A tanker was underway and normal maintenance activities were being accomplished. An engine room crewmember was tasked with opening the steam valve that leads to the deck in order to recirculate excess steam. This had been common practice on board for the last nine years, ever since one of the heavy fuel oil (HFO) bunker tanks had been modified to Marine Gas Oil (MGO). Since that modification, the excess steam could not be sufficiently cooled by the existing dump/drain cooler when the vessel was...