Feature 3 | PROPELLERS AND THRUSTERS TRUST in MARIN for a dynamic solution
MARIN has a long history in research into the performance of thrusters during Dynamic Positioning (DP) operations. Today MARIN is carrying out research on ducted propellers thruster interaction effects in the application of DP systems.
A
s a result of the application of DP systems on many different vessels, there is continued
interest in this subject. Today, newly developed tools enable more detailed measurements and computer simulations of the thrusters on DP vessels. This has lead to new research initiatives to improve the understanding of thruster interaction effects. During DP operations the effective force generated by thrusters can be significantly smaller than would be expected based on the thrusters’ open water characteristics. Tis is a result of thruster interaction
with the hull, current and the wake of neighbouring thrusters. The understanding and quantification of thruster interaction (or thrust degradation) effects is essential for an accurate evaluation of the station- keeping capabilities of DP vessels. At present, thrust degradation effects can be quantified using data available from literature, or by carrying out dedicated model tests. Published data can give valuable insights but often it is too general, or not applicable to the specific design. Model tests, on the other hand, do provide detailed results but they are relatively expensive. In addition, model test results oſten
only become available relatively late in the design process, making it difficult to incorporate the results in the design. Computational fluid dynamics (CFD) calculations could be an alternative method but there is little experience in the application of CFD as an engineering tool for thrust degradation effects. With the rapidly increasing capabilities of CFD models and computer hardware, the time is right for the development of new tools to analyse thruster interaction.
New measuring techniques In the summer of 2009, MARIN received its new stereo-PIV measurement system,
56 PIV measurements of a thruster wake in open water conditions.
PIV measurements of the wake of an azimuthing thruster under a barge.
which has a powerful Class 4 laser with two digital cameras built into a single underwater housing. MARIN carried out a research project to evaluate the possibilities of using the PIV system for measuring the flow velocities in the wake of an azimuthing thruster. Model tests were carried out in MARIN’s Deepwater Towing Tank. An example of the measured wake velocities in open water conditions, showing both cross sections and velocities in the longitudinal plane, is shown above. Results show that the new PIV system is capable of recording the velocity field in the propeller wake, with a level of detail that was not possible before. Tis data is extremely valuable for the validation
of CFD calculations. The model test programme also included measurements with the thruster built into a barge model. Here, the deflection of the thruster wake, due to the presence of the hull and its rounded bilge (Coanda effect), could be captured (see example alongside). Tese first PIV measurements will be used as validation material for CFD calculations. A key issue in these calculations is the correct representation of the shape of the thruster wake.
Research efforts In the coming years, CFD calculations will increasingly be used as an engineering tool for application in offshore hydrodynamics. Examples are the calculation of current and wind loads, viscous effects in wave loads, VIM and the analysis of thruster interaction. MARIN’s research aims are for its CFD studies to be documented into a “roadmap”, in a bid to streamline research and development. Currently, MARIN is starting a Joint Industry Project on the hydrodynamics of thruster interaction,
The Naval Architect July/August 2010
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