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.@en

During berthing operations vessels use bow thruster(s) to improve their manoeuvrability, making them less dependent on the assistance of tugboats. While manoeuvring, the transversal bow thruster jet can reflect on the quay wall and partially be directed towards the bed. At the intersection between the quay wall and the bed, the jet reflects again leading to a highly turbulent and complex flow field (Figure 1). When the bed is left unprotected scour may occur, which can eventually lead to instability of the quay wall (Roubos et al., 2014).

Over the years, the shipping industry has been developing continuously, characterized primarily by the upscaling in size of inland- and sea-going vessels (OECD, 2015; Weenen et al., 2020; Looye, 2021). As a result, vessels have larger draughts, more power and larger thruster diameters leading to higher hydraulic loads on quay walls and bed protections of berthing facilities (Roubos & Verhagen, 2007). To complicate the matter even more, the jet from a transversal bow thruster is confined by the hull of the vessel, the quay wall and the bed. Leading to a complex flow field of the reflected jet and an unknown decay in near-bed flow velocities. Resulting in uncertainties in the design of bed protections, especially in the required width.@en

Propellers of ships generate high velocities adjacent to quay walls, jetties and locks. Generally, a bottom protection is installed in order to prevent instability due to scour. Although design guidance exist, propeller-induced loads are far from fully understood and have predominantly been derived on the basis of model tests. The validation of the existing design methods is lacking, especially for specific types of bow thrusters. In this research, field measurements of flow velocities induced by a 4-channel bow thruster system against a vertical quay wall have been performed. Test results showed a flow characterized by low mean velocities and large fluctuations, with the extent of reflected flow limited to few meters from the quay wall and inflow beneath the suction points playing a role.

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