How much bigger is an ammonia, methanol or LNG tank than a VLSFO tank?

Formula, assumptions, and limits

The calculator holds the ship’s energy fixed, divides it by the new fuel’s heating value to get the mass, divides the mass by the density to get the liquid volume, and divides that by the filling limit to get the tank.

Formula

The energy in the reference tank, then the mass, liquid volume, tank and hull space of fuel $i$:

The fuel-only volume ratio, with no pilot and the same engine efficiency:

• $E$: energy delivered, GJ. From a voyage it is the daily consumption in tonnes times the days times $\text{LCV}_{\text{ref}}$; from the tonnes bunkered, the tonnes times $\text{LCV}_{\text{ref}}$.
• $\text{LCV}$: lower calorific value, MJ/kg, which is the same number as GJ/t.
• $\rho$: liquid density, kg/m³, at the storage temperature shown beside it.
• $K$: filling limit, the share of the tank the liquid may occupy.
• $S$: space factor, hull space per m³ of tank, 1 for no allowance.
• $p$: pilot fuel share of the energy, burned as MGO at 42.7 MJ/kg.
• $\eta$: the engine’s efficiency on fuel $i$ as a fraction of its efficiency on the reference fuel, 1 when they are the same.

Derivation

Lower calorific value is “the amount of heat that would be released by the complete combustion of a specified fuel”, in the words of MEPC.391(81).² Mass for a given energy is therefore energy over LCV, and the mass ratio of two fuels is the inverse ratio of their LCVs: 40.2 ÷ 18.6 = 2.161 for ammonia against VLSFO. Volume is mass over density, which brings in the second ratio: 991 ÷ 681.6 = 1.454. Their product, 3.142, is the fuel-only volume ratio.

The tank follows from the IGF Code’s definition of the filling limit, “the maximum liquid volume in a fuel tank relative to the total tank volume” (paragraph 2.2.16).⁵ The pilot fuel, where an engine needs one, comes out of the same energy and goes to its own MGO tank at 98%.

Assumptions

• Equal delivered energy. The ship needs the same energy at the propeller and the generators whatever the fuel, unless you enter a different engine efficiency.
• Heating values from the IMO set by default: MEPC.364(79) for fuel oils, LNG, LPG, methanol and ethanol, MEPC.391(81) for ammonia, hydrogen, HVO and FAME.^1,2 VLSFO takes the heavy fuel oil row, 40.2 MJ/kg, which MEPC.391(81) names HFO(VLSFO).
• Densities at the conditions printed in the reference table below, not at a common temperature.
• The reference tank and the new tank hold the same energy; endurance uses the daily consumption of the reference fuel.
• No boil-off, no unpumpable residue, no sludge or settling tank volume.

Worked example

A 2,000 m³ VLSFO tank filled to 98% at 991 kg/m³ holds 1,960 m³, or 1,942.36 t. At 40.2 MJ/kg that is 78,082.9 GJ.

Refrigerated ammonia at 18.6 MJ/kg needs 78,082.9 ÷ 18.6 = 4,198.0 t. At 681.6 kg/m³ that is 4,198.0 ÷ 0.6816 = 6,159.0 m³ of liquid, 3.142 times the VLSFO volume and 2.161 times its mass. At the 98% filling limit of IGF Code 6.8.1 the tank is 6,159.0 ÷ 0.98 = 6,284.7 m³.

At 30 t a day of VLSFO, 1,206.0 GJ a day, the VLSFO tank lasts 64.7 days. Filled with ammonia, the same 2,000 m³ tank holds 2,000 × 0.98 × 0.6816 × 18.6 = 24,848 GJ and lasts 20.6 days.

With methanol at 19.9 MJ/kg and 791.0 kg/m³ and 5% of the energy from pilot MGO, the main fuel is 78,082.9 × 0.95 ÷ 19.9 = 3,727.6 t and 4,712.5 m³, and the pilot is 91.4 t of MGO in a 103.7 m³ tank.

Edge cases and limits

• Pressurized ammonia. At 25 °C and 10.0 bar the density is 603.0 kg/m³ and the volume ratio 3.55. MSC.1/Circ.1687 says ammonia fuel “should be stored in a refrigerated state at atmospheric pressure”, so this case sits outside the circular’s base design.⁷
• Liquid hydrogen. MSC.1/Circ.1701 covers liquefied hydrogen in vacuum-insulated type C independent tanks only, and applies IGF Code 6.8 to them.⁸
• The loading limit. For a liquefied gas the 98% holds at the reference temperature, which is set by the relief valve pressure. At a colder loading temperature the limit is LL = 98% × ρR ÷ ρL, lower than 98%; the calculator works it out from two relative densities you enter.⁵
• The 95% cap. IGF Code 6.8.2 allows a higher loading limit for well-insulated tanks “but never above 95%”, and MSC.458(101) repeats that cap in a new 6.8.3 for ships constructed on or after 1 January 2024.⁶
• Space factor. No IMO, EU or IACS text gives a figure for any tank type, so the calculator applies none until you enter one.

Regulatory basis

The heating values are MEPC.364(79), as amended by MEPC.410(84) without change to the table, and MEPC.391(81) Appendix 2, which MEPC 84 left unamended; MEPC.1/Circ.926 adds an ethanol value of 0.0268 MJ/g.^1,2,14 The FuelEU set is Regulation (EU) 2023/1805 Annex II, with liquid biofuels sent to RED Annex III.^3,13 The filling limits are IGF Code 6.8 and the circulars for ammonia, hydrogen, methyl and ethyl alcohol and LPG;^5,7,8,9,10 the LNG filling rate of 0.95 and the 991, 900 and 450 kg/m³ densities are MEPC.411(84) paragraph 4.2.3.2.⁴

Common errors

1. Mixing fuel-only and tank-inclusive ratios. EMSA’s 2022 ammonia study gives LNG 1.8 and ammonia 3.0 against MGO, “not including insulation and secondary barriers”; other sources quote ratios with the tank system in.¹⁹
2. Changing the reference fuel mid-sentence. A ratio against MGO (EMSA) is not a ratio against HFO (ClassNK’s 3.07 for ammonia) or VLSFO.^19,20
3. Using FuelEU’s 41,000 MJ per tonne of VLSFO as a heating value. Annex IV uses it to convert the penalty; the Annex II value is 40.5 MJ/kg.³
4. Taking ammonia’s density at the wrong state: 681.6 kg/m³ refrigerated against 603.0 pressurized at 25 °C is a 13% difference in volume.¹¹
5. Reading a pilot share given by mass as a share of energy. MAN Energy Solutions’ paper on the LGIM methanol engine gives “up to 5% of the MCR fuel consumption in diesel oil mode”, a mass figure; convert a mass share with the two heating values before you enter it.²¹

Heating values and densities of marine fuels

The IMO column takes ammonia, hydrogen, HVO and FAME from MEPC.391(81) and the rest from MEPC.364(79); the FuelEU column takes ethanol, HVO and FAME from RED Annex III, as Annex II directs.^1,2,3,13 NIST densities are saturated liquid at the stated condition.^11,12,15 The 991 kg/m³ for VLSFO is the MEPC.411(84) heavy fuel oil density; a delivered VLSFO is often lighter, and every volume ratio falls in proportion to the density you enter.⁴

Filling limits by fuel and where they come from

Only the liquefied gases have a filling limit in an IMO text. IGF Code 6.8.1 caps a liquefied gas fuel tank at 98% at the reference temperature, and the circulars carry that rule to ammonia (MSC.1/Circ.1687, 6.8.1), liquid hydrogen (MSC.1/Circ.1701, 6.8.1) and LPG (MSC.1/Circ.1666, which applies IGF Code chapter 6 without excluding 6.8).^5,7,8,10

Methanol and ethanol have none. MSC.1/Circ.1621 asks for cofferdams around integral tanks and 600 mm between the tanks and category A machinery spaces, which costs hull space but sets no fill figure.⁹ For fuel oil, HVO, FAME and the alcohols, the calculator uses 98% as a convention, the figure MEPC.411(84) gives oil tanks for the EEDI, and says so beside the result.⁴

Published tank ratios and why they disagree

Three bases appear in print. EMSA’s 2022 study and ClassNK’s Alternative Fuels Insight give fuel-only ratios, against MGO and against HFO (“in liquid form”: LNG 1.89, ammonia 3.07).^19,20 The Mærsk Mc-Kinney Møller Center’s 2024 ammonia design study gives tank capacities: 6,000 m³ of ammonia for the range of 2,000 m³ of fuel oil on a Capesize design.²² A 2019 DNV study for SEA\LNG gives figures “including storage systems”, LNG at about 13 MJ/l against 21 MJ/l for the fuel alone.²³ Against LNG, refrigerated ammonia takes 1.70 times the liquid volume on this page’s figures (3.14 ÷ 1.84).

The calculator keeps the bases apart. The fuel-only ratio depends only on the heating values and densities, which official texts print. The tank ratio adds the filling limit, and the hull space adds the space factor, both of which depend on the design, so the page leaves them to you.