A fairway bottom consists of multiple layers. The deepest of them are essentially rigid, but the layers on top are a mixture of water and mud and therefore behave as (non-Newtonian) fluids. Contact of a ship hull with the rigid bottom layers of a fairway must be avoided because of possible grounding and subsequent damage. At the same time, contact with the fluid mud on the bottom may not cause any direct casualties but results in a considerable change in the ship manoeuvring behaviour. Ship manoeuvrability is crucial in harbours and therefore the majority of captains prefers to avoid any encounter with mud layers for the sake of safety. This, in turn, reduces the effective area of the fairway. Extremely costly dredging procedures are thus often necessary to fulfil the demands of the ship’s manoeuvring in a harbour.
The rheological properties of the mud and the extent to which the hull is submerged in it directly determine the effects on ship manoeuvrability. Therefore, if one could predict in advance if and how far a ship can safely enter a particular mud layer without a considerable loss of manoeuvrability, a significant amount of the resources currently spent on dredging can be saved.
The theoretically correct way of predicting the effect of the contact with the fluid bottom layers on the manoeuvrability of the ship consists of analysing the rheological parameters of the particular mud and simulating the ship manoeuvres in it in order to obtain the corresponding manoeuvring coefficients. However, the collection of the mud samples and their rheological analysis take at least a few days, whereas the properties of the mud may change every day. This makes the described approach inapplicable in practice. Therefore, a faster approach is being developed.
This approach consists of towing a model body through the mud layer and comparing its dynamic behaviour to that in water. This allows for determining the “effective” viscosity and density of the mud layer. After that, a correlation between the effective viscosity and the manoeuvring coefficients should be established to allow for a quick evaluation. However, developing such a correlation is not a trivial undertaking and should involve numerous computational and experimental tests. On the computational side the test should include the simulations of ship manoeuvres (see Figure 1). A number of hull forms and operating conditions has to be considered in order to broaden the applicability range of the described approach.
The research project is currently in its first phase. Different designs of the model body are studied using computational fluid dynamics (CFD) in order to choose the optimal one. The first tests confirmed the applicability of the approach described above.
The simulations of the flow around the model body in water as well as in non-Newtonian fluids replicating the behaviour of the mud are being conducted. A new rheological model has hence been developed in the open-source CFD package OpenFOAM. This model is based on the parameters determined during the rheological analysis of the fluid bottom layers and replicates their stress-strain curve (see Figure 2). The rheological data have been provided by the Hamburg Port Authority.
The research activities of the next project phase will concentrate on the simulation of manoeuvring tests for different types of vessels in order to find the correlation between the manoeuvring coefficients and the effective rheological properties of the fluid. The results of the computational manoeuvring tests will be compared with the experimental data provided by the Waterways Engineering and Research Institute (BAW). This will guarantee the reliability of the developed approach.
The research project is funded by the Hamburg Port Authority.
Hamburg Port Authority (HPA),
Waterways Engineering and Research Institute (BAW),
Hamburg University of Technology (TUHH)
2018 - 2022
Status: current project