<P> 2 . Valves to prevent cement backflow did not close . Weatherford's Auto - Fill float collar, which includes two flapper - type check valves, was installed at Macondo 180 ft above the reamer shoe at casing bottom . The valves are held open by a 2 - inch diameter auto - fill tube to allow the casing to fill with mud while it is lowered down the well . BP's casing installation procedure stated: "slowly increase (mud) pump rates greater than 8 bpm to convert the float equipment (~ 500--700 psi) per Weatherford recommendation". As the flow rate drag force during circulation and cementing was only around 30% of that required, the auto - fill tube was almost certainly not ejected and the flapper valves never closed . Some evidence suggests that personnel on the Deepwater Horizon mistakenly believed that the high pressure needed to establish mud circulation had converted the float collar . </P> <P> 3 . Cementing inadequate . Cementing proceeded a) without flushing the annulus around the shoe track at sufficiently high rate and duration to ensure full circumferential removal of compressed sediment and good distribution of cement, and b) without converting the float collar to activate its two check valves to prevent cement backflow . Cement quality and strength may have been reduced by contamination on its way down the casing, or by mixing with lower density mud in the rathole . There was no cementing evaluation log at Macondo, which may have shown it to be inadequate . </P> <P> 4 . Pressure test wrongly interpreted . Drill pipe was run to 8,367 ft ready for mud displacement . During the "negative pressure test", for which there was no detailed procedure, no flow from the kill line was accepted and 1,400 psi on the drill pipe was ignored . </P> <P> 5 . Rising oil and gas not monitored . While displacing the mud with seawater, reservoir fluids rising up the casing should have been detected by water inflow and mud outflow monitoring before arrival of hydrocarbons at the rig floor, but no reasonably accurate outflow versus inflow observations were made . </P>

What was the cause of the deepwater horizon