Investigation of Hydrodynamic Effects on Controllable-Pitch Propellers during Crash Stop Maneuvers

Due to the increase in global trade, both the number of ship passages and the size of the respective vessels have grown significantly. Especially in areas where limited space is available for navigation, this heightened traffic volume, coupled with the increased vessel sizes, increase the risk of accidents. As a result, the precise quantification of ship maneuverability has gained significance in recent years. Alongside maneuvering properties crucial for regular operations, such as course-keeping ability, turning capability, and turning circle diameter, the behavior of a ship during emergency stop (crash stop) maneuvers has gained great importance. Particularly in situations where evasion is impossible due to limited space, the emergency stop maneuver is the only way to prevent an accident or at the very least mitigate the consequences. During an emergency stop maneuver, the distance that the ship needs to come to a complete stop should be as short as possible. Therefore, it is important to rapidly dissipate the ship’s momentum. As the ship’s hull is designed to have minimal resistance, the propeller remains the only component that can significantly shorten the stopping distance. In this case the propeller operates under ”crashback” conditions, i.e. with positive forward ship speed and opposing propeller force. In order to achieve a backward thrust, the propeller’s direction of rotation is reversed in the case of a fixed-pitch (FP) propeller, or the pitch of the propeller is adjusted in the case of a controllable-pitch (CP) propeller. In the latter case, which is examined in the present work, the adjustment of the pitch leads to a significant distortion of the blade’s sections geometry relative to the inflow. Consequently, the hydrodynamic characteristics and the mechanical loads change significantly. The propeller now generates a force opposing the ship’s direction of motion. This setting results in a situation where the flow around the entire aft ship area becomes highly unsteady and is dominated by large areas of separation. This leads to pronounced fluctuations in propeller thrust and torque, which subsequently impact the entire propulsion system via the propeller shaft. The rapid generation of braking force by the propeller poses a significant load on the entire propulsion system. Based on the type of main machinery, the propulsion engine must undergo rapid load changes. Depending on the propeller geometry, operating points can also be reached where the propeller functions as a turbine, accelerating the rotation of the propulsion system. This can lead to mechanical overload of the drive system components. Especially in the case of electric drive components, the utilized electric motors might be forced into generator mode, potentially resulting in electrical overvoltage and a failure of the onboard power system. A fundamental understanding of the hydrodynamic phenomena occurring at the propeller is therefore of great importance when considering emergency stop maneuvers. As can be deduced from the above, this is particularly true for controllable-pitch propellers. Deeper insights into these phenomena provide the opportunity to include measures into ship and control system design that help to execute emergency stop maneuvers as effectively as possible whilst avoiding negative impacts on the overall propulsion system. This knowledge can significantly enhance the quality of emergency stop maneuver simulations, such as those conducted in a ship handling simulator. Furthermore, the specific loads occurring during emergency stops can already be taken into account during the design of the components, thus avoiding damages. When considering the dynamics of the propulsion system, besides the unsteady propeller loads, other properties such as hydrodynamic inertia and damping are also of high interest. While the hydrodynamics of marine propellers in design conditions are well researched, the emergency stop case has been less studied. This is especially true for controllable-pitch propellers. Ship ma- 1 Chapter 1. Motivation neuvering simulations offer the capability of fast and reliable investigation of complete stopping maneuvers. The quality of the results significantly depends on the accuracy of representing the hydrodynamics of the hull and propeller. The content of this thesis is based in the investigations conducted within the joint research project Opti-Stopp, in which the hydrodynamic, mechanic and electric challenges have been adressed and studied thoroughly in experiments and numerical simulations [1] for a propulsion setup which consists of CP propellers in combination with a diesel-electric engine configuration. In principle, emergency stop scenarios and the resulting special operating conditions for ship propellers can be studied through model or full-scale experiments and using numerical simulations (Computational Fluid Dynamics, CFD). However, except for sea trials, very few emergency stop maneuvers are conducted with ships for research purposes, mainly to protect the propulsion system from unnecessary high stresses. Additionally, measurements on ship propulsion systems are complex. Even in model-scale experiments, ships and propellers are rarely tested outside of design conditions. Furthermore, the limited towing tank dimensions makes free maneuvering experiments with a self-propelled ship model an ambitious task in most model testing facilities. However, the propeller alone can be studied during open water tests at operating points resembling those encountered in emergency stop maneuvers. In such measurements, various inflow conditions to the propeller have to be considered and a high-precision data acquisition system is required to record the propeller loads. Numerical methods for determining ship and propeller hydrodynamics have been state-of-theart for many years. The ongoing development of high-performance processors and large-capacity data storage allows CFD to rapidly and cost-effectively analyze hydrodynamic problems. In combination with powerful hardware, computational software with modern numerical models and methods can also resolve highly transient, turbulent flow conditions. Besides determining integral forces and moments (e.g. on the propeller), computational methods provide the capability to analyze the specific load on individual components over time (e.g. a propeller blade), their fluctuation ranges and statistics. Furthermore, CFD provides insight into local flow phenomena that are very complex or even impossible to capture in full-scale and model tests. Within the scope of this work, the hydrodynamics of the controllable-pitch propeller in crashback conditions are thoroughly examined using numerical methods. Initially, the applicability of high-resolution CFD methods for these purposes is validated and the possibilities and limitations of simulation and computational methods are analyzed. Subsequently, the transient flow around the controllable-pitch propeller is investigated using high-resolution CFD for various pitch angles during open water and behind-ship conditions for relevant operating points in crash stop conditions. The forces and moments on the propeller and aft ship are resolved over spatial and temporal intervals, offering insights into the operation of controllable-pitch propellers in emergency stop situations. The computational results are validated against highfrequency measurements from model tests. In the course of the evaluation, the focus lies on describing the fluid-mechanical phenomena and determining specific propeller and pitch-related parameters for a possible application in simulating the hydrodynamics and propulsion system of the ship.