|Title: Calculation of the Dynamic Positioning Capability in Time Domain in Early Design Stages|
|Written by: Adele Lübcke, Stefan Krüger, Jacob Christiansen|
|in: IMDC, Tokyo 2015|
Abstract: The capability of a vessel to automatically keep its position in open water is so called Dynamic Positioning (DP). Such systems are able to compensate the counteracting environmental forces caused by wind, waves and current. To comply with required conditions, it is necessary to consider this aspect in the early ship design stage. In order to predict the limiting environmental condition a calculation method has been developed by the authors. First, a static calculation is presented in the paper, in which the equilibrium of forces and moments of the environmental load and the thrust of the propulsion systems are solved by using an optimization algorithm. Based on these results a time domain calculation is started afterwards to consider the dynamic effects of wind and waves. Therefore the fluctuating influence of these environmental parameters is modeled by equivalent forces in time domain simulation. The wind forces are determined based on the wind speed, lateral area and the drag coefficients depending on the wind direction. The fluctuation of the wind speed and direction is calculated by a spectrum which is common practice. A similar approach is made for the current forces but the current speed is assumed to be constant. An existing diffraction-radiation sea keeping method is used to account the time varying wave drift forces. The calculation method is intended for each type of vessel and any kind of propulsion plants, such as azimuth thrusters or conventional twin screw configurations. It is implemented in the ship design environment E4, among others developed by the Institute of Ship Design and Ship Safety.