Environmental and Interaction Effects on Propulsion Systems
For the past few years MARINTEK and Rolls-Royce Marine have been developing algorithms and test procedures for prediction of the environmental effects (current and wave) and interaction effects (propulsor-propulsor and hull-propulsor) on propulsion systems.
Manoeuvring and positioning ships and other floating structures are becoming more and more essential in various marine operations. The characteristics of propulsors are usually known for the calm open water case, but there are different interaction and environmental effects that may affect significantly these characteristics.
It is important to consider these effects at the vessel design stage in order to be able to make an optimal selection of propulsor type and dimensions, while they are also crucially important for calculating thrust capability and introducing optimal scenarios in rough seas.
For example, if two thrusters are installed in tandem configuration at a distance of five propeller diameters, the front thruster may well lose about half of its thrust. Similarly, a rotatable thruster beneath the hull can lose about one third of its thrust due to interaction with the hull (Coanda effect), depending on curvature of the hull and distance to the hull.
Figure 1. Thrust loss due to thruster-hull interaction.
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Figure 2. Thrust loss due to out-of-water effect.
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Figure 3. Thrust and torque loss of aft thruster due to the turning angle of fore thruster.
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Figure 4. Thrust and torque loss of aft thruster due to the distance between thrusters.
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Another feature that has come to attention in recent years is dynamic loading fluctuations, which are mostly related to azimuth thrusters and tunnel thrusters. Dynamic loading fluctuations that cause mechanical failure are usually due to ventilation, which in turn is due to waves or relative motions. Dynamic loading fluctuations can vary from 0 to 100 % during a single ventilation cycle.
Experience has shown that despite the availability of theoretical methods for estimating environmental and interaction effects, there are still many cases for which only model tests can deliver satisfactory predictions. However, model tests need to be carefully performed, taking the relevant laws of similarity into account.
MARINTEK and Rolls-Royce Marine have been developing theoretical and empirical algorithms for various propulsor combinations and different environmental and interaction effects. Test procedures have also been developed and complex, unconventional tests are being performed in order to gain insight into unknown effects.
The algorithms are utilised regularly in the simulator and dynamic positioning technologies developed by the Control Department of Rolls-Royce Marine in Ålesund, Norway.
Contact at MARINTEK: Kourosh Koushan
Article in MARINTEK Review 2-2004