Five persons were killed and a dozen others injured in a blast inside an Oil and Natural Gas Corporation (ONGC) tanker at the Cochin Shipyard here in Kerala on Tuesday, police said.Those working at the time of the blast mostly included daily labourers and contract workers. Tuesday being a holiday, no regular workers of the shipyard were present.The blast took place around 11 a.m. in the water tank of the berthed ONGC tanker Sagar Bhushan when some 20 workers were working in the tanker ship.The Sagar Bhushan has been docked at the Cochin Shipyard for a month for repairs and maintenance.High ranking police and top shipyard officials have reached the accident site. Speaking to the media, Kochi City Police Commissioner M.P. Dinesh said: "The death toll from the blast includes some who apparently choked on the smoke after the explosion.A probe has been ordered, Dinesh said.A trade union leader present at a private hospital here said nine workers were admitted there.