Cruise lines analyse pod stress waves

Cruise operators are using stress wave analysis to monitor the condition of azimuthing pod drives and minimise the risk of unscheduled downtime due to bearing degradation. Most installations to date have taken place on ABB Azipods, but the stress wave analysis technology developed by US company Swantech has recently had its first installation on a Rolls-Royce Mermaid pod.

Celebrity Cruises has fitted Swantech’s Swan-branded stress wave analysis technology to the Mermaid-equipped Millennium. Purchase orders for installations on the sister ships Infinity, Summit and Constellation have also been signed and the fitting is ongoing. Celebrity Cruises’ parent company Royal Caribbean Cruises has already installed the Swan system to its Azipod-equipped, Royal Caribbean International-branded Voyager- and Radiance-class ships (it has also fitted Canadian company GasTops’ oil debris monitoring system (Ship Repair supplement, June 2002), on some of these ships for much the same purpose).

Carnival, which operates ships equipped with ABB Azipods, is also using the technology while NCL and V Ships (manager of the Radisson Seven Seas fleet) have also expressed an interest, according to Swantech. The company has also been in contact with ABB on an OEM basis. However Swantech sees a tie-up as unlikely as it could be construed as being a fox watching the chicken coop scenario. “Shipping people prefer for the system to be independent,” explains Brad Cartwright, manager of customer support for Swantech.

The Swan technology uses sensors placed at key points to detect and measure the energy content of ultrasonic sound waves. It uses the data to plot the incidence of damage in a machine. The company says the technology is able to separate high-frequency stress waves emitted by damaged components from the much lower frequency range of operating machinery vibration and audible noise.

The company says the technology features software analysis tools that can highlight the source of damage and warn of its progression as energy levels rise in proportion to the level of damage. The information is used to schedule maintenance or amend operations before catastrophic failure occurs.

Swantech argues that stress wave analysis gives an immediate indication of potential problems, whereas oil debris monitoring requires samples to migrate from the bearings before a potential problem is unearthed. GasTops counters that oil debris monitoring gives a better indication of the degree of bearing damage allowing a more accurate assessment of the bearings’ remaining useful life.

Irrespective of the arguments, Swantech says the data can be readily correlated with other drive information as part of a ship’s regular maintenance programme. It says the analysis enables manufacturers to identify design changes that will enhance the lifespan of their pods and reduce drydocking requirements. It also says that ship’s engineers can use the analysis to identify specific performance and tuning problems that can be addressed during maintenance and to adjust ship’s operations, if necessary, to accommodate specific problems.

For example, early installations of Swan have shown there appears to be a critical shaft speed at which damage is inflicted to the bearings. In the case of the ABB pods, mapping of cruising mode data on one ship has shown that abnormally high bearing damage is inflicted at 125 rev/min. Armed with such knowledge, the operator can avoid operating the pods at this shaft rotational speed (while cruising) to prolong the life of the bearings.

Mapping of data is continuing in order to assess other critical performance criteria that lead to bearing damage, such as during manoeuvring. This takes into account the impact of different azimuthing angles, different shaft speeds and different water depths etc., and is being undertaken for both two and three pod installations. “There are a lot of variables and we’ve only really started to control the easy ones,” confesses Cartwright.

Royal Caribbean admitted in a recent statement, announcing it was using stress wave analysis on its Voyager- and Radiance-class ships, that the design of pods, while elegant, has some drawbacks: “The pod drive’s sealed design makes it difficult to monitor for mechanical health,” it said. “Pods can be costly to maintain, especially if unscheduled downtime is required for repair.”

That the statement was released on its investor relations newswire can be taken as a tacit admission of the damage to its earnings that problems with pods have caused the company. It can also be seen as an attempt to reassure investors that it is minimising the chances of a reoccurrence. The fact that so many other cruise lines have installed, or are considering installing, monitoring technology – be it stress wave analysis or oil debris monitoring – further highlights the current criticality of reliable pod condition data to operations.