Bowthruster-induced flow on the bottom of a vertical quay wall

Abstract

During mooring operations, ships tend to make an extensive use of bowthruster in order to minimize the need for tug assistance. Jet caused by transverse thrusters directly impinges quay walls, and can cause scour on the bed, therefore threatening structural stability of quay walls. Presence of vertical quay walls induces reflection of bowthruster's jet, further complicating the already complex flow field. Despite extensive research has been conduced on free flow, several knowledge gaps are still present regarding propeller induced flow when confined, for instance by a vertical quay wall. In this research, focus is on flow field on the bottom of a vertical quay wall induced by channel-type bowthrusters, which are commonly used for inland vessels. Field measurements have been conducted at the Antarcticakade, Ports of Rotterdam, using inland vessel MTS Vorstenbosch. The vessel is equipped with a 4-channel Veth Jet type bowthruster system. Use of a combination of Acoustic Doppler Velocimeters and Acoustic Doppler Current Profilers allowed measurements of flow velocities on the bottom of the quay. Data from the measurements has been analysed to investigate influence of distance between outlet and quay wall, and keel clearance, on the flow pattern at the bottom. Results have then been contextualized within the literature framework, and their impact on design of bottom protection according to most used guidelines has been assessed.
The results of this field measurement showed mean flow velocities near the quay wall generally in the order of magnitude of 1 m/s, with the exception of one test, where mean flow velocities in the order of magnitude of 2 m/s were recorded. This relatively low mean flow velocities were often correlated with large turbulent fluctuations, leading to values of relative turbulence intensities higher than the ones found in literature, and sometimes even equal to 1. Comparison with the theoretical calculations of velocities according to Dutch and German methods suggested by PIANC, showed both methods to be conservative if compared with data from most tests. Furthermore, it appeared that both formulae’s sensitivity to wall and keel clearance was not reflected by the data. Similarly, results from this measurement showed that the flow generated by simultaneous use of two bowthrusters was characterized by velocities on the bed lower than expected according to the guidelines. Recommendation would be to use either linear superposition or to multiply by square root of n (where n is the number of used propellers) when considering the use of multiple propellers, but this was not reflected by most of the data. However, two of the tests taken into exam represented an exception to these general observations: ADV1, the instrument nearer to the quay wall, recorded velocities higher than the theoretical values for tests 12 (use of bowthruster 2 at high water) and 22 (use of both bowthrusters simultaneously at low water). Results from this study showed how the use of a 4-channel bowthruster system induced a flow on the bottom of a vertical quay wall which is mainly divided in two zones. Near the quay wall is where the highest velocities have been measured, and where the flow is strictly influenced by use of the bowthrusters. There is a return flow beneath the ship, which is dissipated in the space of few meters. Underneath the suction points of the bowthrusters, it is the inflow to determine the flow characteristics on the bed. In this research, the extent of the bowthruster-induced flow was found to be less than 14 m from the quay wall. The instrument hereby located, in fact, didn't record velocities which were affected by the use of bowthrusters. This research represents a step towards filling the knowledge gaps about use of bowthrusters at a vertical quay wall. The unique dataset collected can be used in the future for validating numerical or on-scale models, working for a better understanding of the phenomenon and a more accurate and optimized design of bed protections.