A Cutter Suction Dredger in Operational Condition

A Cutter Suction Dredger in Operational Condition: Implementation of a Cutter Force Function in a dynamic multi-body model

Bakhuizen, Vincent (TU Delft Mechanical, Maritime and Materials Engineering)

Delft University of Technology

Offshore and Dredging Engineering

2018-11-22

Modern large Cutter Suction Dredgers utilize anchors, swing wires, a cutter head and a spud, which enables the CSD to excavate sand and tough rock masses with a relatively high accuracy. A drawback of the relatively stiff is that the ship often encounters problems in certain wave conditions. Several models exist which study the dynamic effects of the mooring system, when the cutter is not being operated. These models have led to innovations in the development of new spud mooring systems which are more reliable and allow the operators to safely work up to the operability boundaries.
From the desire to study innovative concepts for the cutter ladder as well to improve the range of operability, IHC has offered a thesis project to examine a dynamic simulation program of a CSD in operational condition. This project is based on the program DODO which couples the hydrodynamic solutions of a diffraction solving algorithm (DIFFRAC) to multi-body dynamics. The RK4-method is used for the numerical integration of the equations of motion. Although IHC already has several highly detailed cutting models, none of them were used since early on in the project it became clear that they were unsuitable in combination with the multi-body models of CSDs in DODO. Especially the level of detail and the scale size of these programs eventually led to the idea of developing a new and computationally cheaper model. This new Cutter Force Function is based on a case study.
The model for the case study is constructed according to a conventional multi-body method in which the cutter ladder body and the floater body are connected to each other via constraints. The hydrodynamic coefficients and the Froude-Krylov wave forces of the external DIFFRAC solution are applied to the floater body. The transfer functions of the Linear Time Invariant-system are studied to verify the system and to act as a stepping stone for future simulations that include the Cutter Force Function. In the Cutter Force Function an orthogonal spacial discretization is used for a cutter and a seabed matrix which store a template of force unit vectors and the instantaneous surface height of the seabed, respectively. With an elementwise overlay method the cut volumes and their according force vectors are determined for each time step. The Cutter Force Function provides a framework to which clay- and sand cutting models can be added as well as other effects such as bulldozing forces, snow-plough effects, etcetera.
Several time domain simulations have been run to verify the functionality of the Cutter Force Function. The primary results indicated that the function gave adequate signal forms although their amplitudes were not limited to a maximum power. Due to the fact that this could generate energy in the system at moments at which it should not be physically possible, a feedback control loop based on the cutter drive torque should be implemented. The LTI-model of the CSD and the transfer functions are ready to be used for the study of implementing PID-controls on swing wires.

Cutter Force Function
CSD
operability

http://resolver.tudelft.nl/uuid:f3fac801-91b0-488a-a1e2-07c5a78838cd

Student theses

master thesis

© 2018 Vincent Bakhuizen