Operation of thrusters in arctic waters arctic thruster ecosystem

Due to collisions between ice and propeller the drive train of ice breakers in artic conditions is a highly stressed system. Aim of the ArTEco ‘Arctic Thruster Ecosystem’ project is to increase the reliability of vessels when overloads and torsional vibrations occurs. To achieve this aim different load scenarios will be analysed on a test rig located in Tuusula (Finland). Here the WST14 azimuth thruster, which is equipped with measuring instruments, is operated by the VTT and Wärtsilä. To investigate the behaviour of the test rig and the thruster different simulation models are created. These multibody system simulation (MBS) and finite element models (FE) are required to understand the behaviour of the thruster and investigate improvement strategies. Targets of these investigations are the dynamic behaviour during ice contact and the optimization of bevel gears in prospect of safety and efficiency. Therefor estimated propeller loads that occur during ice contact of the gear box housing or loads occur by hitting the propeller blades are used. The simulation models of the thruster regards the flexible structure of the housing and shafts. Using this information the comparison considers natural eigenfrequency correlates to the test rig in Tuusula and the misalignment of the bevel gear can be investigated, validated and an optimisation achieved. For further analysis a simulation model is assembled by TU Dresden and verified by several time based data sets and modal analysis of the test rig. That way overloads and high dynamic loads which can’t be applied on the test rig are evaluable. Besides the dynamic analysis a progress in design phase for bevel gear stages is done. This is achieved by using complex FE models including the elastic bevel gear contact, bearing stiffness, clearances and the support of the flexible housing. The complex load and temperature condition lead to different displacements of the gears. Using simulation based displacement data and the software BECAL [1] a precise contact pattern can be investigated to determine safety factors, damage sum and efficiency. To investigate the possible efficiency improvements of a bevel gear a design process is described. Therefor a variation of macro- and micro geometry were made. Point of interest is to shift the theoretical pitch cone relative to the contact pattern, to reduce the local sliding speed. The combination of profile shifting ℎ1 , pressure ang e and profile crowning modification leads to a significant efficiency improvement

Bevel gear calculation of a vessel drive train with azimuthing thrusters

Increasing requirements to modern vessels concerning performance and ma- noeuvrability led to very complex drive train designs. Therefore azimuthing thrusters became very common in the area of offshore supply vessels, tug boats and specialized research vessels. In order to extend the already huge field of operation and to better understand dynamic effects in such azimuthing thrusters the research project "EraNet HyDynPro" was started. The project focuses on the design of a robust drive train which contains a bevel gear stage. Therefore loads caused by propeller-water-interactions as well as loads while operating under ice conditions will be analysed. Based on this interdisci- plinary project a contemporary way of gear calculation will be shown in this paper. For gear calculation it is necessary to acquire design loads. These can be measured at high costs or be simulated numerically. In the last few years the Multi-Body-System (MBS) Simulation got more and more popular to determine static and dynamic properties of large drive trains. This simulation can contain mathematical models of the propeller behaviour inside the water and under ice conditions as well as models of the electrical motor. Based on this a good prediction of gearing loads is possible. By the knowledge of the gearing load time series a complex tooth contact analysis can be made for every arbitrary point in time using specialized gearing software. Gear tiltings are considered in order to calculate the load distribution on the tooth flanks. By this knowledge a local comparison of the load and load capacity can be carried out. But how to handle the vast variety of load situations during different load cases and long time series? Therefore an appropriate classification of the different occurring states of gear deviations and gear loads will be presented. This way we are able to determine the risk of common gearing failures like pitting and tooth root breakages in detail for individual drive trains under different operating conditions. The paper will present the procedure of this contemporary way of designing gears. By using a simple spur gear set the general idea of simulating the operating conditions of a drive train and the subsequent complex tooth contact analysis will be explained. On basis of this information a suitable approach to assess the risk of different gearing failures is carried out and the results will be illustrated

Possibilities to determine design loads for thrusters

Drive trains of icebreakers in arctic conditions are subjected to high stress levels. Due to collisions between ice and propeller, overloads and torsional vibrations occur. The main objective of the “Arctic Thruster Ecosystem” (ArTEco) project is to increase the reliability of vessels in arctic conditions. In order to achieve this goal different load scenarios are analyzed with multibody-system (MBS) simulation tools complemented by a test rig facility located in Tuusula (Finland). It is operated by the technical research center of Finland (VTT) and Wärtsilä. On the test rig, various loads can be applied to different azimuth thrusters, which are equipped with diagnostic instruments. For further analysis a simulation model is assembled and verified by several time-based data sets and modal analysis of the test rig. Thereby, overloads and high dynamic loads that are unobtainable at the test rig become accessible in simulations. To reduce torsional vibrations, different damping systems, designed at the department of intelligent hydraulics and automation (IHA) Tampere and the VTT, can be analyzed with the MBS-model. Besides torsional vibrations, one objective is the displacement analysis of the bevel gears. Using simulation-based displacement data and the software BECAL [1], a precise contact pattern can be determined, which allows conclusions regarding the required safety factors