First Permanent Magnet Tunnel Thrusters (TT-PM) for Norwegian ship owner Olympic Shipping.
Recently Olympic Shipping has installed the first of Rolls-Royce newly-developed permanent magnet tunnel thrusters into the hull of the OLYMPIC OCTOPUS, a Rolls-Royce designed UT 712 L multifunctional anchor-handling vessel (AHTS).
The ship is built to a classic offshore industry design. Her type classification UT 712 has been around since the 1980s, and is still regarded as a solid workhorse in international waters. Today’s L edition has a new hull design, with a bulbous bow, and meets the requirements for Ice-C and Clean. The design allows for flexibility in the choice of onboard equipment. This flexibility is particularly well exploited in the UT 712 L vessel.
The new thruster design concept is based on a permanent magnet motor in a rim, which drives the propeller in the centre and also frees up space directly above the thruster where traditional tunnel thruster motors are located, making room available for other equipment or alternative use. Compared to traditional tunnel thrusters, the new TT-PM offers an increase in power output of around 25% compared to similar size propellers, lower noise and vibration, and capability for underwater removal, eliminating the need for dry docking. Rolls-Royce president of offshore Anders Almestad said that the TT-PM is suitable for a range of applications in offshore and merchant vessels, where operators will benefit from its high power output and rapid response to power demand, combined with exceptionally low noise levels. "It is capable of running for thousands of hours in intensive operations such as the harsh conditions of the offshore oil and gas fields, where rapidly varying loads and alternating thrust directions are the norm," Almestad said. The permanent magnet motor includes two main parts; a stator that carries a number of electrical coil windings, and a rotor equipped with a number of very strong permanent magnets. A rotating magnetic field is created by the stator that interacts with the fields of the permanent magnets on the rotor, generating force to drag the rotor around, providing mechanical power.
The 800 kW unit has an effective diameter of 1.6 m and is installed as a stern thruster, where it will run for long periods as part of the vessel’s propulsion system used daily and included in the DP2 class dynamic positioning outfit. It operates with variable speed obtained by controlling the frequency of the electrical supply. Rim Drive is a tightly integrated system combining electrical, mechanical and hydrodynamic elements. The electrical motor takes the form of a thin ring. Its stator is fixed in the tunnel and its rotor carries propeller blades that point inwards. Extensive use is made of composite materials and the bearings are water lubricated, eliminating shaft seals and oil-filled gear housings.
Graphs: Courtesy of Olympic Shipping, Rolls Royce