Specialists Develop New Ways for Underwater Ship Care

Rather than booking vessels in to drydock, shipowners are increasingly opting to contract specialist diving teams to carry out the replacement and/or maintenance of propellers and tunnel thrusters. To meet the growing demand for underwater ship care, specialist ship repair and maintenance companies are now expanding their services to facilitate shipowner requirements in more and more locations.

In the last few months, divers from Belgium-based Hydrex, for instance, have carried out more than 25 propeller modifications around the world, primarily to restore the propellers’ balance after blades suffered ice or collision damage.

A ship with bent or cracked propeller blades might experience severe vibrations while sailing, potentially resulting in classification societies demanding repairs before the vessel is allowed to sail on. But by straightening the blades or cropping them, the propeller’s balance can be easily restored.

Prior to a propeller repair, a detailed underwater inspection is performed to obtain the exact parameters of the damage which are then used to make the requisite calculations.

By taking advantage of cold straightening techniques, damaged blades can be straightened underwater, allowing the ship to return to commercial operations without the need to drydock, says Hydrex. The cold straightening machine has been in use for some time now but recently the company has put a new machine into service that is claimed to enhance the technique. It is compatible with the existing model and is used to restore more severely bent propeller blades.

Cropping

However, if propeller straightening is not an option, the affected area on the blade will be cropped to restore the hydrodynamic balance thus achieving the greatest possible efficiency for the vessel.

Both cropping and straightening are done to restore the propeller’s performance to as close to its optimum condition. This can help a vessel that is suffering loss of speed due to an out-of-balance propeller. Propeller optimisation is sometimes also done to restore the performance even if no real damage has occurred; after years of service an engine will often lose some of its performance.

Hydrex not only offers repair services, but can also help customers when they have the need for preventive or other special custom projects. At the end of 2010, for example, preventive modifications were made in Bremerhaven to the blades of three ice-going sister vessels.

When several vessels suffered damage and the propellers needed cropping after the 2009-2010 winter, the owner wanted to find a way to prevent this from occurring to his other containerships. This was done by modifying the blades to a very specific design that made them less prone to damage while keeping the performance of the propeller as close to optimum as possible

However, when a blade is damaged too severely by ice or other floating debris, straightening is not feasible. If it is decided that cropping is the only option, measurements are taken and used to calculate the exact radius of the area to be cropped. This was the case with the propeller blades of a 197m ro-ro vessel. The actual repair job consisted of the four blades being modified underwater one after the other. After the damaged area was marked out and verified, the blade was cropped and its edge ground to give it the correct radius. When the cropping was complete, the blades were polished to make sure that any remaining loss of efficiency would be minimal.

Network

Meanwhile, UMC, the ship maintenance division of V-Ships, has continued to press forwarded with the development of its global network of diving partners. The philosophy that customers want global quality, locally available, at a locally competitive price underpins the investment which has been expanded to include offshore diving partners and a growing number of specialist engineering suppliers. UMC now serves the industry in more than 250 geographical locations.

Although a challenging management task to maintain such an extensive and growing network, it meets a clear customer demand for a one stop, affordable solution for afloat maintenance and repair. Flying in a reduced team size means UMC can offer cost effective solutions to its clients, whilst maintaining the level of quality and expert supervision of critical tasks.

Geometry

Recently UMC carried out in water repairs to a vessel in Durban. A previous inspection of the propeller revealed significant damage had been sustained to three blades of the four bladed 5100mm diameter propeller. UMC mobilised its fast reaction squad and specialised propeller repair equipment from Holland and working with its support partner in the area, who provided the support diving and CCTV equipment, UMC carried out underwater straightening by cold static loading which was successful in restoring all the deflected sections to near design geometry.

The propeller blade edges and repaired surfaces were polished to allow close visual inspection. This close up inspection confirmed there were no micro or macro fractures present. On completion UMC was able to advise that the propeller was considered suitable for use without operational restrictions, till the vessels next drydocking.

Moreover, following a routine propeller polish on the Royal Navy Type 23 Frigate HMS Lancaster, underwater survey photographs revealed cracks to the port propeller blade. Having worked closely with the RN over many years, UMC’s Design and Consultancy Division were well versed in the procedure for their removal and replacement, having helped write the guidelines.

UMC divers began their removal procedure by first recovering the cone to the surface using the dockside crane. Using chain blocks attached to the hull brackets, the propeller was then lifted clear of the water. And in order to ensure that the propeller did not rotate, it was kept at an elevation that ensured that the shaft key remained in the keyway. The propeller was then laid on a flatbed trailer and transported to drydock for repairs.

Working closely with key equipment manufacturers over the last six months UMC has established a suite of procedures and dedicated containerised equipment to support the afloat replacement of tunnel thrusters units. This trend will continue as UMC penetrates further into this market.

Rolls-Royce Marine, too, has now put together a fully equipped mobile propeller blade workshop that can be deployed at the drop of a hat to any location worldwide. Among the multitude of sophisticated technologies, the twenty foot equivalent container houses a seven-axis measuring arm with laser scanner and probes, a 200t C-clamp press, a blade balancing machine, high performance welding and grinding equipment and a PC allowing direct access to the original blade design drawings.

Singapore-based Rolls-Royce Marine says that that underwater propeller repairs can not only save considerable time but also maintenance costs, since there is no need to drydock the vessel.

In the latest edition of In Depth, Rolls-Royce Marine’s customer magazine, the company writes: ‘By arranging to get the blade repair container to the right location on a ship’s route, repairs can be planned into the vessel’s time in port, reducing unnecessary downtime and the time and expense of shipping the blades to a local repair shop where quality and performance could be compromised.’

[mappress]

Source: imarest, July 21, 2011; Image:Hydrex