Inspired by the“casing treatment”in turbomachinery, a rectangular groove structure is applied to the inner surface of ducted propeller to reduce the strength of the tip vorticity and inhibit the cavitation of tip vorticity, which is expected to improve the hydrodynamic and cavitation performance. The unsteady numerical calculation of Ka4-70 propeller operating inside duct 19A, with or without grooves, was carried out by using computational fluid dynamics (CFD) method. The influence of groove structure on tip pressure, tip vortex strength, tip vortex structure and hydrodynamic performance of blades was studied. The results show that the groove structure can significantly change the tip vortex of duct propeller, weaken the strength of vortex, increase the minimum pressure at the tip of blade, and almost have no effect on the propulsion performance. The results provide a new solution for tip vortex control and vibration and noise reduction of the ducted propeller.

Stingers bear the alternating load during operation, and the fatigue failure of the structure can not be ignored. Therefore, based on the wave spectrum and real-time monitoring data, the fatigue problem of a stinger was studied. The comparison and analysis of the calculation results suggest the fatigue damage calculation based on the wave spectrum is conservative due to the strong randomness of the wave load, so this method is suitable for the prediction and evaluation of the fatigue life of the structure in the design stage. However, the fatigue damage calculation based on the real-time monitoring data is relatively more accurate, so it is suitable for real-time assessment of the immediate fatigue damage of the structure. Finally, this paper puts forward appropriate repair and maintenance suggestions accordingly.

LNG cryogenic hoses are used for connecting floating structures to transport LNG efficiently and continuously in the process of LNG deep-sea transportation and unloading. Making sure that cryogenic flexible hoses operate safely and reliably is crucial for the LNG mining system. The main application conditions of LNG cryogenic hoses are ship-ship side-by-side (SBS) unloading, ship-ship tandem unloading, and ship-shore refueling. The hydrodynamic analysis and calculation of LNG cryogenic hoses were carried out according to these application conditions, and sensitivity analysis of the global layout parameters of the hoses was performed. The LNG cryogenic hose with a diameter of 12 inches was used as the object of study. Considering the combined effects of wind, wave and current marine environmental loads, the LNG cryogenic hose was modeled and subjected to finite element calculations, hydrodynamic analysis and computational checks under different application conditions based on Orcaflex software. The critical response was studied, and sensitivity analysis was performed on the global configuration design parameters of the cryogenic hose and the lifting speed. The results show that increasing the hose length and the distance between the connection points leads to an increase in the curvature extreme value and a decrease in the tension extreme value for both tandem and SBS unloading conditions. The lifting speed decreases the tension extreme value and increases the curvature extreme value during ship-shore refueling condition. The study can provide a theoretical basis for the design optimization of LNG cryogenic hoses and has reference significance for the configuration arrangement of FLNG unloading systems.

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.

Underwater stability is one of the important overall performances of deep-sea human occupied vehicles (HOVs). The stability of HOVs is measured by metacentric height (GB), which is closely related to the operation and safety of HOVs. The metacentric height of HOV changes due to the change of ocean parameters and its own ballast during movement. Based on the basic formula of metacentric height and the buoyancy loss calculation method for HOV pressure structure, buoyancy material and hydraulic oil, a mathematical model of metacentric height for HOVs was established in this paper, and a method of underwater stability analysis for HOVs was proposed. Taking “SHEN HAI YONG SHI” HOV as the research object, the underwater stability of HOVs was calculated and analyzed, and the variation law of the metacentric height of HOVs was obtained, which can provide a reference for the design of HOVs.

Based on the discrete module methodology, this paper proposes a numerical method to estimate the mean drift forces on flexible floating bodies. The continuous structure was first discretized into rigid modules connected by elastic beams. The first-order hydroelastic responses were solved by coupling the hydrodynamics on modules and the structural stiffness of elastic beams in the frequency domain. Based on the first-order motions of the modules, the second-order mean drift forces on each rigid module were calculated by second-order multi-body hydrodynamic theory. The motions and mean drift forces of a freely floating flexible barge using the proposed method were verified against the results obtained by the 3D hydroelastic theory based on modal superposition. Moreover, due to the discrete property of the present method for hydrodynamics, the method can be directly extended to inhomogeneous wave conditions.

In order to consider the attenuation effect of the pore medium on the wave propagation deformation, linear resistance, nonlinear resistance and inertial force are introduced in the governing equations of the permeable medium fluid. The exact kinematic and kinetic boundary conditions are used on free surface, and the exact kinetic boundary conditions are adopted on the underwater boundary conditions, and the vertical velocity satisfies the continuity and the horizontal velocity satisfies the momentum equality condition between the free water and the water in permeable medium. Firstly, the three-dimensional Boussinesq-type water-wave equations expressed in two sets of computational velocities with the highest spatial derivative of 3 were derived to suit the wave motion of single-layer permeable seabed. Secondly, Fourier analysis was performed on the newly-presented equations, and the phase velocities and decay rates of the equations were compared with the analytical solutions of Stokes linear waves. The analytical solutions of the equation are in good agreement with the analytical solutions of Stokes linear waves in the range of a dimensionless water depth of h₁/L< 1.0 (deep water wavelength L=gT²/(2π)) at 1% error with a relative water depth of h₂/h₁=0.1-10, which exceeds the range of applicability with any Boussinesq-type model in history. Further, a numerical model of the two-dimensional flume was developed and the numerical model was solved using a prediction-correction-iterative finite-difference method, and a composite fourth-order Adams-Bashforth-Moulton scheme was chosen for time iteration. Finally, the wave evolution over the permeable terrain was simulated and numerical simulations were carried out. Comparison with the relevant experimental results shows a good agreement.

Because of the advantages of good physical conservation and grid suitability along with low demand on computational consumption, VOF is now the main numerical method for the treatment of free-surface in ship hydrodynamics. But for the original VOF method, the problem of interface diffusion is rather serious and results in excessive thickness and insufficient resolution for interface simulation. It will also affect the computational precision of other correlative variables in the flow filed. This problem is especially serious in the simulation of unsteady free-surface flow. In this paper, anti-diffusion VOF method was developed by introducing an artificial convective term into the control equation to restrain interface diffusion and reduce interface thickness. Implicit scheme was adopted for discretization of the artificial convective term to ensure computational stability. The method was tested by typical cases of Zalesak and shear field under different meshes. The results show that the interface thickness is reduced remarkably and the mass error is also reduced obviously. The simulations of 3-dimensional test cases for dam break and flooding in damaged cabin show that the anti-diffusion VOF method can capture the free-surface better and improve computational precision in the CFD simulation of practical unsteady free-surface flow.

To effectively suppress cylindrical vortex induced vibration (VIV), the suppression of cylindrical VIV based on the combined control rod-forced rotation structure was discussed in this paper. Numerical simulations of the cylindrical VIV at low Reynolds number were carried out for comparison and analysis of the flow characteristics, vibration response and hydrodynamic response under different numbers of control rods and rotation rates of the cylinder. The results show that compared with the bare cylinder, the control rods have a positive effect on VIV suppression. For three and five control rods, up to 98% of the cylindrical amplitude suppression can be achieved by adjusting the rotation rate (three control rods, rotation rate 0.4-0.6; five control rods, rotation rate 0-0.2). The amplitude suppression range of the cylinder is identical to the drag reduction range of the cylinder. For the four and six control rods, the effect of rotation on amplitude suppression is small. Vortex shedding on the main cylindrical surface and vortex merging will cause high amplitude fluctuations in the fluid force coefficients, resulting in a high cylindrical amplitude response. For the cylindrical surface with no vortex merging and insignificant vortex shedding, there are no significant fluctuations in the fluid force coefficients and the cylindrical amplitude is significantly reduced.

Conventional bistable wave energy devices have been demonstrated in regular waves by adding adaptive properties to solve the problem of low energy absorption caused by the intrawell oscillation.The power capture performance of an adaptive bistable float wave energy converter (WEC) in irregular waves was studied.The motion equations were established and solved numerically by the fourth-order Runge-Kutta method. The power capture performance of adaptive bistable WEC, conventional bistable WEC and linear WEC was investigated under different device parameters. The results show that with proper device parameters, the adaptive bistable WEC can significantly improve the energy capture performance, and is more suitable for wave energy capture in actual sea conditions than linear and conventional bistable devices.