This can be a very serious safety hazard. It sounds like you have the top head open on a drum that has not been drained, so there is a hazard from the top head as well as the bottom head. You should keep personnel away from both the bottom and the top head until the drum is safely drilled out.
If you have tried steam and water through the fill line and are not able to get the drum to drain, then about the only option is to drain through the bottom head. You do not state what type of bottom head closure you have on your Coker. If you have a DeltaValve or Z&J valve as a bottom head, you could remove people from the drum structure and away from the pit or pad for about 100 meters radius and then open the slide valve. The top head will need to be open to prevent a vacuum from being formed when the water and coke does exit the drum. Once the heads are open, then you can drill a pilot hole through the drum that will drain water, if it has not drained before then. Be aware that this drum may have significant hot spots that could cause a blow out or an eruption, so having the area cleared of people and equipment is important.
If the bottom head is a semi-automatic style, you need to evaulate if the cart can hold a drum full of water. Only if that is true should you attempt to have personnel remove most of the bolts. Some operators have left as many as 8 bolts in the head that have been loosened enough to let the bottom head drop only slightly. Once personel are a significant distance away from the drum should you attempt to lower the bottom head remotely. Once the water has stopped draining through the bottom head, run the cart back up and have the last bolts removed and then again have people a significant distance away before lowering the bottom head completely.
The hazard at the top of the drum is from a boil over. If there is a hot spot in the drum that can cause rapid vaporization of the liquid water, the resulting steam can eject hot water out of the top of the coke drum.
Understand that very serious injuries including fatalities have occurred from hot water associated with drums that did not drain properly so this is a very serious safety hazard. These types of activities should only be attempted after a thorough discussion with all the personnel involved.