The Dynamic Positioning (DP) performance of a ship or semi-submersible is often expressed in a polar plot called a DP capability plot. In an early design stage the DP capability plot is calculated by the so called quasi-static calculation method. A drawback of the method is that it cannot predict the vessel offsets from the mean position in a given environment. In this paper a novel approach to calculate DP capability in an early design stage is presented. Based on the traditional concept of Dynamic Margin, the quasi-static calculation method is extended in order to account for the time variable environmental forces and offset requirements. Force Multiplication Factors (FMFs) dependent on specific DP-offset requirements are introduced to replace the fixed Dynamic Margin. Through time domain simulations, it is shown that a good correlation exists between such FMFs and the extremes of the excitation force impulse peaks. FMFs are calculated for a semi-submersible and a drillship.
One of the aspects in the early stage of the basic design of a DP vessel is the DP capability analysis. In such a study various DP capability and feasibility plots are presented for the different environmental conditions defined in the vessel's specification document. These plots are usually calculated by a quasi-static analysis, which is a very efficient method, but does not include offset requirements. The time variable environmental forces are accounted by a Dynamic Margin - hence the word "quasi". An alternative to the quasi static method is time domain simulations as shown in Brink and Chung (1981) and as proposed by Serraris (2009) and Smogeli et al. (2013). Although modern computational power has increased and such a simulation is no longer considered cumbersome, the problem set-up, number of simulations and the amount of variables is such that it renders the method inefficient for producing capability plots in a practical way. Model tests can also be performed to gain insight in the DP behavior but are too costly and time consuming for the full scope of the DP analysis.
The purpose of this paper is to derive a practical method to calculate the DP capability including offset requirements. Van't Veer and Gachet (2011) presented a dynamic DP approach and discussed the possibilities to correct the quasi static method for the time variable environmental forces. In their paper they presented an initial investigation of the applicability of the proposed method. In the current paper, their method is further extended and verified.
