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.

The new conceptual artificial seabed can effectively improve the adaptability of disastrous marine environmental factors such as strong winds, huge waves and surface currents, but the considerable impact force generated by internal solitary waves is a key factor affecting the operation safety of the artificial seabed. This paper presents a study on hydrodynamic forces and flow field characteristics of the new conceptual artificial seabed under the action of internal solitary waves. First of all, the eKdV equation was used as the theoretical model. A three-dimensional numerical flume was established by using the wave-making method of velocity inlet, and the numerical wave-making of internal solitary waves was achieved. On this basis, the numerical waveform was compared with the theoretical and experimental waveforms, and the feasibility of the numerical method was verified. Finally, hydrodynamic forces exerted by internal solitary waves on the artificial seabed were calculated, the characteristics of forces induced by internal solitary waves of various amplitudes on the artificial seabed were studied in detail. Besides, the velocity field and the vorticity field around the artificial seabed were analyzed. The results show that, with the increase of the amplitude of the internal solitary wave, the drag force, vertical force and lift force on the artificial seabed gradually increase, and the drag force and vertical force are much greater than the lift force. When the internal solitary wave passes through the artificial seabed, both the particle velocity of the fluid and the vortex intensity around the artificial seabed increase. This study provides an effective numerical calculation method for the prediction of internal solitary wave forces and the analysis of flow field characteristics of large underwater engineering structures.

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.

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 damping-corrected potential flow theory was used to study the ship-to-ship transfer system. The CFD numerical model of ship-to-ship transfer system was established and the theory of potential flow was modified by applying viscous damping lid on the free water surface between two ships. The time domain results of ship-to-ship transfer system with and without correction were compared and analyzed. The results show that the calculation results of the motion response of each degree of freedom of the two ships and the force of each mooring or fender without viscosity correction are higher than the corrected results, and that it is very necessary to consider the correction of fluid viscosity.

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.

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.

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.

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.

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.

The significant life reduction of mechanical components during non-proportional loading compared to proportional loading results from the additional damage. In this study, the factors affecting the fatigue life under non-proportional loading were analyzed, and a new non-proportional damage factor was proposed. A novel fatigue life prediction model based on equivalent strain was established with the maximum shear plane as the critical plane. The new multiaxial fatigue life prediction model not only considers the effect of normal stress on the critical plane on the material crack initiation and crack propagation, but also reflects the non-proportional loading on the multiaxial fatigue life by introducing the non-proportional hardening coefficient of the materials and non-proportionality of the load paths. Experimental data under multiaxial loading of five materials were used to verify the accuracy of the novel model, and were also compared with two classical models. The results show that the prediction accuracy of the proposed model is higher.

Sandwich structures have been widely used in fields of sound insulation panels and vibration reduction structure design due to their lightweight, excellent performance in vibration reduction and sound insulation. Usually, sandwich panels in vibration tend to exhibit bending vibration modes in the low frequency band.However, the properties of sandwich skin materials and core materials often differ significantly, resulting in rich vibration modes in the midium and high frequency bands. Therefore, it is necessary to establish a theoretical prediction method for sandwich panel vibration applicable to various frequency bands. In this paper, a new theoretical calculation method for sandwich panel vibration was developed, in which three-dimensional elastic mechanics theory was applied to core material of sandwich panel while thin plate theory was applied to skin material. The theoretical method presented in this paper can reflect the bending deformation mode and dilatational deformation mode of sandwich panels, and allow for normal deformation of sandwich panels. As a result, the method can significantly improve the accuracy of vibration prediction calculation for medium and high frequency bands. Comparison of the calculation results of the theoretical method in this paper with those of other theoretical calculation methods and finite element method by an example proved the effectiveness and advantages of the theoretical method in this paper.

The underwater acoustic radiation of air cushion vehicles (ACVs) is a complex process considering hull structure-aircushion-water three-phase interaction and vibro-acoustic coupling, which has complex transmission characteristics and is the key and difficult issue in the ACV acoustic analysis. With an ACV taken as a computational example, the vibro-acoustic finite element method (FEM) and the analytical method were used to analyze the transmission characteristics of the underwater radiation noise of the ACV. To calculate the underwater acoustic field for mechanical noise sources and airborne sources of lift fans, a vibro-acoustic FEM numerical model considering structure-air-water interaction was established and calibrated by the natural frequency of the hull obtained from the tests. The analytical formulas were used to calculate the underwater acoustic field for airborne sources of other equipment. The overall sound pressure level (OASPL) at points 1 m from the port and starboard sides was adopted as an evaluation index. The research shows that the underwater radiation noise of the ACV is broadband noise. The underwater acoustic pressure calculated by the FEM increases with the increase of frequency, and the pressure is concentrated on the aircushion-water interaction surface at low frequencies. With the increase of frequency, the underwater acoustic field calculated by the FEM gradually presents an irregular interference distribution. The underwater radiation noise caused by the acoustic excitation of equipment like air propellers is mainly near-field and low-frequency noise, accounting averagely for 40% of that of the ACV within the sound radiation range of the equipment. The transmission characteristics of underwater radiation noise of ACVs revealed in this paper has important guiding significance for the vibration and noise reduction of ACVs.

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.