Reliability of the propulsor is vital or shipowners. Failure in propulsion may decrease the safety of the operation, especially in rough seas. It may also lead to discontinuation of the vessel operation and significant reduction of the income provided by the vessel.
Normally, propulsors on ships are maintained during dry-docking of the vessel every five years. However, there is some push from owners and operators to increase dry-docking periods from five to seven years or even more.
In the offshore business, semi-submersible drilling rigs, for example, are not dry-docked at all and their many propulsors, usually eight per rig, are overhauled by demounting a few propulsors at a time. The change-out of propulsor is carried out underwater with the help of divers. The extra replacement unit is changed-in and the demounted propulsor is overhauled (critical components inspected) and put in service rotation.
The Azipod C design has some major advantages to ensure safe and reliable operation over the overhaul period:
– The gearless and simple drivetrain has a minimum of mechanical parts that wear, making overall reliability superior.
– The multistage shaft seal has a primary seal and two backup seals. The leakage from the primary seal can be monitored, giving sufficient planning and preparation time for maintenance in the case of a damaged primary seal.
– The Azipod C motor module is pressurised to avoid any water ingress into the propulsor.
Long-term experience with the Azipod C propulsor is well documented. So far, the longest continuously serving propulsors have been in operation for close to nine years in a rig before an overhaul. Several guests at an offshore oil and gas industry seminar, held in February 2014 at ABB’s Houston Ship Channel workshop facility, were able to see the condition of the critical parts taken out from one of the long-serving Azipod C propulsors. The Azipod CZ propulsor was disassembled one week before the seminar and it came fresh out of operation after a swap-out at the end of 2013. The components showed hardly any wear and tear and most of them could be reused after a thorough inspection and partial reconditioning. See Figure 6 for an example of the critical component condition.
The seminar included a hands-on session where the audience could touch and inspect components such as the thrust, slewing and propeller-end bearing, shaft and slewing seals and the service brake. The strut of the unit and the stator and rotor of the propulsion motor where exhibited for inspection as well. The event was very well received by guests from the oil and gas and offshore industry, including visitors from drilling companies, shipyards, operators, classification societies and designers (see Figure 7).