The world's largest cruise ship, operated by the world's largest cruise line company, is at the center of a $100 million lawsuit, due to bearing failures.

Carnival Corp. (USA), through its Cunard Line division, has sued Rolls-Royce Corp. (UK), claiming the latter's Mermaid pod propulsion systems fitted to Carnival's flagship Queen Mary 2 are inherently defective.

In particular, Carnival claims it was convinced to use the units under false pretenses, because they remain prone to crippling drive bearing failures despite Rolls-Royce assurances to the contrary. The Queen Mary 2 was built around these units.

Carnival is seeking USD $100 million, to recover lost revenue, the frequent need to run slowly, repeated dry dock repairs (Mermaids are generally in-water repairable, but bearing failures require dry docking), and the installation of advanced bearing condition monitoring systems. Carnival is also asking that Rolls-Royce replace the QM2's Mermaid units with newer units equipped with new bearings.

Carnival's complaint goes on to state the four-motor pod drive system fitted to QM2 is inadequate for the purpose, experimental, and prone to a variety of vital system failures -- again, particularly involving the drive units' main thrust bearings.

The four Mermaid units have 28,800 HP (21,500 kW) electric motors. Each has a forward-facing "Kamewa" propeller, with separately replaceable blades.

Powering generators for the electric motors are four 16-cylinder Wärtsilä diesel engines, producing a total of 90,100 HP. Combined with two 67,000 HP GE gas turbines for slow-speed running, the arrangement is termed CODAG (COmbined Diesel And Gas turbine). QM2 is the first non-military CODAG installation.

Carnival claims Rolls-Royce hoodwinked it into "ion, acceptance and continued use" of the QM2's Mermaid systems, saying: "The Mermaids are simply defective and do not function as it was represented that they would. Most importantly, it is now also clear that the defendants have known about this situation all along and that they have deliberately conspired to mislead, deceive and defraud Carnival into believing otherwise."

Pod drive systems offer numerous advantages, such as not taking valuable space inside the ship, relative ease of maintenance, operating efficiency, and maneuverability.

But Rolls-Royce / Alstom's Mermaid and other pod drive systems have a troubled history, particularly when outfitted to large ships operating at relatively high speeds in rough water. The weak link has been the motors' massive thrust bearings, still prone to failure after numerous redesigns.

By massive, we mean two roller bearings, approximately 2-1/2 feet in diameter, carrying each pod's fixed-pitch propeller. The bearing failure mode is under thrust load, exacerbated by driveline shock loads in rough water.

A marine bearing engineer contacted by eBearing termed the pod bearing problems have been, "huge in the maritime community," particularly in light of the fact that all of the pod drive cruise liners ever built have experienced multiple bearing failures with no apparent resolution or lengthening of in-service MTTF (mean time to failure).

The common failure point of origin is raceway spalling, illustrating the pod drive's difficult operating conditions: substantial and rapidly-changing loads experienced while operating at or near maximum operating speed in open seas. Reduced shaft speed and/or smooth water tends to delay the onset of spalling -- the load decreases disproportionately with reduction in speed.

The first evidence of spalling is most often metal shavings picked up by the drive unit's oiling system. Some systems are equipped with monitoring transducers, and in those cases the failure is often detected earlier by means of vibration and predictive failure analyses.

Early or late detection, however, does not change the ultimate course of events. Spalling is a terminal condition which requires the massive pod drive bearings be replaced. To accomplish this repair, the ship must be taken to dry dock so the pods can be taken out of the water.

The QM2's pod condition monitoring installations are MasCon48 systems supplied by Nåiden Teknik, a division of SKF. The MasCon48s monitor vibration, temperature, speed and other parameters, flagging anomalies and giving instructions for correcting existing and/or impending conditions to maintenance personnel onboard. The systems also communicate via satellite for remote maintenance planning and monitoring.

Queen Mary 2 is the world's largest ocean liner and Carnival's flagship. It is also the first four-screw passenger ship built since the SS France in 1961. QM2 required more than eight million man-hours to build, and its final cost was nearly $1 billion. QM2 can carry more than 4,200 passengers and crew in luxury.

Carnival has sued Rolls-Royce in the past, filing a $300 million claim in 2003 for out-of-service costs related to other pod drive propulsion systems in its Millennium class ships, built prior to the QM2. It recovered from Rolls-Royce and Alstom in that case.

The company now says Rolls-Royce lied in claiming that problems Carnival experienced with its other Mermaid systems had been resolved, misleading Carnival into using them for the Queen Mary 2.

In service, Carnival said QM2 has had expensive repairs and out-of-service costs. Bearing failure monitoring equipment has been retrofitted, and the liner has been forced into dry dock for regular service every 2 years and sooner, rather than the designed 5-year intervals.

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