Thordon solves USCG stern tube problem with a solution for the future

Citation

(2007), "Thordon solves USCG stern tube problem with a solution for the future", Anti-Corrosion Methods and Materials, Vol. 54 No. 3. https://doi.org/10.1108/acmm.2007.12854cad.010

Thordon solves USCG stern tube problem with a solution for the future

Thordon solves USCG stern tube problem with a solution for the future

Thordon XL propeller shaft bearings have helped solve a stern tube problem that has plagued the US coast guard ice-breaker Polar Sea since 1992.

Owing to the harsh arctic environment in which it operates, the 13,500t displacement Polar Sea, one of the world's most powerful non-nuclear powered triple-screw icebreakers, suffered repeated corrosion and fretting wear within the shafts. This resulted in damage to the support lands for the stern tube bearing carriers.

The US coast guard effected repairs and repeatedly built up the support rings using a metallic polymer to improve the fit of the bronze carriers to the stern tubes. But this method of repair proved to be only temporary-short-lived and expensive to maintain.

So, in an effort to find a solution, the USCG hired engineering consulting firm LamaLo Technology to investigate the problem. A project was undertaken to develop a permanent repair to prevent the loss of fit between the bearing housings and the stern frame castings.

“We commissioned the study because we had a problem with maintaining the bronze carriers fit” said USCG Staff Engineer James Happe. “Ice-breaking operations and ice impact pounded the carrier shells and when they are loose, this can be a real problem.”

The original arrangement consisted of a bearing housing with dovetail slots in which Thordon XL bearing staves were inserted. The study concluded that the best approach would be to permanently fix the bronze carrier shells into the frame of the stern tube, so they effectively became part of the ship's structure. Such a change would, however, still have to facilitate bearing removal with the shaft still in place. After investigating various alternative arrangements a shortened, double- keyed, segmented, split tube, fully- grooved type bearing was recommended.

These bearing types offer a larger surface area that supports the shafting, resulting in better distribution of the ice loads to the bearing housing and bossing. In addition, these bearing designs provide for a much better condition for hydrodynamic lift than the existing dovetail slot design.

“This would provide the required high ratio of bearing surface to water groove and provide more support for the shaft” said LamaLo's Bruce Cowper. “We also shortened the housing/bearing length in the bossing to ease alignment requirements. But it was Thordon who came up with the double-key bearing design.”

Based on over 20 years' experience with the existing Thordon XL staves on the Polar vessels and acceptable performance in other applications in the USCG fleet, XL was chosen as the preferred material for the new bearings. Thordon XL is approved down to 2:1 L/D ratio and comes in a variety of configurations including staves, full- form tube and split tube. Thordon also provided the bronze double key sets for this project.

“The USCG is a very important customer with a long history of successful Thordon bearing installations so we brought in all our resources to quickly assist in coming up with a solution to this problem” said Todd Terry of Pacific Marine Equipment, the authorised US-based distributor of Thordon Bearings in the vessels' home port of Seattle. “We used the same Thordon XL material in a completely different configuration. This configuration is unusual in that it is a segmented split tube bearing with a double key arrangement so that the upper and lower tube halves can remain secured or be released independently. We provided a flexible bearing solution that allowed the coast guard to solve their bearing carrier interface problems.”

James Happe said: “The Thordon element went pretty much like clockwork and the sea trials this April went well, showing no signs of problems. Underwater divers were also positive about the installation. We are now considering the same system for Polar Sea's sister ship, Polar Star.”

Since, no work on the new bearings is expected for eight to ten years, annual savings in repair/maintenance costs of about $230,000 per bearing are expected. In addition, there will be a reduced risk of unscheduled dockings due to premature bearing or seal failures.

Thordon estimates that over a 20-year period, potential savings of $13 million can realistically be gained by implementing the proposed modifications.

“It's a definite solution for the future” said James Happe.