An Introduction to Ship Fuel Systems

Describing the Layout of the Fuel Oil Storage System

Drawing on the general layout of what a ship's fuel oil storage system would look like.

Who's Responsible for the Fuel Oil System on a Ship?

How Do Ships Refuel?

Pre-Bunkering

Bunkering Operations

Fuel Oil Service System

Internal Fuel Oil Transfers

Getting Fuel to the Engines

Common Fuel Oil Issues Onboard Ships

• Water in the Fuel: Water enters fuel tanks primarily through condensation via tank vents, which allow air exchange to equalize pressure. In humid environments, moisture in the air condenses on tank walls, mixing with the fuel. Water is denser than fuel and settles at the tank's base, where engineers use "kick valves" to drain it, if needed. This process is performed regularly, with engineers collecting the drained liquid in a container until only fuel emerges, ensuring minimal water content in the system.
• Dirty Fuel: Dirty fuel, often received during bunkering, can contain sediments, sludge, or other impurities that render it unusable due to the potential damage it may cause to the system and its components. Fuel samples are collected during bunkering to assess quality, but contaminated fuel can still be loaded, particularly in remote ports with limited fuel oil supply options. If dirty fuel is detected, engineers isolate it by drawing from other tanks and offload it at a waste facility when possible. Alternatively, the fuel may be recirculated through purifiers to remove contaminants, though this is less effective for severely contaminated fuel.
• Biological Contamination: Biological contamination can occur when bacteria or fungi grow at the fuel-water interface in the fuel oil tanks, which produce sediment and clog filters. To prevent this, chemical biocides are added to tanks before bunkering, with dosages tailored to the fuel volume. These treatments inhibit microbial growth, ensuring fuel remains usable and system components are protected.

Components of Marine Fuel Oil Systems

• Transfer Pumps: These include small and large pumps that move fuel between tanks. The smaller pumps handle the low flow-rate demand, while larger pumps support higher flowrates during bunkering evolutions, internal transfers, or offloading of fuel.
• Strainers and Filters: Strainers are located at strategic points throughout the system to protect equipment by removing large debris and anything that may be in the pipe aside from the fluid being pumped. The Fuel Oil filters provide a finer filtering capacity and capture smaller particles. Both strainers and filters work in tandem to protect pumps, valves, engines, and other equipment from damage. Filters are often disposable or regularly cleaned or replaced to maintain flow capacity and a minimum pressure differential across the filter.
• Valves (MOVs): Motor Operated Valves are valves with a motor on them that can be controlled locally, manually overridden using the valve wheel, but are mainly operated using the control console in the Engine Control Room or elsewhere about the ship, as discussed earlier. They are used on many systems on the ship and are vital for any operation involving the movement of fluids.
• Fuel Oil Purifiers (FOPs): Fuel Oil Purifiers use centrifugal force to remove water, carbon, and other contaminants from Fuel and Lube oil systems. They are equipped with sensors to monitor for issues and alert Engineers to any parameters that are out of spec and the equipment may need attention.
• Flow Meters, Pressure Sensors & Gauges, & Temperature Sensors & Gauges: Flow meters track fuel transfer rates, providing a secondary indicator of fuel transfer operations, aside from the rise and fall of Fuel Oil tank levels. Pressure sensors provide indication of system pressure, while temperature sensors monitor fuel conditions, as viscosity and flow characteristics vary with temperature. These sensors are integrated into control systems, providing real-time data to operators. It is also important to note that, for every electronic sensor, there will also be a redundant gauge physically located on the piping for redundancy.

Consequences of Bad Fuel

• Blackout: Blacking out (or losing electricity on the ship) is one of the most serious problems that arises when dealing with bad fuel. Contaminants like water or sediment can cause the Diesel Generators to misfire, overheat, or shut down entirely, resulting in a complete loss of electrical power, known as a blackout. A blackout can disable important systems throughout the ship, which can create a dangerous situation especially if the ship is in heavy traffic or in heavy weather. Hopefully, if the emergency generator is working as expected, the ship will automatically transition over to emergency power until the bad fuel situation can be rectified by the Engineers.
• Losing Propulsion: When bad fuel reaches the Main Propulsion Diesel Engine(s) (MPDEs), it can certainly ruin the day by leading to a loss of propulsion. This is particularly critical during maneuvering in ports, channels, or heavy seas, where the inability to control the ship's movement can result in collisions, groundings, or other accidents. Contaminants such as water or sludge can clog fuel injectors or otherwise disrupt fuel flow to the engine, requiring immediate intervention by the Engineering crew.
• Destruction of Equipment: Bad fuel can cause significant mechanical damage to fuel system components and engines. Abrasive contaminants like dirt or carbon particles can erode fuel pumps, injectors, purifiers as well as other components leading to costly repairs or replacements. Water in the fuel can cause corrosion within the fuel system, weakening pipes, tanks, and fittings, which may result in leaks or even catastrophic failures over the life of the ship. These issues not only compromise system integrity but also increase maintenance costs and downtime, affecting the ship's operational schedule.
• Overconsumption of Disposable Filters and Strainers: Contaminated fuel accelerates the clogging of filters and strainers, which are designed to remove debris from the fuel. When bad fuel is used, these components require frequent cleaning or replacement, significantly increasing operational costs and labor demands. Clogged fuel filters can reduce fuel flow to the engine to the point where the engine trips off on low fuel pressure. Overall, bad fuel causes bad things to happen and extra hours to be put in by the engineering crew to fix these issues.

Maintaining a Fuel Oil Preventative and Corrective Maintenance

• Tank Inspections: Tanks are inspected regularly for structural integrity, focusing on welds, seams, and coatings. Inspections may involve ultrasonic testing to detect thinning or cracks, ensuring tanks remain leak-proof.
• Rust Management: Corrosion is a constant threat in the marine environment. Engineers apply anti-corrosion coatings and cathodic protection systems to mitigate rust. Suspected corrosion is addressed promptly to prevent fuel contamination or tank failure.
• Painting and Preservation: Tank interiors and exteriors are painted with specialized coatings to resist corrosion and chemical degradation. Preservation efforts include maintaining proper ventilation and humidity control to minimize rust formation.
• Component Maintenance: Pumps, purifiers, and valves require routine servicing, such as lubrication, seal replacement, and calibration. Strainers and filters are cleaned or replaced based on usage and differential pressure readings.
• Water Cutting on the Tanks: Inspection of the fluid on the bottom of the tanks via the 'kick valve' will help Engineer determine how much, if any, water is in the bottom of a given Fuel tank.

Fuel Testing and Monitoring

• Fuel Sampling: The engineer responsible for the fuel oil system collects samples during bunkering as well as daily operations. These samples are inspected for water content, sediment, and any other abnormalities that may be present.
• Kick Valves: These valves are used to take a cut off of the bottom of a given tank to inspect for water and assess fuel quality at the bottom of the tank. Regular draining the Service tanks allows for prevention of significant water build up in the tank and protection of the equipment and systems. The Engineer will also be able to tell if an abnormal amount of water is entering the fuel tank and will be prompted to troubleshoot the cause if this is happening.
• Daily Inspections: Engineers typically make daily rounds throughout the Engineroom in the morning and in the evening. During these rounds, the engineering personnel are taking care to make careful observations and take notes of anything out of the ordinary, or any issues that should be marked for further investigation or that need to be addressed.
• Fuel Treatments: Biocides and are added to prevent biological growth at the oil/water interface in each fuel oil storage tank. Dosing of these tanks typically happens before tanks are scheduled to be filled up during bunkering operations. If sludge is observed in the fuel and it is suspected that biological organisms are the cause, the tank may be additionally dosed with the biocidal agent.
• Monitoring Systems: Modern ships employ automated monitoring systems that track tank levels, monitor alarms, and record system performance in real-time. There are many different warning or alarm points in which engineers are notified if the alarms breach a specified threshold. One example of this would be if the fuel filter is clogged up, and the fuel oil pressure downstream of the filter drops below a certain PSI. In this case, there would typically be an alarm point and a shut down point. The alarm point will trigger an engine room alarm, while the shut down setpoint would trigger both the alarm and shut down the engine to protect it.

Fuel Oil Logs

• Bunkering Records – Onloading and offloading of fuel including amount of fuel, date of transfer.
• Transfer Logs: Records of internal fuel transfers, including volumes, source tanks, and destination tanks.
• Daily Reports: Daily Fuel Oil consumption reports (as well as other reports) must be completed by the Chief Engineer.

Different Types of Fuel Oils that Ships Use

• Diesel Fuel: Marine Gas Oil (MGO) is a high-quality, low-sulfur fuel used in smaller vessels or in Emission Control Areas (ECAs) where strict environmental regulations apply. It requires minimal purification but is more expensive than heavier fuels.
• Heavy Fuel Oil (HFO): HFO is a cost-effective, high-viscosity fuel used by large merchant ships. It requires extensive heating and purification to remove sulfur, water, and impurities before use. HFO is restricted in ECAs due to its high sulfur content.
• Liquified Natural Gas (LNG): LNG is an emerging fuel for ships seeking to reduce emissions. It is stored under pressure in specialized tanks and requires dedicated fuel systems, including vaporizers and safety systems to handle its volatility.
• Other Fuels – Some ships keep specific types of fuel on board for certain scenarios. One such example is maintaining a supply of Low-Sulphur Fuel Oil in a dedicated tank. This 'LSFO' is used in certain scenarios and geographic locations and can reduce certain emissions caused by having Sulphur in the fuel.

Interesting Note About Heavy Fuel Oil

Conclusion