SPONSORS OF UNEXPLODED ORDNANCE EXPLOSIVES
As per Crown Estate’s policy a sea bed survey is carried out up to 1km from any proposed offshore wind or tidal construction area or cable runs etc. As part of this survey they are looking for any UXO which in the North Sea would generally most likely be WW2 German air dropped ordnance or British.
STEP-BY-STEP PROCESS An ‘as found survey’ is carried out by ROV which also includes a sonar sweep of the area – this is logged and saved for further use.
Once any potential identified ordnance is found they carry out another ‘as found survey’ and the information gathered is detailed into a spreadsheet as follows...
• Location on seabed to proposed development area, including co- ordinates – GPS waypoints
• Possible type of munition, photographic evidence, still pictures, video, measurements.
• Approx charge weight (by volume) then working out a max NEQ (Net Explosives Weight) as this is what will potentially cause any negative effects on marine mammals / existing pipeline / cables etc.
• Water depth • Sediment / sea bed type • Munitions description • Potential diameter of crater that could be created and estimated released sediment
• Safe range for marine mammals & fish • Injury range for mammals/fish and mortality rate – 25-40%
• Lethal range for mammals & fish – mortality rate 75-100%
• Distance to safety vessels/standby vessels/other third party DSV’s/MV’s etc
• Owner of proposed offshore scheme • Specific Impulse on worse scenario detonation
• Peak & over pressure based on worst scenario detonation
COLLATION
After the above data is collated and firmed up then the actual type/make of ordnance is established, the method of disposal is planned along with marine mammal / fish mitigation procedures, third party safety distances, the technical design and deployment of the donar charge is then planned along with the Environmental Impact Study.
DONAR CHARGE DESIGN
A decision is made on the live explosive charge that will be used to initiate the found UXO – an assumption is made that the UXO is still live and capable of sympathetic detonation by a live initiated donar charge and on both the desktop assessment and the practical operation an estimate of safety distances is calculated.
SUCCESSFUL DESTRUCTION The aim is to sufficiently destroy the UXO so as to not leave any sizeable pieces on the sea bed which in future may still ‘look’ like an UXO even though it may be a piece of inert casing.
UNIQUE DESIGN AND FIRING SYSTEM To achieve this criteria they use their own design of charge with a unique firing system.
They use two conical shaped charges of approx 2kg NEQ each which directionally face each other with the ordnance in the middle. Basically a single conical shaped charge is a directionally functioning charge which produces a molten jet travelling at over 9,000m/s. If you were to place say a 2kg conical charge with a 63* cone angle on top of a block of steel it would easily penetrate into the steel over 3ft-4ft.
TECHNOLOGY BLAST DESIGN TAKE US THROUGH THE PROCESS OF
THE DISPOSAL OF UXO SURVEY OF AREAS
In many cases however the UXO can have become desensitised by salt water ingression and therefore the intention is to use the least NEQ (Net Explosive Quantity) in the donar charge to initiate the UXO. This way, if the UXO fails to detonate NEQ detonating in the water is experienced and the lowest overpressure from the detonation affects marine mammals.
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www.windenergynetwork.co.uk
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