Bunker fuel powers the vast majority of the world’s commercial ships. The term “bunker” originated from the compartments where coal was stored on steamships, but today it refers to any fuel used to propel marine vessels. This marine fuel is fundamentally different from the gasoline or diesel used in cars because it is derived from the heaviest, least refined fractions of crude oil. Powering the fleet that transports over 80% of global trade by volume, bunker fuel’s operational characteristics and environmental footprint have positioned it as a topic of international focus.
Composition and Characteristics
Bunker fuel is primarily a residual fuel oil, the heavy byproduct remaining after lighter petroleum products are extracted during crude oil refining. This residual nature gives it a complex chemical makeup that includes long-chain hydrocarbons, aromatics, and asphaltic compounds. Historically, this composition also meant a high concentration of contaminants like sulfur, nitrogen, and various metals.
One of the most defining physical characteristics of residual bunker fuel is its high viscosity. To be pumped, filtered, and properly combusted in a ship’s large, slow-speed marine engine, this fuel must be pre-heated, sometimes to temperatures exceeding 100°C (212°F). The fuel also possesses a high density, which contributes to its high energy content and cost-effectiveness for long-distance voyages. These specific properties make it suitable for the robust, specialized engines found on large container ships and tankers.
Grades and Classification
The quality and type of bunker fuel are standardized globally under the ISO 8217 specification, which divides marine fuels into two main categories: Residual Fuel Oil (RFO) and Marine Distillate Fuel (MDF). Residual fuels are the heavier grades, such as Heavy Fuel Oil (HFO), which are categorized by their kinematic viscosity at 50°C. The most common residual grades, like RMG 380, are thick, requiring pre-treatment before combustion.
Distillate fuels are cleaner, lighter products more similar to standard diesel, like Marine Gas Oil (MGO). These grades are categorized by parameters such as cetane index and are used in auxiliary engines, smaller vessels, or when operating in environmentally sensitive areas. The ISO 8217 framework ensures that buyers and sellers worldwide have a common language for fuel quality, specifying limits for contaminants like water, sediment, and ash. Recent updates to the standard, such as the 2024 edition, have expanded the categorization to include bio-distillate and bio-residual blends, reflecting the industry’s shift toward cleaner alternatives.
The Bunkering Process
The process of supplying fuel to a vessel is known as bunkering. This transfer can occur in several ways, including ship-to-ship (STS) via a specialized bunker barge, shore-to-ship via a terminal pipeline, or truck-to-ship for smaller volumes. The process begins with extensive safety checks, including pre-transfer checklists and establishing clear communication between the delivery vessel and the receiving ship.
During the actual fuel transfer, flow rates are initially kept low to check connections and ensure the oil is flowing into the correct storage tanks. Continuous monitoring is necessary to prevent spills, overfilling, or pressure anomalies, as a leak poses a severe environmental hazard. To verify the commercial transaction, fuel quantity is tracked using tank soundings or, increasingly, through sealed Mass Flow Meters (MFMs). Fuel samples are also drawn and sealed during the transfer to ensure the quality of the delivered product matches the specifications on the Bunker Delivery Note.
Environmental Regulation and Compliance
The high sulfur content in residual bunker fuel led to significant emissions of sulfur oxides (SOx) and nitrogen oxides (NOx) when burned, contributing to acid rain and air pollution. In response, the International Maritime Organization (IMO) introduced the IMO 2020 mandate, which reduced the global sulfur limit for marine fuel from 3.5% to 0.5% mass by mass (m/m). This regulation applies to all ships operating outside of designated Emission Control Areas (ECAs), where the limit is even stricter at 0.1%.
Ship operators have adopted several strategies to achieve compliance with these new international standards. The most direct method is switching to compliant fuels, such as Very Low Sulfur Fuel Oil (VLSFO) or Marine Gas Oil (MGO). Alternatively, vessels can continue to use high-sulfur fuel oil if they install an exhaust gas cleaning system, commonly known as a scrubber. These systems chemically remove SOx from the engine exhaust before it is released into the atmosphere, allowing the vessel to meet the required emission threshold.