Ship-induced wave forces on a moored ship in the presence of uniform current

Abstract

Due to the increase in ship sizes and traffic, the effect of passing ships on the mooring forces of moored ships is becoming an increasingly more important aspect in restricted waterways, channels, and ports. The objective of the presented work is to investigate the effects of the presence of an ambient current on the hydrodynamic forces on moored ships when another vessel passes through the waterway.

In this research, XBeach-NH in (nonhq3d) mode is used to simulate passing ship effects, corresponding to test conditions as measured in physical model tests carried out at Deltares as a part of the JIP Ropes (Joint Industry Project, Research on Passing Effects on Ships) project (van der Hout and de Jong, 2014). Even though various layouts were tested in the Ropes project; the current paper focuses on the straight channel layout with different combinations of ship velocity and ambient current speed. Results show that XBeach slightly overestimates the draw down effects (water level depression) due to the primary waves, as well as the surge forces. And, the differences in surge forces between XBeach and measurement increases with increasing Froude number. However, sway forces and yaw moments are in better agreement with the measured data, even for higher Froude numbers, though slightly underestimated. This variation in results is consistent in almost all XBeach simulations. Results also indicate that ship velocities relative through water are more important than ship speed over ground in the presence of uniform current. However, in modelling exercises, it is advisable to run simulations implementing actual currents rather than simply adding or subtracting the current velocity to/from ship speed over ground to obtain a representative relative vessel through water, since in the latter case the duration of hydrodynamic force excitation on the moored vessel will not be realistic. Furthermore, simulations show that by only representing the correct relative speed through water in the simulations (and not the correct speed over ground), the surge force & yaw moment magnitude are underestimated in case of counter currents and sway forces are underestimated in case of following currents.