Escort tugs are a relatively recent development within the marine industry. Their role is to control a loaded oil tanker in the event of a failure of its propulsion or steering system. The maximum speed at which an escort tug can take control of the tanker is approximately 10 knots. In order to generate the highest possible towline force at this speed (up to two and one half times the bollard pull), the tug skipper uses the combination of high yaw angle and vectored propulsion system force. These conditions are typically considered to be "off-design" for ships and so little information is known about the hydrodynamics of this situation. The current generation of escort tugs have evolved from practical experience rather than a detailed analysis of the hydrodynamics of the situation. As a result, it is unlikely that tug performance has reached its peak. This report presents some preliminary CFD predictions for the flow around an escort tug and the resulting forces. The report also presents a plan for carrying out Particle Image Velocimetry (PIV) experiments to measure the flow patterns around an escort tug model, based on experience gained with preliminary experiments. The report also discusses the theory of PIV methods and the associated uncertainty of PIV experiments, and what steps will need to be taken to define the uncertainty of the Memorial University PIV system. The outcome of the research will be a unique data set of flow measurements around a yawed hull, where the yaw angle is a required operational condition. The experiment results will be used for understanding the factors effecting escort tug hydrodynamics and for validation of commercial CFD codes for the same flow situations. The validated code will be used to analyze additional hull form configurations and can be used to develop the next generation of escort tugs from sound hydrodynamic principles.
National Research Council of Canada. Institute for Ocean Technology
St. John's, Newfoundland, Canada
Technical Report (National Research Council of Canada. Institute for Ocean Technology), no. TR-2005-15 (2005).