To asses the navigability in muddy navigation areas the nautical bottom concept was introduced, according to the International Navigation Association: The nautical bottom is the level where physical characteristics of the bottom reach a critical limit beyond which contact with a ship’s keel causes either damage or unacceptable effects on controllability and manoeuvrability. The nautical bottom is mostly determined based on a density value as critical limit. The choice of the critical parameter is merely based upon the feasibility to continuously monitor the density of a mud layer, however, it is only a surrogate for the so-called rheological transition. Moreover, according to the nautical bottom criterion knowledge on ship behaviour in muddy areas is also needed. Experimental research A common practice to gather knowledge on ship behaviour is the execution of model tests. One of the major problems when a mud layer is involved is the search for an accurate model for the mud behaviour. Mud behaves rather complexly and is subjected to time dependence. Furthermore its characteristics vary with the depth. The model tests that have been carried out mostly use an artificial mud layer. Because of the time dependence of the mud layer it is difficult or even impossible to repeat several tests under the same natural mud conditions. In most cases an artificial mud layer was used having a constant density and viscosity in function of the depth. One research institute, SOGREAH (1989), included a gradient for the density. The early research programs were however limited in time or technology. MARIN (1976) only carried out some captive manoeuvring runs, while Flanders Hydraulics Research (1984-1989) did not even have a towing tank. Those three research institutes were the only ones that performed experimental research on muddy navigation conditions. Hence, the results of these programs cannot be generalized, however some common interesting observations were made such as the occurrence of undulations of the water-mud interface and the drop in the speed-propeller rate characteristic, which can be ascribed to the decreased propeller efficiency due to the undulations that disturb the inflow of the propeller. An additional problem in the execution of model tests are the scaling effects. Froude’s law is commonly chosen to scale ship models. In this case both model and full scale density are equal. A scaling correction is determined for the ship resistance according to the ITTC 1978 recommendations. To take the effect of the mud layer into account a weighted Reynolds number has been used, based on the amount of wetted hull surface in contact with the mud layer. Other methods are also possible, such as: • The determination of a weighted frictional resistance coefficient based on the amount of wetted hull surface in contact with the mud layer. This however yields larger reductions in case of low density mud layers; • The determination of the resistance coefficient based on the vertical velocity distribution in water and mud layer. The experimental research can be supported by theoretical calculations which show that a shallow water approach can be used to model the effect of a mud layer. Because of the limited character of the research programs carried out in the past, a new research program was initiated, consisting of captive manoeuvring tests in Flanders Hydraulics Research shallow water tank and both fast- and real-time simulation runs. The mud layer was simulated with a mixture of chlorinated paraffins and petroleum. A wide range of viscosities and densities has been tested covering the typical mud viscosities and densities in the harbour of Zeebrugge. The artificial mud layer was however chemical aggressive so that a special coating in the tank was needed. Tests were carried out with three ship models: a model of a 6000 TEU container carrier, a 8000 TEU container carrier and a bulk carrier. Most runs were carried out with the |
