Dynamic Positioning Capability Analysis For DP Vessels

Hydrodynamic interactions of DP thruster system, thrust optimum allocation and analysis of DP capability are presented in this article.The high accuracy locating of dynamic positioning system and the correct prediction of positioning polar capability have a close relation to the correct calculation of propeller losses in all weather conditions. When the propeller worked under the water, there will be complex flow filed around the propeller which leading to attenuation of propeller performance. The formation mechanism of the thrust losses caused by thruster-hull interaction, thruster-thruster interaction, the current, the wave and other factors are discussed in this paper. Using semi-empirical methods based on model test, the thrust losses are calculated, and the measures of how to reduce thrust losses are also discussed.Thrust optimum allocation is the core part of the DPS, so it is important to find a reasonable strategy to determine the magnitude and direction of thrust each thruster generates. Applying the concept of extended thrust, thrust model is established, and based on the nonlinear optimization algorithm, a rational and effective optimal thrust allocation method is presented in this paper. The objective of the thrust optimum allocation is to minimize the fuel consumption for dynamic positioning vessel, other factors such as thrust limitation, forbidden zoon are also taken into consideration. Especially, when rudders are used in DPS, the thrust allocation problem becomes non-convex and therefore difficult to solve. Using convexification techniques and establishing the propeller/rudder model, a thrust allocation method for DP vessels with rudders is presented.The preciseness of DP capability evaluation largely depends on the accuracy of environment load estimation. From the aspect of DP capability, the environment load calculation methods are discussed in this paper. Using the empirical formula, the wind load calculation module is established, and based on the date of series model text, the current load and wave load calculation modules are also established.Finally, DP capability plot is introduced to evaluate the positioning capability. Base on the thrust optimum allocation and capability polar plot program, the wind speed envelopes and the thrust utilization envelopes can be calculated, then the positioning capability of the DPS can be evaluated and analysised.