A FIRE in February on board the tanker British Adventure was likely to have been caused by leaking hydraulic oil ignited by welding work, an enquiry commission was told this week. Leading counsel Ong Hian Sun said in his opening address: ''The most probable cause of the fire was due to atomised hydraulic oil under high pressure discharging through the leak at the coupling on top of the junction box and ignited by the welding work on the supporting bracket for the junction box of the aft power pack.'' The inquiry, which began this week, is expected to last about two weeks. District Judge Ibrahim Burhan is chairing the committee. Two assessors, associate professor Lim Mong King of the Nanyang Technological University, and Marine Department director Teh Kong Leong, are assisting him. About 80 witnesses are scheduled to give evidence. On February 8, a flash fire broke out in the steering gear room located at the aft end of the Bermuda-registered British Adventure, owned by BP Shipping. Of the 11 workers in the room, 10 died of either inhalation of fumes or extensive burns, or both. Mr Ong said: ''Whether the hydraulic system in operation at the time was activated by the aft power pack or the forward power pack is likely to be one of the main issues in contention in this inquiry.'' Among the works carried out on the British Adventure at Jurong Shipyard was the modification of the hydraulic system to connect the forward hydraulic system to the one in the aft. When the fire broke out on the 400,000 deadweight tonne vessel, the high-pressure hydraulic pipeline linking the forward and aft power packs had not been isolated. According to Mr Ong, the Department of Industrial Safety had found that while the handle of the isolation valve at the midship deck was in a shut position, the valve was actually open. It then arranged a test on the hydraulic system on March 9 to determine if the residual pressure in the system could be maintained after the forward power pack had stopped operating. This, in turn, would ascertain if the residual pressure was strong enough to cause a major leak at the coupling above the aft power pack. The department found that the residual pressure dropped ''quite rapidly'' after the hydraulic power pack was switched off. At lower pressures, the quantity of oil escaping from the leak at the coupling would be minimal and less likely to form a flammable mist, said Mr Ong. He added that the test showed that the hydraulic system was likely to have been in operation when the fire broke out.