The Salty Mariner

An Introduction to Ship Fuel Systems

Ship fuel systems circulate the lifeblood of merchant ships, the ship's Fuel supply. These systems deliver the foundational source of energy that powers the propulsion and auxiliary systems, which are critical to keeping the lights on and getting the ship from Point A to Point B. Whether a ship's propulsion system is a Diesel Engine, a Gas Turbine, or Steam plant, the Fuel Oil (FO) systems are specifically designed to ensure reliability, safety, and efficiency across a range of loads for that ship. This article explores the layout, operations, maintenance, and challenges of shipboard marine fuel systems and aims to provide a comprehensive introduction for people who haven't had the awesome experience living a life at sea.

Describing the Layout of the Fuel Oil Storage System

A Ship's propulsion relies on the Fuel Oil (FO) system(s) as the primary source of energy for propulsion and electrical power generation. These systems follow the same patterns across the industry and adhere to some general principles, a few of which are explained in this article.

Diagram of a ship's fuel oil storage system

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

First, we will talk a bit about how the Fuel Oil storage tanks are located throughout the ship. Fuel tanks are strategically distributed from the Bow (front of the ship) to the Stern (back of the ship) to maintain balance and stability. For example, a typical merchant vessel might have 12 large fuel tanks, with six on the Port side and six on the Starboard side. These tanks are labeled sequentially, such as "1P" and "1S" for the forward-most tanks, progressing to "6P" and "6S" for the aft-most pair, following a numbering convention that increments from forward to aft.

The selection of fuel tanks to draw from depends on several factors, including fuel tank levels, tank location, fuel age, fuel quality…etc. Engineers typically prioritize consuming the oldest fuel first, provided it is uncontaminated and suitable for use. Other factors involved in selecting which tanks to use include the quality of the fuel in each tank. For example, if it has been determined that a certain tank (or tanks) has bad fuel, obviously that tank will not be used. Tank selection also plays a role in maintaining or correcting the ship's list (side-to-side tilt) or trim (fore-and-aft tilt). Uneven fuel consumption can shift the ship's center of gravity, causing it to lean to one side or the other. For instance, if forward tanks are depleted faster than aft tanks, the ship may become lighter in the bow, which will affect the trim and stability of the ship. Engineers carefully monitor tank usage to counteract these effects, sometimes transferring fuel or water internally, to different tanks, in order to balance the vessel.

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

The Fuel Oil system is a critical component of a ship's engineering infrastructure. The FO Systems, which consist of the Fuel Oil Storage and Fuel Oil Service systems are managed by a hierarchy within the Engineering department. The Chief Engineer is ultimately responsible for all engineering systems, including the Fuel Oil system, but may not be too involved in the direct day-to-day operation of the system. The Chief Engineer works closely with the Ship's Captain to ensure that all systems operate smoothly and that the engineering team has the resources, tools, and support needed to keep the lights on and make sure the ship can execute its assigned task. This includes overseeing maintenance schedules & shipyard periods, coordinating repairs, and ensuring compliance with safety and regulatory standards.

The day-to-day operation and maintenance of engineering systems, including the Fuel Oil system, fall under the purview of the 1st Assistant Engineer (1 A/E). The 1 A/E supervises the Engineering department, assigns tasks, assists with operations, and monitors system performance to mitigate potential issues before they occur. The 1 A/E's role is pivotal in ensuring that all systems, from propulsion to auxiliary equipment, are maintained in good condition and are operational to the fullest extent possible. One item to note regarding this is that ALL Licensed Engineers are responsible for and will help monitor all engineering systems throughout the ship. Even though Engineers typically have been assigned certain systems, all Engineers will keep an eye out on all systems to make sure there is nothing out of the ordinary, and will help each other out with maintenance tasks where possible.

Within the Engineering team, a specific Engineer is typically assigned a primary responsibility of taking care of the Fuel Oil system, meaning they are directly responsible for its maintenance and operation. This includes overseeing bunkering operations, internal fuel transfers, purifier operations and maintenance, Fuel Oil service system operation and maintenance, and tank level monitoring, to name a few. Given the critical nature of the Fuel Oil system, this responsibility is usually assigned to a Second Assistant Engineer (2 A/E) that will typically have the experience and technical expertise required to maintain and operate these systems. With that being said, some vessels will have a seasoned Third Assistant Engineer (3 A/E) take on this role, particularly if they have demonstrated proficiency and reliability. The designated Engineer ensures that all requirements for the Fuel Oil system such as keeping the Service tanks full, operating the Fuel Oil purifiers, pumps…etc. are met.

How Do Ships Refuel?

Refueling, commonly referred to as "bunkering," is a critical operation performed periodically based on fuel oil tank levels and operational schedules. It requires close coordination between the Engineering and Deck departments to ensure safety and efficiency. The next bit of information will go into the bunkering process and provide a high-level overview of how bunkering evolutions go down onboard a typical ship.

Pre-Bunkering

Bunkering operations require detailed planning to keep everybody safe, prevent Fuel Oil spills, and maintain the ship's stability. The Chief Mate (1st Officer) develops a bunkering plan by calculating the volume of fuel to be loaded into each tank and the sequence of filling to prevent instability or an undesired list. These calculations consider the ship's current load, draft, and trim, as well as the weight and distribution of the incoming fuel. A poorly executed plan could cause the ship to tilt dangerously or put undue stress on the ship's structure.

A required Pre-Bunkering meeting is convened prior to the bunkering operations in which all personnel that will be involved in the evolution are required to attent. This meeting typically includes Deck Officers, Engineers, and other crew members. The Chief Mate thoroughly discusses the bunkering plan, which includes safety & emergency protocols, the amount of fuel to be received and which tanks that fuel will be going to and in what order. Each participant is assigned a specific role, such as monitoring tank levels, operating valves, checking for leaks, or manually checking tank levels. The meeting ensures that all questions are addressed, potential hazards are identified, and everyone understands their responsibilities before the operation begins, as well as ensures that everybody knows what Radio channel to communicate on and that anybody has the authority to STOP the pumping of the Fuel should any potential issues arise during the operations.

Bunkering Operations

Bunkering commences by connecting an external fuel supply hose—typically from a barge or shoreside facility—to the ship's Fuel Oil transfer header. This header is a large pipe with smaller pipes branching off to individual fuel tanks, each controlled by Motor Operated Valves (MOVs). These valves are electronically controlled from centralized control stations on the Bridge, Engine Control Room, or other strategic locations about the ship, with manual valve wheels available as backups in case of electronic or motor failure. The centralized control stations provide real-time data on tank levels, list, trim, and flow rates, enabling the Person in Charge to oversee the operation effectively.

Once the Chief Mate gives the go ahead to start pumping, fuel transfer begins at a low flow rate to verify flow and ensure correct valve alignment to make sure the fuel is being directed to the intended tanks. While pumping, crew members monitor the system and communicate via radio, confirming rising tank levels and checking for issues like leaks or anything out of the ordinary. Once the setup is confirmed, the ship instructs shoreside personnel to increase the flow rate, accelerating the pumping process. As tanks approach their target fill levels, valves for subsequent tanks are opened, and the filling sequence continues per the bunkering plan. The operation concludes when the planned fuel volume is loaded, or the supply is exhausted.

Throughout bunkering, the crew vigilantly monitors for leaks, inspects sounding tubes, and uses sounding tapes to manually measure fuel levels. These measurements are reported to the Control Room and cross-referenced with the electronic Tank Level Indicators (TLIs) to ensure accuracy. Upon completion, all valves are closed, the supply hose is disconnected, and the system is secured, marking the end of the bunkering evolution.

Fuel Oil Service System

With bunkering complete, the fuel resides in the FO storage tanks but is not yet ready for engine consumption. It must first be transferred to the FO service tanks via an internal fuel transfer process.

Internal Fuel Oil Transfers

Internal fuel transfers move fuel from storage tanks to other storage tanks, or to the fuel oil service system tanks, which directly supply the ship's engines with fuel. For a diesel-powered vessel underway, fuel is consumed by the Main Propulsion Diesel Engine(s) (MPDEs) and Diesel Generators (DGs). For example, a ship consuming 15,000 gallons of diesel daily will require the Engineers to perform transfers once or twice daily to maintain service tank levels.

Before reaching the service tanks, fuel passes through Fuel Oil Purifiers (FOPs), which operate at high rotational speeds to separate contaminants like dirt, water, and sludge using centrifugal force. The purified fuel is discharged into the service tanks, ensuring a clean supply for approximately 12 hours of operation. This purification step is critical to prevent engine damage and maintain efficiency.

Getting Fuel to the Engines

Fuel Oil supply pumps draw fuel from the service tanks, pressurizing the FO service system to meet the demands of the MPDEs and DGs, respectively. The system is designed with excess flow capacity to ensure consistent pressure, even under varying engine loads. A network of sensors, flow meters, valves, strainers, and filters clean the fuel, ensuring it is at the rated pressure and is as clean as possible before reaching the engines. Unconsumed fuel recirculates back to the service tank.

Common Fuel Oil Issues Onboard Ships

Fuel quality issues, such as water contamination, dirty fuel, and biological growth, are common challenges that require proactive management to protect engine performance and ship safety.

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

Marine fuel systems are comprised of several common components, each designed to ensure reliable fuel delivery and quality:

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.

For most of the major components listed above such as the Pumps, Purifiers…etc., there are multiple of each component. These components are rotated on a routine basis to ensure that the running hours are close to even on both. Depending on the ship's op tempo, maintenance is usually performed on the offline equipment when it is appropriate.

Consequences of Bad Fuel

The use of contaminated or poor-quality fuel can have severe consequences for a ship's operations, safety, and equipment longevity. Bad fuel, whether contaminated with water, dirt, or biological organisms, can disrupt critical systems and lead to blackouts and/or equipment damage. Below are the primary consequences of using 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

Preventative and corrective maintenance are both essential to keep machinery and systems running in tiptop shape. With regards to the Fuel Oil system, the below items are a few of the tasks that must be done in order to stay on top of maintenance for the fuel system:

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.

Corrective maintenance addresses issues like abnormal trends, strange noises, failures of online equipment, valve malfunctions…etc. Engineers will try to maintain detailed maintenance logs to track service intervals and identify recurring problems with equipment.

Fuel Testing and Monitoring

Fuel Oil testing and monitoring protocols ensure fuel quality and system reliability:

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

The maritime industry (in the United States at least) is extremely regulated. There are numerous international maritime authorities as well as authorities within the United States that demand meticulous record keeping for the shipping industry. Below is some of the information that must be recorded and maintained by a ship:

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.

Logs are maintained in both paper and digital formats, with entries verified by the Chief Engineer or designated officers. Many of the logs are kept in the Oil Record Book (ORB), which is a legal ledger that is used to keep track of the level of different fluids on the ship such as Oily Waste, Waste Oil, Fuel, and Lube Oil. The Ship's Engineering and Deck Officers can and have been held legally liable if inconsistencies are found in the Oil Record Book. The entries in the ORB are critical for regulatory audits, such as those conducted by port state control or classification societies. Log keeping also supports fuel efficiency initiatives, as engineers analyze consumption patterns to identify optimization opportunities.

Different Types of Fuel Oils that Ships Use

Ships use a variety of fuels, each with unique properties, handling requirements, and regulatory considerations:

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.

Environmental regulations do have an impact on the type of fuel a ship uses. For example, in parts of the United State, ships are required to use Low-Sulfur Fuel Oil (LSFO) to reduce emissions resulting from fuels containing Sulfur. Another example is that ships are not allowed to use Heavy Fuel Oil (HFO) when within certain distances of land and are forced to change over to a lighter fuel such as Diesel Oil before arrival. Due to this concept, many vessels carry multiple fuel types, switching between them based on operational region, distance from shore, and regulatory requirements. Transitioning from one type of fuel to another involves a meticulous process that must be adhered to in order to prevent any interruptions in the ship's operations such as loss of propulsion or power.

Interesting Note About Heavy Fuel Oil

Heavy Fuel Oil (HFO) is used because it is one of the most energy dense types of fuels, or the highest BTUs per pound (BTU/lb). However, HFO is NOT a liquid at ambient temperature, and heat must be applied to it and the temperature maintained above a certain threshold so that the viscosity can be lowered enough to allow the fuel to be pumped. In other words, it needs to be kept hot or else it will ruin all the systems and machinery that hold it.

Conclusion

In this article, we briefly discussed how a Ship's Fuel Oil system is laid out, and how it is maintained on a day-to-day basis. With that being said, this is only the tip of the iceberg with respect to talking about Fuel Oil systems onboard ships. Books could be written (and ARE written) on this topic alone. This information described a typical fuel system on a ship with a Diesel engine propulsion system. The other main types of propulsion systems, Gas Turbines and Steam plants, are similar in overall design but do vary a bit due to the differences between the types of engineering plants. We will look forward to writing articles about those types of systems as well. If you enjoyed this article, please send us a message and let us know!