Entwicklung eines automatischen Positioniersystems für Schiffe mit Voith-Schneider-Propellern: Numerische Simulationen von DP-Operationen unter Berücksichtigung realer Betriebsverbindungen

Project Acronym
Project Title
Numerical Simulation of Dynamic Positioning Manoeuvres under Realistic Operational Conditions
Funding Code
Principal Investigator
Contractor Organization(s)
The industrial utilization of coastal waters experiences continuous growth. Along with the installation and maintenance of offshore installations, the demand for special crafts and platforms is increasing. To assure safe and efficient offshore operations the vessels are equipped with highly sophisticated dynamic positioning (DP) systems. To evaluate the capability of dynamic positioning and for the reason of approving DP systems for specified offshore operations, classification societies, ship owners and charterers, methods for the assessment of positioning capability are needed. In the past, static assessment methods represented the state of the art. Herein, the maximum available thrust in specified directions is evaluated against wave forces of second order, constant wind and current forces. This procedure neglects all dynamic effects arising from non-steady external loads as well as the dynamics of the propulsion plant and control systems itself, leading to unrealistic results. Indeed, the real positioning performance of the DP vessels were significantly lower than expected. Intelligent, tailor-made control strategies and highly reactive manoeuvring systems, e.g. Voith-Schneider Propellers could help to increase the position-keeping performance in real ship operation. Therefore, reliable methods for the simulation-based design of the DP systems and assessment of the position-keeping capability are required by the industrie.
Besides efficient and reliable procedures for the prediction of environmental loads from wind and current, a numerical method for simulation-based position-keeping capability assessment is developed in DPOpera. The time-domain simulation tool is based on impulse-response-functions, which allows the application of efficient frequency domain-based boundary element methods to determine the hydrodynamic forces acting on the vessel. Techniques for the unsteady consideration of environmental loads are implemented as well as the dynamic modelling of control systems and propulsion plants. All developed methods, procedures and tools are integrated in an evaluation framework allowing the participants to perform holistic project-based position-keeping assessments based on time-domain simulations.

For the testing of the designed control strategies and validation of the developed methods, a large amount of experimental reference data is collected during DPII. This includes wind tunnel experiments at the TUHH, comprehensive model tests at the Potsdam Model Basin and full-scale measurements.
The research project DPOpera focusses on the development of a numerical framework for the time domain simulation-based assessment of the positioning capability. As ship motions in seaway are of stochastic nature, long simulation timespans or multiple realizations of same seaways are needed. Therefore, an fast and reliable numerical method to simulate ship motions in waves under arbitrary external loads is developed and validated. Hydrodynamic radiation forces are determined from results of efficient linear frequency domain boundary element methods, whereas important non-linear contributions, e.g. viscous damping and restoring forces, are directly modelled in time-domain.

Within DPOpera, also experimental and numerical investigations of wind loads acting on modern DP-relevant ship forms are conducted. This includes also measurements at the institutes slow-speed wind tunnel to obtain valuable validation data. Turbulent CFD simulations are used for detailed analysis of the wind flow around the ship’s superstructures. For a later consideration of unsteady wind loads in the DP simulations, efficient coefficient-based methods for fluctuating wind modelling are developed and implemented.

Beside wave drift forces, also current loads have significant impact on the positioning performance of DP actuated vessels. Therefore, current induced loads are also determined by viscous CFD simulations. To obtain the best reasonable compromise between computational effort and accuracy, different influencing factors and modelling approaches are and evaluated.

To obtain deeper insights on the thrust generation at the Voith-Schneider-Propellers during DP- Operation, detailed numerical investigations are performed, e.g. concerning scaling effects and thruster wave interaction. The findings are also implemented into the DP simulation framework.

For the validation of the developed simulation-based capability assessment method, extensive model test at the Potsdam model basin are conducted in DP-II. In a later phase of the project, also full-scale measurements are planned.