Since it became widespread in 2007 in response to increasing oil prices, slow-steaming has already made an impact on greenhouse gas emissions from shipping.

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.

By Alvin Forster, Loss Prevention Executive, North P&I Club

Above article has been initially published in North P&I Club's website and is reproduced here with author’s kind permission.

The views expressed in this article are solely those of the author and do not necessarily represent those of SAFETY4SEA and are for information sharing and discussion  purposes only.


About Alvin Forster, Loss Prevention Executive, The North of England P&I Association Limited

Alvin is a qualified Chief Engineer and has attained a Chartered Insurance Institute Advanced Diploma in Insurance (ACII). He joined the loss prevention team at North P&I in 2011. Prior experience includes 12 years at sea as an Engineer Officer on merchant vessels, a marine surveyor with a major consultancy and a technical superintendent within a shipping company. Following the announcement by IMO that the reduced global sulphur cap will take effect in 2020, Alvin has keenly followed developments, working closely with North’s defence lawyers to develop guidance for North’s members as well as revising North’s loss prevention guide on preventing marine fuel claims.