The integrity of a vessel’s stern tube seal system is critical to safe and efficient ship operations. Stern tube seals prevent lubricating oil from leaking into the sea and stop seawater from entering the stern tube assembly. When these seals become damaged or worn, operators face increasing oil leakage, water ingress, environmental risks, and potentially severe propulsion system damage.

Traditionally, repairing or replacing stern tube seals often required an expensive and time-consuming drydocking. Today, however, advanced underwater repair techniques make it possible to perform permanent seal repairs while vessels remain afloat and operational. Hydrex has become a global leader in this field, offering turnkey underwater stern tube seal repairs anywhere in the world.

The challenges of stern tube seal repairs 

Replacing stern tube seals is rarely straightforward. Different vessels have different seal arrangements, stern tube configurations, and shaft dimensions. In many cases, liners are worn or grooved, which further complicates the repair process.

If damage is left untreated, the consequences can escalate rapidly. Oil leakage can increase, environmental compliance can be compromised, and water ingress can threaten the reliability of the propulsion system. Acting quickly is therefore essential.

Hydrex addresses these challenges by performing underwater seal replacements using its flexible mobdock system. This technology makes it possible to create a dry underwater environment around the stern tube assembly, allowing diver/technicians to work in drydock-like conditions without removing the vessel from service.

The Hydrex flexible mobdock solution

A cornerstone of Hydrex’s repair methodology is the flexible mobdock. This portable underwater habitat isolates the repair area from the surrounding seawater and creates a dry workspace around the stern tube seal assembly.

By using this approach, repairs can be carried out while cargo operations continue, allowing ships to maintain their schedules and avoid unscheduled drydockings. Hydrex performs these repairs in cooperation with OEMs and can handle all seal types and configurations.

Another important advantage is rapid mobilization. Hydrex maintains fast-response centers with ready-to-deploy lightweight equipment, enabling teams to mobilize worldwide at short notice.

Case studies

Double stern tube seal repair on RoRo vessel in Tasmania

One of the most notable projects involved a double underwater stern tube seal repair on a RoRo vessel berthed in Burnie, Tasmania. The vessel was suffering from oil leaks on both stern tube seal assemblies, requiring immediate intervention.

Hydrex mobilized two flexible mobdocks simultaneously, allowing both assemblies to be repaired underwater at the same time. After arriving on-site, the diver/technicians established a monitoring station and carried out detailed underwater inspections.

The teams cleaned the assemblies, installed the mobdocks, and removed the split rings for cleaning. The damaged seals were then replaced one by one in cooperation with an OEM technician. After successful leakage tests, the mobdocks were removed and the rope guards reinstalled.

The operation enabled the vessel to avoid an expensive drydock visit while remaining on schedule, despite the remote location of the repair.

Emergency stern tube seal repair in Rotterdam

Hydrex also completed an emergency stern tube seal repair on a 170-meter tanker in Rotterdam under severe time pressure.

After receiving the inquiry the technical department issued a quotation within two hours, and the repair operation was scheduled immediately. Two teams of diver/technicians worked in day and night shifts to minimize repair time.

The operation included underwater inspection, removal of the rope guard, shaft wear-down measurements, installation of the habitat, and replacement of all four stern tube seals. The stern tube assembly was subsequently tested and reassembled successfully.

The entire repair was completed ahead of schedule, allowing the tanker to return to service without a drydock interruption.

Container ship repair in Algeciras 

A 225-meter container vessel in Algeciras required urgent underwater seal replacement after suffering an oil leak. Hydrex mobilized lightweight equipment from its fast-response center and rapidly deployed a diving team to the vessel.

After conducting a thorough underwater inspection, the team removed the rope guard and cleaned the seal assembly before installing the flexible mobdock. Working in dry conditions underwater, the diver/technicians replaced the damaged seals and carried out successful leakage tests before reinstalling the rope guard.

Because the operation was completed underwater and on-site, the vessel was able to continue its schedule without the delays associated with drydocking.

Cruise vessel repair in Brest 

Hydrex also carried out an underwater stern tube seal replacement on a 150-meter cruise vessel in Brest, France. The vessel had a tight operational schedule and could not afford an unscheduled drydock stop.

The Hydrex team mobilized quickly and set up a monitoring station alongside the vessel. After inspection and rope guard removal, the flexible mobdock was installed to create a dry underwater workspace.

The damaged seals were replaced efficiently, enabling the cruise vessel to continue its itinerary without disruption. This project highlighted the importance of underwater repair solutions for passenger vessels operating on strict schedules.

Semi-submersible offshore platform in Mexico 

In Veracruz, Mexico, Hydrex teams carried out a complex underwater seal replacement on both stern tube assemblies of a semi-submersible offshore platform.

The operation involved replacing twelve seals in total — six on each assembly. Diver/technicians first performed inspections and installed flexible mobdocks around the assemblies. The damaged aft and forward seals were then removed and replaced one by one.

After successful leakage testing and reinstallation of the assemblies, part of the Hydrex team remained on standby to ensure everything operated correctly before demobilization.

This project demonstrated Hydrex’s ability to handle large-scale and technically demanding seal repair operations in offshore environments.

Conclusion

Stern tube seal failures can have serious operational and environmental consequences if not addressed quickly. Hydrex’s underwater repair methodology enables permanent seal replacements to be carried out safely and efficiently while vessels remain afloat.

Using flexible mobdocks, highly trained diver/technicians, and rapid worldwide mobilization capabilities, Hydrex has successfully completed complex stern tube seal repairs on tankers, container ships, cruise vessels, offshore platforms, and RoRo vessels across the globe.

These case studies demonstrate how underwater repair technology allows shipowners to avoid drydockings, minimize downtime, and keep vessels operating on schedule — all while ensuring reliable, long-term repair quality.