Scale model in 6.5 m high beam waves, water depth 21.3 m
In comparison with deep waters, the shallow water waves encountered in the area are significantly steeper and exhibit much higher crests. These dangerous waves are well-known to the crews sailing frequently in the area.
Scaling in our Offshore Basin These environmental conditions were recreated at a scale of 1 to 63 in MARIN’s Offshore Basin. A special feature of our basin is the movable floor, which made it possible to adjust the water depths to match those encountered on the shipping routes along the Wadden Islands.
Bastien Abeil, Project Manager, explains: “Adjusting the water depth in the basin allowed us to reproduce the wave hydrodynamics that are observed at sea along these shallow routes.” MARIN prepared a scale model of a ULCS similar to the MSC Zoe. Particular attention was given to the loading condition of the model, so that it was representative of a typical vessel
of this type. Tests were conducted at different water depths, representing both the southerly and northerly routes which enabled the ship’s behaviour along the two routes to be compared. Additionally, different wave heights were considered, so that the consequences of a more severe storm could be observed. MARIN also did extensive calculations and simulations to broaden the range of ship and wave conditions and talked to nautical specialists who have sailed containerships in this area, he adds.
Based on these investigations MARIN came to the conclusion that a combination of the following four phenomena could lead to ULCSs losing containers:
1. ULCSs have a very large beam of around 60 m which makes them very stable; when a force is applied to them they will return to their upright equilibrium position relatively quickly. This results in a short natural period during which the ship starts to roll
as it is brought into motion by an external force. 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 in such north-westerly storms. As a result, roll resonance may occur, causing heeling angles of 15 to 20 degrees. Such extreme roll amplitudes combined with high ship stability can cause large accelerations and forces being applied to the containers that can exceed safe design values.
2. In these beam waves, ships do not only roll from side to side, but also heave up and down many vertical metres. With a draught of around 12 m in a water depth of only 21 m, there is very limited under keel clearance between the ship and the seabed - actually less than 10 m. As a result of the combined rolling and heaving, wide and deep ULCSs can touch the seabed. When this happens, shocks and vibrations can occur in the ship, containers and lashings.
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