Safeguarding methanol-fuelled engine cylinders

One of seven methanol-fuelled tankers with cylinders protected by Chevron's BN20 oil and analysis, and MAN's ACOM system One of seven methanol-fuelled tankers with cylinders protected by Chevron's BN20 oil and analysis, and MAN's ACOM system

A white paper exploring the lubrication demands onboard seven methanol carriers has cast a light on cylinder condition challenges for dual-fuel, two-stroke engines.

The report by Chevron Marine Lubricants, titled ‘Methanol and Marine Lubricants in a Lower Sulphur, Lower Emissions Future’, looks at the operation of methanol-fuelled ME-LGI engines from MAN Diesel & Turbo onboard two ships co-owned by Marinvest Shipping. It describes how Chevron's Taro Special cylinder lubricants and DOT.FAST analysis service support the successful operation of Mari Jone and Mari Boyle – two of the first ships in the world powered by methanol-fuelled low speed engines.

Ian Thurloway, brand and marketing manager, Chevron Marine Lubricants, says: “ECA and IMO 2020 regulations mean that shipowners and operators are increasingly turning to the use of alternative marine fuels to meet the challenges of a rapidly changing legislative and operational landscape.”

The paper, produced in cooperation with MAN, Marinvest and the charterer of the vessels, Methanex Corp, is the first in a series of white papers planned by Chevron focusing on ‘innovations and developments impacting the shipping industry’.

The challenges of dual-fuel operation are daunting and the ME-LGI engines onboard the series of seven methanol carriers, all delivered in 2016, are a case in point.  They are designed to burn heavy fuel oil, low-sulphur diesel and methanol. Even when running on methanol they require a small amount of fuel oil, around 5% of total consumption, for pilot fuel.

BASE COMPLEXITY

That variety of fuels translates to a more complex range of engine operating conditions than ever before. This becomes apparent in the cylinder lubrication requirements of the engines: in modern engines heavy fuel oil requires a high base number (BN) lubricant to counter cold corrosion, while low-sulphur fuels such as methanol would require a low BN oil.

Loading bunkers in a wide range of ports complicates matters further, as the sulphur content of HFO alone varies from 1.8% to 3.5%. The engines could be burning 95% methanol, along with either a low- or high-sulphur pilot fuel; they could be burning a 0.10% sulphur distillate fuel for ECA compliance; they could be burning up to a 3.5% maximum sulphur heavy fuel oil; or they could be burning a mix of 70% methanol along with either a high- or low-sulphur oil product.

“This makes it very tricky to know to which level to limit the cylinder oil to the liners, and what the BN of the cylinder lubricant has to be,” says Fredrik Stubner, director ship management, Marinvest Shipping. “In fact, one of the biggest concerns when we ordered the engines early on was the effect that dual-fuel operation has on the liners, and it is something no-one could really answer.”

Chevron account manager Bert Van Cleemput worked with Marinvest to develop a lubrication package for these pioneering vessels, and attributes the success of the project to Chevron’s DOT.FAST programme. The service provides analysis of used oil to detect indicators of cylinder damage. Early on in the process the system indicated areas for improvement in feed rate settings and engine hardware.

“When we look at optimizing the feed rates and lubrication of two-stroke engines, the two main parameters are BN and iron,” says Van Cleemput. “DOT.FAST onboard analysis delivers immediate feedback on cylinder running conditions, and provides an early indication of elevated levels of both abrasive and corrosive engine wear. At the same time, it helps optimise the cylinder oil feed rate and minimise the build-up of abrasive deposits, cylinder oil consumption, engine fouling, and the risk of scuffing.”

TIMELY ANALYSIS

Detailed onshore analysis is also part of the DOT.FAST programme, but according to Stubner it is the onboard analysis that allows for critical and timely interventions in the case of a problem.

“It allows us to see on a daily basis the iron content and the BN number of the breakdown oil,” he says. “It tells us immediately whether to adjust the cylinder oil feed rate up or down accordingly. There is no need to wait for shore analysis that would mean you lose time and miss the opportunity to act promptly to avoid excessive wear on the piston rings.”

As an extra precaution against cylinder wear, and to safeguard against human error with cylinder oil selection, the vessels also employ MAN’s Automated Cylinder Oil Mixing (ACOM) system to ensure the right cylinder oil dosage at all times. This system, which is now standard on new engines, blends oils to create the required BN depending on an analysis of the fuels being used.

“When we are talking about zero-sulphur fuel you want a low BN cylinder oil, like 25BN Taro Special HT LF, but in our case we can use methanol along with high-sulphur fuel oil where a 140BN oil might be better,” explains Stubner.

Chevron’s current recommendation for two-stroke, dual-fuel engines burning methanol is a the 25BN lubricant, Van Cleemput confirms. Supported by Chevron’s scrapedown oil analysis service and MAN’s automated mixing system, the partners in this pioneering methanol fuel project have safely navigated the challenges of lubricating dual-fuel engines.

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