Development of a numerical model for Cutter Suction Dredgers

Abstract

A Cutter Suction Dredger (CSD) is a vessel that is used to cut hard soils with precision. It is moored stiffly through a spud pile while the soil is being cut. This, in combination with wave loads, causes the whole system to be dynamically challenging and nonlinear. Previous attempts to model a CSD in operation have been limited to the frequency domain. However, in order to be able to include these nonlinear effects occurring during the operation of a CSD, a time domain analysis is necessary. The hydrodynamic forces in the time domain analysis will be calculated using Ansys AQWA. The external forces which occur during operation will be calculated by a custom script written in Python. The script consists out of 3 modules; a spud module, a winch module and a soil module. The spud module is responsible for calculating the mooring forces on the spud pile in order to maintain its position. The spud has been modeled as a beam with 3 pinned supports. This linear model has been expanded to include the option to take the presence of a flexible spud carriage into account. This means that if the buffer of the flexible carriage is activated, the stiffness of the system changes accordingly in order to reduce the loading on the spud with increasing displacements. The winch module is responsible for determining the force required to achieve the swing around the spud at the desired velocity. Within the module, the application of a PID controller ensures that the tension in the side wires is adjusted dynamically in order to maintain a stable swing velocity throughout the whole process. Tuning the controller correctly is critical to avoid unnecessary tension peaks while at the same having a sufficiently quick response to sudden changes. The soil module is responsible for calculating the reaction forces on the CSD as a consequence of the cutting of the soil. The soil is characterized by the specific energy characteristic, which enables the module to be applied to all types of soil, from soft clay to hard rock. The 3D force vector on the cutterhead is then calculated using the volume cut and the rotational torque of the cutterhead. Furthermore, the module keeps track of where the soil has been cut, adjusting the new height as the cutterhead passes. This enables to realistically create a time series where the volume cut during each timestep will vary due to the oscillations of the cutter. The final result is a model in 7 degrees of freedom which can dynamically respond to the nonlinear reactions caused by cutting soil, wave loads, mooring through a flexible carriage and varying side wire forces. This model is now ready to be used to investigate the effects of different combinations of soil types and wave conditions.