A multi-stage process for reducing the environmental contaminants in an ISO8217 compliant Feedstock Heavy Marine Fuel Oil involving a core desulfurizing process and a ultrasound treatment process as either a pre-treating step or post-treating step to the core process. The Product Heavy Marine Fuel Oil complies with ISO 8217 for residual marine fuel oils and has a sulfur 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 is also disclosed.