New Delhi - Port authorities in Chennai have impounded a BW LPG vessel and a local ship carrying heavy fuel oil, and detained their crews, a spokesman for the port said on Friday, after their collision last week caused an oil spill affecting marine life and local fishing. About 20 tonnes of heavy fuel oil leaked and a complete clean-up is expected to take eight to 10 days, according to an Indian coast guard spokesman. BW Maple, with a total capacity of 82,000 cubic metres of liquefied petroleum gas, was half full when it collided near Chennai with the Indian ship Dawn Kanchipuram on Saturday. The port spokesman said the sludge - a mixture of oil, water and sand - has travelled over 18 miles, polluting the Marina Beach, one of the world’s longest. Reuters Television footage showed black layers of oil floating near the shoreline, with buckets being used by volunteers and coast guard officials to clean up the sludge. No one at Darya Shipmanagement Pvt Ltd., owner of the Indian vessel or the Oslo-listed company BW LPG were immediately available for comment.