WAGENINGEN, 26 JUNE 2020 – In the night of 1 to 2 January 2019, the large container ship MSC Zoe sails in a southwesterly storm in the southern shipping route above the Dutch Wadden. As a result, the ship loses 345 containers, which leads to major pollution of the sea and the Wadden Islands. The Dutch Safety Board therefore asked the applied research organizations Deltares and MARIN to cooperate in an investigation. Two questions were central to this: what could cause container loss above the Wadden and how can we prevent this in the future?
Deltares determined the wind, current, water levels and wave conditions at the time of the disaster on the basis of detailed calculations. Arne van der Hout, senior advisor port and waterways at Deltares: “That night, the water depth on the route was between 21 and 26 meters. There was a north-westerly storm, up to wind force eight, almost perpendicular to the sailing route above the Wadden. Large waves with a significant wave height of six and a half meters came across the ship, occasionally generating waves of 11 meters high. These conditions occur there on average once or twice a year. “Due to the shallow water above the Wadden, the waves are steep with high peaks. They break regularly, with the top of the wave falling forward at high speed. These “ground seas” above the Wadden Islands are notorious among seafarers who know the area well.
MARIN accurately simulated the conditions set by Deltares at 1:63 scale in its model test facilities. For this MARIN made a test model of a very large container ship like the MSC Zoe. MARIN also did extensive calculations and simulations and spoke to nautical specialists who have sailed in this area. Based on this research, MARIN concludes that the following four mechanisms together can lead to the loss of containers above the Wadden Islands:
Sixty meter wide container ships like the MSC Zoe are very stable. This makes them want to quickly return to their upright balance position when they are unbalanced by a force.
This results in a short “own period” during which the ship will swing by itself when pushed against it. In the current generation of large container ships, this natural period can be between 15 and 20 seconds, close to the wave periods above the Wadden in northwestern storms. This creates “winding”, which allows ships to tilt up to 16 degrees. This results in large accelerations and forces on the containers and lashings that threaten to exceed the safe values.
In these transverse waves, the ship not only swings back and forth, but also dumps meters up and down. With a large draft of about 12 meters in the only 21 meters deep water, large container ships have little space between keel and sea bottom, less than 10 meters. The wide and deep ship can therefore touch the seabed through the combination of swinging and dipping. If this happens, severe shocks and vibrations will occur in the ship, containers and lashings. They can break as a result.
In the very shallow water above the Wadden Islands, the high breaking waves can crash against the side wall of the ship, causing the water to spurt up to the containers, 20 to 40 meters above the sea. This is called “green water” because it is solid sea water, not flying white foam. This massive green water then violently hits the bottom and side of the containers. They can therefore damage themselves, or entire piles can be pushed over like dominoes. When MARIN compares the places where many green water impacts occur with the damage to the containers on the ship, it seems plausible that green water plays a role in the loss of containers.
Finally, the hull of the ship also suffers severe wave impacts from these breaking waves. They too can cause vibrations throughout the ship, which can damage the lashings of containers.
To avoid this type of container disaster in the future, it is important to look at all ship types and sizes that sail through this busy area. For smaller ships, the four mechanisms described also occur, but their sensitivity is different and so are the weather limits for safe sailing. Bas Buchner, director MARIN:
‘Based on shipping traffic above the Wadden, MARIN has therefore advised the Ministry of Infrastructure and Water Management to conduct further research into three container ships: in addition to very large container ships of almost 400 meters like the MSC Zoe, a shorter and narrower’ Panamax ‘of almost 300 meters and smaller ‘Feeder’ of more than 160 meters. A relevant advice because shortly after that, on February 11 this year, the feeder ‘Rauma’ lost seven containers above the Wadden. MARIN’s research is aimed at ensuring that those ships can sail there safely with their crew and cargo, so that no more containers go overboard. We do this for the shallow southern route directly above the Wadden, but also for the somewhat deeper northern route. Based on this, the government can determine which policy is needed: advice to ships and their crews by the Coast Guard, or closing 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. ‘
Esther Germanus, corporate communication MARIN, via firstname.lastname@example.org of M: +31 (0)6 29 72 17 43
Mariska van Gelderen, corporate communication Deltares, via email@example.com of M: +31 (0)6 13 67 13 70