Vessel speed reduction
Slow steaming is the practice of reducing maritime vessel speeds. By operating ships at a speed significantly lower than their maximum speed, the required fuel is reduced. This results in reduced CO2 emissions stemming from ship operations. Different ship types benefit differently from slow steaming.
There are various ways in which lower speeds can be achieved. Ship operators can have incentives to slow down as this reduces their fuel costs, in particular if fuel prices are high and freight rates or revenues are low. However, ship speeds can also be regulated, either globally, unilaterally as a condition of entry into a port or as a condition to navigate in coastal waters, or bilaterally between ports in two or more states.
Speed limits could be absolute or expressed in averages, in which case the average speed over a certain route should not surpass a certain limit and hereby leaving room for speed optimisation. Speed limits could be gradually lowered over time, so as to allow shipyards to add the extra ship capacity that would be needed. Speed limits could also be differentiated according to vessel type or main cargo types.
Market-based mechanisms, such as a fuel levy, which would increase fuel prices, can induce slow steaming and resulting emissions reductions. However, the link between carbon pricing and speed reductions is not undisputed.
As there is an exponential relationship among speed, engine power and fuel oil consumption, slow steaming can reduce the amount of fuel required to operate a ship, and resulting CO2 emissions, more than proportionately.
Estimates from the literature show, for example, that:
- a speed reduction of 10% can translate into a 27% reduction of engine power requirements and a 19% reduction in overall engine power required to cover the same distance
- speed reductions of up to 30-50% (depending on ship type) compared with design speeds can reduce CO2 emissions up to 40%
- if the overcapacity in shipping markets in 2009 had been used for slow steaming, emissions by bulkers, tankers and container vessels could have been reduced by 30% compared with 2007.
In general, lower speeds are more effective for achieving CO2 emissions reductions if design speeds of ships decrease as well.
The implementation of lower speed limits for maritime vessels has no direct costs.
However, operating at lower speeds means increased travel times, which implies higher staff costs to cover the same transport operations and a reduction in the frequency of service where no mitigating actions are taken.
Slow steaming can lead to a reduction in other pollutant emissions.
Slow steaming may require ships to run their engines in suboptimal conditions, which could create issues such as reduction of engine efficiency and lifespan and potentially relatively higher emissions of nitrogen oxides and particulate matter. Newer vessels are designed to operate at lower speeds, effectively avoiding this problem.
Slow steaming increases voyage duration. As a result, goods and/or passengers take longer to reach their final destination, which, depending on the value of the good/the value of time of the passenger, can impact mode choice and potentially result in a shift to less sustainable modes of transport.
ITF (2021) Transport Climate Action Directory – Vessel speed reduction
https://www.itf-oecd.org/policy/vessel-speed-reduction
ITF (2018) Decarbonising Maritime Transport Pathways to zero-carbon shipping by 2035, OECD Publishing, Paris, https://www.itf-oecd.org/sites/default/files/docs/decarbonising-maritime-transport-2035.pdf
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Psaraftis, H.N. and Kontovas, C.A. (2013) Speed Models for Energy-Efficient Maritime Transportation: A Taxonomy and Survey, Transportation Research Part C: Emerging Technologies 26, 331–351. https://www.sciencedirect.com/science/article/pii/S0968090X12001246
Rutherford, D., Mao, X., Osipova, L. and Comer, B. (2020) Limiting engine power to reduce CO2 emissions from existing ships. https://theicct.org/sites/default/files/publications/Limiting_engine_power_02112020_0.pdf
Zis, T. and Psaraftis, H.N. (2019) Sustainable shipping: a cross-disciplinary view. https://www.springer.com/gp/book/9783030043292
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