Benefits Of Slow Steaming On Emissions Might Be Grossly Overestimated!

Credit: Ojas Narappanawar/Pexels

The emission benefits of slow-steaming may have been “grossly overestimated”, according to studies by Simpson Spence & Young, corroborated by research from Clarksons last week.

Ship Speed And Fuel Consumption

Although it has been assumed that ship speed has a cubic relationship with fuel consumption, with consumption increasing exponentially as speed increases, real-world evidence has shown this largely academic assumption does not necessarily pan out in reality. This threatens to undermine many of the maritime industry’s long-held beliefs, and even brings into question the assumptions underlying the IMO CII regulation. “The reason for the misconception is that calculations are based on textbook speed-consumption curves, which have an exponential growth across an entire speed range,” said Clarksons lead analyst Jon Leonhardsen on Friday.

Simpson Spence & Young (SSY) head of research Roar Adland referred to work on the subject by himself and colleagues in 2020. That study found the so-called ‘cubic law’ – a function of ship speed and hull resistance – is really only true “near the design speed of vessels” , he said. “The fuel consumption curve is flatter than the cubic rule at current sailing speeds and below, which makes further speed reduction much less impactful,” he explained. “Go below 9-10 knots and you may actually get the inverse effect, increasing emissions per tonne-mile.

In Longer Term

However, in the longer-term, Ms Stausbøll admitted, slow-steaming to comply with CII and other emissions regulations will increase the amount of ship capacity needed for the same amount of cargo, adding: “Ultimately yes, slow-steaming is a problem because it requires more capacity.” This means that slow-steaming, though it might decrease emissions from individual ships in some cases, actually increases the number of ship engines burning fuel and emitting carbon, per given unit of cargo. This will cause an overall rise in CO2 emissions from the sector.

Mike Wackett, sea freight consultant for The Loadstar, said: “The re-emergence of the debate on the questionable benefits of super slow-steaming comes at a bad time for carriers endeavoring to convince their customers that the deployment of additional ships per loop to absorb their surplus capacity, and the subsequent increase in transit times for shippers, will cut emissions.”

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Source: Theloadstar