Why are some EALs failing in stern tubes?

PANOLIN business development manager Phil Cumberlidge separates fact from fiction regarding environmentally-acceptable lubricants and looks at performance issues affecting stern tube lubricants.

I think it is fair to say that synthetic (highly refined) mineral oils perform better than conventional mineral-based (petroleum) oils and we are prepared to pay more for the higher performance, longer lifetime and cleaner operations that the former offers.

Why then, given the accepted performance gains of synthetic over conventional oils, do stories abound about 'troublesome' and 'failing' environmentally-acceptable synthetic oils, some of which are suspected of causing stern tube bearing failures and damage to stern tube seals?

Firstly, let us define what an environmentally-acceptable lubricant (EALs) must do: it must have a minimal effect on marine life and therefore it must biodegrade; it must have a minimal toxic effect and not accumulate in the organs of marine life, to avoid being transferred up the food chain. The EAL must also perform well in the machinery it is lubricating.

From an environmental perspective, the US Environmental Protection Agency (EPA) has led the way in this area with its vessel general permit (VGP). This requires that vessels entering US waters must use EALs in "oil-to-sea interfaces" and classifies three lubricant types as meeting biodegradability requirements:

  • Triglycerides (HETG - a natural ester, otherwise known as vegetable oil).
  • Polyalkylene glycol (HEPG - otherwise known as PAG, or polyglycol).
  • Synthetic esters (HEES – two classes: fully saturated esters and unsaturated esters).

Polyalphaolefins (HEPR, otherwise known as PAOs, synthetic hydrocarbons, or 'man-made' mineral oils), are not included in the EPA's list of biodegradable base oils in its environmentally-acceptable lubricants document EPA 800-R-11-002 Section 2, of November 2011, or in its presentation Overview of the Final 2013 VGP of 6 November 2013, as they are only biodegradable in low viscosities. However, technically, the International Standard ISO 15380 (for hydraulic lubricants, industrial oils and related products) adds HEPR to the other three lubricant types, as ISO 15380 is not involved with the environmental performance of lubricants, only with technical properties and conversion recommendations from mineral-based oils.

Some EALs are not solely made from one base oil type; they can be blends of PAO and ester (typically unsaturated), to combine minimum performance and give the minimum requirement for biodegradability. Alternatively, they can be a mix of saturated ester to provide performance requirements and unsaturated ester to reduce cost – the lower performing unsaturated ester being lower in cost than the robust saturated ester.

So, how does an original-equipment manufacturer and consequently, a vessel owner, choose the right EAL for their equipment to provide optimum operational performance and reliability, and to support the scheduled operational availability of the vessel?

Reliability and long life come from the combined high performance of the base oil and the additives selected. Issues such as those listed below have become evident with some types of EALs:

  • Increasing oil viscosity due to poor thermal stability, causing equipment overheating.
  • Decreasing oil viscosity due to shear instability, causing equipment damage.
  • Varnish and gumming of the system, due to thermal degradation of the oil causing loss of machinery efficiency.
  • Slime and bad smells due to oxidation, water contamination and bacterial growth.
  • Corrosion of equipment internals due to the hygroscopic nature of the lubricant used.
  • Overheating of propulsion systems, questioning oil film thickness and frictional characteristics.
  • Difficult filterability, separating of sea water, particularly from emulsifying oils.
  • Additive solubility issues in cold conditions, where valuable performance additives come out of solution, clump together and get filtered out of the base oil.
  • Difficult change-over from existing oils in equipment, due to immiscibility/incompatibility with existing mineral or other EAL oil.

These problems result in more frequent system and lubricant monitoring, increased maintenance, more lubricant top-ups, system draining and refilling, increased costs for the transport and disposal of used oil and add to the cost of replacement parts. These are the obvious direct costs of using inferior performing/life oils. Other hidden costs include the paperwork to administer all these monitoring/inspection/testing/re-ordering/logistics operations, and lost revenue due to equipment downtime/failure – and the reputational damage of late deliveries of consignments.

Currently, the EAL world is in turmoil; reports abound of poor performance of EALs in equipment where mineral oils were previously proven to work.

Stern tube lubricants sometimes have a short life of only two to two and half years before acidity of the lubricant increases, viscosity dramatically increases, or the stern tube is contaminated with stinking black sludge that blocks seal oil-feed pipes and bearing oil lubrication washways.

Many vessel owners who adopted EALs early in the current VGP2013 lifespan have fallen foul of these lower performing, short-life EALs. Most are changing to the better performing EALs. An announcement earlier this year by DNV GL of a joint development project with several marine insurers and the University of Sheffield, which will investigate the increase in stern tube bearing failures, will shed light on whether EALs are to blame. If so, it will be interesting to see which base oil lubricant types are prone to operational failure. Slower steaming with larger, heavier propellers requires the highest oil film performance from the stern tube lubricant.

Fully saturated synthetic esters are leading the way in terms of reliable, high performance and long-life marine lubricants for stern tube/CPP and thruster propulsion, rudder and bow thruster steering and vessel stability systems, such as fin stabilisers and ballast control. They have the benefit of being environmentally considerate and reducing a vessel's carbon footprint. Documented monitoring of PANOLIN-saturated synthetic esters shows that in all areas of fluids and lubricant use – stern tube, hydraulics and gearboxes – the robust chemistry of saturated ester technology can enable up to 10 times the operational life of mineral oils, when combined with best management practice for lubricants, which includes looking after the oil and having regular sample checks.

It is the lubricant manufacturer's job to tell customers about the key performance indicators of their lubricant(s) – so consult them and ask them what specific EAL base oil technology they use. If they say "We use ester technology", ask if it is natural, unsaturated, or a blend of saturated and unsaturated ester, or if it is the 'real thing' – a fully 100% saturated synthetic ester.

The correct selection of the type of EAL base oil, coupled with complimentary additive technology and good housekeeping of the EAL's condition, can ensure an EAL specifically designed for the stern tube application performs well.

Source: Marine Propulsion & Auxiliary Machinery, December 2018
Author: Phil Cumberlidge, Business Development Manager of PANOLIN GREENMARINE