Operators, engine designers and oil suppliers join to examine cold corrosion

28 Aug 2014
Prominent industry figures drawn from shipowners, ship managers, engine designers and lubricant suppliers gathered to discuss the cold corrosion issue

Prominent industry figures drawn from shipowners, ship managers, engine designers and lubricant suppliers gathered to discuss the cold corrosion issue

ExxonMobil and The Motorship recently hosted a round table discussion on cold corrosion in Hamburg, prior to the 36th Propulsion & Emissions Conference.

The event, at the Atlantic Kempinski Hotel, was attended by a number of shipowners and ship managers, with representatives of the oil company and an engine company offering their views. All of the shipping companies present were familiar with cold corrosion, although not all had actually experienced the problem.

The discussion was chaired by Lars Robert Pedersen, deputy secretary general of BIMCO, with special responsibility for technical and operational affairs. Mr Henneberg began by pointing out that, his company was operating mostly older ships so had not seen the problem first hand, but expcted to learn a lot that could be applied to a newbuilding programme involving some 60 ships, with new-technology engines.

Mr Koerber said his company was currently running 65 ships with no major problems so far, but had observed some evidence of cold corrosion in some RT-flex engines, which were being closely monitored. He regarded his company as one of the pioneers of slow steaming, going down to 10% load or even less.

Kjeld Aabo, customer support director MAN Diesel & Turbo, explained that cold corrosion is part of the normal process in the liners, and when under control it is acceptable – even desirable (see panel). He admitted that the way in which, in certain engine types and under certain conditions, cold corrosion leapt out of control which had taken MAN and the other engine designers rather by surprise, but working in conjunction with the oil companies, new cylinder oils with high total base numbers, typically BN100, are being launched to the market. These have proved effective in dealing with cold corrosion, while alongside this development, MAN has been looking at achieving increases in liner temperature, through redesign of the liners and cooling systems on new engines and introducing modification of the cooling system at some existing ships. Service experience of these measures, in conjunction with BN100 oils, are presently showing perfect results. Mr Aabo added that MDT’s specification now calls for BN100 oil to be used as the standard for later engines, such as the MAN B&W Mk8 onwards. It is possible to use BN70 oils at increased feed rates for some engine applications, but this, in his opinion, is neither as cost-effective nor as sustainable a solution as BN100 for Mk8 onwards engines.

Mr Pedersen asked about other new technologies being introduced in response to forthcoming, and more stringent, emission limits, such as EGR (exhaust gas recirculation) and whether these might bring further challenges for engines. Mr Aabo said that this was a possibility, but the main task was to get all existing engines running at more favourable temperatures on high-BN oil. The monitoring of the cylinder condition by sweep tests of the cylinder lube oil enables cylinder condition diagnosis and allows an optimised cylinder lube oil feed rate to be securely determined. The lower sulphur fuels, which are to be mandated as tighter SOx regulations come into force, will further change the situation, meaning that it remains essential to constantly monitor the engine oil condition.

Several oil suppliers had introduced ‘intermediate’ cylinder oils, hoping to supply the market with one cylinder oil for all operating patterns. But these have proved less effective than hoped for in dealing with the challenges of operating in ECAs, with low fuel sulphur levels, and when both slow steaming and running at normal loads has been required. Mr Koslowski pointed out that one of the newbuild ships for which Peter Döhle is responsible, had ‘killed’ the main engine on the yard sea trials on such intermediate oil.

Mr Koerber asked why the engine designers had been caught out by cold corrosion. He felt that it should have been evident that higher pressure and longer stroke at lower revolutions could cause difficulties. He also saw difficulties persuading chief engineers on his ships that rather than working hard to keep engine coolant temperatures low, as had always been the case previously, they should do the opposite – and actually heat the liners. This, he said, seem an odd practice to marine engineers. He felt that engine designers need to issue advice or service letters about resetting alarms etc, because most shipboard systems will shut down engines should the temperature rise to, say 93°C. The letter should also explain the reasons for raising the temperatures, because many onboard engineers would consider the engine designer was mad to specify running at temperatures close to boiling point.

Steve Walker, Field Engineering Manager for ExxonMobil Marine Fuels & Lubricants, concurred. He felt that crew reactions would be somewhat unfavourable. Mr Koerber said that a recent meeting of his officers had discussed the issue, and engine designers must address the issue of crew objections. Furthermore, would raised cooling water temperatures require new treatments and additives for the coolant?

Mr Aabo agreed that the engine companies need to make the situation clear, and although there was no evidence at present that higher temperatures could bring their own difficulties, his company would bear in mind the possible need for different treatments. He agreed that, with the benefit of hindsight, the engine designers could have been better prepared. But with large engines, such things do not become obvious until they are in service, at sea. It has to be acknowledged that the efficiency of the engines, especially at part load, has been considerably improved. The increased cylinder pressure which has enabled higher efficiency is an important factor in the increased level of cold corrosion - and hence wear - seen in the liners of some of these new engine designs.

The discussion moved on to the way in which the engines were being used. They had not originally been designed for slow steaming. Their original configuration covered quite different operating patterns, so the designers would not have allowed for prolonged operation at low load. Mr Aabo said that trade-off between NOx emissions and fuel consumption had changed the load points and the operating diagram anyway, making it even less relevant to optimise the engine for slow steaming.

ExxonMobil’s Steve Walker explained that cold corrosion is a complex issue and often the level of corrosive conditions can vary from engine to engine – even engines of the same model, used in similar operating patterns.

“The traditional way of selecting a cylinder oil was based solely on fuel sulphur content,” explained Mr Walker. “But the new operating challenges mean that shipowners must take a number of other factors into account – these include vessel speed, engine load, engine model and whether there have been any modifications to enable running at low load, as well as fuel sulphur level.

“But it’s not just a matter of choosing the right oil – it is vital to implement an engine and oil condition monitoring programme to help identify the right base number and feed rate to use.”

Consulting an expert is key to choosing the right oil in the first place. The Mobilgard range includes both 100BN and 70BN solutions, and its field service engineers can recommend the best choice for given engines and operating conditions.

Mr Walker said that virtually all of the lube oil suppliers had introduced a 100BN cylinder oil to the market in response to the cold corrosion challenge. “But there are significant differences between the oils,” he said. He explained that ExxonMobil’s Mobilgard 5100 100BN cylinder oil is a premium cylinder oil that helps optimise the performance of two-stroke diesel engines operating on HFO. It has been formulated to help protect engines from cold corrosion, causing premature liner wear when operating at low load conditions, meaning that liner temperatures can fall below the acid dew point.

“Mobilgard 5100 has been developed for the optimum balance between cost, feed rate and engine cleanliness, not only to neutralise acid formation, but to control deposits and provide oxidation stability, even during ultra-slow steaming and at low feed rates,” Mr Walker explained. Both MAN Diesel & Turbo and Wärtsilä had issued no-objection letters in respect of Mobilgard 5100.

However, Mr Walker explained that the most important factor was for operators to understand exactly what is happening inside the engine, and proper monitoring is the only way to ensure that oil is being fed at the optimum rate – enough to provide adequate lubrication but without waste or other adverse effects of over-lubrication. “It is important to note that feed rate requirements will vary with fuel sulphur, engine load, humidity and cylinder oil quality,” he said.

He explained that ExxonMobil had worked closely with OEMs to understand the problem and develop a solution, and is continuing to do so. There was no universal answer – field trials had shown that cold corrosion has to be addressed on a case-by-case basis, hence the emphasis that the company places on monitoring. Indeed, MAN stipulates a cylinder condition monitoring programme for its engines at risk of cold corrosion.

The key to getting the feed rate right was to determine the correct ACC (adaptive cylinder control) factor, using sweep tests on scrape down oil. Originally low feed rates resulted in clean pistons, with higher feed rates having the potential to cause piston crown deposits which could result in liner wear. However, more recent trials have shown deposits forming even at lower feed rates. This, said Mr Walker, was because at low feed rates the cylinder oil additives can become fully depleted.

So it is vital that any engine that could be subject to corrosion, which includes older models operating under slow steaming as well as the latest longer-stroke engines, is monitored for wear. ExxonMobil and the leading OEMs advocate scape-down analysis to ensure feed rates are set at the optimum level.

“ExxonMobil’s Mobilgard cylinder condition monitoring service has recently been upgraded to include a specific cold corrosion test, and to bring online a database of nearly 200,000 scrape down samples,” said Mr Walker. The test, carried out onboard, analyses the BN level and the ferrous iron content in scrape down samples, and now includes a test to monitor the level of iron salts. “It is important to note that only by carrying out the three onboard tests can operators fully understand the condition of the cylinder oil and the engine,” Mr Walker concluded.

Mr Walker was one of several who recalled that slow steaming was not new – in the days of steam he had been instructed to run at ‘most economic power’ (MEP) and 55%-60% load was common. However, now, engines can be running for prolonged periods at what their designers considered, at the time, to be virtually manoeuvring speed. He had been told recently by a chief engineer that his ship had been operating at 9% engine power. “So we are now operating in an exploratory world,” he added.

Mr Koerber said his experience was similar – steam-powered tankers often operated at part load, with the governing factor being the speed at which the ship would still steer, i.e. about 7 knots.

Mr Toll offered the perspective from a ship manager’s point of view. He believed that, when it came to slow steaming and dealing with any consequent problems, managers needed to approach these in a different way from owners .

Mr Walker said that he had been told by older people in the industry that “nothing ever changes in shipping.” However, emissions legislation has altered this view, and is bringing about changes and problems not experienced before.

The discussion turned to older engines, for which Mr Aabo felt that the use of intermediate BN cylinder oils for these engines was perfectly satisfactory.

“ExxonMobil hopes soon to introduce a product to the market with a lower base number, suitable for use with low sulphur fuels,” said Mr Walker. “The problem with just reducing base number is that this loses some of the oil’s detergent properties, so an alternative formulation is required.”

Mr Schubert added that it is unknown how long a ship could operate on low sulphur fuel with a high base number cylinder oil, as many will have to do in ECAs, making oil condition monitoring essential.

Mr Schüler said that his company’s container ships could bunker in three different ports in a single voyage, taking on fuel with a different sulphur content each time. Additionally, on the same voyage the ship could run at any engine load in the range of 5% to 95%. “So we need a cylinder oil that can cope with all of these conditions. This did not seem to be a problem some two and a half years ago, when several oil companies offered a ‘universal’ oil that was said to cope with the full range of conditions – now, it seems, one oil is definitely not enough,” he commented.

“Maersk, several years ago, proposed using one base oil for its whole fleet, and, by carrying out on-board analysis, could vary additives and feed rates to make sure that the engine was properly protected,” he recalled.

Both Mr Aabo and Mr Walker had worked with Maersk Fluid Technology on the onboard equipment used to adjust the lubricating oil to suit the fuel type. Although the company tried hard, and succeeded, to make it work, other shipowners were reluctant to take up the system. Mr Henneberg thought this was because owners would prefer to buy the right oil rather than have crews try to blend lubricant themselves on board.

Mr Toll said that when a new ship is designed and built, nobody looks at how all the systems work together. Mr Aabo agreed, citing the example of shipyards incorporating their own heating/cooling systems which sometimes make it impossible to maintain stable temperatures. One example is the design of systems for heating the fuel oil before passing through the centrifuges, which often results in fluctuating temperatures and levels of fuel cleanliness.

Mr Henneberg said that in his experience the Asian yards are, in general, poorly prepared for forthcoming emissions regulations; although it is known that changes will have to be made to the ships, the yards do not seem ready.

Mr Koerber said that his company had adopted the latest scrape-down analysis which measures iron corrosive particles – few such systems can do this yet. Mr Walker said it is critical for ships to have good test kits onboard, while Mr Schubert pointed out that because crew quality is variable, tests should be made simple.

Mr Toll pointed out that when crews change frequently, as is usually the case, then service letters are not of much value on board.

Mr Koerber raised the point of lubricants for use with 0.1% sulphur fuel, to which Mr Aabo replied that it highlighted a challenge, as with no ships currently using such a fuel it is impossible to carry out sea trials. Laboratory tests with low sulphur fuel and low BN oil had shown that deposits could build up on the pistons, so more information is needed. Mr Walker agreed that it has proved impossible to find the lowest fuel sulphur content that can be used in conjunction with 100BN oils. An earlier estimate was 1.5%, but even this could well cause difficulties with deposits.

Mr Koerber and Mr Koslowski agreed that it was likely that there would be more surprises still to come, while Mr Henneberg felt that having to cope with several different cylinder lubricants onboard would cause problems. Mr Schubert said that for older engines the answer was to use intermediate BN oil and adjust feed rates to find the optimum level, keeping above the dew point.

Mr Aabo said that water and humidity did indeed make a difference. Scuffing problems in the past had partly been caused by water droplets landing at the liner walls. Mr Koerber pointed out that he had experienced problems with the design of water mist catchers on K90 and K96 engines; fortunately the ships were fitted with liner temperature indicators so the effects of changes to the feed rate were evident.

Mr Koerber said that all his ships used a lubricity additive in the fuel, because in his view not all fuel oils are fit for their intended purpose, and the lubricity problem will be greater with lower sulphur fuels, and fuel pumps will need to be overhauled more often. “For too long,” he said, “ships have been regarded as ‘unifuel products’”.

Mr Pedersen summed up the discussion with five ‘take away’ points.

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