In order to study the propeller cavitation and induced pressure fluctuation in non-uniform wake, a multi-field synchronous measurement system was used to carry out tests in a cavitation tunnel for a highly skewed propeller (HSP) and explore the influence of advance coefficient and cavitation number on the propeller cavitation performance and fluctuating pressure characteristics. The results show that the propeller cavitation appears in the test mainly in three types: back cavitation, face cavitation and tip vortex cavitation. Low-frequency pressure fluctuation is highly correlated to the cavity volume oscillation. Under non-cavitation condition, the first blade passing frequency is the main component of pressure fluctuation while the amplitude of higher blade passing frequency component can be ignored by comparing the contributions of each blade passing frequency component of the whole pressure fluctuation. Under cavitation condition, the unsteady cavitation has a great impact on the pressure fluctuation. The higher blade passing frequency components can be obviously observed, and the amplitude of main frequency pressure fluctuation increases significantly.The variation of pressure will become more observable with the increase of intensity of cavitation evolution process. The higher order blade frequency component of pulsating pressure increases obviously when tip vortex cavitation is present.

It is inevitable to use various high-strength materials with brittle characteristics in the construction of ship structures. In order to study the fracture and crack propagation behavior of marine brittle materials, a coupling model of FEM and peridynamics was proposed based on the peridynamic theory. Firstly, the long-range force attenuation effect correction was considered on the basis of peridynamics. Then, the sharing node method was used to couple the FEM with the improved peridynamics, and a new fracture criterion was derived. Finally, the accuracy of the coupling model was verified by three examples. The results show that the coupling model improves the computational accuracy of the traditional peridynamic model greatly, and eliminates the“surface effect”, and it overcomes the FEM singularity when dealing with discontinuities. The calculation results of the coupling model are in good agreement with the experimental results, and the present model is feasible to study the fracture of marine brittle materials.

The mooring system is the key structure of a floating platform, which plays an important role in ensuring the safe production of the platform. It is of profound significance to understand and evaluate the safety status and risk level of the mooring system in time. Aiming at the difficulties of real-time in-situ detection of mooring lines under the platform field operation states, a reliability assessment method of catenary mooring system based on prototype monitoring information is proposed in this paper. Compared with the conventional Monte Carlo simulation, the proposed reliability assessment model can improve the calculation efficiency to meet the requirements of real-time reliability analysis. Firstly, based on the catenary equation, a numerical simulation analysis of the forcing behavior of the mooring line was performed considering the influence of the current load. Then, a reliability assessment modelling method for strength analysis and fatigue analysis of mooring system was proposed based on Enhanced Monte Carlo (EMC) method. Finally, based on the prototype monitoring data of a semi-submersible platform in the South China Sea, the reliability assessment of the mooring line was performed by taking into consideration the impact of the corrosion. The simulated results indicate that the present real-time reliability assessment method could provide a superior ability for the guidance of the safety assessment and maintenance of catenary mooring system.

Under the welding process specifications that meet the structural strength requirements, welding energy input and welding sequence result in different welding residual stress and deformation, which significantly impact the typical bidirectional stiffened plate structure’s vibration and acoustic radiation. In order to explore the influence of welding process parameters on the vibro-acoustic characteristics of typical bidirectional stiffened plate structures, the accuracy of the welded structure test method was verified by combining numerical simulation and experiment, several tests of stiffened plate structures were carried out regarding the modal, underwater vibration, and acoustic radiation under different welding energy inputs. The results show that under the same welding sequence, different welding energy inputs have a large effect on the natural frequency of thin plates and a relatively small effect on the natural frequency of thick plates. For stiffened thick plate structures, under the same welding sequence, with the increase in welding energy input, the impact on the natural frequency shows a trend of first decreasing and then increasing, and the overall vibration acceleration level and radiated sound pressure level in the same frequency band decrease first and then increase. Under the symmetrical welding sequence, the optimal welding parameters cover a welding current of 200 A, a welding voltage of 25 V, a welding speed of 3.02-3.06 mm/s, and a welding energy input of 167 J/cm. This study can provide guidance for the design of low-noise processes for acoustic stealth of ships and marine structures.

The propulsion mechanism and swimming performance are of great significance to the construction of fish migratory channels. By using computational fluid dynamics method combined with overlapping mesh technology, the two-dimensional fish autonomous swimming was simulated by compiling the UDF program for controlling fish body swing, analyzing the evolution process and parameter changes of fish body pressure field distribution and inverse Carmen vortex street structure, carrying out the changes of fish swimming performance and fish body force under different parameters of tail swing frequency, tail swing amplitude, fish body shape and tail fin size, etc., and revealing the swimming mechanism of fish in the process of autonomous swimming. The results show that: (1) the fish body’s tail fin periodically swings back and forth under the formation of anti-Carmen vortex street, which is the main source of the fish body forward thrust, and with the increase of tail swing frequency and tail swing amplitude, the fish body tail vortex street length and vortex street strength gradually increase, while the effect of tail swing amplitude on the vortex street width is greater; (2) with the increase of swinging frequency and swinging amplitude, the horizontal mean coefficient of synergy and the maximum lateral force coefficient increase, which makes the fish obtain a larger swimming speed, but the increase of mean coefficient of synergy and swimming speed is more obvious when the swinging frequency has been changed, and the increase of maximum lateral force coefficient is more obvious when the swinging amplitude has changed; (3) with the increase of body width index, the horizontal average coefficient of force gradually decreases, and the swimming resistance to be overcome increases, which makes the swimming speed of fish gradually decrease, while the maximum lateral force coefficient gradually increases; (4)and with the increase of caudal fin index, both horizontal mean coefficient of force and maximum lateral force coefficient increase, which leads to the gradual increase of swimming speed of fish. The results of the study can provide a support for fish habitat restoration.

A sea-launching ship will have complex nonlinear motion response under the environmental loads and launch impact load, which has a vital effect on the safety of rocket launching process. Based on the three-dimensional potential flow theory, this paper focuses on the motion responses of a sea-launching ship under the action of wind, waves, and impact loads in different sea states. And the effects of the dynamic position system and the launch ignition time on the motion of the launch ship were investigated. The results show that the launch impact load has a great influence on the pitch motion of the ship, especially when the wind/wave direction is parallel to the ship direction. The existence of dynamic position system will increase the roll of the launch ship. And the launch ignition time can be selected at the moment when the pitch motion of the launch ship is about to reach the extreme point, so as to reduce the impact of the launch ship's motion on the rocket attitude during taking-off.

In this paper, numerical simulation of projectile’s underwater motion process with tail-slaps was studied based on CEL (coupled Eulerian-Lagrangian analysis) method. The stability principle of projectile’s motion with tail-slaps was studied. Some parameters, such as initial angle of attack when tail-slaps occurred, maximum angle of attack, and instability critical angle of attack were used to characterize the projectile’s tail-slap. The relationship between stability and angle of attack was revealed. The location change of pressure center caused by projectile’s tail-slap was studied, too. The results showed that the static stability margin of projectile was enough to meet its stability requirements. Finally, three constraint criteria for the motion stability of projectile, including the requirement for the size of cavity, the relative location of center of pressure and center of mass, and the static stability margin of projectile, were obtained. The location and area of tail-slap can be used to control the static stability margin of projectile, some suggestions for the design of projectile’s shape were proposed, too.

Based on the analysis of the lubrication performance of the eccentric stern bearing, an elasto-hydrodynamic coupling lubrication model for the local wear and stiffness of the stern bearing was established. The joint program of finite difference method and finite element method was compiled to solve the elastic deformation of the bearing, and the mass conservation boundary condition was used to replace the Reynolds boundary condition. The effects of local wear depth, bearing elastic modulus and other factors on the hydrodynamic pressure, liquid film thickness, cavitation area and friction law of the bearing were discussed in detail. The results show that when the local wear depth of rigid body bearing is lower than the threshold value, it is beneficial for bearing lubrication. When the local wear depth exceeds the threshold value, the maximum hydrodynamic pressure, friction force and cavitation area increase significantly. The influence of bearing elastic deformation on the calculation results cannot be ignored. Elastic deformation and local wear exist at the same time, and the change law is basically consistent with the change trend of local wear of rigid body bearings.

Fatigue problem as a common failure form in the engineering field has been widely concerned. The fatigue damage-crack size can be obtained by the fatigue analysis method based on fracture mechanics, but the calculation is relatively complicated. In this paper, aiming at the spectrum analysis based fatigue analysis of ocean engineering structures, the stress intensity factor (SIF) spectrum under random loading conditions of the same hot spot through genetic algorithm wavelet neural network (GAWNN) was established, and the network training with the SIF obtained from finite element analysis was conducted. The results show that the model can predict the SIF spectra under random loading conditions well. The method proposed in this paper can considerably reduce the repetitive finite element calculation and provide a reference for the fatigue life prediction of engineering structures under random load conditions by applying crack propagation method. Finally, combined with the unique crack growth rate curve model, the rapid prediction of crack growth under random loading conditions was realized.

Ocean thermal energy conversion is one of the research hotspots of marine renewable energy in recent years. Free-hanging water intake pipes are the key structure to extract deep cold seawater. At present, the vortex-induced vibration (VIV) response characteristics of free-hanging pipes in deep sea currents are not clear yet. In this paper, model tests of a free-hanging pipe under uniform flow were carried out, and the strain response of vortex-induced vibration was measured by the fiber Bragg grating strain sensor. The amplitude and frequency characteristics of the free-hanging pipe were investigated by modal analysis and wavelet transform data processing methods. It is revealed that the maximum amplitude of the VIV displacement response of the free-hanging pipe under the uniform flow mainly occurs at the bottom. The dominant frequencies in inline (IL) direction is basically two times that in cross flow (CF) direction. However, in the conditions where the modal transition occurs, the dominant frequencies in IL and CF directions are the same, accompanied by obvious "traveling wave", "multi-frequency response" and "time-sharing" phenomenon. In addition, the Strouhal number of the overhanging pipe model in CF and IL under uniform sea currents are 0.15 and 0.30, which are slightly smaller than the results of flexible risers hinged at both ends. This value may serve as the parameter input for the vortex-induced vibration prediction of free-hanging pipes.