Type of project:  

• Investigate and rectify the cause of gearbox housing bolts shearing and coming loose on Wartsila gearbox 
• Carry out a VSD (Variable Speed Drives) condition survey of the vessels Danfoss and ABB Drives. 
• Rudder and rudder stock bearing overhaul, including seals. 

Customer name: UK Offshore vessel fleet operator 

Location: Great Yarmouth/MMS Hull 

Initial enquiry: MIT (Marine & Industrial Transmissions) asked to attend the offshore support vessel to investigate why the gearbox housing bolts on the vessel’s port side gearbox were shearing and coming loose. The team attended while the ship was alongside in Great Yarmouth and inspected with a borescope, reviewing the condition of the bearings, clutch plates, and gears. This was when we identified the bearing spinning in the housing as the potential root cause for the bolt shearing. Following this attendance, an overhaul of the gearbox and alignment checks on the shaft line was recommended at the next available date. 

Following the team’s initial attendance, we were then asked to attend to carry out the recommended works on the gearbox whilst in the dry dock at MMS, Hull. In addition to this work, we were also asked to carry out a variable speed drive survey of all the vessel’s drives and an overhaul of the vessel’s rudder and rudder stock bearing, including seals. 

Project description: 

During a routine docking at Great Yarmouth, our MIT team was tasked with uncovering the cause of the vessel’s starboard gearbox housing’s loosening and shearing bolts.  

Upon close inspection, the engineers discovered shearing on five input side bolts and multiple leaks across the gearbox. The team began by removing the inspection hatches and checking the PTO gear and input meshing before inspecting the bearings, clutch plates, and gear with a borescope. This gave us a closer look at the internals, revealing wear on all bearings consistent with the gearbox’s age. However, one finding caught our eye—the Input FWD rear Bearing displayed signs of spinning within its housing, a likely culprit for the bolt failures. Which confirmed our thoughts from the initial borescope inspection. 

Following the inspection, our engineers submitted a detailed report with recommendations, which included a full gearbox overhaul, including bearings, seals, and gaskets, and the correction of the housing to stop the suspected bearing movement causing the bolts to shear. 

Following the report, the operators of the platform supply vessel advised that they would be dry docking the ship in the next quarter and asked us to attend to carry out the recommended work. They also asked us to carry out a rudder and rudder stock-bearing overhaul, including seals. In addition, we were also asked to carry out a condition survey on their variable speed drives, including 5 x Danfoss VLT drives and 1 x ABB ACS drive. 

Gearbox repair: 

Once the Yard team had installed and tested the lifting point, our engineers were able to carry out the task ahead safely.  

Our approach was methodical: the team started by removing all the pipework and sub-assemblies, ready to be resealed. 

Next, the coupling was removed from the alternator to the PTO shaft on the gearbox before removing the PTO shaft and top half housing, which revealed two sheared bolts and fretting on both faces, as well as wear to the thruster washer on the input shaft. This wear on the thrust washer could have been one of the reasons for the shearing bolts. Due to a long lead time on a new thrust washer from Wartsila, we had to manufacture a new thrust washer. Whilst a third party off-site did this, the team continued the job. They faced a particular challenge with removing three stubborn sheared bolts and repairing elongated dowel holes. With some ingenuity and hard work, we completed the required repair. The custom-manufactured thrust washer and strategic use of Belzona to fill in the damaged dowel holes were among the critical steps taken to ensure the longevity and reliability of the gearbox. The final touch was a thorough rebuild, following the manufacturer’s guidelines closely. 

Rudder overhaul 

The work on the rudders started with a visual examination of the port and starboard rudders, as well as their internal and external components. This initial inspection revealed significant play in the Becker fins, particularly on the starboard side, signalling the need for immediate action. 

We started by dismantling the Tenfjord steering gear and meticulously marking each component to ensure accurate reassembly. We discovered loose brackets within the chamber and a damaged yet functional nylon spacer. The inboard seals, tested with feeler gauges, proved to be in excellent condition, thus avoiding immediate replacement. 

Next, our engineers turned their attention to the port side and discovered that the port side valve block required replacement. This involved comprehensive rewiring and the installation of new hydraulic fittings and filters. The shipyard assisted in removing the rudders, which revealed poor grease conditions and discoloured stocks, clear indicators of seal failure due to water ingress. 

Our investigation found the seal boxes and liners in dire condition, with heavy wear and stepping. These components were critical for maintaining the seal integrity and required prompt replacement. 

We noted that all Becker fin bushings were also worn, contributing to the excessive play in the fins. Although the pins exhibited wear, they remained serviceable. However, we advised monitoring them for future maintenance. 

Using nitrogen N2, we achieved a precise shrink fit with custom-manufactured phosphorus bronze bushings. The upper bushings fit perfectly, and upon greasing, we prepared for the reassembly of the seal boxes, which underwent a thorough overhaul. 

After fitting new liners and greased seal boxes, we meticulously reattached the rudders, adhering to the manufacturer’s guidelines for torque and positioning. Functional tests confirmed correct rudder operation, with minor adjustments to the Becker fins. 

Danfoss Drive survey 

Our visit to the FS Kristiansand also included a Danfoss Drives survey, which revealed that the vessel’s drives were overdue for a service upgrade. It was clear that a retrofit or replacement of the ageing drives was imperative to maintain operational efficiency, and all recommendations for upgrades were detailed in the report submitted following the survey. 

Conclusion 

During the comprehensive sea trials, both the gearbox and the rudders performed well. The gearbox highlighted over 85% gear contact, and the newly installed components, like pressure gauges and needle valves, performed well, ensuring confidence in the ship’s rejuvenated performance. Simultaneously, both rudders operated seamlessly within the specified pressure parameters. Despite an unrelated electrical fault requiring external attention, all mechanical aspects of the rudders functioned without fault, further affirming the success of the repairs.  

This case study confirms the importance of the pre-docking survey as part of the MyDriveTrain Preventative Maintenance service package, which allows us to identify any potential issues that may come up as part of the docking. Id emerging issues, reduces downs, plan dry docking all parts and materials. 

To find out more about our comprehensive MyDriveTrain lifecycle services, please get in touch