On behalf of the Towage and Salvage Union (URS) based in Antwerp, Belgium an assessment is made of the hydrodynamic interaction forces acting on a tug sailing freely in the vicinity of the bow of a large container vessel.
Together with an increase of carrying capacity of cargo vessels – especially container carriers – the installed power has increased significantly, which results in an increased dead slow speed and therefore an increase in minimum speed at which these vessels sail. These ships thus travel at greater speed in the narrow canals and rivers that provide access to the ports. Although modern tugs are more powerful than their predecessors, their size has not increased at the same rate. Therefore, the tug has to travel at a higher Froude number than before at the moment the towline has to be passed, which may create a potentially dangerous situation for both the tug as well as the crew.
To pass the towline successfully, the tug has to spend considerable time at a constant station close to the forward shoulder or the bow of the cargo vessel. Due to the restricted water depth and the relatively high velocity, the flow around the bow of the container vessel is highly two-dimensional, which causes an amplification of the interaction effects (sway force, yaw moment) experienced by the tug.
To assess these interaction forces acting on the tug while sailing close to the bow of a large container vessel, a series of towing tank test have been performed in the Towing Tank for Manoeuvres in Shallow Water at Flanders Hydraulics Research in Antwerp, Belgium. During these tests, the model of a 33 meter azimuth stern drive tug is towed together with the model of a 200 meter container vessel.
During these tests the longitudinal position, the transverse position and the relative heading of the tug were varied systematically to assess the influence of these parameters on the interaction forces. All tests are performed in a stationary condition; the relative position of the two models is not varied during a run. In each configuration, the sway and surge forces, the roll moment and the yaw moment were measured. The model of the tug was free to heave and pitch. This series of experiments has been performed at multiple velocities.
The interaction forces registered during the model tests are then incorporated in a time-domain simulation, by treating them as extra body-forces, to assess the steering action needed to keep the tug at a fixed station. The fast-time simulation program is able to change the thrust of both thrusters as well as the azimuth angle. Mutual interaction forces between thrusters and thruster-hull interaction is not incorporated since it is outside the scope of this research. The mathematical model describing the manoeuvring characteristics of the tug is based on model experiments done at Flanders Hydraulics Research and modelling available from literature and Ghent University.
By simulating different positions at which the towline is passed, an assessment can be made of the most suitable position to perform this action. It is as well possible to determine a suitable approach velocity and approach.