According to truthout the Department of Interior's federal Minerals Management Services fined BP in 2004 for shutting off a similar warning system on another rig.
...MMS fined BP $25,000 in February 2004 for another incident that took place a year earlier on the Diamond Offshore drilling rig. MMS said "the rig's Gas Detection System was bypassed with ongoing drilling operations being conducted."
http://www.truthout.org/whistlelower-bps-other-offshore-drilling-project-gulf-vulnerable-catastrophe59027The release of natural gas from working wells is apparently a common phenomenon. Since it's odorless and heavier than air, any gas that comes through the well will settle around the platform undetected.
Perhaps only survivors of the accident could tell if Deepwater had a natural gas alarm system that was working at the time of the accident. If I were an amoral petroleum company, I'd be persuading their silence through any means available.