Decay of bow thruster induced near-bed flow velocities at a vertical quay wall: A field measurement

Abstract

During berthing operations of large vessels, the bow thruster jet deflecting on the quay wall and the bed can lead to high flow velocities near the bed. This may scour the bed when it is left unprotected, causing instability of the adjacent quay wall. Due to the complex flow field of the reflected jet, the decay in near-bed flow velocities perpendicular to the quay wall is unknown. This results in uncertainties in the design of bed protections, especially in the required width.

 In this research, the decay of the near-bed flow velocity perpendicular and parallel to the quay wall induced by a 4-channel bow thruster is studied. Field measurements have been conducted in the North Sea Port of Gent with one of the largest Dutch inland vessels. The near-bed flow velocities have been measured at multiple distances from the quay wall.

For the flow velocity measurements four main parameters have been varied: the applied bow thruster power, quay wall clearance, number of thrusters, and the lateral distance between jet axis and measurement sensors.

The highest flow velocities were measured near the quay wall, rapidly declining while moving away from the quay. Comparison of the measurement results to the Dutch and German guidelines generally leads to the conclusion that these guidelines are conservative. In addition, the dependency of the velocity on the total travelled distance by the jet as given in the Dutch method is not reflected in the measurement results. Furthermore, fundamentally different outcomes on the influence of the quay wall clearance on the near-bed flow velocity have been found.

When the measured near-bed flow velocities are used as the sole input to calculate the required bed protection, significantly smaller rock sizes and asphalt mattress thickness would be necessary to withstand the hydraulic load of the jet in comparison to current guidelines. Further studies with different vessels and direct measurement of the efflux velocity of the thrusters are recommended.