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The Cussons Marine Hydrodynamic Division were required to supply their client, National Research Council Canada, Institute for Ocean Technology (NRC IOT) with a Planar Motion Mechanism (PMM) to replace an existing PMM.
Installed in either the 90m ice tank or the 200m towing tank the PMM is used to test ship models in ice and open water. The PMM allows a model to move in exact, pre-programmed patterns while forces, moments and motions around the model are recorded.
The PMM is installed in conjunction with a towing carriage, which is a large structure that straddles a long tank of water. It sits on rails that run parallel to the water tank so that the carriage can traverse up and down the length of the tank. The PMM is fixed to the carriage such that the long sway girder is normal to the direction of travel and a sway carriage is allowed to move up and down the sway girder. The sway carriage has a ‘tow post’ protruding out of the bottom to which the ship model is attached.
Cussons Technology required IDAC to evaluate the design of the sway carriage assembly using finite element analysis (FEA). The analyses were carried out using the FEA software suite ANSYS Workbench.
The sway carriage assembly was supplied to IDAC in SolidWorks format. This was then transferred to the finite element (FE) module, workbench design simulation (WBDS), where a FE model was prepared for analysis. A three-dimensional, linear structural analysis was considered for the study. 3D eight-noded solid-shell elements and 3D 20-node solid elements were used to mesh the sway carriage assembly geometry.
Components of the sway carriage assembly were connected using standard ANSYS contact elements. All contact was specified to be linear bonded contact. The material used in this analysis was plain carbon steel and it was assumed to behave elastically and homogeneously for the purpose of the design evaluation.
Two loadcases were analysed at each of the two restraint scenarios. The structure was fully restrained at eight points for both restraint scenarios. The graphic to the left shows the positions of the eight restraint points for one of the scenarios.
The loadcases were as follows:
- Normal usage – a gravitational load was applied to model the self-weight of the structure and forces and moments were applied at the foot of the tow post
- Emergency deceleration – again a gravitational load was applied to model the self-weight of the structure, and a force was applied at the foot of the tow post in order to model the deceleration
In addition to the above analyses a modal analysis was also carried out for the model in its two restraint conditions. This was required to evaluate the minimum natural frequency of the sway girder. The modal analysis was performed with the corresponding constraints for each restraint condition and a gravitational load to model the self-weight of the structure.
The results requested for each load case were displacements and stresses of the main sway carriage structure and the minimum natural frequency from the modal analysis.
It was concluded from the stress analysis of the sway carriage assembly that the current design, subjected to loading and restraint criteria specified by Cussons Technology, was unlikely to resonate or fail during its operation.
The analysis that IDAC undertook showed that the PMM system that would be structurally reliable when it went into operation at the National Research Council Canada – Institute for Ocean Technology. The alternatives to using FEA to verify the structural integrity of the PMM such as hand calculations would have been inaccurate or overly cautious thus compromising the design in terms of weight. Investigating the PMM using FEA analysis allowed accurate and early verification of its performance in a short period of time allowing Cussons Technology to install their equipment with confidence.
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