Study of container losses in shipping lanes above the Wadden Islands
Published: 27 June 2020
Deltares provided a description of the wind, current, water levels and wave conditions at the time of the disaster on the basis of detailed calculations. Arne van der Hout, a researcher at Deltares: ‘The water depth in that shipping lane was between 21 and 26 metres on the night in question. There was a north-westerly storm, with a force-eight wind blowing almost directly across the shipping lane above the Wadden Islands. The vessel was hit with crosswise waves with a significant wave height of six and a half metres, with some waves being up to 11 metres high. These conditions are found here once or twice a year on average.’ The shallow water above the Wadden Islands means that the waves are steep with high peaks. They break regularly and the top of a wave then falls forward at a high speed. These ‘ground seas’ above the Wadden Islands are notorious among seafarers who know the area well.
From wave conditions to ship simulations
The conditions identified by Deltares were simulated precisely by MARIN to a scale of 1:63 in its model testing facilities. MARIN made a test model for this purpose of a very large container vessel like the MSC Zoe, completed calculations and simulations, and spoke to nautical specialists who have sailed in this area. The conclusion on the basis of those tests were that a combination of four mechanisms can result in container loss above the Wadden Islands:
1. Sixty-meter-wide container vessels like the MSC Zoe are very stable. That makes them inclined to return quickly to their upright position when they are unbalanced by a force.
This results in a short ‘roll period’ when a force is exerted on the vessel. For the current generation of large container vessels, this roll period can be between 15 and 20 seconds, which is close to the wave periods above the Wadden during north-westerly storms. As a result, vessels may start to heel up to 16 degrees. That generates high accelerations and forces on the containers and lashings that may exceed safe values.
2. In these beam seas, a vessel will not only roll back and forth but also heave several metres up and down. With a large draught of approximately 12 metres in waters that are only 21 metres deep, there is little clearance between the keel of large container vessels and the seabed: less than 10 metres. Due to the combination of roll and heave, the wide and deep vessel may therefore come into contact with the seabed. When that happens, the vessel, container and lashings will be subjected to severe shocks and vibrations with which they cannot cope.
3. In the very shallow waters above the Wadden Islands, the high breaking waves can crash against the side of the vessel, causing the water to spurt up as far as the containers 20 to 40 metres above the surface. This is called ‘green water’ because it is solid sea water rather than white spray. This green water then collides violently with the bottoms and sides of the containers. The containers can be damaged as they collide with one another. Alternatively, entire stacks can be pushed over like dominoes. When MARIN compared the locations of many green-water impacts with the damage to the containers on the vessel, it found that green water may well play a role in the loss of containers.
4. Finally, the hull of a vessel is also subjected to hard wave impacts from these breaking waves. They too can cause vibrations throughout the vessel that can damage the lashings of containers.
To prevent container disasters of this kind in the future, it is important to look at all the different types and sizes of ships sailing through this busy area. The four mechanisms described here also apply to smaller vessels but the way those vessels are exposed to the forces is different, as are the weather limits for safe sailing. Bas Buchner, MARIN director: ‘On the basis of the shipping traffic above the Wadden Islands, MARIN has therefore advised the Ministry of Infrastructure and Water Management to conduct additional studies of three container vessels: in addition to very large container vessels measuring almost 400 metres such as the MSC Zoe, a shorter and narrower ‘Panamax’ of almost 300 metres and a smaller ‘Feeder’ of more than 160 metres. The relevance of this recommendation was demonstrated shortly after when, on February 11 of this year, the feeder “Rauma” lost seven containers above the Wadden Islands.’ The aim of this study is to ensure that these vessels can also sail safely with their crew and cargo so that no more containers are lost overboard in either the shallower southern or deeper northern shipping lanes above the Wadden Islands. The results can be used as a basis for a government decision about the appropriate policy: recommendations for vessels and their crews from the coastguard, or closure of a specific shipping lane in certain conditions. MARIN will share the results as soon as this study has been completed.
For more information, please contact:
Esther Germanus, MARIN corporate communications, M: +31 (0)6 29 72 17 43.
Mariska van Gelderen, Deltares corporate communications, M: +31 (0)6 13 67 13 70.