SPONSORS OF UNEXPLODED ORDNANCE
UNEXPLODED ORDNANCE A RECOGNISED THREAT
The potential for UXO within proposed offshore wind developments has become a recognised threat in recent years. Risk reduction to ‘As Low as Reasonably Practicable’ (ALARP), is not only a legal requirement, but is also a consenting, insurance and project financing issue, and as such can become a serious obstacle to the overall development of the site.
SEABED SURVEY
An early stage in ensuring the consenting area is free from UXO involves a detailed seabed survey using a high-resolution side scan sonar, and a magnetometer positioned close to the seabed.
The magnetometer is used to detect variations in the total magnetic field of the underlying seafloor and sub-seabed geology on the basis of anomalies in the Earth’s magnetic field, caused by the presence of ferrous iron on, or below, the seafloor.
Materials high in ferrous or ferric compounds may include wreck debris, pipelines, sites of archaeological interest, volcanic rocks, and clearly, UXO.
OSIRIS PROJECTS – A CASE STUDY Osiris Projects were commissioned to perform such a survey for a major Round 2 development.
A WW2 50kg bomb was identified by specialist UXO consultants to be the smallest threat on-site, which required the magnetometer data to have a maximum lateral spacing of 5m, and a maximum vertical height from seabed of 3m.
SPECIALIST PROCEDURE The company developed a simple but effective tow configuration allowing multiple magnetometers to be deployed simultaneously whereby each magnetometer is deployed in a ‘piggy- back’ arrangement behind a side scan sonar fish and controlled from separate winches.
This allows the towing height of each fish to be controlled independently and equipment deployment and recovery time is drastically reduced compared to a towed array setup. Each magnetometer is fitted with a USBL beacon for accurate and independent positioning.
On this project, the magnetometer towing configuration delivered a mean fish height of 2m above seabed - exceeding the client specification - and the accurate positioning allowed areas of infill to be quickly identified resulting in almost complete coverage of the site.
OUTCOME
Dive investigations, conducted by specialist commercial diving contractor, corroborated potential targets identified by Osiris Projects, and fortunately the site was free from UXO.
Osiris Projects
www.osirisprojects.co.uk Click to view more info DATA
Data was processed and interpreted using Oasis Montaj with UX-Detect extension, which allows accurate localisation and analysis of UXO targets which were then or interpreted by the company’s specialist processing geophysicists.
In-depth profile-by-profile analysis of the data, as opposed to automated target picking algorithms, ensured that all anomalies were analysed and that many geological anomalies could be rejected on the basis of their morphology, thus narrowing the number of relevant targets delivered to the UXO Consultants.
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www.windenergynetwork.co.uk
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