I doubt they needed an alarm....the movement of the explosion must have been enough to alert everyone. After that...it was finding the best exit..and a place to jump ship....
The alarm would presumably have given them warning of something that wasn't right, prior to the explosion. A failed valve, or sudden pressure change, or a failed system of some sort - anything that might have been a precursor to the explosion. Had any type of significant event occurred, and had the crew received adequate warning, they could have reacted by either manually making corrections or shutting down the rig, or by evacuating quickly.
Of course, this is assuming that the event was preceded by a non-catostrophic event that could have been corrected. And, it's assuming that the alarm system is working properly.