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Andy

Incident involving Pont-Aven 29 April 2019

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It is not unusual to replace engines via a cut out in the hull side once in dry dock. Depending on the design o the vessel, this can be far quicker than removing vertically through several decks. Perhaps on a ferry design, the vertical solution is practical up to the level of the lowest vehicle deck?

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2 hours ago, Cabin-boy said:

OK, thanks. How does such a switch affect an engine? Is it a dramatic temperature or pressure change that leads to such a failure? And if so, are they likely to be carefully checking the other three engines for signs of similar problems? Ed. 

MGO has to be kept cooler than ULSHFO due to the difference in viscosity. Switching is usually done through a manual or auto 3 way valve at high power levels over a very short space of time. This can cause gassing in the fuel system if the MGO overheats.

SOLAS  regs mean that ferry's have separate fuel tanks which is fair enough although this can aggravate the process, boilers and steam production heats HFO, coolers are used to maintain a lower temp for MGO... initial premixing is governed by the viscosity control system which controls the temp of the preheaters in the fuel system, as the MGO levels become purer the temps are reduced to stop the above mentioned gassing.

Doing the opposite switching to HFO can cause clogging if the temps are too low, the build up is an explosive risk.

Many older vessels only have the one tank and premix the fuels once the HFO falls to around 15/20% so, by the time the switchover needs to occur residual fuels have been suitably diluted with distillate - or by the time it reaches the valves the chance of volatility has been removed. The overriding problem with this process is the time it takes so it's not realistic for short sea ferries.

Back to viscosity... if MGO is also overheated it looses it lubricating properties,  it's the opposite for HFO, too cold. Engines seize.

You then have to consider the emissions. PA has both scrubbers and SCR's. Scrubbers need high temps to remove SOx, and create sulfuric acid, catalytic reduction for NOx  needs cold temps. If the scrubber gas is too hot it reacts with the ammonia in the catalysers. 

That's (NH4) SO4 and that's is exothermic.

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