Following the MSC Zoe disaster, the Dutch Safety Board asked research organisations Deltares and MARIN to cooperate in an investigation. The key research questions were: what could cause container losses above the Wadden Sea and how could we prevent this in the future? To answer these, model tests took place to recreate the situation.
On the night of 1 to 2 January 2019 the large container ship MSC Zoe sailed the southern route above the Dutch Wadden Sea in a northwesterly storm. As a result, the ship lost 345 containers, leading to major pollution of the sea and the Wadden Islands.
Based on detailed calculations, Deltares was able to determine the wind, current, water depth and wave conditions at the time of the disaster. Arne van der Hout, senior advisor port and waterways at Deltares: ‘The water depth on the route that night was between 21 and 26 metres. There was a northwesterly storm, with winds up to Beaufort 8, almost perpendicular to the route above the Wadden Sea. Large beam waves with a significant wave height of 6.5 metres were coming toward the ship, occasionally creating waves of 11 metres high. These conditions occur on average once or twice a year’.
Due to the shallow water above the Wadden Sea, the waves are steep with high crests. Regular breaking occurs, resulting in wave crests falling forward at high velocity. These dangerous shallow water waves above the Wadden Islands are notorious among seafarers who know the area well.
MARIN accurately mimicked the conditions determined by Deltares at scale 1:63 in its model test facilities. For this purpose MARIN made a test model of an ultra large container ship like the MSC Zoe. MARIN also did extensive calculations and simulations and spoke with nautical specialists who have sailed in this area themselves. Based on this research, MARIN came to the conclusion that the following four mechanisms together could lead to the loss of containers above the Wadden Islands:
- Sixty-metre wide container ships such as the MSC Zoe are very stable. As a result, they quickly want to return to their upright equilibrium position when they are brought out of balance once a force is applied to them. This results in a short “natural period” at which the ship starts to roll as it is brought into motion by an external force.
For the present generation of ultra large container ships, this natural period can be between fifteen and twenty seconds, close to the wave periods that occur above the Wadden Sea during northwesterly storms. As a result “roll resonance” can occur, causing heeling angles of up to sixteen degrees. This results in large accelerations and forces on the containers and lashing rings that can exceed safe design values.
- In these beam waves, the ship does not only roll from side to side, but also heaves up and down many vertical metres. With a large draught of about twelve metres, in a water depth of only 21 metres, there is very limited under keel clearance between the ship and the seabed, less than ten metres. Due to the combination of rolling and heaving, a wide ship with a large draught can hit the seabed. When this happens, shocks and vibrations can occur in the ship, the containers and the lashings. Lashings can fail as a result.
- In the very shallow water above the Wadden Islands, breaking waves can hit the side of the ship, resulting in a large upward jet of water reaching the containers, which are twenty to forty metres above the surface of the sea. This is called “green water”, as it is massive sea water, not just white foam in the wind. This massive green water hits the bottom and sides of the containers, which can damage them. Yet, complete stacks can also be pushed over like dominoes.
When comparing the locations on the ship where green water impacts are observed, with the damaged rows of containers on the ship, a clear correlation can be seen. It is therefore probable that green water played a role in the loss of containers.
- Finally, the hull of the ship was also hit by breaking waves. This can result in vibrations throughout the ship, damaging containers and lashings.
These four mechanisms are explained in the following video (text continues after the video).
Research into other vessel types
To prevent such container disasters in the future, it is important to look also look at other ship types and sizes sailing through this busy area. For smaller ships, the four mechanisms described also occur, but their sensitivity will be different as will be the limiting weather conditions for safe operations.
Bas Buchner, director of MARIN: ‘Based on the shipping traffic above the Wadden Sea, MARIN has therefore advised the Ministry of Infrastructure and Public Works to carry out further research into three container ships: in addition to very large container ships of 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.’
This advice proves relevant as on 11 February of this year, the feeder Rauma lost seven containers above the Wadden Sea. MARIN’s investigation is aimed at ensuring that these ships, too, can sail this route safely without containers being lost.
Buchner adds: ‘We do this for the shallow southern route directly above the Wadden Sea, but also for the deeper northern route. On this basis, the government can determine which policy is required: advice to ships and their crews by the Coast Guard, or the closure of a route in certain circumstances. As soon as this extensive research is completed, we will disseminate the results widely, because MARIN wants to contribute to a cleaner and safer sea’.
A more extensive article on the research carried out by MARIN will be published in SWZ|Maritime’s July/August issue, to appear at the end of August.
Picture: Scale models of an ultra large container ship, panamax and container feeder in the test basin at MARIN.