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At first glance, refuelling a vessel may appear to be no more than delivering the required bunker fuel on board. In reality, the operation involves many interdependent factors, from the vessel schedule and fuel specification to meteorological conditions, transfer pressure, sample management and environmental safety.
Particularly when it is not operationally practical for a vessel to berth at a terminal solely to take fuel, or when refuelling is planned at an anchorage, transferring fuel directly from a marine tanker to the receiving vessel provides an important solution.
In the maritime industry, this operational model is known as Ship-to-Ship (STS) bunkering. It is based on the bunker tanker safely coming alongside the receiving vessel and transferring fuel through a controlled transfer system.
However, STS bunkering is not simply a matter of bringing two vessels alongside and starting a pump. A safe operation is planned well before transfer begins and continues until the final delivery documentation has been completed.
What Is Ship-to-Ship Bunkering? Ship-to-Ship bunkering is the direct transfer, from a supply vessel to a receiving vessel, of bunker fuel required for consumption in the receiving vessel’s own engines and boilers.
Depending on the operational setup, the vessel carrying and delivering the fuel may be referred to as a bunker tanker, bunker vessel or, in widely used industry terminology, a bunker barge.
There is a small but important distinction in this terminology. Although the expression “bunker barge” is widely used in the industry, not every marine craft engaged in bunkering is technically a barge.
A self-propelled vessel classed as a tanker may also be used for bunker operations. Technical documents should therefore reflect the vessel’s actual type and class characteristics.
STS bunker deliveries are most commonly carried out at anchorages, port approach areas or in waters approved for the operation by the relevant coastal or port authority.
Accordingly, the term “offshore refuelling” does not mean that fuel transfer can be performed in any sea area. The operating location must be suitable in terms of navigational safety, environmental conditions, vessel traffic and local regulations.
The Difference Between STS Bunkering and STS Cargo Transfer Because the term Ship-to-Ship can be used for different types of maritime operations, it can sometimes cause terminological confusion.
An STS cargo transfer operation, in which commercial petroleum cargo is transferred between two oil tankers, is not the same operation as the ship-to-ship delivery of bunker fuel for a vessel’s own consumption.
This distinction is also clearly reflected in the regulatory framework.
MARPOL Annex I, Chapter 8 regulates STS provisions for the transfer of oil cargo between oil tankers at sea. Regulation 40, however, excludes bunkering operations from the scope of that chapter.
It is therefore not correct to assess a bunker delivery under the same regulatory framework as an STS cargo operation conducted under MARPOL Annex I, Chapter 8.
This does not, of course, mean that bunker operations are unregulated. Depending on the nature of the operation, requirements and procedures may include MARPOL Annex VI provisions on fuel quality and delivery records, SOLAS requirements relating to fuel safety and fire safety, vessel Safety Management System procedures, flag-state requirements, port- or coastal-state rules and applicable local operating procedures.
Current STS guidance published by OCIMF, CDI, ICS and SIGTTO is primarily intended for ship-to-ship transfers of petroleum, chemical and liquefied-gas cargoes. Although it does not create the same direct regulatory framework for bunkering, it remains an important technical reference for understanding industry good practice in areas such as risk assessment, human factors, communications, manoeuvring alongside and transfer safety. The current second edition of the guide was published in 2025.
How Is an STS Bunker Operation Planned? One of the most critical stages of a safe bunker operation is the planning completed before any fuel hose is connected.
The first step is to match the receiving vessel’s fuel requirement with the product carried by the supply tanker and with the tanker’s technical capabilities.
The required fuel type and quantity, the vessel’s ETA, the operating location, the position and connection dimensions of the receiving vessel’s bunker manifold, estimated draught and freeboard, the tanks to be filled and the acceptable transfer parameters are evaluated in advance.
The fuel specification must also be defined clearly at the ordering stage. A description such as “marine gas oil” or “fuel oil” does not always provide a sufficiently precise technical definition. The applicable fuel standard and relevant product characteristics should be stated explicitly in the order terms.
At the operating location, wind, waves, current, visibility and marine traffic are assessed. These parameters directly affect whether the bunker tanker and receiving vessel can safely remain alongside one another.
Safe Approach and Mooring Are Fundamental to the Operation The bunker tanker’s approach to the receiving vessel is one of the most sensitive manoeuvring phases of an STS delivery.
A fendering arrangement suitable for the operation is used to prevent direct hull-to-hull contact and to absorb loads created by relative vessel movement. Fender capacity and positioning should be compatible with vessel size, side structure and the environmental conditions expected during the operation.
After the approach, the bunker tanker is secured to the receiving vessel using an appropriate mooring arrangement.
Head and stern lines, spring lines and other mooring components do more than simply keep the vessels connected. They must control movement caused by wind, current and waves and maintain the relative position of the two vessels throughout the operation.
The mooring system therefore continues to be monitored after transfer begins. If weather or sea conditions move outside safe operating limits, it may be necessary to reduce the bunker transfer rate, suspend the operation temporarily or terminate it altogether.
What Checks Are Completed Before Fuel Transfer? Before bunker transfer starts, the vessel and bunker tanker must mutually verify the operating procedures. The communications channels, initial transfer rate, planned maximum transfer rate and pressure, tanks to be used, tank-change procedures and the method for stopping transfer in an emergency are clarified between the parties.
Emergency communications methods and the emergency-stop procedure must be confirmed in a form understood by both parties.
Preparations are also made for the risk of environmental pollution. Depending on the operating procedure and vessel arrangement, drip-tray areas beneath the bunker manifold are checked, deck drains are managed so as to prevent fuel from reaching the sea, and the equipment required to respond to a potential spill is kept readily accessible.
It is also important to control activities unrelated to fuel transfer that could create an ignition risk during the operation.
The objective of these preparations is not merely to respond after an incident occurs, but to reduce the probability of an incident as far as reasonably possible.
Why the Bunker Manifold and Transfer Hose Matter Once the vessels have been safely positioned, the bunker tanker’s fuel hose is connected to the receiving vessel’s bunker manifold.
Before connection, the hose is checked for physical condition, compatibility with the product and suitability for the operation. Flanges, gaskets, connection components and the manifold arrangement are also inspected.
The hose must not be exposed to excessive tension or improper bending as the vessels move. Where necessary, support arrangements are used to control the hose’s weight and movement.
At the same time, the receiving vessel’s valve line-up is verified. It must be clear which tank or group of tanks will receive the fuel, and sufficient available capacity must be confirmed before transfer starts.
The governing principle is straightforward: the bunker tanker’s pumping capacity does not determine the receiving vessel’s acceptance capacity.
Transfer rate and manifold pressure must be set with due regard to the receiving vessel’s piping system, tank-venting capacity and operational limits.
Why Is Fuel Transfer Started in a Controlled Manner? At the beginning of a bunker operation, transfer is generally started at a low, controlled rate.
The purpose is to confirm that the system has been configured correctly.
Once it has been verified that the fuel is entering the intended tank, there are no leaks at hose or flange connections, manifold pressure is normal and the tank system is functioning as expected, the transfer rate may be increased to the operating level agreed by both parties.
Throughout the transfer, the manifold, hose connections, tank levels and quantity transferred are monitored.
As the receiving tank approaches its planned filling level, the operation moves into the topping-off phase. Reducing the transfer rate in a controlled manner at this stage allows tank level to be monitored more precisely.
Overfilling a tank is one of the most serious environmental risks in bunkering. Fuel escaping through the tank vent system or other openings onto the deck and subsequently into the sea can result in a significant pollution incident as well as operational and legal consequences.
The final part of the operation must therefore be managed just as carefully as the start of the transfer.
Key Risks in STS Bunkering Operations Ship-to-ship fuel transfer requires multiple risks to be managed within the same operation.
Key risks include contact between the vessels during approach, excessive loading of mooring lines, damage to the bunker hose, leakage at flange connections, transfer pressure rising to an unsuitable level and overfilling of the receiving tank.
Changing weather conditions during the operation are another factor that must be taken into account.
In addition, controlling fire and ignition sources is critical when petroleum-based bunker fuels are being transferred.
Technical equipment alone, however, does not make a bunker operation safe.
A misunderstood instruction, a stop command issued too late or a breakdown in communication between the receiving and supplying parties can create serious risk even where all equipment is functioning correctly.
For this reason, contemporary maritime safety practice treats human factors - including human performance and decision-making - as an important part of operational risk management. The fact that the 2025 edition of the STS Transfer Guide includes a dedicated section on human factors illustrates the growing importance of this approach across the industry.
Why Is ISO 8217 Important for Bunker Fuel? It is as important for the product delivered to meet the required technical specification as it is for the fuel to be transferred safely on board.
One of the principal international technical references for marine fuels is ISO 8217.
The current version, ISO 8217:2024, defines general requirements and specifications for fuels used in marine diesel engines and boilers. In addition to petroleum-derived fuels, the current edition also covers certain synthetic and renewable hydrocarbons, fuels containing FAME where permitted, and certain blends of these products.
A bunker order should therefore specify not only the commercial product name but also the technical specification agreed between the parties.
A significant proportion of fuel-quality disputes can arise because the product specification was not defined clearly enough before delivery or because the representative sample was not managed correctly.
How Is a MARPOL Fuel Sample Taken? For relevant fuel deliveries subject to MARPOL Annex VI, a representative fuel sample is an important regulatory element alongside the Bunker Delivery Note.
IMO’s current MSC-MEPC.2/Circ.18 sampling guidelines provide that the primary sample used for the MARPOL delivered sample should be taken at the receiving vessel’s bunker manifold throughout the bunkering period. The sample should be composed so as to represent the average characteristics of the total fuel delivered.
Once the operation has been completed, the sample must be sealed appropriately and identified with the required information.
This sample is not merely an administrative formality. It provides an important reference where later verification is required that the fuel complied with MARPOL Annex VI and, where applicable, SOLAS requirements.
Bunker Delivery Note and Traceability The physical completion of a bunkering operation does not mean that the delivery process is complete.
One of the operation’s core documents is the Bunker Delivery Note, or BDN.
For fuel oil supplied to relevant vessels under MARPOL Annex VI Regulation 18, the BDN is one of the primary records of fuel delivery. It must contain the information specified in MARPOL Annex VI Appendix V.
On applicable vessels, the BDN must be retained for at least three years after fuel delivery.
The MARPOL representative sample for the fuel delivered must remain under the vessel’s control until the fuel is substantially consumed and, in all cases, for at least 12 months from the date of delivery.
This chain of records forms the basis of traceability in bunker operations.
Correctly linking the source, quantity, characteristics and sample of the delivered product is important both for operational quality and for regulatory compliance.
How Do the 2026 SOLAS Amendments Affect the Bunkering Process? The effect of bunker fuel on vessel safety is being addressed increasingly strictly alongside fuel quality.
Amendments to SOLAS Chapter II-2 that entered into force on 1 January 2026 introduced new provisions intended to ensure that fuel oil supplied to ships complies with SOLAS flashpoint requirements.
Before bunkering, the vessel must be provided with a declaration signed and certified by the fuel supplier’s representative. The declaration confirms that the fuel supplied complies with SOLAS regulation II-2/4.2.1 and identifies the test method used to determine the flashpoint. The principal objective is to prevent fuel oil that does not meet SOLAS’s general 60°C flashpoint safety requirement from being supplied to ships.
This development shows that documentation in modern bunker operations is becoming increasingly important not only for quantity and commercial delivery, but also for vessel safety.
What Are the Advantages of Bunker Delivery by Marine Tanker? One of the main advantages of bunker delivery by marine tanker is operational flexibility.
For a commercial vessel, making a significant deviation from its existing route or berthing at an unsuitable terminal solely to take fuel can create both time and operating costs.
Where the necessary permits are in place and operating conditions are suitable, bringing the bunker tanker to the vessel can help integrate refuelling more efficiently into the vessel’s existing voyage plan.
Marine tankers can also offer different logistical advantages from road-tanker operations, particularly for high-volume deliveries.
In addition, tank segregation, pumping capacity, manifold arrangement and transfer equipment on board the tanker can enable it to serve different operational profiles.
Where different products may be carried on the same tanker, however, segregation and contamination control become even more important.
It is not sufficient for the product name alone to be correct without considering previous cargo, tank condition, transfer lines and valve configuration. The fuel must retain its required characteristics throughout the entire logistics chain until it reaches the receiving vessel.
Which Matters More in a Bunker Operation: Speed or Control? Time is a critical operating parameter for commercial vessels. In bunkering, however, efficiency cannot be measured solely by how quickly the transfer is completed.
If an excessively high transfer rate exceeds the receiving vessel’s operating limits, the risk created can far outweigh the few minutes saved.
True operational efficiency means delivering the correct fuel, in the correct quantity, to the correct vessel, within the planned time window and within defined safety limits.
The success of a well-managed bunker operation therefore depends on three elements being achieved together: ● The right product, ● the right planning ● and a controlled transfer.
In addition to having the fuel available on time, it is important to coordinate the tanker schedule with the receiving vessel’s ETA, prepare documentation in advance, maintain uninterrupted vessel-to-vessel communication and monitor environmental conditions throughout the transfer.
Marine Refuelling Solutions with Asmira Marine Asmira Marine provides vessel and yacht refuelling services backed by bunker-sector operating experience developed within Asmira Group since 1977. Its current service network includes physical supply points extending from Gemlik to Alanya, while its fleet of specially equipped road and marine tankers provides 24/7 supply solutions aligned with vessel operating schedules.
Asmira Marine’s marine delivery capability was further strengthened in 2025 with the addition of Kumbor, a new-generation double-hull marine tanker, to the fleet. In the company’s current corporate materials, Kumbor is positioned as one of the key fleet investments enhancing its operating capacity for marine tanker deliveries.
In bunker operations, it is not enough simply to transport the fuel physically to the vessel; the delivery chain must also be controllable and transparent. According to Asmira’s current bunker documentation, fuel deliveries carried out by marine tanker are supported by independent supervision mechanisms.
A strong vessel-refuelling operation is not created merely by having a sufficient quantity of fuel available. The fuel must be sourced from the right point, transported by a suitable tanker, coordinated with the receiving vessel’s voyage schedule and delivered through a safe transfer process.
This is where the real value of Ship-to-Ship bunkering becomes clear: rather than changing the vessel’s operation to suit fuel supply, well-planned fuel supply is adapted to the vessel’s operation.