Numerical And Experimental Study On Motion Responses Of LNG Ship Model

With increasing demand for energy and the enhancement of environment consciousness, the natural gas as a clean energy has drawn more and more attention. For long-distance offshore transportation of liquefied natural gas, undoubtedly, LNG vessel is the most effective and most economical choice. Seakeeping performance as an important indicator of the safety performance of the ship, violent motion in waves affect its operating efficiency and safety performance, hence its seakeeping performance should be taken into account during the progress of designing and operating. The movement response of the LNG ship in wave can be obtained by numerical calculation, which can provide a reference for the design. It needs to consider the impact of tank sloshing during operating progress of the LNG ship, on the mean time, the calculated results can provide motion load when studying sloshing loads on the liquid tank. In present paper, based on the three-dimensional time-domain Rankine source method,the motion responses of LNG ship with forward speed in waves were studied. The content as follows:Firstly, numerical verification of Rankine source method. The motion responses of the Wigley II and III ship in regular waves are calculated with the application of Rankine source panel method. Two kinds of linearized free surface method, including N-K(Neumann-Kelvin) linearization and D-B(Double-body) linearization method, are taken account in the calculation. The calculated results were compared with experimental results, shown in good agreement with the experimental results, which proving the reliability of the method. For the large midship section coefficient like Wigley II ship, the results of DB linearization method are closer to the experimental results than N-K linearization method. For slender hull form(e.g. Wigley III vessel), the calculation results at the speed of Fr = 0.2 are in good agreement with the experimental results, while exist acceptable error at the speed of Fr = 0.3 and 0.4 during the peak area. In general,the results of Double-body linearization method is more reliable.Secondly, based on Rankine source method, the motion responses of LNG ship in waves were calculated. In this paper, under different wave direction and forward speed, the motion of LNG ship such as pitch, heave, roll and other motion response are calculated in time domain with linear method or nonlinear methods. The motion responses are transformed into the frequency domain by the Fourier transformation. The response curve of the LNG ship at the forward speed of 19.5 knot is consistent with the forward speed of 17.5 knot, which indicates that the impact of the speed on motion response is not very obvious. Wave direction has significant influence on motion response. Heave, roll and sway are the maximum in the case of transverse wave, while pitch is the maximum in the case of head sea. Nonlinear factors have weakly effect on the results of the calculation when the wave length is short, while in the long wave length, the motion shows certain nonlinear response.Lastly, according to relevant requirements, the seakeeping experiment of LNG ship mode is conducted and then the spectrum analysis method is applied to predict motion response with calculated results. The calculated results are compared with the experimental results in this paper. The variation of the calculated result and experimental result is fairly consistent, which giving further evidence for the feasibility of Rankine source method used to calculate ship motion responses. Based on spectrum analysis method, significant motion amplitude under different sea level is obtained from the calculated result by using JONSWAP spectra.