Keywords

1 Introduction

1.1 Background

In the city of IJmuiden in the Netherlands, recently a new navigation lock “Sea Lock IJmuiden” is built, providing larger vessels access to the Port of Amsterdam. This new lock is currently the largest navigation lock in the world. In preparation to the operation this new large lock, more insight in nautical procedures and mooring line forces due to levelling and lock-exchange process was required. Therefore, prior to its opening on 26th of January 2022, a field measurement campaign was conducted in the North Lock of IJmuiden in 2020, which at that time was the largest operational lock of the IJmuiden lock complex.

The IJmuiden lock complex forms a boundary between fresh (North Sea Canal side) and salt water (North Sea side), see Fig. 1. For the IJmuiden Locks, density currents generate the most dominant loads on moored vessels during the lockage process. The physical process of density currents in navigation locks has been already extensively studied and described by e.g. Vrijburcht (1991). These effects were carefully investigated in physical scale model research to assess the design of the Sea Lock IJmuiden (Van der Hout 2017; Kortlever 2018; Van der Hout 2018; Nogueira 2018). However, in order to translate theoretical force criteria used in the design of a lock to practical nautical procedures, more information on the present-day operation was needed. During the field test campaign described in the present paper, mooring line forces and the density distribution in the lock chamber were measured simultaneously to provide better understanding of the governing hydraulic processes.

Fig. 1.
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IJmuiden lock complex, with the names of the most important elements indicated (Google Earth).

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In anticipation of mooring forces that were expected to be larger in the new Sea Lock IJmuiden, due to its larger dimensions and expected vessel sizes, than presently in the North Lock (van der Hout 2018), it was investigated whether innovative mooring solutions could be applied in a lock and whether it would help to increase safety of the lock operation. For this, the ShoreTension system (see Sect. 1.3) was tested in a field test campaign. Although it is a proven system in port settings, it was not known yet how such a dynamic mooring system would perform in a navigation lock.

1.2 North Lock of IJmuiden

All tests described here have been performed in the North Lock of IJmuiden. The lock chamber of the North lock has a length of 400 m between the outer gates and a width of 50 m. The bed level is located at 15 m below NAP (Amsterdam Ordnance Datum, which is almost equal to mean sea level). The lock is equipped with short culverts for levelling (see Fig. 3). At the canal side the water level remains relatively constant and varies around NAP-0.4 m. At the sea side of the lock the water level varies with the tide, with typical tidal ranges of NAP ± 1 m.

1.3 Shore Tension System

ShoreTension is a dynamic mooring system that has been developed for port operations and is already successfully applied worldwide at several terminals. The working principle of the ShoreTension system is that a cylindrical shaped unit, attached to the bollards on a quay wall, exerts a constant tension to the ship’s mooring lines. This requires no energy except for an external hydraulic system which only needs to be used once to initialize ShoreTension at the correct pretension. After that, the cylinder of ShoreTension hydraulically moves along with the force variations in the mooring line. When the force in the mooring line becomes larger than a certain selected threshold value, the cylinder extends, thereby reducing the force in the line. Once the force in mooring line drops below a preset minimum value, the cylinder contracts again, thereby increasing the line force. Because of this dynamic operation, much control over pretension is achieved. The ShoreTension units are equipped with own, relatively stiff (Dyneema) mooring lines that need to be secured on board of the ship. This implies that a different mooring protocol needs to be followed than usual, as normally the ship’s mooring lines are given out to be connected to the bollards on the lock wall. The used ShoreTension system has a Safe Working Load (SWL) of 150 Ton in fully extended position and a SWL of 100 Ton during normal operation. ShoreTension is equipped with sensors measuring the forces in the mooring rope and measurement data can be saved autonomously to a data file.

1.4 Objectives of the Test Campaign

The test campaign aimed to provide insight in the possibilities to increase safety of the mooring operation in a sea lock using an innovative mooring system like ShoreTension. It was meant as a pilot to gain real-life experience with the performance of the system. Next to insight in the applicability of the working principles of such a dynamic mooring system, also practical aspects were investigated during the tests. So, practical questions like “How long does it take to connect a ShoreTension unit?”, “What protocol should the lines men follow to have a safe operation?” or “What mooring configuration works best?” have been addressed during the test campaign.

This paper provides an overview of the performed tests and will present insight in the most important hydraulic processes leading to mooring loads in sea locks. It also shows possible benefits and limitations of applying the ShoreTension system in lock operations. The knowledge obtained within this project and presented in this paper aims to make future operation of navigation locks safer. First, some general information on the density of fresh and salt water that is present on both sides the IJmuiden lock complex is provided, and it is discussed how external factors may cause variation in density differences. Next, the measurement setup using the ShoreTension system is presented. Lastly, the results of the measurements are provided and the suitability of the ShoreTension system in lock operation is discussed.

The research project was performed as part of TKI-Delta Technology program and was supported by Deltares, Dutch Pilot Association, ShoreTension, Arcadis and Rijkswaterstaat.

2 Density Differences Over the Lock

In preparation of the mooring force measurement campaign it was important to have a better understanding of actual density differences that are present over the lock in daily conditions. Although much historic monitoring data is already available in the North Sea Canal -mostly measured somewhat further away from the IJmuiden lock complex- and some data was available for the outer approach harbor on the sea side, these datasets spanned different time periods and simultaneous measurements on both sides of the lock were never performed before. Therefore, it was still unknow how variable the density close to the lock would be and how the density difference over the lock itself would be influenced by several external processes. As the strength of the density current during a lock-exchange depends on the density difference and since density currents generate the dominant hydraulic forces on moored ships in the North Lock, it is important to know how large the density difference over the lock complex can be. The density difference over the lock complex provides an upper limit for the actual density difference that can be present over a lock gate when opening.

To get more understanding of the local density variations over the lock, a monitoring campaign of three weeks was conducted prior to the measurements with the ShoreTension system (Van der Veen 2021). At two locations, one in the outer and one in the inner approach harbor of the North Lock, the density of the water was measured close to lock gates. At each location CTD sensors (sea Sect. 3.2) were deployed at several water depths, varying from 3 to 5 sensors in the vertical depending on measurement period. The vertical distribution of sensors was chosen such that a reasonable estimate of vertical density profiles was obtained and variations in average densities could be captured adequately. Such stationary density measurements give valuable information in temporal variations of the density, but due to the limited number of sensors available, the spatial resolution that could be obtained was quite coarse. Therefore, the stationary density measurements were complemented with manual profile measurements, by lowering and raising a CTD sensor on a rope in the water at specific time instances. These density profiles were mainly used to verify the parameters derived from the stationary measurements, but of course, instantaneous profile measurements provide less information on larger scale temporal changes in density.

The main conclusions of the monitoring campaign can be summarized as follows:

  • The mean density difference that was present over the lock was around 14 kg/m3, which was in line with expectations (Kortlever 2018). The density difference over the lock varied between a minimum value of 11–12 kg/m3 and maximum value of 17–18 kg/m3, with somewhat larger density variations on the sea side than on the canal side.

  • The wind had the largest effect on the density distribution in the approach harbors, with a stronger influence for the larger wind speeds. For wind blowing in offshore direction (from east to west), the mean density in the outer approach harbor was much higher than for onshore (westerly) wind directions. The main reason for this was thought to be that the upper fresh water layer is pushed towards the sea by the easterly wind, while westerly winds push the upper fresh water layer towards the lock complex and thereby inducing a larger mixing layer in the upper part of the water column. Large scale upwelling and downwelling processes could also play a role, although that could not be conclusively confirmed by the obtained dataset.

  • Smaller influences on local density differences were identified for tidal variations and water discharged through the nearby discharge sluices. The levelling process itself mainly causes short scale variations in the density distributions in the approach harbors, due to the levelling and opening of the lock gates (in the order of 30 min – 1 h).

Within the monitoring campaign, that had a relatively short duration, already quite some variation in density difference over the lock was observed. Although the obvious influence of the wind was well captured by the measurements, longer scale variations due to e.g. varying discharges of the North Sea Canal related to wet/dry periods, water availability and water control decisions could not be captured. As the density balance in this water system is known to be quite complex, it is expected that variations in density on the canal side will to a large extent be influenced by these processes with longer time scales. It was therefore surprising that the largest density differences that were measured in this monitoring campaign (18 kg/m3) were already close to the upper value of the density difference (20 kg/m3) that was chosen as design value for the new Sea Lock IJmuiden (Kortlever 2018).

The density monitoring campaign provided valuable background information on the magnitudes of forces to be expected during the measurement campaign and it provided insight in the main parameters influence forces during a lock-exchange most.

3 Measurement Setup

3.1 Mooring Force Measurements

The mooring force measurement setup has been described by Hoff (2021) in more detail; here the most important elements are reproduced. All vessels that were measured were moored with 4 mooring lines (forward and aft hawser and forward and aft spring). The forces in the mooring lines were measured using two traditional load cells with a capacity of 100 Ton and two ShoreTension units. The load cells were not available during the full test campaign, so in some tests not all mooring lines loads were measured. Figure 2 shows the four mooring layouts that were (planned to be) used during the test campaign.

In configuration A, the ShoreTension units are connected to both hawsers. In configuration B, ShoreTension is used on both springs. In configuration C, ShoreTension is used on the forward spring and the aft hawser, and in configuration D, the ShoreTension units are connected to the forward hawser and aft spring.

The units can be operated in different operation modes, in which specific settings can be chosen to ensure optimal performance. In the performed tests, a constant pretension of 10 Ton was selected. If the force in the mooring line would drop below this value, the cylinder would heave-in the line until the required pre-tension was obtained again. If the force in the mooring line increases however, an upper limit was specified at which the ShoreTension cylinder starts to pay out, thereby reducing the force in the line again. In the performed tests, 40 Ton was selected as upper working limit, ensuring that the SWL of bollards and hawse-holes would not be exceeded during the tests.

Fig. 2.
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Schematic measurement setup mooring line loads for the four mooring configurations (A-D), reproduced from (Hoff 2021).

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To secure the ShoreTension unit on the quay wall, it was connected to a storm bollard present at the North Lock, located approximated 15 m away from the lock chamber wall. On the opposite site of the unit, a mooring line is connected to the movable part of the cylinder and this line was secured to the deck of the ship. However, the mooring line is not directly tied from the ShoreTension unit to the ship but guided via a “standard” bollard located close to the lock wall. A sleeve was used to reduce friction and wear and tear on the lines. A picture of the used setup is shown in Fig. 3.

Because the ShoreTension mooring line is not directly connected from the cylinder to the ship but guided via a bollard as shown in Fig. 3, the force measured by the ShoreTension cylinder will be lower than the actual mooring force exerted on the bollard due to friction at that bollard. To assess these friction losses, dedicated calibration tests have been performed. In those tests, the ShoreTension line was connected directly to a load cell and different line angles around a guiding bollard were tested. By comparing the loads measured by the ShoreTension unit and by the load cell, a relation between line angle and friction was derived. During all mooring tests the line angles were estimated and documented, and in post-processing the measured line forces are corrected for the friction loss. In the presented results, the forces that are exerted on the bollards are shown, which are a factor 1.1–1.7 higher than the forces measured by the ShoreTension unit depending on the applied line angle.

Fig. 3.
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Setup of the ShoreTension system (orange cylinder). Note that the number of people in the picture is not representative for the personnel needed to operate the system, as this picture was taken during one of the first “instruction” measurements.

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3.2 Density Measurements

To measure density currents in the lock chamber during the operation of the lock, six CTD-divers were mounted in the lock chamber. These CTD-divers measure Conductivity, Pressure and Temperature simultaneously (Van Essen 2016). The measured conductivity and temperature are converted to density, using the UNESCO-1981 sea water formulation (UNESCO 1981). The divers logged autonomously, storing the data in their internal memory. The divers were taken out of the water after approximately 4 days to download the data, to verify operation and accuracy using a calibration liquid, to time-synchronize, to clean the sensors and to restart the and remount the sensors in the lock chamber again. A sample interval of 10 s or 30 s was used, depending on the type of sensor (two types were available: a somewhat older type and newer type with larger internal memory capacity, hence a shorter sample interval could be used). A sample interval of 30 s was thought to be sufficient to capture the large-scale density currents during a lock-exchange process, which has a typical time scale of 30 min - 1 h.

On each side of the lock chamber three sensors were attached to a vertical rope that was kept in place by a weight on the bottom. On the west (sea) side of the lock chamber the sensors were positioned 17 m from the gate and on the east (canal) side of the lock the sensors were positioned 5.5 m from the gate (Fig. 4). The vertical positions of the divers are given in Table 1.

By measuring at different heights in the water column, insight is obtained in density gradients in the lock chamber. Although the spatial resolution is limited, the difference in (mean) density between the east and the west side of the lock chamber can be used as indication for the density difference over the length of the ship. This density difference is expected to have a direct correlation with the longitudinal force acting on the ship.

The measured pressure was used to determine the water level in the lock chamber. The pressure measurements were corrected for atmospheric pressure variations, using a separate barometric pressure sensor mounted on the lock chamber wall above the water.

Table 1. Position of the CTD divers
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Fig. 4.
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Schematic measurement setup density sensors.

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4 Measurement Results

4.1 Mooring Forces

The measurement campaign had a duration of 15 days. Within that period, mooring line forces were measured of 21 large vessels that passed through the North Lock. Ten vessels were inbound, i.e. sailing west to east from sea to canal, and eleven vessels were outbound, i.e. sailing east to west from canal to sea. All vessels were moored on the north side of the lock, as the ShoreTension units were only deployed at that side of the lock.

Table 2 provides an overview of all vessels and maximum measured line forces. The highest forces were generally measured as a results of density currents during the lock-exchange after opening the lock gate. In some tests however, largest peak loads in the lines occurred during the mooring procedure when fastening the mooring lines. The mooring configuration that is used follows from the definition provided in Fig. 2. The load cells and ShoreTension units are labelled “east” and “west”, referring to their deployment location. Depending on the sailing direction of the vessel, this refers to aft or bow of the vessel, i.e. “east” refers to forward lines for inbound vessels and to aft lines for outbound vessels (and vice versa for “west”).

4.2 Discussion on the Measured Forces Using Mooring Configuration A

The forces in the ShoreTension system are generally higher than in the conventional mooring lines measured with load cells. Since the ShoreTension mooring lines are much stiffer than the conventional mooring lines, most forces on the vessel are absorbed by the former lines. Next to that, also the pretension in the ShoreTension lines was typically larger. In several tests, the maximum line force in the conventional mooring lines even stayed below the set pretension in the ShoreTension lines (>10 Ton). During previous measurements by Meijering (2018) in the North Lock of IJmuiden, it was already observed that pretension in conventional mooring lines can vary strongly and that pretension is generally lower than theoretically desired. He also observed that largest vessel motions during levelling typically occurred for vessels with low pretension.

Table 2. Performed tests, with in/out indicating inbound or outbound sailing direction. In the grey columns the maximum line forces are shown, with the forces measured by the ShoreTension units marked ST, the line forces measured by the load cells marked LC.
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With some exceptions, highest forces are measured on the east side. This was expected (Van der Hout 2018), since for inbound vessels the dominant density forces during a lock-exchange is directed backwards (towards sea) with a strong bow-out moment, pushing the bow of the vessel away from the lock wall. This creates largest line loads in the bow hawser (STeast). For outbound vessels, the dominant density force is in longitudinal direction, pushing the vessel forward (also towards sea). This creates highest forces in the front spring and aft hawser. Because the ShoreTension line was typically stiffer than the conventional mooring line and because of the higher pretension, also for outbound vessels largest forces were measured by the east ShoreTension system.

Most tests are performed using mooring configuration A, in which ShoreTension is connected to the bow and aft hawser of the vessel. This configuration worked quite well and provided most flexibility in the placement of the ShoreTension units. Due to the used line angles of the hawsers, both transversal and longitudinal forces exerted on the vessel could be absorbed by the ShoreTension lines.

In almost all tests with mooring configuration A, resulting vessel motions during levelling and lock-exchange were absent or small, i.e. less than 0.5 m. The only exception is test 2, in which the setting of the east ShoreTension unit was accidently incorrect, and consequently the cylinder already started to pay out the line at a force lower than 40 Ton. As a result, driven by the density current of the lock-exchange, the vessel started to move towards the outer approach harbor, leading to a strong increase of the force in the aft hawser (up to 63 Ton). Although test 2 is not representative for the correct working of the ShoreTension system, it shows how high line forces can become once a ship starts moving, thereby easily exceeding safe working load of conventional mooring lines.

4.3 Discussion on Other Mooring Configurations

In three tests, mooring configuration B was used, in which ShoreTension was connected to the spring lines. This configuration gave also good control over vessel motions and line forces but was less flexible than configuration A. Due to the line angles in configuration B, the ShoreTension lines could absorb longitudinal forces well, but transversal forces less effectively.

In test 7, only one ShoreTension unit was connected to the bow hawser, since for inbound vessels the highest force due to the lock-exchange is expected on this bow line. The setup with one ShoreTension unit showed not to be practical, because the pretension exerted by the ShoreTension unit was so high that it pulled the aft of the ship off the fenders. Apparently, the crew on board of the vessel was not expecting such (large) pretension in the mooring lines and slacked the aft lines during tensioning of the ShoreTension system, or the hold capacity of the winches was not high enough on this relatively small vessel to keep the aft lines under proper tension. As a result, the vessel started to move forward during the test and the test was aborted.

In theory, mooring configuration C and D could work very well for outbound and inbound vessels respectively. In those situations, both ShoreTension units would work in the same direction, opposing the direction of the dominant density force, i.e. which is always directed towards the (saltier) sea side. However, after the experience in test 7 (and test 2) it was decided not to apply these mooring configurations, as the risk existed that the deck crew would slack the conventional mooring lines too soon, potentially leading to unwanted, uncontrolled vessel motions.

4.4 General Observations

In all tests, the forces measured by the ShoreTension units stayed below the limit of 40 Tons, so the system did not have to automatically pay out the line to reduce line forces. Due to friction at the guiding bollard, the actual forces in the mooring line itself could higher than measured at the cylinder and did exceed 40 Ton (as shown in Table 2). It means that, would a vessel have been moored with conventional mooring lines only, with similar pretension values, the SWL of the conventional mooring line would be exceeded. (In lock design, typically a value of 50%–55% of the minimum breaking load of a mooring line, MBL, is used as SWL). In 10 of the 21 tests, the SWL of conventional mooring lines would have been exceed with line forces reaching up to 181% of the SWL of the conventional mooring lines on board of the vessel. It should be mentioned however that for test purposes the mooring lines were intentionally released later than normal to be able to measure the development of forces during the lock-exchange. In normal operation, mooring lines might have been released earlier for the vessel to sail out of the lock. Also, when forces in mooring lines increase, the crew may decide to pay out the lines or which breaks may start slipping, thereby reducing line forces. This may however result in unwanted ship motions.

4.5 Relation to Density Differences

In 7 tests (Test 13 – Test 19), density measurements were available simultaneous with force measurements. Based on the three sensors on each side of the lock chamber, the density distribution in the vertical could be estimated at those positions. The depth averaged density was also determined for both sides, and hence the mean density difference between the west and the east side of the lock could be quantified.

Figure 5 shows a typical measurement result. It shows the results of test 15, the handysize tanker Cielo di New York, an inbound sailing vessel, using mooring configuration A. Around 11:50 the vessel sails into the initially salty lock chamber from the west side, visible by the small disturbance in upper density on the west side of the lock. After the west gate closes (red rectangle in the upper two panels), the water level in the lock chamber is adjusted to the lower canal level. During this levelling process, mooring lines forces of the aft hawser (STwest) and forward spring (LCeast) reduce slightly, the force in the bow hawser remains constant (STeast). The force in the aft spring (LCwest) was too small to be accurately measured by the load cell and remained below 2 Ton during levelling and lock-exchange.

When the east gate opens (grey rectangle in the upper two panels), fresh water flows into the lock chamber; first only visible at the east side of the lock, reaching the west side of the lock approximately 10 min later at 12:20. The incoming fresh water initially concentrates in the upper part of the water column propagating over the outflowing saltier bottom layer. Only after reflection at the west gate, the density distribution becomes more homogeneous spread over the vertical. The increase in density difference over the length of the lock chamber (purple dotted line in the bottom panel of Fig. 5), is followed by a strong increase of the force in the bow hawser (STeast). The forces in the other mooring lines remain similar or decrease.

Around 12:24 the mooring lines are released, and the vessel can sail out of the lock chamber. Around 12:35 a disturbance of the density measured by the lowest sensor on the east side is observed. This is thought to be the moment that the vessel passes that sensor location, thereby inducing mixing over the vertical.

The strong increase of the bow hawser force as a result of the lock exchange in test 15 was in line with expectations from previous research (Van der Hout 2018). The mean density difference between both approach harbors in this test was around 12 kg/m3, with a mean density of 1020 kg/m3 in the outer approach harbor and 1008 kg/m3 in the inner approach harbor. During the lock-exchange, the mean density difference over the lock chamber reached up to 8 kg/m3, which is a good indication for the density difference that developed over the length of the vessel. As hydrodynamic forces during a lock-exchange scale with the density difference over the vessel (Van der Hout 2018), mooring line forces may increase in conditions with larger density differences. As described in Sect. 2, density difference larger than 12 kg/m3 are not unusual and may occur frequently in IJmuiden.

In all tests, a clear relation between the density difference over the lock chamber and mooring lines forces was found, confirming that the density current is the dominant forcing mechanism in the North Lock. For conciseness, not all measurement results could be reproduced here, and only test 15 is provided as illustration for the quality of data that has been obtained during the measurement campaign.

Fig. 5.
figure 5

Combined density and mooring force results for test 15, Cielo di New York, inbound, mooring configuration A. Top panel: contour plot of the density distribution on the west side of the lock. Density values are linearly interpolated between the sensor positions, indicated with the horizontal black dotted lines. Constant extrapolation to water surface and lock bottom is applied. The red rectangle indicates the period that the west gate closes, the grey rectangle the period that the east gate opens. Middle panel: idem, for the east side of the lock chamber. Bottom panel: measured forces (solid lines, left axis) and difference in mean density between east and west side (purple dotted line, right axis).

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4.6 Use of the ShoreTension System in a Navigation Lock

An important objective of the test campaign was to gain practical experience with the ShoreTension system in navigation lock operation. As described above, during the test campaign it could be clearly identified which mooring configuration worked best: configuration A, with ShoreTension units connected to both hawsers. After connection of the lines, vessel motions were minimized due to proper pretension, and mooring line forces stayed comfortably within safe working load limits of the system. The people operating the system and performing the tests indicated that they experienced a positive effect on mooring safety, mainly because it provided more control over desired pretension in the mooring lines. Experience during the test period learned that line handling by vessel crews during lock passage is not always optimal and that it is often the cause for potentially dangerous situations, e.g. high line loads or unwanted vessel motions, as also reported by Meijering (2018).

The use of a system like ShoreTension however comes at a cost, because it takes more time to connect mooring lines than using conventional mooring lines. This is partly because the lines men and the vessel crew do not have experience with the system yet and need to adjust to new working procedures, and partly because the system was not optimized for lock operation yet. During the test period it was monitored how much (extra) time was needed to connect the ShoreTension lines. The results are shown in Fig. 6. For the first 4 tests no information was recorded, but for the following tests a learning curve was observed. This is illustrated by an indicative trendline, which shows that after a few tests to build experience with the system, the process went quicker. For the last 10 vessel passages, the time needed did not decrease much further, and remained on average around 10–11 min. If further time reduction is desired, the system should be optimized for lock operations.

Fig. 6.
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Time needed to connect the ShoreTension lines

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5 Conclusions

The results of the measurement campaign show that hydrodynamic forces during a lock-exchange in North Lock IJmuiden are large and that under daily conditions safe working load limits of conventional mooring lines may be exceeded, potentially leading to line breakage or unwanted vessel motions. In normal operation, this will usually not lead to large problems since mooring lines are released quickly after opening of the lock gate and the vessel is able to sail out of the lock. However, if something goes wrong, e.g. a vessel must wait, or a mooring line is stuck in a winch, this can lead to dangerous situations.

The ShoreTension system was tested for the first time in lock operation. Using this system showed to have several benefits: proper pretension could be applied to the mooring lines, thereby preventing large vessel motions during levelling and lock-exchange. Since it is a dynamic system, the risk of exceeding safe working loads is much smaller than for conventional mooring lines as lines loads are reduced automatically by extending the hydraulic cylinder. Furthermore, the hold capacity of the ShoreTension unit is larger than the break hold capacity of the winches on board of vessels passing the North Lock IJmuiden, thereby providing better control of the moored vessel behavior. Especially under critical conditions, e.g. for large vessels or large density differences, the use of the ShoreTension system can therefore improve safety of lock operations. During the test weeks, the number of ships was limited, and mostly relatively small vessels with a beam up to 32 m were tested. It is expected that for larger vessels, the ShoreTension system could even be more beneficial, since hydrodynamic forces will increase for larger vessels.

The downside of using the ShoreTension system is that it cost more time to connect the mooring lines than using conventional mooring lines. In the test campaign the additional time needed for connecting the system was approximately 10 min per vessel. This might however be reduced in the future if the system configuration is optimized for the use in locks. Also, when using the ShoreTension system a different mooring procedure needs to be used, since the mooring lines now need to be fastened on board of the vessel instead of on the lock wall. During the test period, the system could be deployed flexibly, but not all locks will have a similar bollard configuration as the North Lock, so mooring layout may need to be optimized for other navigation locks.

All in all, a successful test period in the North Lock of IJmuiden was performed. The dynamic mooring system ShoreTension was successfully tested in a practical lock operation for the first time, showing its potential to increase safety of lock operations. The density measurements provided valuable insight in the governing hydraulic processes during levelling and lock-exchange, and the most influential external drivers for density differences over the lock complex were identified.