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Ocean cruise ships continue to be plagued, to an increasing degree as more units are put in service, by bearing failures in the latest pod-drive technologies.
Celebrity Cruise Lines' Summit liner has been forced to curtail a May 7 cruise due to a bearing failure in its starboard pod drive. A subsequent trip was cancelled, and the boat is currently in dry dock in Victoria, British Columbia for repairs.
The Millennium-class Summit is equipped with two Rolls-Royce Mermaid pod propulsion systems. Two large roller bearings, approximately 2-1/2 feet in diameter, carry each pod's fixed-pitch 18-foot propeller. The bearings are apparently failing under thrust loading.
At least three cruise liners are currently out of service around the world in dry dock, having failed pod drive bearings replaced.
A marine bearing engineer contacted by eBearing termed the bearing problem, "huge in the maritime community," particularly in light of the fact that all of the pod drive cruise liners have experienced multiple bearing failures with no apparent resolution or lengthening of in-service MTTF (mean time to failure).
Celebrity Summit is in Victoria Shipyard, NCL's Norwegian Star and Pride of Aloha are also in dry dock or bearing replacement -- Pride of Aloha has standard drive, but the starboard propeller bearings have failed. A new sister ship to Pride of Aloha, Pride of Hawaii, was recently delivered late due to pod drive bearing failures it experienced during pre-delivery tests.
While Summit is equipped Mermaid systems, Norwegian Star and Pride of Hawaii are driven by competitor ABB's Azipods.
Each Mermaid pod propulsion system pulls the ship via a 19.5 MW electric motor turning the 18-foot propeller. The electric motor is contained within the pod, completely submersed, and has infinitely variable speed control. Most importantly, the two pods can be rotated through 360 degrees, providing thrust in any direction. The propellers normally point forward, but their infinite speed adjustment and infinite directional adjustment allow the ship to be steered in any direction at any speed up to 24 knots. The propulsion pods not only allow the rudder to be eliminated, but putting the power unit in the pod frees up substantial space onboard.
According to our sources, there is commonality across virtually all the bearing failures which even cuts across the pod drive designs, indicating a potentially fundamental bearing design and application problem for all pod drives.
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 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.
In the case of the Summit's most recent failure, spalling was detected when filters in the bearings' pressurized oiling system began to show metal particles. According to a company spokesman, Celebrity did not know the extent of the bearing problem indicated by the oil analysis until a full day later, while the ship was en route to its destination with a full complement of paying passengers.
The spokesman defended Celebrity's decision to continue, despite the obviously developing problem, saying, "It's a question of, 'When does that become problematic?'"
It became problematic the next day, when Rolls-Royce's Mermaid bearing engineers informed the Summit's maintenance staff the shaft speed would need to be reduced by a further 20%. Designed to run at 24 knots, Summit had already been running at 22.5 knots as a precaution. The new instructions called for another 20% reduction, to just 17.5 knots.
Because in-service loading is not linear, reducing the prop shaft speed by 20% reduces the spall-creating load on the bearings by more than 20%, although how much more is a function of design, operating environment, and the condition of the bearings. Under full power, the motors are capable of using 19.8 megawatts to swing the propellers.
Now, in order to finish its run up the west coast of the United States to Alaska on time, Celebrity decided it had to eliminate Summit's port calls in Seattle, Washington and Sitka, Alaska.
Sitka is particularly popular with cruise line passengers, and its elimination reportedly caused significant backlash from the nearly 2,000 passengers. Celebrity responded with a shipboard credit of $200 per stateroom, which did little to satisfy passengers who paid as much as $4,000.
The May 20 7-day Alaska cruise has been cancelled altogether, at significant expense to Celebrity -- refunds of all prepaid tickets for that cancelled cruise, plus issuing vouchers for a free cruise to those ticket holders. Adding the cost of dry dock, sailing time, and other factors, the cost of a drive bearing failure quickly runs into several million dollars. In litigation, Celebrity has cited direct and indirect costs ranging up to $14 million per episode.
In 2003, Celebrity filed a $300 million suit against Rolls-Royce / Alstom, claiming its Mermaid pod drive units are fundamentally flawed and that repair attempts were, "nothing more than an elaborate charade to cover up the truth."
Alstom Power Conversion, which supplied the Mermaid system electronics, settled with Celebrity several months ago for USD $36 million. The suit against Rolls-Royce is still pending; because the companies involved are all multinationals, the suit has created jurisdictional issues.
Military applications for pod drives have been developing slowly. The UK's navy shipbuilding head said, "The principal worry with pod engines is the shock loading." Shock loading a pod drive shaft tends to move it out of alignment, exacerbating the bearing load. While commercial pod drive motors continue to use roller bearings, military application designs are specifying sleeve bearings; the change should improve shock resistance and overall load-carrying capacity.
| Product Model | Inside Diameter | Outside Diameter | Thickness |
| GRAE50NPPB bearing | 50 | 90 | 30.2 |
| GE45KLLHB bearing | 45 | 85 | 42.9 |