Super-Slow Steaming: VTI Turbocharger Retrofit Pay-Off

By George Backwell at November 01, 2013 23:20
Filed Under: Marine Diesel Engines

What to do when the tanker markets are depressed? Maersk Tankers has taken a hard look at bunker fuel consumption, which on their Very Large Crude Carriers (VLCC) makes up about 85% of the voyage costs. They found that the key to improve earnings is super slow steaming, which they’ve taken to a whole new low with giant VLCC’s ambling over the oceans at speeds as slow as 8.5 knots.

Super slow steaming requires engine load to be decreased down to 10%, which is equivalent to 50% speed, but the company recently decided to further economise on fuel consumption (and not by reducing the speed of its ships yet more) with the retrofit of a Variable Turbine Inlet system (VTI) onboard the 318,000 dwt crude oil carrier Maersk Ingrid. Manufacturers, Mitsubishi Heavy Industries Marine Machinery & Engine (MHI-MME) say that their VTI turbocharger system has been been fitted to more than ten new buildings, but never before retrofitted to an existing unit, a job that took just four days during the Maersk Ingrid’s first dry-docking.

Photo courtesy of Maersk Tankers

VTI technology & slow steaming
Ship’s engineers normally employ turbocharger cut-out during slow-steaming, so that in an engine equipped with several turbochargers one of them is stopped at slow engine speed. Then, when say one of four turbochargers is stopped, exhaust gas is distributed to the remaining three, which serves to increase the scavenging air pressure and improves fuel efficiency. MHI-MME set out to redesign the turbocharger hardware to improve on that operational procedure and came up with  their VTI technology.

This system which was developed for the manufacturer’s MET turbocharger series, incorporates a gas inlet passage, which is equipped with an on-off valve. This changes turbine capacity  in  two  stages  by  admitting  the exhaust gas into two separate  concentric segments  of  the  nozzle  ring.  When the vessel is operating in slow steaming mode, the scavenging air pressure of the main engine is increased by closing the on-off valve, thereby decreasing the fuel consumption rate and CO2 emission levels.

VTI turbocharger components: Image courtesy of MHI-MME

Opening & closing nozzle throat: Image courtesy of MHI-MMSE

Maersk Ingrid's main engine is a Wärtsilä, type 7RTA82T diesel fitted with Mitsubishi MET71MA turbochargers, and after the VTI retrofit NOx emission levels were measured during sea trials. The results confirmed that the vessel was compliant with IMO Tier 2 exhaust gas emission regulations; moreover MMH-MME  demonstrated that the  expected  fuel consumption reductions during low-load operation could also be achieved without sacrificing performance in the high-load range.

Tests are said to have showed that the fuel consumption reduction was  around 1.6 - 3.2%  in  a  load  range  of  10 to  50% MCR and the manufacturers consider that the pay-back period for the retrofit will be attractively  short. 




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