Stern Tube Seal Repair and Propulsion Overhaul on an Offshore Supply Vessel (OSV)

Dry Dock Enquiry

MIT was asked to attend an Offshore Supply Vessel (OSV) in dry dock in Ulsteinvik, Norway, to replace the shaft seals on the forward (inboard) and aft (outboard) seal boxes using replacement parts supplied by the customer.

The vessel operator had identified issues affecting the integrity of the stern tube sealing arrangement and required urgent intervention to maintain propulsion reliability and protect the dry dock schedule.

Type of project:
Stern tube seal repair and propulsion overhaul

Customer:
UK Offshore Supply Vessel fleet operator

Location:
Ulsteinvik, Norway

Primary scope:

• Bonding and replacement of port and starboard outboard and inboard stern tube seals
• Replacement of seal box liners
• Inspection and refurbishment of associated driveline components

Project Planning and Scope Definition

The project began with an initial meeting between the MIT service team, the shipyard supervisor and the vessel superintendent.

This stage established the scope of work, confirmed safety requirements and allowed presentation of the Risk Assessment and Method Statement (RAMS) before physical work commenced.

Following approval, our engineers organised specialist tooling and customer-supplied spares to support the repair.

Because vessel downtime carries significant operational impact in offshore operations, planning focused on ensuring the work could progress efficiently while retaining flexibility if additional damage was uncovered during disassembly.

Stern Tube Seal Removal and Technical Findings

During disassembly of the port and starboard seal boxes, our engineers identified excessive wear on the seal box liners.

What initially appeared to be a seal replacement developed into a wider propulsion repair.

The findings were discussed with the superintendent and the decision was taken to extend the scope to include full liner replacement.

Further inspection identified:

In areas of the aft seal track the damaged surfaces were restored to support correct sealing performance and long-term reliability utilising parts supplied by the customer.

Shaft Extraction, Workshop Overhaul and Reassembly

The next stage required removal of the propulsion shafts.

Working with the shipyard team, the port rudder was removed to create clearance for shaft withdrawal. Engineers disconnected the inboard coupling and control rod before extracting the propeller shaft.

The same process was repeated on the starboard side.

Once removed, the seal boxes were transferred to the workshop where they were:

• Disassembled
• Cleaned
• Inspected
• Reassembled using customer-supplied seals, gaskets, bolts and replacement liners

During overhaul, corrosion inside the sealbox chambers was addressed to restore a secure sealing surface.

The refurbished seal boxes were returned to the vessel and reinstalled.

Propeller shafts were reinserted and aligned to Simplex tolerances and guidelines, with liner runouts checked and confirmed before final assembly. Bolts were secured using lock wire to maintain integrity under operational vibration.

Commissioning and Final Checks

The final stages focused on restoring lubrication and confirming seal performance.

Work included:

• Removal of propeller plugs for venting
• Filling of stern tubes with fresh oil
• Installation and filling of inboard sealbox tanks
• Inspection of all sealing boundaries

No oil leaks were observed following completion.

The shipyard then completed rudder installation while the MIT team provided technical support and supervised advancement of the cone in accordance with Kongsberg specifications.

Although a live sea trial could not be completed due to unrelated issues with the vessel’s engine coolers being rectified by a third-party contractor, all mechanical remediation and validation activities were completed to schedule.

Customer Feedback

The vessel superintendent commented:

“Nothing but high praise for the attitude, communication and flexibility of the team and as always it was a pleasure to work with. They adapted and completed the job over and above scope and as professionally as any team.”

Preventative Stern Tube and Propulsion Maintenance

This intervention reinforced the value of detailed inspection during dry dock periods.

What began as a planned seal replacement expanded into liner replacement, corrosion repair and shaft extraction once equipment condition became visible.

Projects like this highlight why propulsion maintenance depends not only on replacing failed parts, but understanding the wider interaction between shafts, seals, alignment and operating conditions.

MIT supports vessel operators with:

• Stern tube seal replacement
• Propulsion overhauls
• Tailshaft repairs
• Dry dock support
• Marine repair and breakdown response
• Shaft alignment and commissioning support

Speak to our marine service team to discuss upcoming dry dock works or in-service repairs.