• Marine

Subsea can be a minefield

James Fisher has devised an innovative solution to a potentially explosive offshore problem for the renewables industry.

In recent months, progress on building offshore wind farms in parts of the North Sea have been hampered by the discovery that many sites are littered with unexploded ordnance (UXO) dropped by Allied and German planes, ships and submarines during World War II, but a James Fisher team is quickly growing a strong reputation as an effective 'bomb-buster' in the area, having devised a solution to save customers time and money.

Earlier this year, renewables company Vattenfall called in Fendercare Subsea Services (now James Fisher Subsea) to work with German partner company, GSS Hansa Offshore to help clear an area called Sandbank, 90km off the coast of Germany.

A previous survey had identified a number of metal objects on the seabed or buried up to 2.5m deep in the sand and the team was asked to relocate and excavate them and dispose of any UXOs safely.

'Our first task was to launch ROVs to find out which of these objects could actually be UXOs. The buried objects were cleared, carefully, using a 'zip pump' to suck away the sand and we then called in a specialist to identify them and advise on an appropriate course of action,'

says Martyn Muntingh.

Adrian Dann, formerly Lt Commander with the Royal Navy, and one of the leading technical authorities on UXO disposal in the UK, was called in to help with detection and detonation.

Of the 120 targets, 28 turned out to be unexploded ordnance which had to be safely detonated with minimal impact to sea life in the area.

The project received high praise for speedy completion - the team exceeded identification and detonation targets, identifying as many as 12 devices in one day. Significant time and money was saved by the choice of vessel which performs both identification and disposal allowing the team to switch quickly between the two as required.

The operation was coordinated by project services manager Gary Hitchcock, and overseen by technical diving director, Bob Macmillan, who explains how the UXOs were exploded with minimal disruption to sea life,

"We use high frequency sound waves to ward off marine life, then set up a 'bubble curtain' (a hose laid on the seabed to encircle the site, which sends off a wall of air bubbles) to clear the blast radius and prevent damage to the local area."