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DNV GL INVESTIGATES STERNGEAR FAILURES WITH EALs

Friday, October 11, 2019  (Comments: 2)

Classification society DNV GL has published the results of an extensive investigation into a surge of stern tube bearing failures which have occurred since 2014, to determine whether the performance of environmentally acceptable lubricants (EALs) is inferior to traditional mineral oils used in stern tubes.

The introduction of the Vessel General Permit (VGP) requirements in 2013 resulted in a sudden and massive uptake of environmentally acceptable lubricants (EALs) as stern tube lubricants for ships. This trend coincided with a reported widespread increase in the number of stern tube bearing failures, and it was inevitable that the maritime industry started to question the actual lubrication performance of the EALs.

DNV GL joined with insurers the Swedish Club, the Norwegian Hull Club, Gard and Skuld in 2017 in a joint development project (JDP) ito investigate the topic and map potential differences in lubrication capabilities of EALs and mineral oils.

Most failures were found to happen early in the lifetime of the vessels, and during operating conditions known to inflict significant shaft deflections in way of the aft bearing, such as extreme turning in the upper speed range.

From a design perspective, lubricant viscosity must be chosen correctly to ensure that hydrodynamic lubrication is given in all continuous operating conditions. The DNV GL study has, however, discovered two specific viscosity-related characteristics where EALs differ from mineral oils. These properties will affect the safety margin of the aft stern tube bearing in certain critical operating conditions.

Firstly, the pressure-viscosity coefficient of typical stern tube EALs is different from that of the same-grade mineral oil. The practical consequence of this difference is that under typical high-load operating conditions an EAL will operate with a lower true viscosity in the minimum oil film thickness region. This results in a reduced safety margin for an EAL installation in high-load running modes.

Secondly, the viscosity index is significantly higher for EALs compared to mineral oils. This means that EALs will operate with a much lower viscosity in the lower temperature region. Again, the safety margin will be reduced in those types of operation.

DNV GL concludes that to retain the safety level either the specified viscosity of the EAL should be increased, or the aft stern tube bearing design should be optimised to increase the shaft-bearing contact area and reduce peak pressure in the lubricant film. Based on this conclusion, DNV GL has revised the main class shaft alignment rules to achieve an equivalent safety margin for the aft stern tube bearings intended to be operated with an EAL.

DNV GL recommends that for:

  • Vessels with applicable DNV GL design rules from July 2019 or later – new shaft alignment rule criteria is mandatory
  • Vessels with applicable design rules older than July 2019 – not mandatory (but recommended to switch to the next higher viscosity grade above design specification)
  • All vessels – ensure seal compatibility and carry out proper system monitoring in accordance with oil and seal makers‘ recommendations.

Reader Comments (2)

Are the manufacturers of EALís involved with DnV GL in further development and modification of their respective lubricants to improve viscosity stability and lubricity?

By Michael Cheyne on Tuesday, October 15, 2019

Was this study carried out in colaboration with any of the EAL manufacturers . Seal manufacturers already provides compatible EAL grade.

By Md Shamsul Alam on Thursday, October 17, 2019

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