A multi-stage process for the production of an ISO 8217 compliant Product Heavy Marine Fuel Oil from ISO 8217 compliant Feedstock Heavy Marine Fuel Oil involving a Reaction System composed of one or more reactor vessels selected from a group reactor wherein said one or more reactor vessels contains one or more reaction sections configured to promote the transformation of the Feedstock Heavy Marine Fuel Oil to the Product Heavy Marine Fuel Oil. The Product Heavy Marine Fuel Oil has a Environmental Contaminate level has a maximum sulfur content (ISO 14596 or ISO 8754) between the range of 0.05 mass % to 1.0 mass. A process plant for conducting the process for conducting the process is disclosed that can utilize a modular reactor vessel.