Scale models of a large containership, panamax and feeder in the test basin at MARIN.
Investigations into the 2019 loss of 345 containers from MSC Zoe have indicated that the vessel’s high stability was a contributing factor, and other vessels may face similar risks
MSC Zoe was sailing on the southern route to Bremerhaven, Germany, in the Terschelling-German Bight traffic separation scheme along the Dutch Wadden Islands on 1 January 2019 when the containers were lost overboard. The Dutch Safety Board engaged the Deltares research institute and the Maritime Research Institute Netherlands (MARIN) to assist in its investigation, and the report has now been released.
Deltares determined that the water depth on the southern route that night was between 21 and 26 meters. There was a northwesterly storm with winds up to Beaufort 8 almost perpendicular to the route. Large beam waves with a significant height of 6.5 metres were coming towards the ship, resulting in extreme wave heights of up to 11 metres. Under these conditions, such waves are steep with high crests. Regular breaking occurs, resulting in wave crests falling forward at high velocity.
The environmental conditions determined by Deltares were modelled accurately at a scale of 1:63 by MARIN at its testing facilities. MARIN investigated a test model of an Ultra Large Container Ship (ULCS) like MSC Zoe at this scale, conducted expert interviews and also analysed vessel data including Voyage Data Recorder audio.
Recordings of equipment moving back and forth on the bridge were used to determine when the most intense rolling occurred, and GPS data also indicated a swaying motion in the track of the vessel. Inclinometer readings onboard were not considered a reliable measure of roll angle; MARIN research has previously determined that mechanical inclinometers are sensitive to accelerations and under severe motion will provide a reading of a combined gravity angle due to transverse and vertical accelerations.
Based on its research, MARIN concluded that four factors led to the loss of the containers:
60-metre wide container ships like the 395.4 meter MSC Zoe are very stable. When a force is applied to them they tend to return to their upright equilibrium position quickly. This results in a short natural period during which the ship starts to roll in response to external forces. For the present generation of ULCSs this natural period can be between 15 and 20 seconds, close to the wave periods that occur above the Wadden Islands during northwesterly storms. As a result, roll resonance can occur, causing heeling angles of up to 16 degrees. So, although they are stable, these large container ships can roll steeply. This causes large accelerations and forces being applied to the containers that can exceed safe design values.
Various factors including loading condition are important, but MARIN concluded that that a ship with lower stability than MSC Zoe has a lower chance of seabed contact as a result of rolling.
In the beam waves experienced by MSC Zoe, the ship rolls from side to side and also heaves up and down many vertical metres. With a large draft of around 12 metres in a water depth of only 21 metres, there is less than 10 meters under keel clearance. As a result of the combined rolling and heaving, a wide ship with a large draft can touch the seabed. When this happens, shocks and vibrations can occur in the ship, containers and lashings. The lashings can fail as a result.
No damage to MSC Zoe‘s hull and bilge were observed after the incident, but MARIN indicated that contact could not be ruled out, as soft contact with the seabed may not necessarily result in detectable damage.
3. Green Water
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 20 to 40 metres above the surface of the sea. This green water is massive sea water, not just white foam in the wind, and it can damage the bottom and the side of containers and can also lead to complete stacks of containers being pushed over like dominos.
MARIN concluded that green water impacts probably played a role in the loss of the containers which were stacked on MSC Zoe‘s deck up to the eighth tier. Modelling indicated green water was concentrated on six container bays around the wheelhouse (bays 26 to 46), and this is where most containers were lost.
The hull of the vessel was hit by breaking waves. This can result in vibrations throughout the ship, damaging containers and lashings. There were six moments of container loss, and MARIN stated that they can be considered as independent events with different combinations of the four factors involved.
The conditions that MSC Zoe experienced occur once or twice every year in the area. In 2019, the Dutch Safety Board issued an interim warning that container ships dimensioned similar to MSC Zoe could experience contact or near contact with the seabed in the southern route in specific wave and wind conditions.
However, MARIN has now urged the Ministry of Infrastructure and Water Management to conduct further investigations into three ship types: ULCS with lengths of up to 400 metres, like the MSC Zoe, a shorter and narrower Panamax, nearly 300 metres long, and a smaller container feeder with a length of 160 metres. The importance of testing smaller ships was underscored when the 180-metre feeder Rauma lost seven containers off the Dutch coast on 11 February 2020.
The MARIN investigation has also concluded that both the northern and southern shipping routes near the Dutch Wadden Islands are subject to comparable route-specific risks for large container ships. The investigation revealed that the wave conditions on the northern shipping route were comparable with those on the southern route, so large, wide container ships on the northern shipping route could also experience extreme ship motions, green water and slamming. The northern shipping route is deeper than the southern one, so contact with the seabed is not likely.
The southern and northern shipping route have been set by the IMO, and the Netherlands has no independent authority to impose binding regulations or to adjust the location of the routes. The recognition of the Wadden Sea as a Particularly Sensitive Sea Area by the IMO does however offer the affected coastal states (the Netherlands, Germany and Denmark) some opportunities for proposing protective measures for shipping to the IMO.