By Angel Gonzalez Of DOW JONES NEWSWIRES HOUSTON (Dow Jones)--The vessels drilling relief wells at the BP PLC (BP) spill site are demobilizing and could leave the Deepwater Horizon site Friday ahead of a tropical storm, the head of the federal oil spill response team said. In a teleconference early Friday, retired U.S. Coast Guard Admiral Thad Allen said that the two Transocean Ltd. (RIG) drillships are in the process of pulling thousands of feet of riser pipe back into their decks before they can leave. "They haven't physically left the scene," he said. Another vessel that is connected to the blow-out preventer at the damaged subsea well, the Q4000, is also disconnecting and aims to leave later Friday, he added. The ships' operators will place the vessels in a safe place during the passage of Tropical Storm Bonnie, which is expected to occur sometime between Saturday and Monday. Bonnie, which became the second named storm of the Atlantic hurricane season late Thursday, is prompting the evacuation of the flotilla fighting the oil spill anchored at the site where the Deepwater Horizon rig sank last April, killing 11 people and unleashing a massive oil spill. The evacuation could delay the killing of the leaking Macondo well through a relief well, originally targeted for mid-August, by up to two weeks, placing a cloud over what had been a successful week for BP. The company managed to contain the flow of oil last week and secured authorization from the U.S. government to begin preparing for a new method to permanently stop the spill by flooding the well from the top with drilling fluid, a method that would be more quick than the relief well. Allen said on Friday that the evacuation could last 48 hours or less, but he wasn't specific on when the drillships could resume laying the final casing for the relief well. He said it would take 48 hours to lay the casing, and two days after that BP could proceed with the new method, dubbed "static kill." Allen reiterated that the well's flow will remain contained during the evacuation, and that vessels operating the subsea robots that keep a close eye on the well will be the last to leave and the first to return. Some of these vessels may remain in place through the storm if conditions allow, he said. If not, responders will conduct aerial and satellite surveillance to make sure no oil is leaking into the Gulf. -By Angel Gonzalez, Dow Jones Newswires; 713-547-9214;angel.gonzalez@dowjones.com (END) Dow Jones Newswires July 23, 2010 11:45 ET (15:45 GMT)