Slow steaming would increase emissions from LNG engines

As calls come for the regulation of ship power rather than speed to reduce emissions, support has emerged from an unlikely source – a rebuff to allegations of miscounting methane slip.

As reported, Norwegian research institute SINTEF has claimed that methane emissions are accounted for incorrectly in a report commissioned by LNG lobby groups SEA/LNG and the Society for Gas as a Marine Fuel (SGMF). SINTEF chief scientist Dr Elizabeth Lindstad argued that the use of unburned methane measurements gathered at a high engine load provide an artificially low estimation of emissions. Instead, the measurements should reflect the higher methane emissions generated at the lower engine loads currently adopted by the global fleet under slow steaming.

But in an addendum to the original report released last week, consultancy Thinkstep argues that LNG-fuelled vessels would not be likely to engage in slow steaming as gas engines are more efficient at higher loads. They burn the least fuel (and produce the least emissions) at loads of around 65-80% compared to 50-65% for diesel engines.

The authors note: “Running LNG fuelled engines on low load points would be neither environmentally friendly nor economically beneficial.”

Thinkstep uses this difference to justify its use of methane measurements at higher load points. But the argument also implies that speed limits, such as those proposed by some IMO member states as a means to cut emissions, could be counter effective for gas-fuelled ships.

The stance lends support to proponents of restricting ship power rather than speed to reduce greenhouse gas (GHG) emissions. The latest advocate of restricting power is shipowner association BIMCO, which has submitted a proposal to IMO’s intersessional working group on reducing GHG emissions. However, the power restrictions in BIMCO’s proposal are linked to target speeds for each vessel type. To minimise emissions, these target speeds would need to be updated for LNG-fuelled vessels to represent the higher optimal engine load point – with the result that LNG-fuelled ships would be allowed to sail faster than those with diesel engines.

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