The development of the potential flow solver for the CAE software of a ship requests a reliable method to solve the potential flow. The numerical method of the diffraction force with a three-dimensional time-domain panel method was studied. With a three-dimensional time-domain Green function introduced, its Rankine part was calculated by Hess & Smith’s method while its free-surface memory part was calculated by the method of Beck team from the University of Michigan, followed by the derivation of the diffraction impulse function in the mathematical expressions for making the program. Then the source method was used to calculate the source and the diffraction potential, and the diffraction potential force was obtained by integrating the pressure around the floating body. Finally, the diffraction force and Froude-Krylov force were calculated with the Wigley I ship. The verification was carried out by comparing the results with the published experimental and numerical results. The method and the code in this paper are reliable for developing the potential flow solver of the CAE software and predicting the nonlinear stability failure models in waves.

This paper proposes a hydrodynamic mathematical model of a tethered underwater robot system by introducing boundary conditions and coupling relations into the existed governing equations for umbilical cable systems. A feed-forward and feedback control method was used for adjusting the length of the umbilical cable while the incremental PID algorithm was applied on regulating rotating speeds of propellers for establishing the integral hydrodynamic and control model of a tethered underwater robot. The experimental validation and hydrodynamic responses under the two control manipulations were simulated numerically. The simulation results showed that the proposed model was valid and reliable. In the depth control, the maximum errors of pitch, roll and submerged depth of the underwater robot between simulation and experiment are 2°, 1° and -50 mm respectively. The errors of trajectory tracking simulations in X direction and Z direction are 10% and 15% respectively. The motion in Z direction of the underwater robot is determined mainly by the feed-forward and feedback control strategy for the cable, and the motion in X direction of the underwater robot is primarily controlled by the PID algorithm for regulating the rotational speeds of the duct propellers. The hydrodynamic loadings on the robot are influenced by the flow fields around the robot, and the changes of the flow fields are determined by the changes of the robot velocity and the propellers rotating speed.

High-precision monitoring of the thrust of the propulsion shafting is of great significance for the ship rapidity prediction, hull-engine-propeller matching, and health management of the shaft. However, for the super-long propulsion shafting, because of its large span, small slenderness ratio (ratio of the radius to the length of the shaft) and large thrust, the shaft will produce severe bending-longitudinal coupling nonlinear deformation, which will have an important influence on the thrust measurement based on deformation information such as strain or displacement. Therefore, the bending-longitudinal coupling nonlinear effect of the propulsion shafting on the thrust measurement accuracy was studied. Considering the Von Karman nonlinear displacement-strain relationship, a nonlinear mechanical model of the propulsion shaft was established by using the Hamilton variational principle combined with the finite element method. The bending-longitudinal coupling effect on the displacement, strain and thrust measurement errors was studied. The results show that at low rotation speeds, the bending-longitudinal coupling effect is weak and has little effect on the thrust measurement. However, at high rotational speeds, ignoring the bending-longitudinal coupling effect will cause large measurement errors (the error can reach 13.95% at 240 r/min). Besides, the closer the measuring point is to the propeller, the stronger the bending-longitudinal coupling effect will be, and the larger the thrust measurement errors will be. Therefore, arranging the measuring point near the thrust bearing at the front end of the shaft can effectively reduce the measurement errors caused by the bending-longitudinal coupling effect. The research results have guiding significance for the thrust measurement of super-long shafts with large spans and small slenderness ratios.

Offshore wind energy resources is richer than land wind energy, and water depth of the continental shelf in China's waters increases slowly as the distance offshore increases. Based on the characteristics, how to optimize the design of mooring systems to adapt to the water depth conditions in China is one of the major problems encountered in the development of floating wind turbines. In this paper, a 5MW-OC4 semi-submersible floating wind turbine was used as the research object, the floating wind turbine was moored by suspended chain lines, and the frequency domain and time domain calculations of the floating wind turbine were performed under 40 m water depth in a sea area of Bohai Sea using SESAM software. Mooring accessories were used separately and in combination for parameter sensitivity analysis, and then the mooring system was optimized by combining buoys and clump weight blocks. The results of the study show that under shallow water conditions, the combination of mooring fittings has the same effect on the overall response of the floating wind turbine as changing the same mooring parameters when used alone, but the effect of changing the mooring parameters on the optimisation of the performance of the mooring system is more obvious when used in combination, the optimisation difference in the mean value of the counterweight block position parameters when used in combination can reach 15.7%, and the optimisation difference in the longitudinal oscillation, longitudinal rocking and the tension response are all within 10% of each other. The optimisation difference of longitudinal oscillation, longitudinal rocking and tension response is basically within 10%. Therefore, choosing a reasonable combination of accessories can significantly change the overall characteristics of the floating system and affect the safety and cost of the system.

Polar ship icing is formed when supercooled droplets from the air or seawater fall onto ships. After the crystal nuclei in the supercooled droplet are formed, they solidify and release heat, so that the temperature returns to the freezing point and forms an ice water mixture, which is called recalescence. This process is the beginning of the freezing stage. The phase field method was used to simulate the recalescence process of water under different undercooling conditions, and the icing morphology and ice-phase proportion after recalescence were studied. The results show that the undercooling has a certain effect on the results of recalescence. The dendrite growth rate in the process of recalescence is fast at first and then becomes slow. The proportion of ice phase increases with the increase of undercooling. When the undercooling increases from 30K to 45k, the proportion of ice phase increases from 11.92% to 29.17%.

Mn25Al7 steel is a new type of lightweight and high-strength marine steel, and the fatigue properties of Mn25Al7 steel have not been studied in relevant experiments. In this paper, the fatigue tests of typical nodes such as base metal, butt welded joint and T-welded joint of Mn25Al7 steel were carried out, and the fatigue grade curve of the typical nodes was obtained based on the nominal stress method and the hot stress method, respectively, and compared with the fatigue grade curve of the existing standard. The fracture morphology of the three samples was observed, and the crack propagation law and fracture mechanism were analyzed. The test results show that the fatigue life of Mn25Al7 steel base metal is higher than that of ordinary steel designed by the specification. The specification underestimates the fatigue life of butt weld joints of base metal and smoothed toes, but can accurately evaluate the fatigue life of T-weld joints. By comparing and analyzing the difference in damage rate of the three specimens, the fracture morphology was further systematically analyzed. It is found that the initial crack source and welding residue will reduce the fatigue strength. This study can provide a theoretical basis and experimental support for the prediction of fatigue life of marine high-strength steel.

Two-dimensional numerical simulation was conducted to investigate the characteristics of fluid-induced vibration of a D-section prism with two degrees of freedom at an attack angle of 90°and a mass ratio of 2.6. The RANS equations were solved with the SST k-ω turbulent model closure. Uniform acceleration of inlet velocity and Newmark-β method were incorporated. Firstly the sensitivity analysis of the grid and time step in the present numerical model was carried out, then the comparisons with published experimental results were made to validate the existing numerical model. Then, a systematic analysis of response amplitude, vibration frequency, hydrodynamic coefficient, wake vortex shedding mode and average position-offset was made. The D-section prism exhibits combined response of VIV and galloping modes at a reduced velocity range of U_r=8-14, with vortex shedding pattern alternating between 2S and S+2S. The response amplitude of the two-degree-of-freedom prism is often stronger than that of the one-degree-of-freedom one. The lift force is inclined to the straight side of the section, and more than one frequency multiplications were found for the lift force. The average position-offsets of both cross-flow and downstream direction have maximum values exceeding one characteristic length.

Free surface impact of complex structures, such as airdrop torpedo, the diving of unmanned underwater vehicles, and the water impact of high-speed ships, has always been a research hotspot in the field of ocean engineering. This paper presents the simulation of water impact of complex structures by combining the ghost cell method and the gradient augmented level set method (GALS). A time semi-implicit finite difference method was used to solve the incompressible Navier-Stokes equations, the ghost cell method was used to enforce the no-slip boundary conditions by interface reconstruction, and the gradient augmented level set method was used to capture nonlinear free surfaces such as wave overturning and jet flows. The slamming of a two-dimensional cylinder at constant speed was simulated to validate the accuracy of this numerical method by comparing the present results with the experimental data. Upon simulation of the slamming of a two-dimensional hull section, the variation rules of the impulsive pressure, the motion response, the pressure distribution, and the free surface motion with respect to the relative impact angle were analyzed. Also, some typical impact phenomena were observed such as the flow separation, the jet flow, and the ventilation for a small impact angle.

As wind speed and wave height are the main loading parameters in offshore facility operations, their accurate prediction is of great importance. In order to solve the problem of wind speed and wave height prediction with complex and changeable characteristics, a wave height forecast model was established based on prototype monitoring data and Long-Short-Term Memory (LSTM) neural network. Firstly, the correlation analysis of wind speed and wave height was carried out based on prototype monitoring data. Then, a one-step-ahead wind speed forecast model and wave height forecast method were established based on LSTM neural network. Different prediction models with different time intervals (t=0.5 h, 1 h, 3 h) were built to verify the accuracy. Finally, a joint prediction model based on two forecast models was obtained with a prediction error of only 0.12 m at the time interval of 0.5 h.

The air lubrication drag reduction is a flow control method for energy-saving and carbon reduction. Under the background of “carbon peaking, carbon neutrality” strategy in China, the air lubrication drag reduction is essential for the development of low-carbon and zero-carbon ships since it can effectively reduce the ship resistance. In this paper, the background of the air lubrication drag reduction technology both at home and abroad is introduced. The definition and classification of air lubrication drag reduction are clarified based on the air-water flow regime and drag reduction mechanisms, i.e. dispersed bubble drag reduction and continuous air layer drag reduction. The progress of dispersed bubble drag reduction and continuous air layer drag reduction in two-phase flow regimes, drag reduction characteristics and mechanisms are summarized respectively. Finally, the status of air lubrication drag reduction is summarized and the future opportunities in this field are discussed.