To reduce the risk of contamination, a backflow preventer can be fitted. A more complex valve that accomplishes much the same purpose is the backflow preventer. Pumps on the rig normally deliver pressure to the blowout preventer stack through hydraulic lines. An annular blowout preventer uses the principle of a wedge to shut in the wellbore. BP representatives suggested that the preventer could have suffered a hydraulic leak. While drilling the oil well, surface pressure control is provided by a blowout preventer (BOP). The American Bureau of Shipping last inspected the rig's failed blowout preventer in 2005. The blowout preventer was damaged in a previously unreported accident in late March. Later that day a replacement blowout preventer was placed on the well. An attempt was made to activate the blowout preventer, but it failed. As of May 2010 it was unknown why the blowout preventer failed. A blowout preventer may also simply be referred to by its type (e.g. ram). Two categories of blowout preventer are most prevalent&colon; "ram" and "annular". Several individual units serving various functions are combined to compose a blowout preventer stack. In 1924 the first successful blowout preventer was brought to market.