The full report can be downloaded here. Image Credit: GCMD

• New report released Wednesday, Quantity assurance in marine biofuel supply chains, completes GCMD's three-pillar biofuel assurance framework.
• Warns FAME blending can cut viscosity below a mass flow meter's (MFM) validated range
• Blend ratios such as B24 and B30 are typically supplier-declared, not independently verified
• Centre says blend viscosity should be checked against the meter's range at actual transfer conditions

Blending FAME into conventional bunker fuel can reduce the blend's viscosity enough to potentially take it outside the validated operating range of the mass flow meter (MFM) installed on the bunker barge, says the Global Centre for Maritime Decarbonisation (GCMD).

The warning came as part of a new report on quantity assurance in marine biofuel supply chains, released on Wednesday.

The report explains that the applicable MFM measurment range for an individual bunker barge is determined by the conditions under which its MFM system has been calibrated and validated.

For example, an MFM installed on a residual-fuel bunker barge that predominantly supplies grades such as RMG 380 may be calibrated and validated over a viscosity range of approximately 30-150 mm²/s.

"While the meter itself may technically be capable of measuring fuels outside this range, the use of a significantly lower-viscosity fuel would fall outside the barge’s validated operating envelope unless the MFM system has been appropriately calibrated, validated, and approved for that additional range," says GCMD.

Further, blend ratios, such as B24 or B30, are typically supplier-declared rather than independently verified, it says.

Under-reporting blend ratios also has GHG accounting and compliance consequences, and the renewable fraction of a blend determines its green premium.

GCMD recommends assessing the final blend's viscosity under actual transfer conditions against the meter's operating range, managed either through transfer-temperature control or by using MFMs validated for broader ranges.

The wider report sets out how shipowners can verify that a biofuel delivery contains both the quantity and the renewable content they contracted for, drawing on the centre's end-to-end supply chain trials under commercial operating conditions.

GCMD also cautions against over-reading test results, noting that neat FAME may contain up to 3.5% non-ester material and that laboratory methods have finite precision, so a measured result that differs from a declared blend ratio does not necessarily indicate under-delivery.

The report completes GCMD's three-pillar assurance framework for marine biofuels, covering quality, quantity, and GHG emissions abatement.

"Buyers need confidence that they have received both the fuel and the renewable content they paid for," said Lynn Loo, CEO of GCMD, in a statement.

Biofuel bunkering volumes in Singapore and Rotterdam grew from negligible levels in 2020 to almost two million mt in 2025, according to the report.

The full report can be downloaded here.