New Sloshing Severity Prediction And Evaluation of Sloshing Effect on Ship Added Resistance in Waves

Abstract

Engineering practice presents some impact of sloshing and ship motion interaction on both sloshing severity and ship added resistance in waves. This paper aims at finding a proper way to estimate sloshing severity, and then attempt to figure out the effect of sloshing on added resistance through experiment. First, the concept of sloshing severity RAO (SSR) is introduced. From comparisons with experimental data of a 3D regular model test, the wave elevation on the liquid free surface as the initial SSR is adjusted and a new SSR is generated. Nonlinear combinations of the new SSR can give sloshing severity index under irregular sea states. After validation, the sloshing severity of a liquefied natural gas floating production storage and offloading (LNG FPSO) is predicted under a low filling condition. Then, an LNG carrier with one prismatic tank is modeled for free running tests in regular head seas in the deepwater towing tank. For each wave case, the revolution rate of propeller, the ship speed, and corresponding torque and thrust are measured, and the added resistance is thereafter obtained by subtracting the still water resistance from the mean total resistance on assumption of a constant thrust deduction. The experimental results are comparatively discussed in conditions of sloshing and ballast weight. From the present observation, this proposed prescreening methodology and generated sloshing severity index is expected to be applicable to the selection of severe sea states for sloshing loads analysis. Besides, under current model test conditions, internal sloshing affects ship motion, and the added resistance in waves decreases accordingly while the energy consumption decreases. This phenomenon shall be taken into account by shipbuilding industry. Liquid cargos may have influences on the resistance compared to solid cargos used in sea trail, in turn a change of ship power.