Have you ever considered what happens to a sealed drum when you change altitude during transport?  During a recent drilling job for Stratus, the staff loaded drums at the office located at about 30-feet above MSL then drove these drums up into the mountains to a project at about 4000-feet above MSL.   Know what happened when the first drum was opened?  As the drum ring was being loosened/removed, the lid popped off the drum and flew a few feet in the air.  WOW.  No one was hurt luckily.   Something easily overlooked and needing consideration during your risk assessments.   How would you mitigate the pressure risk associated with drum transportation to different elevations?