Harry Thierens testified at the U.S. Coast Guard and Bureau of Energy Management joint investigation of the April 20, 2010, Deepwater Horizon oil rig explosion in the Gulf of Mexico. In his testimony he said that changes to the blowout preventer device hurt efforts to manually operate it days after the explosion. He also said that after several failed attempts to get the blowout preventer to work, he learned that its security function was replaced by a testing device not designed for well control.
This hearing was the third day of the fourth session of hearings of the Deepwater Horizon joint investigation.