Stern tube overhaul on Offshore Support Vessel

Customer Name: Offshore Platform Supply Vessel Company 

Date: May 2023

Type of project: Field Service – Stern tube overhau

Project Location: MMS Shipyard, Hull, United Kingdom

Initial Enquiry: 

 A UK operator of Offshore Platform Supply vessels contacted us regarding high bearing temperatures being recorded in the vessel’s stern tube. One of our service team members attended to the ship to survey the vessel and understand the scope of work. Upon interviewing the Chief Engineer, it was discovered that the stern bearings overheated due to loss of lubrication fluid. This caused the bearing faces to become seized to the shaft momentarily, leading to accelerated bearing wear on the stern tube bearing faces. As a result, the bearing material migrated through the stern tube lubrication system, which was verified in the lubrication oil sample reports. It was decided that the vessel required the tail shaft to be removed and the stern tube bearing replaced during the planned upcoming dry dock. 

Project description:  

• Remove and install new forward and aft Svenska stern tube bearings.
• Supervision of all starboard prop shaft and rudder work.
• Supervision of port propeller and rudder work. 
• Laser alignment of starboard gearbox PTO shaft to the alternator.  

Before work commenced, a service team member visited the vessel whilst it was alongside in Scotland and carried out a survey to establish the scope of work. To carry out the work, it was established that the job required the removal of the stuffing box and the muff coupling from the starboard shaft. During the visit, the ship’s drawings and information concerning stern tube bearing measurements and shaft assembly, including the CPP hub and push-pull rod were obtained to enable our engineers to prep for the vessel’s dry docking.  

On the 24th of April, the engineers from AtZ and our sister company MIT attended the vessel in dry dock at MMS, Hull. They collaborated closely with the Marine Super-Intendent, Dockyard Manager, dockyard staff, and ship’s crew to carry out the work. The first step was the removal of the rudder from the vessel on the starboard shaft; this was necessary to gain access to the propeller. The team then removed the propeller from the CPP hub, which was subsequently sent away for crack testing, polishing, and balance. The seal box was then let go on the aft of the stern tube, and our engineers drained the remaining stern tube oil and safely contained it. 

Work then transferred to the inside of the ship, where the muff couplings were released. Specialist high-pressure jacking equipment was used to remove from the taper on the shaft and to give access to the push-pull rod. Next, the push-pull rod was disconnected, as were the stuffing box and pipework, along with the forward stern tube seal. 

The next preparations involved removing the shaft from the vessel. AtZ/ MIT engineers were assisted by Dockyard staff to carry out this task. Multiple pad eyes were welded to the ship’s hull, allowing the engineers to guide the shaft out carefully. Once the shaft was removed, it was discovered that significant wear was present on the bearing landing of the shaft, which was not part of the original scope. AtZ/ MIT had to work closely with third-party suppliers and shipyard staff to find a company that could repair the shaft to DNV classification standards. Eventually, they identified a company in Holland, who was approved by class to carry out the repair.  

With the shaft diameter now reduced, the new stern tube bearings needed to be undersized to suit the new shaft dimensions. MIT facilitated this and turned them around in just three days, in time for the shaft return from Holland. 

The team then proceeded to remove the old bearings from the stern tube using specific tooling that the AtZ/ MIT engineers manufactured in our workshop. The process required a pressure of 30 tonnes to remove the bearings from the tube. After removing the bearings, the engineers thoroughly cleaned and inspected the stern tube for measuring purposes.   

Prior to the shaft’s return and in preparation for the stern tube bearings to be machined, the team took measurements of the stern tube and checked the bearings’ inside and outside diameters. This was crucial to ensure that the newly machined bearings were perfectly aligned with the shaft and stern tube dimensions, with the correct clearance and interference fit. 

Once the new bearings were manufactured the team began the work to reinserting the bearings. To do this they applied a pressure of 10 tonnes using specialist equipment. Throughout the procedure, stern tube temperatures were recorded, and the DNV class surveyor correctly observed and agreed upon all measurements.  

The stern tube bearing alignment was checked and witnessed by class. The old bearings were retained for potential rework for use as ship spares. This is advantageous for the customers, as the ship is one of three identical vessels in the fleet.  

Afterwards, the shaft was refitted into the vessel and reconnected using the reverse procedures of disassembly. The seal boxes were newly assembled on the forward and aft bearings, using Cedervall branded seals. Wartsila engineers were present to build and install the seal boxes, as specialist bonding was required. The process was completed successfully, and everything is in working order. 

Finally, the team assembled the CPP hub using the new parts, blade feet, and blade control. 

During the overhaul, the AtZ/ MIT engineers had to take out and inspect the rudders on the starboard and port sides. Unfortunately, they found some damage to the becker fin bearings, pins, seal liner, and linkage. So, new bushes, liners, and linkages were installed, and pins were welded and machined back to suit.  

In addition, it was discovered that there were signs of wear to the hub and cranks on the Starboard propeller hub – however, this wear was light and could be addressed at a later date.  

Once all parts were overhauled, the starboard gearbox PTO shaft was laser aligned to the alternator to ensure optimal alignment and perfect operation. 

Conclusion: Once all the work was carried out a sea trial took place. The captain was pleased with the vessel’s operation and could see a considerable reduction in stern tube-bearing temperature. The CPP reaction time was good, and no leaks were observed.