Analysis of ROV thrusters and small marine propellers at specific rotational speeds
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Thrusters are vital for the functionalities of remotely operated vehicles (ROVs). The development of thruster design is a trade-off between cost, thrust force, physical weight and size. Furthermore, it is known that problems with thrusters are a common fault in ROVs. As a result, this study is performed on different thruster configurations to highlight important aspects in the design of thruster systems. This includes the analysis of different marine propellers with 4 to 6 blades and with a diameter of 300 to 400 mm. A six degree of freedom model is created in OpenModelica to investigate vibrations and bearing responses in thruster systems. The model consists of a marine propeller, a shaft, and two bearings, and is applicable for simulating various steady-state cases. The results of the simulations return displacements in the axial, horizontal, and vertical directions, which are further used to investigate vibration amplitudes and bearing life. The marine propeller contributions to the OpenModelica model are based on propellers from the Wageningen B-screw propeller series and open water tests of this series. The hydrodynamic added mass and damping elements are calculated from different sets of regression equations depending on the number of blades on the propeller, blade area ratio, and pitch ratio. Meanwhile, the mean thrust and torque are obtained from open water test data of the relevant propeller. The mean thrust and torque are then further used to calculate the dynamic forces and moments from the marine propeller. The bearing life of the bearings in the thruster is highly dependent on the axial load acting on the bearing, i.e., the thrust force. Moreover, if the propeller is not balanced then high centrifugal forces can occur, resulting in severe forces in the radial direction that can be of concern regarding the bearing life. Furthermore, the thruster and bearing design should be related to the maximum thrust force desired from the thruster. It is possible to use different propellers with the same design, to change the RPM-thrust force configuration, or to change the vibratory properties of the thruster system. However, the maximum thrust force for which the original thruster was designed should not be exceeded. Furthermore, the vibrations in the thruster system depend on the bearing configuration, stiffness, unbalance, and the propeller type.
Master's thesis in Offshore Technology: Marine and Subsea Technology