This study focuses on the optimization of ship dimensions by considering hydrodynamic performance in waves. In actual seaways, a ship experiences speed loss due to environmental loads by waves and wind. Therefore, along with calm water resistance, speed loss in waves should be considered in the hull form design in order to improve operational efficiency in waves. However, a trade-off may be needed between total resistance on the ship and the speed loss in waves. To address this problem, Non-dominated Sorting Genetic Algorithm II, which is a multi-objective optimization method, is used to minimize the total resistance on a ship in seaways and the speed loss by additional resistance. In the optimization process, added resistance is predicted using a numerical method based on slender-body theory, Maruo's far-field formulation, and an empirical formula for added resistance in short waves. The speed loss in waves, which can be expressed by a weather factor (f w ), is estimated using power-speed curves. This article introduces some examples of the sensitivity analysis of added resistance and speed loss in waves to the variations of ship dimensions. Finally, the optimization solutions on a Pareto front set are compared to a basis ship in terms of hull form, and the corresponding hydrodynamic performances are evaluated.
This study considers the evaluation of ship operational performance in real sea states using a time-domain approach. The current seakeeping-maneuvering coupling approach consists of two modules. First, in the seakeeping module, the time-domain three-dimensional Rankine panel method is applied to compute wave-induced forces and resultant ship motion. To validate this module, the computational results for wave drift force are compared with the existing experimental data for various forward speeds and regular wave conditions. Second, in the maneuvering module, the equations of motion with 4 degrees of freedom that are based on the Maneuvering Modeling Group are solved to simulate the ship navigation. The computed seakeeping and maneuvering values are immediately transferred between the two modules in the time domain, and so they are directly integrated. By applying this coupling method, a free-running simulation for a ship navigating along a given route is performed. The trajectory tracking method based on a proportional–derivative-based rudder control is adopted for straight course-keeping. Not only the speed loss but also the attitude for route maintenance is evaluated for various environmental load conditions. The simulation results are validated by a comparison with those of the existing free-running model test. Based on comparisons, environmental load effects and resultant quantities on operational performance are discussed.
An enhanced body-force propeller model is developed to consider propulsion effects without solving the actual propeller geometry in ship maneuvering problems. Application to the KCS turning circle test in calm water (starting at the self-propulsion point) is conducted using the computational fluid dynamics solver, snuMHLFoam, which was developed on OpenFOAM-plus. Based on the original Hough and Ordway model, the non-uniform advance velocity of the propeller is considered using the local velocity on the inflow plane, to compute the local advance coefficient for different parts of the propeller disk when the propeller works behind a hull. The original overset algorithm is revised by introducing more flexible hole-patch definitions for the hole-cutting procedure and an iterative procedure for the donor-searching procedure to remove invalid donors. The motion decomposition of ship and rudder motions with the revised overset is implemented in order to handle body motions effectively. Self-propulsion and turning circle tests for KCS ships are successfully conducted in calm water. The predicted results, including the PI control results, turning trajectory and parameters, ship motions and velocities, forces and moments, and flow and vortical structures are illustrated and compared with the benchmark experimental data, as well as the numerical results obtained using the Maneuvering Modeling Group (MMG) model. The results indicate that the developed body-force propeller model can provide more reasonable predictions of the propulsive performance when the propeller works behind a hull. The snuMHLFoam solver, which is coupled with motion decomposition using the revised overset grid methodology, is validated to be effective and reliable.
This paper introduces an outlier analysis which can improve the convergence of the statistical analysis results of sloshing model test data. The paper classify possible outliers in the sloshing model test into three categories and present a treatment method for each outlier. The developed outlier analysis is adapted to the model test results for the cargo of the liquefied-natural-gas (LNG) carrier in operation. The results of the present new method are compared with those of the conventional procedure, particularly focusing on long-term sloshing prediction. Through this study, the effectiveness of the present method is observed, and it is found that the present method provides is robust and reliable results in the application of experimental data for load prediction.
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