Supporting Fletcher Group With a Complete Drivetrain Return to Service During Dry Dock

MIT Group was appointed by Fletcher Group to support a major dry dock work scope on the FS Aquarius, a twin-shaft offshore supply vessel. The project centred on a full overhaul of the vessel’s port main gearbox, alongside bearing inspection, shaft line installation, sealbox installation, commissioning support, and post-dry dock vibration analysis. 

The vessel was fitted with a Lufkin HS2800HG-K-PTO-S gearbox and twin shaft lines. The work was completed during dry dock at Dales Marine Leith. It required close coordination between MIT engineers, the vessel superintendent, the shipyard, machining specialists, and bearing suppliers.

The overhaul was required due to age-related bearing wear and early-stage gear tooth damage. PTO coupling wear and an oil leak from the clutch oil delivery system were also identified. The objective was to return the gearbox to Class-compliant service condition, with gear mesh, bearings, lubrication, and alignment restored in line with OEM requirements. 

Dry dock projects of this type often develop once equipment is opened and internal components can be inspected properly. In this case, the planned gearbox overhaul became a wider programme of inspection, repair planning, specialist machining, component refurbishment, shaft line installation, and final verification before the vessel returned to service. 

Project Overview

Customer: Fletcher Group

Vessel: FS Aquarius

Vessel Type: Offshore Supply Vessel

Location: Dales Marine Leith

Gearbox: Lufkin HS2800HG-K-PTO-S

Work Completed: Port Gearbox Overhaul, Shaft Line & Sealbox Installation, and Vibration Testing

Verification: Sea Trials and Post-Dry Dock Vibration Analysis

Outcome: Gearbox Returned to Healthy Operational Condition

The Requirement & Pre-Dock Planning

MIT was requested to overhaul the port main gearbox during dry dock and support installation of the vessel’s port and starboard propshafts, rudders, and sealboxes.

The objective was to return the gearbox to a reliable, Class-compliant operating condition in line with OEM requirements. Before the vessel entered dry dock, MIT reviewed operating history, vibration data, and oil analysis trends to help define the scope of work. 

Planning also covered the practical requirements of the overhaul, including access, tooling, lifting arrangements, and component handling. Parts and consumables were reviewed in advance so that bearings, seals, gaskets, PTO coupling components, clutch system seals, and gearbox oil requirements could be planned before work began. 

This preparation gave the project a more controlled starting point once the vessel entered dry dock and the gearbox was opened for inspection. 

Gearbox Strip Down & Inspection

Once the vessel was in dry dock, MIT engineers began removing the external gearbox components and housing sections to access the internal arrangement. 

As the gearbox was stripped, damage was identified to the PTO shaft white metal bearings and the associated bearing surfaces. Further inspection also revealed damage to the drive shaft, including seal track, that required repair. 

In addition to this, further disassembly revealed excessive wear on multiple bearing journal areas, which were likely a result of the bearings moving within the housing. A retaining nut was also found to have come loose, releasing tension in the shaft. 

These findings reinforced the importance of allowing for discovery work during dry dock. Although the planned scope was a gearbox overhaul, the full repair requirement could only be confirmed once the gearbox had been opened, measured, and assessed.

Bearing & Thrust Pad Inspection

White metal bearings, thrust pads, and carrier rings were removed for specialist inspection as part of the overhaul. 

The inspection of these confirmed that the bearings were generally serviceable, but the white metal bores showed wiping. Some components also showed evidence of static misalignment, with uneven wear visible in the bore. The thrust pad assemblies showed side wear from the pads rubbing against the carrier rings, with scoring and wiping also visible on the white metal faces. 

This inspection stage was essential in determining which components could be refurbished and where further corrective work was required before the gearbox could be rebuilt. 

Developing the Repair Route 

Once the Lufkin gearbox had been stripped and measured, the bearing journal damage was found to be too significant for a standard rebuild. 

MIT reviewed the available repair options with the specialist teams involved in the project. The agreed route was to machine the bearing bores and use bespoke spherical roller bearings with a modified outside diameter, allowing the damaged housing areas to be recovered for reuse. 

Measurements were taken before and after machining to confirm that the bores were back within the required tolerance. The lubrication pathways were also checked, with corrections made as needed to ensure oil could reach the bearings.

Rebuild & Reassembly

Following machining and component refurbishment, MIT began the gearbox rebuild. 

Firstly, the housing sections were cleaned thoroughly before reassembly to remove swarf and contamination. The drive and input shafts were  then rebuilt, clutch operation was tested, and the shafts were returned to the vessel for installation. 

During reassembly, MIT installed the output shaft, white metal bearings, thrust pads intermediate drive shaft, and input shaft, supported by the gearbox housing sections. Gear backlash was set and fine-adjusted, with good tooth contact confirmed before final assembly. 

The housing faces were cleaned and sealed, and fasteners were torqued to the required specification. Bearings and retaining nuts were also marked in position to support future inspection and condition monitoring.

Shaft Line & Sealbox Installation

Alongside the gearbox overhaul, MIT assisted with the installation of the port and starboard propshafts, rudders, and sealboxes. 

The propshafts were installed, and the couplings were fitted to the required push-up measurement. Sealbox runout was checked and recorded on both shaft lines, with all readings within the required manufacturer tolerance. 

The port and starboard stern tubes were filled and pressure tested to 0.3 bar before being left overnight. With no leaks found, the installation was ready for the next stage of commissioning.

Commissioning & Final Checks

Once the gearbox rebuild and shaft line installation were complete, MIT supported commissioning checks with the onboard engineering team. 

The alignment of the alternator to the transmission was checked and found satisfactory. During the refit process, an access issue was identified with the PTO coupling. After review, a local machining solution was agreed, allowing the coupling to be installed and torqued correctly. 

The transmission was then filled with oil and prepared for operational testing. 

A seal housing modification was also completed to resolve an oil leak issue identified during commissioning. The modified arrangement provided a more robust sealing solution before the vessel proceeded to final trials. 

Return to service was supported by final commissioning checks, including leak checks, pressure and temperature monitoring, clutch operation, sea trials, and post-dry dock vibration analysis. 

Sea Trials & Post-Dry Dock Vibration Testing

The gearbox was initially run alongside the quay before the vessel proceeded to sea trials.

During trials, the gearbox load was increased, and key operating values were monitored. Pressures, temperatures, and general operation were confirmed to be correct, with no leaks observed. The gearbox operated correctly, and the vessel’s Chief Engineer was happy with the outcome. 

Post-dry dock vibration analysis was then completed across both the port and starboard main gearboxes. Measurements were taken across the input shaft, drive shaft, PTO shaft, and outboard shaft under varying load conditions. The vibration analysis confirmed that both gearboxes were in a healthy operational condition. The data showed expected gearbox signatures, including shaft speed harmonics and gear loading, with no indicators of concern.

The Outcome

The dry dock programme was completed with the port gearbox rebuilt, the shaft line and sealbox installation completed, and the vessel successfully tested before return to service. 

This project demonstrated MIT’s capability across the full dry dock drivetrain process, from initial inspection through to final return-to-service testing. Key areas included marine gearbox strip down and rebuild, bearing and thrust pad assessment, shaft line installation, sealbox runout checks, specialist repair coordination, commissioning support, and post-dry dock vibration verification. 

Planning a Dry Dock Project? We Can Help from Inspection to Return to Service

Dry dock projects often uncover additional repair requirements once equipment is opened for inspection. In this case, a planned gearbox overhaul developed into a wider scope involving bearing refurbishment, specialist machining, shaft line installation, sealbox setup, and post-dry dock vibration verification.

MIT supports vessel operators throughout the dry dock process, delivering gearbox overhauls, drivetrain inspections, shaft line work, alignment, commissioning, and condition monitoring to help vessels return to service safely and efficiently.

If you are planning dry dock maintenance, contact MIT to discuss your drivetrain requirements before the vessel reaches the yard.