A Cutter Suction Dredger in waves

Abstract

The CSD650 is the largest cutter suction dredger produced by Damen Dredging Equipment (DDE) in Nijkerk. To provide for a wider range of clients, the CSD will be used in more coastal conditions. Therefore, it is important to study the behaviour of a cutter suction dredger in waves. In this context the following research question is formulated: ’What are the operational limits of a CSD650 in waves and, how do harsh environmental conditions affect the important structural parts of a CSD650 over time?’ To be able to answer these research questions, the possible failure mechanisms are stated. Secondly, a linear frequency domain model is build that incorporates all forces acting on the CSD. A twelve degree of freedom model is built in MATLAB and uses DELFRAC potential software to calculate the frequency dependent added mass, damping and wave forces. All the other forces on the CSD are stated as linearized inertia, damping or reaction forces. The ladder of the CSD is treated as a Morison element to calculate the wave forces on the ladder as for the hydrodynamic forces. When the whole equation of the system is known, the equation of motion can be solved to get the Response Amplitude Operators of the system. Using transformation matrices the RAOs of the centre of gravity of the barge can be transformed to the RAOs of the spud cage. When the mechanical properties of the spud pole and soil interaction are known, the relation between spud motions and stresses can be determined. This relation allows for calculating a stress RAO for the spud pole. In combination with any given wave spectra, so called stress spectra can be determined. Using the statistical characteristics of these stress spectra, important statistical quantities for the stress response can be derived. Firstly, the probability that a stress cycle amplitude exceeds the yield stress which leads to plastic deformation of the spud pole. With the help of fatigue curves and the Miner rule the fatigue damage, caused by different combinations of significant wave height and peak period, can be calculated with this spectral method. However the model seems to overestimate the values of the fatigue damage and yield stress exceedance probability. It is therefore advised to validate the model with real life measurements.