You are correct in saying that this scenario could be very dangerous. This was the case at the Equilon Anacortes Coker in 1998. They unheaded the drum without quenching with water and the resulting incident caused 6 fatal injuries. There have been a number of presentations on this incident and there are always multiple factors that contribute to fatal injuries.
As a way to prevent the same sort of incident, water quenching the coke drum before deheading to removed the stored energy is imperative. If the feed line is plugged and steam and water are not able to enter the drum through the normal flow paths, add water to the top of the drum. Cover the coke bed with water and let the water boil away. Do this as many times as required to keep the water level over the top of the coke/tar.
The slide gates used for bottom heads from DeltaValve or Z&J make opening the bottom head from a remote location possible. An exclusion zone should be set up around the pit where the coke/water mixture will be routed when the bottom head is opened up. If the coke drum quench water is not completely drained, the potential for a flood of water into the pit is possible, so keep the area around the pit clear of personnel.
If you do not have slide gates for bottom heads, this complicates the draining of the drum and removing coke. Frank Tracy of P66 made a good presentation on this subject at the 2016 Galveston coking.local conference.
