A process for reducing environmental contaminants in a Feedstock Heavy Marine Fuel Oil compliant with ISO 8217:2017, Table 2, RMG or RMK, for bulk properties except for a sulfur content as determined according to ISO 8754 and except, optionally, for one or more of a content of vanadium, nickel, iron, aluminum and silicon as determined by the respective ISO standards, the process comprising: mixing a quantity of Feedstock Heavy Marine Fuel Oil with a quantity of Activating Gas mixture to give a feedstock mixture consisting of the Feedstock Heavy Marine Fuel Oil and the Activating Gas mixture; the feedstock mixture is brought up to the process conditions of temperature and pressure and introduced into a first vessel; contacting the feedstock mixture with one or more catalysts to form a Process Mixture from said feedstock mixture; receiving said Process Mixture and separating a Product Heavy Marine Fuel Oil liquid components of the Process Mixture from the gaseous components and by-product hydrocarbon components of the Process Mixture; and discharging the Product Heavy Marine Fuel Oil, thereby reducing the sulfur content to less than 0.5 wt % as determined according to ISO 8754, wherein said Activating Gas is selected from mixtures of nitrogen, hydrogen, carbon dioxide, gaseous water, and methane, such that Activating Gas has an ideal gas partial pressure of hydrogen (P H2) greater than 80% of the total pressure of the Activating Gas mixture, and wherein the ratio of the quantity of the Activating Gas to the quantity of Feedstock Heavy Marine Fuel Oil is in the range from 44 Nm 3 gas/m 3 (250 scf gas/bbl) of Feedstock Heavy Marine Fuel Oil to 1,780 Nm 3 gas/m 3 (10,000 scf gas/bbl) of Feedstock Heavy Marine Fuel Oil, wherein the total pressure in the first vessel is between 1.7 MPa (250 psig) and 20.7 MPa (3000 psig), wherein the indicated temperature within the first vessel is between 260°C (500°F) and 482°C (900°F), and wherein the liquid hourly space velocity within the first vessel is between 0.05 oil/hour/m 3 catalyst and 1.0 oil/hour/m 3 catalyst. mixing a quantity of Feedstock Heavy Marine Fuel Oil with a quantity of Activating Gas mixture to give a feedstock mixture consisting of the Feedstock Heavy Marine Fuel Oil and the Activating Gas mixture; the feedstock mixture is brought up to the process conditions of temperature and pressure and introduced into a first vessel; contacting the feedstock mixture with one or more catalysts to form a Process Mixture from said feedstock mixture; receiving said Process Mixture and separating a Product Heavy Marine Fuel Oil liquid components of the Process Mixture from the gaseous components and by-product hydrocarbon components of the Process Mixture; and discharging the Product Heavy Marine Fuel Oil, thereby reducing the sulfur content to less than 0.5 wt % as determined according to ISO 8754, wherein said Activating Gas is selected from mixtures of nitrogen, hydrogen, carbon dioxide, gaseous water, and methane, such that Activating Gas has an ideal gas partial pressure of hydrogen (P H2) greater than 80% of the total pressure of the Activating Gas mixture, and wherein the ratio of the quantity of the Activating Gas to the quantity of Feedstock Heavy Marine Fuel Oil is in the range from 44 Nm 3 gas/m 3 (250 scf gas/bbl) of Feedstock Heavy Marine Fuel Oil to 1,780 Nm 3 gas/m 3 (10,000 scf gas/bbl) of Feedstock Heavy Marine Fuel Oil, wherein the total pressure in the first vessel is between 1.7 MPa (250 psig) and 20.7 MPa (3000 psig), wherein the indicated temperature within the first vessel is between 260°C (500°F) and 482°C (900°F), and wherein the liquid hourly space velocity within the first vessel is between 0.05 oil/hour/m 3 catalyst and 1.0 oil/hour/m 3 catalyst.