The term “Bunkers” refers to either the fuel oil used by ocean going ships or the tanks they are stored in. It is derived from an historic reference to coal bunkers used in the days of steam and sail. The great majority of the bunker fuels used by the world’s merchant fleet are residual fuel oil. This is also true for the vast majority of large diesel engines operated on land. By definition, residual fuel oils are the products remaining from the refinery processes after all the distillate or lighter fractions have been removed.

Bunker fuel is delivered by pipeline, boat or barge. One common theme is that the world of commercial marine fuel supply is totally separate from the world of recreational marine fuels. Shipping has a higher volume and low-cost incentive to arrange deals with refiners to produce tailored marine fuels that are most cost effective for their engines. They use fuels that involve less refining (leaving the sulphur in the fuel), and therefore lower cost than other fuels.

For ocean going ships and to a large extent, land based users also, the type of fuel depends on three things:

  • Engine design and fuel system used
  • Cost of the fuel
  • Availability in the bunker port
  • Sulphur content (where restricted for operation in Emissions Control areas)

Bunker Fuel Terminology

There are two basic types of marine fuels – distillate and residual. A third type is a mixture of these two, commonly called “intermediate”. Distillate fuel is composed of petroleum fractions of crude oil that are separated in a refinery by a boiling or “distillation” process. Residual fuel or “residuum” is the fraction that did not boil, sometimes referred to as “tar” or “petroleum pitch”. Fuel for marine use in engines and boilers has the following types and grades:

Fuel Type & Fuel Grades Common Industry Name

  • Distillate DMX, DMA, DMB, DMZ Gas Oil or Marine Gas Oil
  • Intermediate IFO 180,IFO 380, Marine Diesel Fuel or Fuel Oil
  • Residual RMA-RML Fuel Oil or Residual Fuel Oil

Bunker Fuel Oil Specifications (ISO 8217)

The grades are summarised in ISO 8217 (2010) and the usage generally restricted within the requirements of IMO Marpol Annex VI, Regulation 14.

Marine Distillate Fuels
Test
Unit
Test method
Limits
Grade
ASTM
IP
ISO
DMX
DMA
DMZ
DMB
Viscosity at 40°C mm²/s (cSt) D445 71 3104 max. 5.5 6 6 11
min. 1.4 2 3 2
Density at 15°C kg/m³ D1298 160 3675 or 12185 max. 890 890 900
Cetane index 4737 380 4264 min. 45 40 40 35
Sulfur mass % D4294 336 8754, 14596 max. 1 1.5 1.5 2
Flash point °C D93 34 2719 min. 43 60 60 60
Hydrogen sulfide mg/kg 570 max. 2 2 2 2
Acid number mg KOH/g D664 max. 0.5 0.5 0.5 0.5
Total sediment hot filtration mass % 375 10307-1 max. 0.1
Oxidation stability g/m3 388 12205 max. 25 25 25 25†
Carbon residue, micro mass % D4530 398 10370 max. 0.3 0.3 0.3 0.3
Cloud point °C 219 3015 max. -16
Pour point
Winter quality °C D97 15 3016 max. -6 -6 0
Summer quality °C D97 15 3016 max. 0 0 6
Appearance Clear and bright
Water volume % D95 74 3733 max. 0.3
Ash mass % D482 4 6245 max. 0.01 0.01 0.01 0.01
Lubricity* µm 12156-1 max. 520 520 520 520†
* This requirement is applicable to fuels with a sulfur content below 0.05%.
† If the sample is not clear and bright, the test cannot be undertaken and the limit shall not apply
Residual Marine Fuels
Test
Unit
Test method
Limits
Grade
ASTM
IP
ISO
 
RMB30
RMD80
RME180
RMG180
RMG380
RMG500
RMG700
RMK380
RMK500
RMK700
Viscosity at 50°C mm²/s (cSt) D445 71 3104 max. 10 30 80 180 180 380 500 700 380 500 700
Density at 15°C kg/m³ D1298 160 3675 or 12185 max. 920 960 975 991 991 991 991 991 1010 1010 1010
CCAI -Calculated- max. 850 860 860 860 870 870 870 870 870 870 870
Sulfur mass % D4294 336 8754, 14596 max. -Statutory Requirements-
Flash point °C D93 34 2719 min. 60 60 60 60 60 60 60 60 60 60 60
Hydrogen sulfide mg/kg 570 max. 2 2 2 2 2 2 2 2 2 2 2
Acid number mg KOH/g D664 max. 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
Total sediment aged mass % 390 10307-2 max. 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Carbon residue, micro mass % D4530 398 10370 max. 2.5 10 14 15 18 18 18 18 20 20 20
Pour point    
Winter quality °C D97 15 3016 max. 0 0 30 30 30 30 30 30 30 30 30
Summer quality °C D97 15 3016 max. 6 6 30 30 30 30 30 30 30 30 30
Water volume % D95 74 3733 max. 0.3 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Ash mass % D482 4 6245 max. 0.04 0.07 0.07 0.07 0.1 0.1 0.1 0.1 0.15 0.15 0.15
Vanadium mg/kg 501, 470 14597 max. 50 150 150 150 350 350 350 350 450 450 450
Sodium mg/kg 501, 470 max. 50 100 100 50 100 100 100 100 100 100 100
Aluminium + silicon mg/kg D5184 501, 470 10478 max. 25 40 40 50 60 60 60 60 60 60 60
Used lubricating oil           The fuel shall be free of ULO.
Calcium + Zinc mg/kg 501 or 470 A fuel shall be considered to contain ULO when either one of the following conditions is met:
Calcium + Phosphorus mg/kg 500 Calcium > 30 and zinc > 15 or calcium > 30 and phosphorus > 15
To communicate effectively in a speciality field like “marine-fuels” it is necessary to be clear on the bunker fuel terminology used in this industry. Distillate fuels are commonly called “Gas Oil” or Marine Gas Oil; residual fuels are called Marine Fuel Oil or Residual Fuel Oil; and intermediate types are called “Marine Diesel Fuel”, or Intermediate Fuel Oil (IFO). While the term “diesel fuel” for land based automobile and truck use is 100% distillate, in the marine industry Marine Diesel Fuel is the blend of distillate and residual oils (intermediate types). The 100% distillate type fuel in the marine industry is the Marine Gas Oil (implying that it was boiled into a gas, then condensed into a liquid).

One common theme is that the world of commercial marine fuel supply is totally separate from the world of recreational marine fuels. Diesel fuel for marine applications depends on three things: engine used, cost, and availability. Commercial marine businesses have the higher volume and low-cost incentives to arrange deals with refiners to produce tailored marine fuels that are most cost effective for their engines. Commercial marine businesses arrange for fuels that involve less refining (leaving the sulphur in the fuel), and therefore lower cost than other diesel fuels, delivered by pipeline, boat or barge.

The product that is supplied as DMA is sometimes specially manufactured at local refineries with a higher sulphur content and lower price than land-based fuels rebranded for marine use. When that supply is short of demand, “Home-Heating Oil” (No 2 fuel oil) is used, provided the flash is above 60ª C, which it normally is. Similarly, DMC is sometimes supplied by local refineries from “cycle oil” (lower boiling point than No 2 fuels), and is sometimes blended in the supply terminal from DMA and residual fuel oils. DMA is the common fuel for tugboats, fishing boats, crew boats, drilling rigs and ferry boats. Ocean-going ships that take residual fuel oil bunkers also take distillate fuels for use in auxiliary engines and sometimes for use in port. The common fuels are DMC, IFO-180 and IFO-380, depending on the specific engines in service. DMB is infrequently specified, and is not available in all ports. Where it is not available, DMA is supplied, sometimes in a barge that has transported DMC or IFO (hence, a “dirty” cargo hold that would contaminate DMA).

In contrast to commercial marine businesses, recreational boat owners have no volume leverage, and less cost-reduction incentives to arrange for special fuels. With the large overlap in boiling range with other distillate fuels, the sources of recreational marine fuels are often land-based fuels that can meet or exceed all specifications of the marine fuel