versions, a retractable azimuthing thruster can work in any orientation when extended but when retracted into its tunnel housing performs as a normal tunnel thruster. Norwegian manufacturer Brunvoll claims to have installed the first tunnel thruster in 1965 although there may be competing claims from elsewhere in the world. Tunnel thrusters have proved their worth over time in increasing manoeuvrability and – arguably their main purpose – reducing tug usage and associated expense.


Despite Asian dominance in shipbuilding, thruster manufacture and development remains mostly a European specialisation but with some US involvement. Asian manufacturing is not altogether missing as Kawasaki with its Rexpeller thrusters has been around for some time and, after installing its first in-house produced tunnel thruster in 2005, Hyundai Heavy Industries has branched out into azimuthing thrusters. In China also, companies such as NGC Marine and Wuxi Ruifeng Marine Propulsion are producing both tunnel and propulsion thrusters. Asian experience with thrusters has produced one claimed ‘first’: in early 2012, South Korean shipbuilder DSME claimed to have carried out the world`s first on-shore installation of an azimuth thruster. The ship involved was Heerema’s 50,568dwt pipe lay construction vessel Aegir. According to DSME, this new shore based installation method accelerated the construction schedule by almost six months when compared to the traditional underwater installation process. After 50 years of use for both propulsion and manoeuvring, thrusters are now quite a mature technology but improvements and innovations are still being made for all types. Most recently


this has involved rim drive


and permanent magnet technology, propeller blade form and modifications in duct shape aimed at achieving greater efficiency. Other recent developments in thruster technology have been the development of versions for use in ice and contra-rotating propellers, both of which come together in the Steerprop system for a ro-pax ferry being built in Italy by Fincantieri for Société des traversiers du Québec (STQ).


The number of thrusters in service is testament to their reliability and robustness but, as with all machinery, problems and faults will occur. Condition monitoring and condition-based maintenance are growing in importance in the marine industry and thrusters have not been ignored with products being developed specifically for use with thrusters and for the occasions when things do go wrong, there are also new developments in underwater repair. MP


www.mpropulsion.com


Hydrex engineers guide a thruster's hub onto a workboat (credit: Hydrex)


Special service speeds up repairs


As more vessels are fitted with thrusters, the likelihood of a breakdown or damage needing urgent attention


will inevitably


increase. Underwater repairs of all types are quite commonplace today and one of the world’s leading specialists, Antwerp-based Hydrex, has recently launched a service aimed at operators of ships fitted with thrusters of any kind. Hydrex is well known for using its mobdocks to facilitate repairs to hulls and the experience gained is behind the new service. The company claims it was the first to show that it was possible to remove and then replace thrusters fast by creating a dry environment underwater. Using mobdocks to seal off the thruster tunnel, with an access shaft protruding above the water, work teams accessed the thruster tunnel and removed or repaired the thruster within the tunnel in complete safety. Hydrex has developed this technology further using lightweight flexible mobdocks designed to be easily transported around the world.


In its new permanent thruster repair and


replacement system for offshore related vessels and units, each vessel will carry its own custom-designed mobdock supplied by Hydrex as part of the service. The mobdock can be included in the planning for a newbuild, installed on a unit going to drydock or constructed and brought onboard at any other suitable time. With such a system on standby any repair work to the thruster that may arise can be dealt with much faster and more easily.


Some of the thruster related repairs recently carried out by Hydrex include an 86m research vessel in Congo that needed the stainless steel belt in one of its thruster tunnels replaced. The belt is installed around the perimeter of the thruster tunnel at the location of the thruster blades where the impact of the cavitation caused by the movement of the blades is the most severe and is designed to give extra protection against cavitation damage. When this suffered cracks, the underlying steel was exposed to cavitation and the belt needed to be replaced to prevent the thruster tunnel from getting damaged too severely. To facilitate the repair underwater, Hydrex designed an open-top cofferdam that was constructed in a local workshop in Pointe- Noire under the supervision of the company’s diver/technicians. At the same time a regular shaped second cofferdam was also built. Using the cofferdams and having drained the tunnel, the old damaged belt was removed and replaced with a new stainless steel belt over a period of five days. More recent repairs involved bow thruster


blade replacements in situ on three container ships in Rotterdam and removal and later replacement of the complete bow thruster units on two other vessels in Rotterdam and Tacoma. In the case of the thruster removals these were done by first removing the blades and then the thruster itself. With the vessels sailing on normal operations between removal and replacement it was also necessary to seal off the tunnel from inside the vessel.


Marine Propulsion I April/May 2014 I 67


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