Condition monitoring notations extend survey intervals

Two new notations from DNV GL offer potentially unlimited survey intervals for monitored water-lubricated tail shaft systems

DNV GL explains the basis of new notations offering unlimited survey intervals for water lubricated tail shaft systems, dependent on condition monitoring.

Editor’s introduction

On 18 January DNV GL launched new rules allowing users of water-lubricated tailshafts to extend survey intervals indefinitely, on the provision that they adopt suitable condition monitoring practices.

According to one key supplier, Thordon Bearings, the move (and similar moves from other class societies) represent the culmination of gradual improvements in water lubrication technology over the past two decades or more. That development has seen many early condition challenges – mainly caused by the corrosive attack of seawater on tailshaft bearings – overcome to the point where water-lubricated tailshafts are today viewed as essentially equivalent to conventional, oil-lubricated shafts.

Craig Carter, director of marketing and customer service, Thordon Bearings, notes: “Early users sent divers down for wear readings, but then began to see consistent wear over ten, 12, even 15 years. So they were able to say to the class societies, if the wear is consistent and predictable, why not allow longer survey intervals.”

The improvements in recent years include innovations in new coatings and materials that do not allow ‘wicking’ (the spread of corrosion down the tailshaft) when damaged, as well as in monitoring methods. Carter notes that the extended intervals from class societies – in combination with US requirements for environmentally-acceptable lubricants - are leading to a rapidly increasing market interest. In Thordon’s case that is emblemised by its first order in the large containership segment, for two 3,600 teu Jones Act vessels being built by Aker Philadelphia for Matson Navigation Co.

In the article below, DNV GL outlines the mechanics of its new class notations.

Water-lubricated tail shaft systems have assumed prominence recently with the increasing global focus on the environment and the stringent requirements for oil-to-sea interfaces in the US Environmental Protection Agency’s Vessel General Permit. Operators are expected to make an informed choice when it comes to selecting the type of stern tube lubrication and accompanying classification follow-up intervals for water-lubricated systems in comparison with oil-lubricated systems.

DNV GL therefore now offers two notations for water-lubricated tail shafts - TMON (closed loop water) and TMON (open loop water) - allowing unlimited intervals between tail shaft withdrawal surveys based on condition monitoring of the tail shaft, bearings and lubricant system. The new notations have been derived as a result of extensive internal research projects, risk assessments to identify respective failure mechanisms, historic experience and an evaluation of evolving industry designs and monitoring systems. Discussions with prominent designers and manufacturers were an integral part of the process to evaluate the practical feasibility of compliance with the requirements.

As long as the monitoring and inspections carried out during periodical DNV GL surveys do not reveal any unacceptable deterioration in the condition of the tail shaft, bearings and lubricant system, the tail shaft withdrawal survey will not have a pre-determined interval. The propeller connection survey, however, is carried out as an independent survey based on the type of connection – every five years for keyed connections and every 15 years for keyless and flanged connections.

The enhanced focus on the design and follow-up requirements for condition and performance-monitoring systems offers several benefits to ship owners and operators:

* Unlimited tail shaft withdrawal survey intervals;
* Condition-based maintenance to save time and costs;
* Effective management of risks;
* Optimum component and system condition;
* Early identification of deterioration within safety margins; and
* Environmentally friendly systems.

Customer focus

Tuva Kristine Flagstad-Andersen, head of the machinery and systems section at DNV GL’s group headquarters in Oslo, explains: “Water-lubricated systems are here to stay and put increasing responsibility on class societies to support the industry by providing a reliable means of assessing the condition of the shafts, bearings and lubricant system while considering the impact of survey regimes on operators. The introduction of condition-monitoring assisted surveys with unlimited tail shaft withdrawal survey intervals, without compromising safety, is seen as a major breakthrough in this regard.”

Flagstad-Andersen adds: “The optional class notation requirements focus on maintaining the integral components of the tail shaft system in an optimal condition through an effective design and in-service monitoring regime. Remedial actions based on the early identification of any deterioration in condition and performance act as critical supporting measures.”

The TMON (closed loop water) notation has already been requested and evaluated for the Western Queen, a tanker owned and operated by P&O Maritime. Robin Reed, group technical services manager, P&O Maritime, Corporate (Dubai), explains that ensuring the optimal condition of the shafting system by effective monitoring while deriving the best benefits from a classification perspective was one of the main reasons for the company’s choice of the notation.

“We expect the monitoring regimes to complement the enhanced design criteria and reflect the representative condition of the system in the operational phase to assist us in initiating subsequent preventive actions and planned maintenance,” says Reed.

Technical considerations

Condition-monitoring rules for propulsion shafting systems are critical to the effective management of risks, while allowing operators to derive the best operational benefits. DNV GL rules consider shaft corrosion, consequential fatigue resistance and bearing-shaft interaction as the most predominant factors in this regard.

Main design criteria

Corrosion

* Approved corrosion protection of the shafting and system;
* Passivating properties of the lubricant system for closed loop systems have been defined;
* A salinometer for closed loop systems, as a warning against sea water contamination;
* Alternative means of inspection for some open loop system designs (low grade stainless steel and liner/coating combinations).

Bearing-shaft interaction

* Remote sensors for trending the wear down of the aft tail shaft bearing;
* Approval requirements for bearings, shaft coatings and wear-down sensors;
* Stringent lubricant quality, flow and monitoring requirements in all the shaft's operating conditions (including stopped mode);
* A stringent focus on shaft alignment irrespective of shaft size.

Non-toxic nature of closed loop system lubricants

* Tested in accordance with selected ISO standards, when exposed to aquatic organisms

Pre-defined acceptance criteria

* For follow up in service

System integrity

* In-place replacement provisions for shaft sealing elements for the respective systems (closed or open).

Corrosion protection, closed loop

The tail shaft is protected against corrosion by an approved selection of material or equivalent arrangement and is supported by a passivated lubricant system, tested periodically for quality and wear elements. The passivation helps to minimize corrosion of the tail shaft and stern tube system.

Corrosion protection, open loop

The tail shaft is protected against corrosion by an approved selection of material or equivalent arrangement.
Stainless steel shafts with a PREN (pitting resistance equivalent number, reflecting the extent and nature of alloying) of 34 or higher are considered practically immune to corrosion and have an extremely high resistance to pitting corrosion.

Arun Sethumadhavan, principal engineer in DNV GL’s Oslo-based machinery and systems section for fleet in service, explains: “We intend to attract industry attention to the use of shafts with a high PREN by highlighting this distinction in the rules while making the choice of shafting material at the design stage. Shafts protected by a continuous corrosion-resistant one-piece sleeve also have a comparatively lower level of risk of corrosion resulting from a breach of the sleeve's integrity.

“In some designs with an increased risk of shaft corrosion or pitting - typically low grade stainless steel with a PREN under 34 and shafts with a coating and/or multi-liner combination - we utilize the alternative means of inspection (including boroscopes, inspection covers etc) to address the corrosion-related fatigue risks during a scheduled dry docking of the vessel. This also takes into account the longer duration for vessels with approved extended dry docking schemes. Designs involving A-brackets and struts with open shafts have also been given due consideration.”

Bearing-shaft interaction

The design conditions, including the selection of bearings and the quality and flow of the lubricant, should provide a satisfactory shaft-bearing interaction. Synthetic bearings normally reveal a poor shaft-bearing interaction through an accelerated bearing wear pattern accompanied by an increase in heat generation from the bearing.

The wear down of the aft tail shaft bearing is required to be monitored and trended using reliable remote static sensors which provide a reading whenever the shaft is stopped. Closed loop systems should also monitor the differential lubricant temperature across the bearings.
The efficacy of temperature sensors fitted in synthetic bearing materials, which are inherently poor conductors of heat, to reflect the representative bearing surface temperature, is questionable and hence not a requirement.

Follow-up in operation

The factors below represent the main considerations for follow-up in operation:

* Periodic trending of the aft tail shaft bearing wear-down measurements
* Periodical testing and analysis of the lubricant system quality and trending of corrosion/wear elements (for closed loop systems)
* Annual DNV GL surveys for the class notations
* Alternative means of inspection during scheduled bottom surveys in dry dock (for low grade stainless steel shafts (PREN of less than 34) and shafts protected by a coating and/or multi-liner combination)

Conclusion

DNV GL considers the new tail shaft monitoring notation with unlimited shaft withdrawal survey intervals to be one of the most important milestones in the evolving focus on water-lubricated systems.