Modifying waterways for larger ships

The region surrounding Lake Mälaren requires a good infrastructure in order to meet the needs of both industry and society. Freight transportation in the region is both by land (road and rail) and water (shipping). Both the road and railway networks support a heavy load of traffic, which is why the Swedish government is making investments to improve the shipping infrastructure of the Lake Mälaren waterways. A project to increase shipping traffic so that larger ships can be accommodated has been initiated. The navigational aspects will be investigated in two steps: first, introductory ship simulations using a desktop simulator and second, verifying real-time simulations in a Full Mission Simulator. SSPA was asked to perform the introductory study, which involves identifying critical passages and preliminary testing of potential fairway modifications.

Today, the maximum size of ships travelling on Lake Mälaren is 124 m in length and 17 m for beam. The intention is to increase this to a length of 160 m and a beam of 23 m. In order to accommodate larger tonnage, both Lake Mälaren and the Södertälje lock and canal would need to be dredged. At present, the intention is to increase the depth of the canal section north of the lock to 8.2 m and the depth of the Mälaren fairways to 8.4 m.

The latter depth provides a nominal clearance of 1.4 m to allow for effects due to squat, wind and turn-induced heel angles. Squat effect is particularly prevalent in shallow water. When a vessel moves quickly through shallow water, an area of lowered pressure is created, which causes the ship to be closer to the seabed than would otherwise be expected. The phenomenon becomes more pronounced the shallower the water is. SSPA developed a way of calculating the squat effect based on a comprehensive model test study that was carried out as a Joint Industrial Project called SWABE.

The study covered both the manoeuvring aspects and the risk of grounding in the shallower areas of both the canal and lake. Two tests were conducted using the SSPA Compact Bridge Simulator (CBS) and two local pilots participated.

Test ships

Two ships were chosen for the study. The first, Coral Ivory (length: 116 m) represents typical vessels that currently travel in the waterways of Lake Mälaren. The second was a container ship (length: 160m, beam: 23 m). It represents the maximum size for ships travelling on the Lake Mälaren waterways in the future. At present, the max draught on Lake Mälaren is 6.8 m and Coral Ivory was tested at this draught. The ambition is to increase the max draught to 7.0 m and this was tested using the container vessel. Both ships were tested in both ‘loaded’ and ‘ballast’ state. Södertälje Canal is more or less protected from strong winds, so this is where the ‘loaded’ state was tested. The Mälaren fairways were used for testing the ‘ballast’ state, i.e. higher winds and more requirements on manoeuvring. 

Critical areas

Five areas were initially identified as being more or less critical for the navigation of larger ships:
1 Södertälje Lock and Canal
2 Fairway outside Enköping with focus on Hjulsta Bridge and narrow bends at Tedarö
3 Fairway into Västerås and its deep draught port
4 Passage through Kvicksund Bridge
5 Fairway into Köping with focus on the narrow bend at Sjöboda and the turning basin in Köping Port

Compact Bridge Simulator (CBS)

SSPA’s Compact Bridge Simulator has physical controls for main engine, rudder and bow thrusters. Operational information is presented graphically, showing such data as ship speed, transverse velocity in fore and aft end of the ship, turning rate, rudder and thruster indicators, etc. The operator also has a bird’s eye view of the surroundings, which are projected onto the wall.

Simulations

The two pilots from Mälaren spent a total of four days in the simulator on two separate test occasions. The first time, simulations focused on identifying the critical areas. Once that was completed, efforts focused on preparing modifications in specifically selected areas, before new simulations were carried out. 

Based on the conditions that would exist after dredging, it was found that manoeuvring of the large container vessel was feasible in both Lake Mälaren and the Södertälje Canal, provided that the ship is equipped with the right type of rudder. The biggest challenges were not actually the sharp bends at Tedarö and Sjöboda. Rather, the most critical locations were identified as the very narrow passages at Hjulsta Bridge and Kvicksund Bridge. When passing at these places, the speed of the ship was lowered to approximately 5-6 knots, thus exposing it to large drift angles when there were side winds.

Accordingly, there may be some wind restrictions for these locations. However, based on the available wind statistics, this is not expected to have more than a marginal effect on traffic.

The dynamic bottom clearance, both in the canal and the shallower areas of Lake Mälaren, is another issue that must be addressed when considering larger ships. Besides the squat effects, the dynamic bottom clearance is also affected by the heel of the ship. Both wind and sharp turns may, depending on the stability of the ship, provide more or less significant heeling angles.

The diagrams above show that for speeds below 10 knots, the squat is quite moderate at less than 0.4 m. For a container ship, with a low metacentric height, the ship may heel significantly when making sharp turns and at high speeds. A six degree heel, for instance, will lower the bilge by about 1.2 m for a 23 m beam ship.

Photos and illustrations

Södertälje Lock. Photo: Tommy Gardebring, Swedish Maritime Administration.

Passage at Hjulsta Bridge. Photo: Tommy Gardebring, Swedish Maritime Administration.

At Södertälje, the lock must be both widened and lengthened. The suggested new size is 190 x 25 m, the latter figure thus meeting the max size ship with beam of 23 m. Furthermore, the bends in the canal north and south of the lock must be widened somewhat. The movable bridges at Hjulsta (position 2 in the chart above) and Kvicksund (4) are particularly narrow passages, with opening widths for the ships of about 35-40 m. The narrow bends at Tedarö (2) and Sjöboda (5) were also identified as being difficult for larger ships.

The tracked plot represents a transport through Kvicksund when there is a strong northerly wind.

Illustrations: Estimated squat for a 160 m long container vessel operating at three different water depths: 8, 10 and 12 m. 

Take a look at the video clip "Mälarprojektet" (only in Swedish) by the Swedish Martime Administration/Sjöfartsverket.

The question of the expansion of Södertälje Canal and the route through Lake Mälaren to allow passage of larger ships has been an issue for a number of years. By allowing larger vessels, transport costs can be lowered for companies transporting goods through the ports of Lake Mälaren.

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