Speed Limits v Optimisation

Its apparent effectiveness in reducing the intensity of CO2 emissions from a vessel has led to calls for it to be extended further as one of the means of reaching the IMO’s targets on greenhouse gas emissions.

In recent months, environmental activists have lobbied for mandatory speed limits in the hope that lower ship speeds equate to less emissions. But is a speed limit the answer? Or is it an oversimplified solution that lacks nuance?

The case for speed limits

There are two simple factors to support the introduction of mandatory speed limits. Firstly, it would be relatively simple to implement. Secondly, it is likely to have some positive impact on CO2 emissions.

Supporters of speed limits have proposed a two-tier system; one limit for container ships – based on maximum average speed – and an absolute limit for all remaining vessel types.

The case against

To minimise the intensity of CO₂ emitted by a vessel – usually measured in grams of CO₂ per tonne-mile – the vessel’s engines and plant should be operating at maximum efficiency. The speed at which this occurs is known as its optimum speed and this will vary from vessel to vessel, depending on vessel design and engine type.

It can therefore be argued that a more impactful measure is for all vessels to run at their optimum speeds. For some vessels, this optimum speed could be greater than an imposed speed limit, so sticking to the limit could produce a greater CO₂ intensity than breaking it.

Some South American countries have already raised concerns on speed limits. It is seen as a barrier in their ability to trade competitively to Asia, in particular the export of perishable cargoes. If voyages are longer, grain cargoes such as soya beans are at higher risk of spoiling.

Other impacts of slowing down

Reducing speed, whether limited by regulation or simply running at optimum efficiency, can have a knock-on effect that negates the vessel’s gains in CO₂ emission intensity.

If the world fleet slows down, this can result in reduced capacity. Good news for those affected by low freight rates, but would it lead to more ships needing to be built? This might impact the desired effect of reduced total emissions. But the counter argument is that it would lead to the building of more efficient vessels.

There are also concerns that speed limits will impact scheduling, leading to delays in ports.

Drivers for optimisation

In addition to reduced CO₂ emissions, optimisation results in reduced fuel consumption. This isn’t limited to speed optimisation; it also applies to routing, scheduling and planning of maintenance. Overall performance optimisation benefits an owner through reduced fuel costs, benefitting also a time charterer if they have an obligation to provide the fuel.

There is a potential for the drive for efficiency to be undermined by very low fuel prices. This could be addressed by the authorities introducing market-based measures to reduce CO₂ emissions such as carbon trading or a bunker levy.