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.

The cyclic void growth model (CVGM) based on microscopic fracture mechanism is an effective method to analyze and predict ultra-low cycle fatigue (ULCF) fracture. In this method, the void growth index and damage degradation parameters are important parameters to control the crack propagation process. Due to the influence of production technology, the void growth index and damage degradation parameters of different batches of steels are often unfixed, which leads to the insufficient accuracy of the ultra-low cycle fatigue fracture analysis. In order to solve this problem, smooth round bar, smooth notch round bar and notch samples were used to carry out experiments, the ultra-low cycle fatigue characteristics of steel were studied, and the cracking mechanism and damage evolution law of the samples were explored. Secondly, the VUSDFLD program based on the cyclic void growth model was written, and the finite element analysis was carried out based on the results of test to calibrate the void growth index and damage degradation parameters. Finally, the ultra-low cycle fatigue fracture of notched samples was studied, the crack initiation and crack propagation rates were analyzed, and the ultra-low cycle fatigue fracture life of the samples was predicted. The results show that the fracture process of ultra-low cycle fatigue matches well with the experimental results, which is suitable for the prediction of ultra-low cycle fatigue fracture life.

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.

In order to effectively reduce the ‘window effect’ in the sound field reconstruction process of near-field acoustic holography (NAH) and accurately measure and locate the surface sound source of a submarine by using the limited measuring aperture, a patch NAH method based on two-level iteration was proposed. Firstly, patch NAH based on orthogonal spherical waves was improved, and a patch NAH method based on two-level iteration was proposed. Then, the reconstruction results of the two NAH methods were simulated and compared, and the influence of extrapolation error and extended measurement points on the reconstruction accuracy was studied. Finally, the experimental study of patch NAH based on two-level iteration was carried out in an anechoic tank using a small aperture holographic measuring surface. The results show that the patch NAH based on two-level iteration proposed in this paper can greatly reduce the ‘window effect’ and ‘aperture repetition effect’ errors caused by the limited measurement aperture, which verifies the advantages of the method and provides an important reference for NAH precise measurement of large size sound sources.

Sandwich plates, as a new structure, have the advantages of strong design, light weight, high specific stiffness, good performance of vibration reduction and sound insulation, and have been used in various engineering structures. However, there are many types of sandwich structures, and it is difficult to theoretically analyze the sound insulation performance of complex sandwich structures. New approximate analysis methods are urgently needed. In this paper, based on the equivalent method of spring, torsion spring and concentrated mass, the sound-vibration coupling control equation of a C-type sandwich panel was established for the marine C-type sandwich panel. Using the spatial harmonic expansion method, the expression of the sound transmission loss of the C-shaped sandwich structure was derived. The influence of the system parameters of the sandwich panel on the sound insulation performance of the structure was discussed. The results show that the material properties and the thickness of the sandwich panel structure have greater influence on the sound insulation performance of the C-type sandwich panel structure. The finite element simulation was used to verify the validity of the theoretical analysis.

The current Level 2 vulnerability criteria assessment method for the surf-riding/broaching stability failure mode of the IMO Second Generation Intact Stability Criteria only applies to ships using conventional propellers. However, waterjet propelled ships are also prone to this stability failure mode. Therefore, it is necessary to establish a regulatory assessment method for such a ship type as well. This study established a mechanical model for waterjet propulsion systems through the analogy to the model for propellers. The Level 2 vulnerability assessment method applicable to waterjet propelled ships was proposed and validated based on the actual pump data and sample ship calculations. A specific assessment method for waterjet propelled ships was proposed to overcome the limitation on propulsion type, which not only makes the regulation more complete, but also provides technical support for the stability safety assessment of waterjet propelled ships.

Based on the boundary data immersion method (BDIM) and the fluid-structure coupling algorithm for rigid body motions, a numerical program for the vertical water-entry of a sphere was developed. The accuracy and effectiveness of the numerical method were verified by comparison with the experimental results. Based on the analysis of the numerical calculation results, the cavity development and corresponding flow structures with different impact velocities during vertical water entry, as well as the development of the splash were obtained. Different vortex identification criteria were adopted to investigate the development of vortex structures. The results show that the impact velocity of the sphere has a significant effect on the closure of the splash, the cavity shape and the evolution of the flow structures inside the cavity. The Q criterion can identify the complex vortex structure more accurately, and the entry velocity of the sphere will affect the vortex intensity inside the cavity.

In order to select and optimize the skin friction reduction scheme for high-speed underwater vehicles, a better technical scheme of skin friction reduction test, the expression method of characteristic parameters and the technical way to improve the measurement accuracy were put forward in this paper. The feasibility of the above scheme and approach was demonstrated by the actual skin friction reduction test of PEO. The results can provide support for the selection and optimization of the skin friction reduction scheme for high-speed underwater vehicles.

The hull form modelling progress in ship design is significantly relied on the parent hull database and the professional designers well trained with CAD software, and it is usually a time and experience costly work. The conditional generation of ship hull with both geometrical and locational features by training an artificial neural network was concerned by this paper. The geometrical feature means the overall shape variety of ship designs like bulbous bow, stern shaft, etc., the locational feature means the shape difference between stern, front and mid-body of ships. Firstly, a conditional deep-convolutional generative adversarial network (CDC-GAN) was constructed to distinguish the geometrical and locational features individually; Secondly, the CDC-GAN was well trained to learn and generate these features with different resolutions and categories, from easy to hard; In the end, the training cost and performance of networks were compared and concluded to prove the capability of CDC-GAN in solving ship hull form generating issues. This paper is based on authors’ previous investigation with regular GAN, and it provides a further exploration about the potential of CDC-GAN in ship design.

Effective use of numerous wave energy resources in the ocean remains a problem to be solved. Due to the wave conditions for low periodic and small wave height in the South China Sea, a model combining a split heave point absorber with a tension leg platform (TLP) was proposed. Through a physical model test, the hydrodynamic characteristics and wave energy capture efficiency of the split float were measured and analyzed, then compared with the whole float. And the optimal wave energy capture of the split float was discussed. The results show that the split float presents different hydrodynamic characteristics, and the energy capture efficiency of the device for small periodic waves is greatly improved in the low sea state, but the pitch motion of the platform is significantly increased in the specific wave direction, which requires further optimization of the tension leg system.