larger vessels aren’t always the order of the day. But generally, “with the Round 3 wind farms there is the need for vessels that have longer endurance and better sea handling capabilities,” offers Birchall. “Our latest vessel, the Ivero has the capability to carry vessel and survey crew of up to 16 people for an endurance of 21-25 days, which means it can be on site 24/7 for 3-4 weeks saving our clients the considerable costs of transit time to port and from port to re-supply and the possibility of missing weather windows which is a real consideration in European waters.”
Indeed it is, especially winters. “At the worst times we were down to working one day a week,” says Hamilton, of a recent survey. “In these situations there is really very little you can do but wait for the weather to improve and grab every opportunity to get out surveying even if it’s only for a couple of hours. With the time restrictions for completing these surveys, pausing the survey over the winter months isn’t always an option anymore.”
While the issue of accurate positioning remains the biggest challenge, another is onboard data backup. “These sites by their very natures generate terabytes of data that has all to be stored and backed up onboard and then transferred so data is not lost,” says Birchall. “We use dual raid arrays to back the data up onboard and have strict procedures backed up by ISO9001 on the moving and transport of data.”
To process these large volumes, EMU uses Geosoft Oasis montaj with the Geophysics and UX-detect modules to process and interpret the magnetometer data. “EMU chose Oasis montaj for its processing capabilities, ability to handle large data sets and the fact it is relatively easy to learn to use,” says Hamilton. “We use the Geophysics module to filter the magnetometer data to initially remove heading and diurnal variations. And then, for UXO data sets, to remove large scale geological features. We then use the UX-detect module to grid the data, select targets and run the target analysis.”
For general reports Hamilton says Oasis montaj has been used to produce analytic signal colour maps to highlight where targets are present. “These are normally exported as geotiffs along with a CSV of targets for presentation in ArcGIS,” he says. “However on one project we are using Oasis montaj as the mapping software to produce pdf charts with the analytic signal, potential UXO targets, exclusions zones, line plans and proposed installations all overlaid. The individual components were then output as shapefiles or geotiff's for display in their webGIS system.”
Gardline too uses Oasis montaj similarly. “We filter the data in Oasis montaj to remove background noise and diurnal variation etc,” says Birchall. “Then calculate the analytic signal which is a 3D gradient. We use Oasis montaj with the UX detect module.” The main reason they chose it was essentially that clients now trust in it and request that it be used for analysis. “It means that we can do all the processing of the raw data and provide the whole project to a client for them to double check without the hassle of understanding our formats etc,” he says. “If the client asks for it then we give it to them, and they are asking for Oasis montaj.” He says they produce a plot of the analytic signal, then use the UXO detect module to provide the client with a list of contacts and size of anomalies and estimated burial depth.
Birchall says he likes Geosoft because “the analysis of the data is very quick and accurate and we can combine that with our sonar interpretation to link contacts on the sonar with anomalies identified on the magnetometer, and can offer feedback to the vessel quickly once the data has been dropped off.”
To assist with interpreting anomalies, DONG Energy has developed its own surrogates. “Where we anticipate what kind of UXO we expect to find in an area, we have made surrogates of them,” says Lauridsen. “This is the key for successful data interpretation. We do a survey of the surrogates and use that setup.” Surrogate UXOs are physical objects with the same dimensions and mass, made from standard steel. “Due to our time schedule and challenging accessibility, it’s not possible to get an old bomb,” he says.
Gardline employs a similar tactic, using it for client demos too. “We have created a test site where four metal objects of known size have been deployed and we can run our kit over them to prove to a client that it is working and so can move on to the survey,” says Birchall. “The key is the accuracy of positioning, and proving that positioning is what it says it is. That’s the whole point of the test site. And we can use Oasis montaj to model the data collected against a known baseline.”
Once locations are identified, it’s usually easiest and less costly to avoid suspected UXO rather than remove them. “Mostly we just go around the targets,” says Lauridsen. “We don’t send a diver unless we have to. We’ll route the cables around or move the foundations. We only need to move them a few metres often, for a clear area.” Obvious findings of UXO are of course reported and EOD activities initiated to eliminate the threat due to the potential movable nature of the UXO.
Sometimes however diver or ROV surveys need to be carried out on objects and anomalies noted within a certain radius of piling locations. But, especially in places like the southern North Sea with its dynamic environment, this poses problems of currents and visibility, driving up costs. “I know of instances where divers have done sweeps of an area using finger tip searches or wire sweeps and missed things because of the nature of the seabed,” says Birchall. “Mega ripples can be up to a metre high and snag search wires in areas where a diver can barely see their hand in front of their face.”
Despite all the challenges, the value of a proper UXO survey is clear for the offshore wind farm industry. “There is a maxim within the survey industry,” says Birchall: “you pay for a survey whether you do it or not.”