Safe operation of tugs within close proximity to the forward and aft regions of ships

When tugs are used for ship assist manoeuvers, hydrodynamic interaction effects between them can adversely affect the safety and handling of the tug. During such manoeuvers, tugs need to frequently change their location and drift angle with respect to the ship in order to provide the required assistance. Such variations can adversely affect the tug’s hydrodynamic interaction forces and moments, thus making it vulnerable to collisions or run-overs.

In order to safely and effectively operate tugs in these situations, it is essential that the operators are aware of the adverse interaction effects under different operating conditions and locations, enabling them to take necessary precautions and corrective actions to mitigate the dangers. To date, however, most of the data available in the public domain are limited to an ‘ideal’ tug assist situation, where the tug is operating parallel to the ship. This study discusses the hydrodynamic interaction effects on tugs operating at drift angles ranging from zero to 90 degrees relative to the ship, when located around the forward and aft regions of the ship and at progressively increasing lateral separation between the vessels. The study was conducted using Computational Fluid Dynamics (CFD) simulation models which were validated against experimental measurements obtained at the Australian Maritime College model test basin.

The non-dimensionalised interaction effects were used to create Hydrodynamic Interaction Region Plots (HIRP) to identify the variation of the coefficients with respect to the tug drift angle and the relative distance between the vessels. The results demonstrate that the safest approach to the ship with the least interaction effects is at a tug drift angle of less than 15 degrees. In addition, once the tug reaches the desired position relative to the ship, it is advisable to maintain a parallel course with the ship to avoid substantial longitudinal forces and yaw moments that can adversely affect the tug’s manoeuvrability.