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.

The large number of internal solitary waves in the South China Sea seriously threaten the safety of offshore engineering operations. In this paper, the vector form intrinsic finite element (VFIFE) method was used to analyze the dynamic characteristics of deep-sea steel lazy wave risers (SLWR) under the action of internal solitary waves. The dynamic model of a deep-sea steel lazy wave riser considering soil reaction in touchdown zone and internal solitary wave load was established. The internal solitary wave load was solved according to the mKdV equation. Fortran calculation program was compiled based on vector form intrinsic finite element beam element. The displacement extremum and variation trends of tension and bending moment of a riser under different incident angles were analyzed. The dynamic characteristics of a steel lazy wave riser and a simple steel catenary riser under internal solitary waves were compared. The results show that under the action of internal solitary waves, the steel lazy wave riser will have a greater displacement response, especially the upper section will have a greater horizontal displacement. Compared with the simple steel catenary riser, the steel lazy wave riser has a more significant tension variation amplitude and a more complex bending moment variation trend.

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.

Prediction of ice force acting on the hull is a key to assess the navigation performance of ice-going ships. An ice-going ship was used to conduct the manoeuvring oblique test, the circular synthetic ice made of polypropylene was adopted, the forces of hull in water and medium and high floe ice concentration region were given by model test, the characteristics of force under different speeds and drift angles were investigated, the data repeatability was analyzed based on typical conditions, and the synthetic ice model test technology was developed. The model test results show that increasing ice concentration, speed and drift angle will cause the larger force acting on the ship, the contact area of the ship and ice is larger in higher ice concentration and larger drift angle, the repeatability of data is also better.

The cooling system can be carried out by the flow pressure difference through the outside protrusion of an underwater vehicle. This system can prevent the low line frequency pipe noise from the pump’s excitation. However, the protrusion is external, generating the hydrodynamic noise and destroying the acoustic stealth of underwater vehicles. Two protrusion models with a rectangular inlet and a circular inlet were created. The flow field and sound field were numerically calculated. The generation property of the hydrodynamic noise from the two protrusions on the condition of different flow velocities was analyzed. It is indicated that the total radiated sound power of the protrusion with the rectangular inlet is slightly lower than that with the circular inlet. To further reduce the hydrodynamic noise, the protrusion with the rectangular inlet was carried out by the flow control through the serrations. The noise reduction effect of the protrusion with the rectangular inlet by the leading-edge serrations is better than that by the tail-edge serrations. The optimized parameters of the serrations were summarized. To validate the numerical calculation results, four protrusion models were designed and fabricated, and the experimental tests were done in the gravity low-noise water tunnel. The results show that the protrusion with the rectangular inlet by the serrations optimization has a better noise reduction effect in the frequency range from 10 Hz to 2000 Hz. The results in this paper can provide some references for the low-noise design of protrusions of underwater vehicles.

The defects of a submarine pipeline caused by laying, non-uniform seabed support and impact of falling objects are often distributed in many places. In order to simplify the analysis, the buckling model of a pipeline with multiple elliptical defects in the axial direction is generally simplified as the pipeline with uniform elliptical defects or the pipeline with a single local elliptical defect in the axial direction. However, there are few studies on the differences of these three models and their application in engineering. Based on the numerical simulation, this paper presents a study on the buckling and instability behavior of the pipeline with three kinds of elliptical defects. The RIKS method was used to solve the buckling pressure of the pipeline, and compared with the calculation results of DNV-OS-F101, focusing on the analysis of the excitation position and influence parameters of the buckling and instability of the pipeline with multiple elliptical defects in the axial direction. The results show that the buckling pressure of the pipeline model with different elliptical defects is obviously different, and the order is the buckling pressure of the pipeline with uniform elliptical defects < the theoretical solution of DNV-OS-F101 < the buckling pressure of the pipeline with multiple elliptical defects < the buckling pressure of the pipeline with single local elliptical defects. For the buckling analysis of the pipeline with multiple elliptical defects in the axial direction, if the pipeline model is simplified with axial uniform elliptical defects, it is more conservative. However, if the pipeline model is simplified with axial single local elliptical defects, it is more dangerous. Besides, the buckling pressure of the pipeline with multiple elliptical defects is affected by the size of elliptical defects, the distance between elliptical defects and the length of elliptical defects.

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.

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.

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.