Rig removal may leave heavy metal in the sea

September 28, 2001

Removing disused oil rigs may damage the environment more than leaving them in place, researchers have found.

Seabed landers sent to the bottom of the North Sea detected lower than expected levels of heavy metals escaping from polluted mud into the water.

The study, by a team at the Dunstaffnage Marine Laboratory, Oban, is the first to investigate the problem in situ and it suggests the government is wrong to propose that all steelwork be stripped away during decommissioning.

Graham Shimmield, director of the Scottish Association for Marine Science, concluded there was the potential for higher levels of pollution during rig removal.

Several hundred thousand tonnes of waste have accumulated on the bottom of the North Sea around many of the 250 oil and gas installations.

These drill cuttings are made up of drilling mud, lubricants, detergents, corrosion inhibitors and other chemicals as well as fragments of the underlying rock. The mixture contains heavy metals that have a severe impact on seabed life.

With many North Sea oil and gas fields reaching the end of their useful lives, concern has grown about the environmental impact.

Professor Shimmield's team used remotely operated vehicles to place tripod-mounted landers the size of refrigerators on the seabed. Sensors on the landers allowed the scientists to measure subtle differences in chemical levels in the water between mud particles. They found that in stable conditions, negligible quantities of heavy metals seeped into the overlying seawater. This might have been because seabed bacteria converted soluble metals into an inert mineral form rapidly.

In the lab, the scientists disturbed cuttings mud for 50 hours and detected a tenfold increase in metal concentration in the water.

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