This May will mark 20 years since Thordon Bearings’ water-lubricated propeller shaft bearings were first installed onboard Princess Cruises’ Grand Princess and Disney Cruise Line’s Disney Magic. Since those very first installations, the cruise sector has become a major proponent of the conventional water-lubricated propeller shaft design. In total, 32 oceangoing cruise ships now operate with a Thordon arrangement and there are 11 more on order.
Not only are cruise ship owners choosing Thordon’s systems because they are becoming more environmentally conscious, but they’re also implementing them because they are reliable and they equate to reduced operational expenditure and maintenance. During their years of continuous service, none of the cruise ships that feature Thordon’s water lubricated propeller shaft bearing systems have experienced downtime, cancellations or changes to cruise itineraries due to propeller shaft bearing or seal failure.
“Theoretically, a well-designed oil-lubricated shafting system should outlast the operational life of any vessel, but this depends on the quality and type of lubricating oil used and regular seal maintenance,” says Richard Vie, a retired chartered engineer who was involved in designing Grand Princess and subsequent Princess newbuilds in his former role as vice president of Technical Development and Quality Assurance in Carnival Corporation’s Corporate Shipbuilding division.
“When we built Grand Princess the risks we were addressing were unscheduled dry docks (there were not many dry docks that could accommodate a ship of this size at the time) and oil pollution,” Vie adds. “The cost benefit analysis included, as best we could, the expected lifetime of the bearings and we assumed one replacement throughout the life of the ship. Even with this cost figured in, the benefit [of Thordon’s water-lubricated conventional shaft system] was still overwhelming.”
Over the course of the past 20 years, Princess Cruises and Disney have closely monitored the bearing wear-down rates on both Grand Princess and Disney Magic. Published data shows that the propeller shaft bearing clearances – in other words, how far the shaft has dropped from its original build alignment – for both ships are within the classification societies’ maximum allowable wear-down rates.
“With water-lubricated systems the issue of how long it will last depends on the water quality and how many shaft revolutions there are in the ship’s life,” explains Vie. “As we can see from the Grand Princess experience, Princess decided to replace the original bearings with new [during a major refit, rather than waiting for special survey]. It does sound as though the Inconel shaft liners performed very well.”
After 18.5 years of Grand Princess’s continuous service, Princess Cruises decided to replace the bearing – again with Thordon COMPAC bearings – during a scheduled dry dock in December 2016.
“During the vessel’s scheduled drydocking in 2013, class surveyors found the COMPAC bearings still fit for purpose, but recommended changing them at the next dry dock in 2019,” says Andy Wright, fleet operations director for Technical Operations at Princess Cruises. “We decided to replace all four bearings in 2016 during Grand Princess’s extensive refit at the Vigor floating dock in Portland, Oregon. Despite our apprehension at working on equipment that had remained untouched and under water for many years it went very smoothly indeed with no issues.”
Meanwhile, Disney Magic continues to operate with the original polymer bearing. During her last shaft inspection in September 2015, surveyors recorded a bearing wear down of seven millimetres, which is significantly below Lloyd’s Register’s 10.5 millimetre maximum allowable clearance. This was 17 years after the system was first installed.
“Based on our cruise installations to date, a seawater-lubricated propeller shaft system will typically have a wear life of 18 to 20 years depending on the operational profile,” says Craig Carter, Thordon’s director of marketing and customer service. “For all the cruise ships where Thordon COMPAC is installed, none have required replacement due to wear, no shafts have been withdrawn and no corrosion issues have occurred. This is why it is the most reliable and pollution-free propulsion design for the cruise industry. It’s reliability is unmatched in cruiseship operation.”
As the design of the COMPAC system allows the water lubricated bearings to be removed, inspected and replaced without shaft withdrawal, the major classification societies have now introduced shaft condition monitoring notations for water lubricated shaft bearings. Provided that certain conditions are met, this can allow extended shaft withdrawal periods of 18 years or longer.
Although Thordon’s water-lubricated propeller shaft is reliable and environmentally sound, the system can initially cost slightly more than oil-based alternatives. This could be a possible factor in some shipbuilders’ reluctance to offer the solution as standard, according to Carter.
“The water-lubricated propeller has fewer components and is simple to install, so shipbuilders who do not offer it as standard are really missing an advantage, especially in these environmentally conscientious times,” he says. “Seawater-based shaft systems are less time consuming to install than an oil-lubricated system with complicated air seals. There is only one shaft seal, pipe work and wiring are minimal and there is no header tank to top up, which can reduce install and operational costs.”
One major advantage of Thordon’s system is the fact that it has no aft seal. This component is the most susceptible to damage in an oil-lubricated propeller shaft configuration. Typically, aft seals require maintenance every three to five years. Even if they are not damaged, the rubber sealing elements wear down as operating hours increase and seals start to leak oil. A typical closed-oil lubricated system contains 1,500 to 3,000 litres of mineral oil and when this leaks, it impacts the environment. In addition, seawater passing into the closed system can emulsify the oil, leading to shaft bearing failure.
“Aside from the environmental issue of oil leaking out, water ingress can affect the viscosity of the lubricating oil,” says Carter. “This means shipboard crews have to drain down the contaminated oil and top up with fresh, which can be expensive. You don’t have these problems with water-lubricated shafts. A seawater-lubricated system can eliminate this unnecessary expense, negate the regular maintenance or emergency repair of these seals, and mitigate against any off-hire time and unexpected itinerary change.”
Carnival has faced these issues in the past, says Wright. “We have experienced seal problems on vessels with oil-lubricated shafts, resulting in one vessel having to emergency dry dock as there was water ingress,” he comments. “Compared to the oil-lubricated propeller shafts of other vessels in the Princess Cruises fleet, there has been no issues with the Thordon water-lubricated configuration. To me, it’s a no-brainer. A water-lubricated shaft means there is less to worry about.”
Taking a cruise ship the size of Grand Princess out of service for an emergency dry dock can add a considerable cost, which can quadruple for podded ships if special tools or replacement parts are required. As such, water-lubricated shafts could have the edge on a cruise ship propelled by pods, which are no less immune to bearing and seal failures than the oil-lubricated shaft.
In a paper presented to the Royal Institution of Naval Architects in 2007, Fincantieri executives concluded that Lavini and Lorenzo Pedone concluded that a passenger ship with a rounded skeg hull shape, twin six-blade fixed-pitch propeller and a seawater-lubricated shaft line, with appendages optimised using computational fluid dynamics, would be comparable to a cruiseship with podded propulsion. Oil leakage is avoided, while the intermediate bracket provides a stiff shaft configuration that reduces bearing mechanical and thermal stresses, especially during manoeuvring or crash stops.
Another issue that may prompt more cruise ship owners to consider conventional seawater-lubricated shafts is the steep rise in bearing and seal failures on ships with conventional closed-propeller shaft systems where environmentally acceptable lubricants (EALs) have been used. This is such a concern that global classification society DNV-GL recently established a working group to study performance and compatibility of EALs on conventional metal bearings. The classification society also suggested that the introduction of new propulsion system designs, such as single stern tube bearing installations and larger and heavier propellers operating at lower revolutions per minute, also had a negative impact on the oil-lubricated propeller shaft bearing.
Certainly, faced with the prospect of environmental fines, reputational damage and limited dry dock facilities, it made sense 20 years ago for some cruise lines to opt for the seawater-lubricated propeller shaft. “We are pleased to say, they still do,” says Carter. “After 20 years of proven cruise industry service, it is difficult to deny the reliability and cost saving-attributes of a water-lubricated arrangement.”
Since those first installations, Thordon has supplied water-lubricated bearing systems to 32 cruise ships with 11 more on order that have specified the arrangement. Of the 18 cruise ships in the Princess fleet, 13 operate with the Thordon system as will three newbuilds, while all the cruise ships in the Seabourn, Disney and Viking Cruises fleets benefit from the arrangement. Other operators using conventional Thordon seawater lubrciated bearing systems include MSC Cruises, P&O Cruises, Oceania Cruises and Regent Seven Seas Cruises.