AEA Technology has devised a system to deal with an emerging problem of North Sea decommissioning – how to deal with tens of thousands of tonnes of contaminated drill cuttings that have built up at the base of offshore platforms.
While attention has focused on removing the steelwork – particularly in the wake of the Brent Spar controversy – the piles of oil-contaminated waste rock from drilling operations will become a serious issue if it is decided that steel platforms must be entirely removed.
Oil-based muds used in the drilling process pose the environmental threat, with up to 45,000 tonnes of contaminated cuttings having accumulated around the bases of some installations.
Bryan Taylor, technical director at the UK Offshore Operators’ Association, says there are some installations with piles 20m high.
With some signatory countries to the Oslo and Paris sea dumping conventions showing signs of softening their stance on total removal of all platforms, it seems likely that any final accord to be reached later this year will require complete removal of around 30 large steel platforms from British waters.
Where this is to be done, the present method of leaving cuttings to pile up so a crust eventually forms cannot be adopted, as removing the steel structure around it would break the crust and disperse the contamination.
Taylor said the industry was reluctant to commit itself to recovering the piles without a proven and economic means of doing so. `If the costs were reasonable, more people would be prepared to do it,’ he said.
AEA Technology believes it has come up with the answer. Its proposed solution employs a subsea crawler, guided by a remotely operated vehicle. The crawler has a cutting head to break the crust, and a pump which sucks up a mixture of pile and water (in a 15:85 ratio) and transfers it to the surface for treatment.
On the platform, the mix is broken down to its constituents of water, oil and rock chippings. The water is processed to contain no more than 15 parts of oil per million and poured back in the sea, the oil is put in drums, and the excavated material is mixed into slurry for reinjecting under the seabed.
Jack Stewart, sales engineer at AEA Technology, said the system could remove a 40,000-50,000 tonne pile in four to six months for under £5m, compared with the £70m bill for removing a large steel platform.