Overhaul interval extended beyond 42,500h
‘CSCL Europe’; one piston has achieved over 42,000h despite slow steaming and low oil feed rates
ExxonMobil reports that its lubricant experts have been working together with engineers from Athens-based Danaos Shipping to extended a piston overhaul interval from 16,000 hours to 42,575 hours.
Trials were carried out on containership CSCL Europe’s Doosan-MAN B&W 12-cylinder K98MC-C main engine, running with Mobilgard 570 cylinder oil, where an overhaul revealed that cylinder number 11 was in remarkably good condition, with a negligible liner wear rate, despite slow steaming and low oil feed rate.
The vessel concerned is chartered to China Shipping Container Lines. Experts from Danaos Shipping and ExxonMobil systematically monitored the condition of the engine throughout the overhauling process. The results were said to be particularly significant because the containership has been slow steaming since 2009, averaging 15.6 knots with engine loads as low as 30% to 40%, and operating with a reduced cylinder oil feed rate.
Since delivery in 2004, CSCL Europe has been lubricated with Mobilgard 570, which ExxonMobil claims can reduce piston wear and deposits and extend overhaul intervals in two-stroke diesel engines. The ship is equipped with MAN Diesel & Turbo’s Alpha adaptive cylinder oil control (ACC), a system that controls the dosage of cylinder oil according to the amount of sulphur entering the cylinder with the fuel. Danaos Shipping uses ExxonMobil’s cylinder condition monitoring (CCM) programme to detect changes in cylinder condition so that engineers can safely reduce feed rates without increasing wear.
ExxonMobil states that the ship’s engineers gradually extended the overhaul period on successive pistons in the 12-cylinder engine. Overhauling started at 16,000 hours and continued at extended intervals based on the good condition of the pistons, rings and liners. In cylinder number 11, the overhaul was done at 42,575 hours. Fuel sulphur level averaged 2.62% throughout the period. Just before cylinder number 11 was overhauled, all monitoring parameters were within normal range, based on the results of scavenge air port inspections. A graph of liner wear rate over time shows that the rate in fact fell from a high of 0.02mm/1,000h to a rate at time of overhaul that was said to be negligible.
The overhaul revealed that even at 42,575 hours, cylinder number 11 was in good condition, suggesting that the interval could have been extended still further. Liner wear rate, piston ring wear and the ring groove clearance were all well below the thresholds identified by MAN Diesel & Turbo. No evidence of scuffing was found.
Danaos Shipping used the Alpha ACC to reduce the amount of Mobilgard 570 cylinder oil from a factor of 0.34g/k/Wh x S% to the current minimum factor of 0.2g/k/Wh x S%, which equates to 0.6g/KWhr (the minimum setting recommended by MAN B&W) at normal loads. According to ExxonMobil, the benefits of using Mobilgard 570 can become even more pronounced at feed rates below 0.6g/KWhr.
“We operate a high-quality fleet of 56 container ships with a reputation for reliability and cost-efficiency,” said Danaos Shipping’s technical manager Dimitrios Vastarouchas. “We have seen that Mobilgard 570 can help significantly extend piston overhaul intervals and for us the benefits of this technical achievement are financial and environmental.”
ExxonMobil says that Mobilgard 570 is formulated with high-quality, heavy neutral basestocks, rather than the more common bright stocks, which have less thermal and oxidative stability. To optimise the oil’s performance, balanced individual additive components are used rather than an additive package. One of these additives is a patented synthetic thickener technology to increase viscosity, important for engine designs that incorporate higher combustion gas pressures and temperatures, higher stroke/bore ratios and reduced cylinder oil feed rates. The oil is said to contain proprietary performance-enhancing detergents, dispersants, anti-oxidants and anti-corrosive additives.
In addition to helping extend piston overhaul intervals, Mobilgard 570 is claimed to increase engine reliability, providing thermal stability and acid-corrosion protection. It also promotes cleanliness and reduces the amount of waste cylinder oil requiring disposal, thus allowing ship operators to be more environmentally responsible.
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