LNG spot cargo specification: GHV, Wobbe index, methane number and what a buyer must check

An LNG spot cargo specification is not a grade in the way that EN 590 or Jet A-1 is. Liquefied natural gas is specified as a composition range, and the properties the buyer pays for and the terminal accepts, gross heating value, Wobbe index and methane number, are calculated from that composition. A buyer confirming a spot cargo under a master agreement has a short window to check that the cargo fits the receiving terminal and the downstream market.

What an LNG spot cargo specification contains

A confirmation notice for a spot cargo typically states the loading terminal, the vessel, the delivery window, the quantity in MMBtu with a tolerance, the price in USD per MMBtu and the quality in terms of the master agreement. The quality clause usually sets:

• Gross heating value (GHV): a minimum and maximum, expressed in MJ/m³, Btu/scf or kWh/m³ at stated reference conditions
• Wobbe index: a minimum and maximum, the interchangeability figure that determines whether gas burns correctly in the downstream appliances
• Methane number: a minimum for markets where gas is burned in reciprocating engines; EN 16726 sets a minimum of 65 for grid gas in Europe
• Nitrogen: a maximum in mol %, commonly around 1 mol %, because nitrogen adds no energy and raises boil-off
• Hydrogen sulphide, total sulphur, mercury and carbon dioxide: maxima in mg/m³, ng/m³ or ppm, set to protect the terminal and the grid
• Ethane, propane, butanes and pentanes-plus: maxima in mol % that keep the Wobbe index inside the band

GHV, Wobbe index and methane number explained

Gross heating value

GHV is the energy released on complete combustion of a unit volume of gas with the water of combustion condensed. It is calculated from the composition measured by gas chromatography (ISO 6974) using the method in ISO 6976. Reference conditions matter: 15 °C/15 °C, 25 °C/0 °C and 60 °F/14.73 psia give different numbers for the same gas, and the contract must state which pair applies.

Wobbe index

The Wobbe index is the GHV divided by the square root of the relative density. Two gases with the same Wobbe index deliver the same heat through the same burner orifice, which is why grids regulate on Wobbe rather than on GHV. The EASEE-gas common business practice for European entry points sets a Wobbe band of 13.60 to 15.81 kWh/m³ and a relative density of 0.555 to 0.700, and national rules such as those in the UK and Spain are narrower. A rich cargo from a wet-gas source can exceed the band and require nitrogen ballasting at the terminal.

Methane number

The methane number describes knock resistance in gas engines, analogous to octane in petrol. It falls as ethane and heavier components rise. A cargo acceptable for a grid and power station may be unsuitable for a market supplying gas engines or LNG-fuelled vessels, so the minimum should be stated whenever the end use includes engines.

Rule: an LNG cargo is paid for in energy and accepted on composition. The buyer must confirm both the GHV range for the invoice and the Wobbe and contaminant limits for the terminal, using the unloading-port analysis, before the confirmation notice is signed.

Quantity and quality measurement

Custody transfer follows the GIIGNL LNG Custody Transfer Handbook and ISO 10976 for measurement on board. The volume is taken from the ship's calibrated tank gauging before and after transfer, corrected for trim, list and temperature. Density is calculated from composition and temperature, and the energy delivered is volume times density times GHV, less the gas returned to the ship. Sampling follows ISO 8943, with continuous vaporisation and online chromatography, and the certificate of quantity and quality is issued by the terminal and the independent surveyor.

What a buyer must check before confirming

1. Terminal entry specification: compare the cargo's expected composition, obtained from the loading terminal's recent analyses, against the receiving terminal's Wobbe, GHV, nitrogen and sulphur limits. Lean US Gulf Coast cargoes and rich cargoes from wet-gas basins sit at opposite ends of most bands.
2. Ageing allowance: a long voyage removes nitrogen and methane through boil-off and raises GHV and Wobbe. Check the arrival composition, not the loading composition, against the band.
3. Ship-shore compatibility: the vessel must have a valid compatibility study with the receiving jetty and, for FOB cargoes, with the loading jetty. This is a precondition, not a formality.
4. Reference conditions and units: confirm that the GHV in the price formula and the GHV in the terminal specification use the same reference conditions, or the invoice and the acceptance test will disagree.
5. Delivery term: Incoterms 2020 does not contain DES. Spot confirmations use DAP or the term defined in the master agreement, with title and risk passing at the unloading flange.

Summary table

A genuine seller provides recent loading analyses without hesitation. Comparing them against the terminal access code before confirming avoids the most expensive mistake in spot LNG: paying for a cargo the terminal cannot take.