Transverse Mooring Forces Due to Asymmetrical Filling in a Lock with Longitudinal Culverts and Side Ports

Abstract

In lock design, several geometries are proposed for the levelling system with the aim of insuring a smooth and fast levelling operation. For high and medium lift lock, longitudinal culverts with side ports located in the lock walls are often chosen because this system distribute the flow along the entire length of the lock chamber. Nevertheless, when one of the culvert (valve) is out of order, the flow is asymmetrical what induced significant transverse mooring forces, especially during filling operations.

Field measurement on such a lock (225 m long, 25 m wide and 13.5 m lift) demonstrated transverse water slopes significantly larger than the admissible criteria during asymmetrical filling operation. Further investigations on a physical scale model highlighted the driving effect of the side port jets on the rolling flow. Detailed measurements were performed on the scale model covering: (1) water surface slope; (2) transverse velocity distribution; (3) transverse mooring forces and rolling angle of a vessel located in the lock chamber. It was concluded that the size of the outlets of the side ports has a major impact on the transverse forces. For smaller outlets at a given discharge, the flow velocity increases and more impulse is transferred to the rolling flow, resulting in larger forces on the vessel.

1 Introduction

The design of the levelling system of a navigation lock results from a compromise between three main constraints: (1) reducing the levelling duration; (2) minimizing the mooring forces acting on the vessel in the lock chamber; (3) keeping the lock culvert design simple and not too expensive.

Criteria for allowable mooring forces can be found in the literature and in some national regulations (PIANC WG 155 2015). These criteria cover in all cases the maximum longitudinal forces and sometimes also the transverse forces. Physical processes resulting to longitudinal forces are well documented. Notably, for through-the-head filling systems, different sources of force are identified: (1) translatory waves and longitudinal water slope; (2) impulse difference along the vessel length; (3) jet effect on the bow; and (4) friction on the vessel hull. On the other hand, only limited information on the physical processes involved in transverse forces can be found.

The present investigation focuses on locks filled through longitudinal culverts and side ports located in the lock walls. In normal operation, the filling discharge is distributed all along the lock chamber symmetrically between the two longitudinal culverts. The flow spreads from symmetrical side ports. Both jets face along the central axis of the lock and energy is efficiently dissipated (Fig. 1a). When one filling valve is out of order, e.g. due to maintenance operations, the lock is operated asymmetrically. The jets flowing from the ports on the operating side spread on the whole lock width. This generates a rolling flow in the chamber lock and significant transverse forces (Fig. 1b).

Fig. 1.

Cross-section of the lock chamber - flow pattern for symmetrical (a) and asymmetrical (b) filling operation

To reduce transverse forces during asymmetrical filling operation, the valve opening time can be slowed down. However, it is usually not possible to find an opening schedule of the valve which allows decreasing the forces drastically with an acceptable duration for the leveling operation. Then, in addition, operational guidelines have to be applied: only one boat is allowed on the width of the lock chamber and it has to be moored on the side of the operational culvert (then the vessel is held against the wall due to transverse currents).

Figure 2 illustrates an incident that occurred at the lock of Lanaye in Belgium (225 m long, 25 m wide and 13.5 m lift), while these guidelines were note applied and the mooring conditions were not optimal. Ships A,D and G were moored on the side of the inoperative culvert and had to face significant difficulties. Site measurements revealed transverse slope of the free surface up to 3 to 5‰ depending on the valve opening schedule (Savary et al. 2019) and important gyratory movement in the lock chamber. The velocity profile measured at the middle of the lock chamber, centered on a port side is illustrated at Fig. 3, a transverse velocity higher than 2 m/s is observed near the free-surface.

Fig. 2.

Asymmetrical filling at the lock of Lanaye

Fig. 3.

Velocity profile at the lock of Lanaye

Additional investigations were realized on a physical model to measure the velocity field, the pressure around the boat, the transverse forces and the transverse slopes (water free-surface and ship) for different configurations. The measurements illustrate the influence of some parameters like the opening schedule of the valves, the position of the ship in the lock chamber and the size of the ports. The present paper focus on this experimental campaign and its results from the master thesis of El Ouamari I. and Lenaerts M. (2019) and Bertin B. (2021).

2 Physical Model and Experimental Set up

The physical model is located in the facilities of the Hydraulic Research Laboratory depending on Service public de Wallonie (Belgium). The model is a reproduction of a lock levelled through longitudinal culverts with side ports at a 1/25 scale (Fig. 4a). The lock chamber (9 m long, 0.72 m wide 0.58 m lift) is connected on each side to a longitudinal culvert (24 cm × 18 cm) through 20 ports (1 cm × 8 cm or 4 cm). The culverts are equipped with butterfly valves allowing to level the lock with different opening schedules.

The upstream and downstream reaches are equipped with large weirs to maintain a constant level during the lockage process.

Some experiments are realized with a vessel in the lock chamber. The boat dimensions are close to those of a Vb vessel at 1/25 scale (7.2 m long, 0.5 m wide, 0.3 m height), its total weight is 602.7 kg. The vessel is held in a fixed transverse position by means of two vertical bars crossing the structure (red arrows at Fig. 4a. The vessel is connected to the bars by means of a specific structure allowing rotation and vertical translation of the vessel (Fig. 4b). The mooring forces are deduced from the transverse and longitudinal forces applied by the vessel on the bars (deformation gauges). The system allows to consider elastic and unstretched mooring lines. In the present study, mooring lines are considered rigid and stretched.

Fig. 4.

View from upstream of the physical model (a), view from above of the specific structure connecting the vessel to the vertical bars (b)

The gauges used for the measurements are:

• Ultrasonic water level gauges (Baumer Unam18U6903): measure the water level in the lock chamber and the reaches (US in Fig. 5). In the lock chamber, during lockage, they allow to calculate the evolution of the average water level and the discharge.

• Differential pressure gauges Yokogawa EJA110E): are used to estimate the free-surface slope in five cross-sections along the lock chamber (P_dif at Fig. 5). They are also used to estimate de hydrostatic pressure around the vessel. The range of the gauges is ± 50 mmH₂O with a 0.02 mmH₂O precision.

• Cable position sensors (ASM WS10SG): are placed at the bow and the stern to measure the trim and rolling angles of the vessel.

• Deformation gauges (TEDEA HUNTLEIGH): measurements from three deformation gauges (range 30 N, precision 0.006 N) are used to measure the forces on the vessel (F at Fig. 5).

• Electromagnetic velocity gauges (PEMS WL Delft Hydraulics): are used to measure the velocity field in cross-sections of the lock chamber. One probe (range ± 2.5 m/s, precision 0.01 m/s) is used, the same test is realized several times and the probe is displaced at each test to obtain the complete velocity field.

Fig. 5.

Location of gauges

3 Forces, Slopes and Rolling Angle of the Vessel

Regarding the longitudinal forces during the levelling process, in many cases, the free-surface slope in the lock chamber is representative of the hawser forces because hydrostatic forces are the main components. However, the presence of a ship in the lock chamber during the levelling has an influence on the measured water slope. Longitudinal water slopes during levelling without a ship in the lock chamber will be lower than during levelling with a ship in the chamber (PIANC WG155 2015). As an order of magnitude, the presence of the ship increases the slope up to 30% for inland navigation locks depending on the ship and the size of the lock. Then the threshold value used as a reference to check that the levelling is safe will depend on what is measured or calculated (forces > free-surface slope with ship > free-surface slope without ship).

Regarding the transverse direction, in the specific case of asymmetrical filling, tests were realized on the scale model. Figure 6 illustrates the results and the difference between the measured free-surface slopes with and without ship in the lock chamber, force and rolling angle of the vessel. To be compared, all the values are expressed in ‰. As expected, the total transverse force is higher than the slope with a vessel. In this case, the transverse free-surface slope without ship is not representative of the transverse forces. The rolling angle is related to the transverse force, but of opposite sign as illustrated in Fig. 7.

The transverse free-surface slope presented in Fig. 6 is deduced from the difference in piezometric level measured against the lock walls. Additional tests were realized with a measurement made on both side against the vessel, the resulting slope is higher than the one plotted in Fig. 6 and closer from the measured forces.

Fig. 6.

Asymmetrical filling – comparison between measured transverse force, free-surface slope (with and without boat) and rolling angle (El Ouamari and Lenaerts 2019)

Fig. 7.

Cross-section during asymmetrical filling (Bertin 2021)

4 Velocity Field

The velocity field was measured in two cross-sections, one centered on a side port and the other between two side-ports. Figure 8 illustrates the development of the gyratory movement during the asymmetrical filling process for the cross-section centered on a side port.

Tests were realized with and without a vessel in the lock chamber and for two ports size (1 cm × 8 cm and 1 cm × 4 cm). Figure 8b illustrate how the presence of the vessel influence the velocity field. Due to the dimension of the boat the depth of the gyratory movement is reduced.

Fig. 8.

Velocity field – cross section centered on a side port without (a) and with (b) a vessel (Bertin 2021)

The flow velocity coming from the side ports induces an impulse transfer to the static water of the lock chamber creating a rolling flow. To quantify this transfer, the discharge crossing different areas of the cross-section (Q1 to Q4 at Fig. 9) is roughly estimated from the integration of the velocity measurements and compared to the discharge coming from the ports Qp for the 2 geometries of the side ports. The results obtained for the maximum discharge are in Table 1. For a similar valve opening schedule, the discharge going through the ports at a given time is smaller for smaller outlets. To make possible the comparison between both geometries, the velocities measured for the smaller ports configuration was multiplied by a factor in order to have the same discharge at the ports. When the dimensions of the ports are smaller, the discharges are higher, showing that the impulse transfer is more important.

Fig. 9.

Division of the cross-section (El Ouamari and Lenaerts 2019)

Table 1. Estimated discharge from integration of velocity measurements

5 Configurations Impacting the Transverse Forces

Among all the parameters that could have an impact on the transverse forces on the vessel, the influence of the valve opening schedule (3 schedules tested), the position of the vessel in the lock chamber (3 positions tested) and the dimensions of the side ports (2 width tested) was experimented.

5.1 Valve Opening Schedule

Figure 10 illustrates the measured hydrographs for 3 linear opening schedules of the valve (opening in 30 s, 60 s and 90 s).

Fig. 10.

Hydrographs for asymmetrical filling for 3 opening schedules of the valve (Tv) (El Ouamari and Lenaerts 2019)

Figure 11 illustrates the measured transverse force related to the different opening schedules. Opening the valve slower is quite efficient to reduce the transverse force with a limited impact, in this case, on the filling time. If the opening time of the valve is doubled (from 30 s to 60 s), the maximum transverse force decreases by 20% for a filling time 4% longer. If it is tripled (from 30 s to 90 s) the decrease in the transverse force is 30% and the increase of filling time is 11%. In this case, the efficiency of an increase of the valve opening time is related to the fact that initial value (30 s) is small regarding the filling time. If the valve opening time is further increased, the gain regarding the transverse force will become smaller and the cost on the filling time will become higher. An optimum has to be found.

Fig. 11.

Transverse force for 3 opening schedules of the valve (Tv) (El Ouamari and Lenaerts 2019)

5.2 Position of the Vessel

Figure 12 illustrates the measured transverse force for 3 positions of the vessel in the lock chamber. The results confirm the operational guidelines which recommend mooring the vessel on the wall located on the side of the functional culvert (valve). The transverse force is divided by 2 when the vessel is moored against the wall located on the side of the functional culvert in comparison with a mooring on the other wall.

Fig. 12.

Transverse force for 3 vessel positions in the lock chamber (El Ouamari and Lenaerts 2019)

5.3 Dimensions of the Side Ports

Figure 13 illustrates the hydrographs for an asymmetrical filling corresponding to 2 geometries for the outlet of the side ports (1 cm × 8 cm and 1 cm × 4 cm). As expected, for an identical opening schedule of the valve (linear opening in 30 s), due to an increased head loss, the filling operation is slower when the side ports are smaller.

Fig. 13.

Hydrographs for asymmetrical filling for 2 side ports geometries (El Ouamari and Lenaerts 2019)

As explained in Sect. 4, for smaller outlets at a given discharge, the flow velocity increases, and more impulse is transferred to the rolling flow, resulting in larger forces on the vessel. In the present case, when the width of the ports decreases, the discharge also decreases but it does not allow decreasing the transverse force significantly (Fig. 14) due to the increased impulse transfer.

Fig. 14.

Transverse force during asymmetrical filling for 2 side ports geometries (El Ouamari and Lenaerts 2019)

6 Conclusions

The experimental results highlight the behavior of the flow and the induced transverse forces applied on a vessel during the asymmetrical filling (one of the valve out of order) process of a lock with longitudinal culverts and side ports.

The results indicates that, especially in the transverse direction, one should be careful using the free-surface slope to estimate the force. Moreover, the free-surface slope is significantly impacted by the presence of a vessel in the lock chamber.

The transverse force can be decreased acting on the opening schedule of the valve, an optimization is necessary to avoid to long filling time. Usually it is not enough, it is important to apply the operational guidelines consisting in mooring the vessel at the wall located on the side of the operational culvert, it significantly limits the transverse forces. For safety reason, several boats must not be moored side by side. Reducing the area of the side ports is not efficient to reduce the transverse force.

Finally, as another mean to decrease transvers forces not treated in the present paper, some dissipation structures or baffles can be placed at the outlet of the ports to decrease the rolling effect of the flow.