To investigate the impact of extreme waves, based on the physical model test in a wave tank, this paper aims to study the characteristics of wave impact pressure and impulse generated by focused waves with different breaking stages on a plate-square column structure. Wavelet denoising, empirical mode decomposition, and local weighted linear regression methods were used to process the experimental data. The wave pressure at different parts of square columns was obtained, and the temporal and spatial distribution features of wave pressure were explored. Based on the time integral of wave impact pressure, the time-varying characteristics of the pressure impulse at the typical part of the structure were analyzed. Additionally, the effects of wave breaking stage, initial air gap, and trim angle on the spatial variation of pressure impulse were discussed in detail. The results show that the maximum impact pressure impulse generated by the focused waves is significantly affected by the breaking stage, the initial air gap, and the trim angle. Waves with crest curling or even premature breaking usually produce larger impact pressure impulses. Meanwhile, the plate-square column structure with small initial air gap and positive trim angle is subjected to a larger impact pressure impulse. This study provides a valuable reference for further investigation of the impact of extreme waves on semi-submersible platforms.

The trajectory planning of unmanned ships is an important part of unmanned ships autonomous navigation. A trajectory planning method for unmanned ship dynamic collision avoidance was proposed based on improved PRM algorithm and event triggering mechanism. Firstly, PRM and A* were combined to obtain the PRM-A* algorithm, and a trajectory planning model for unmanned ships was established based on the PRM-A* algorithm. A grid map was established based on the current environmental situation, the PRM-A* algorithm was used to plan the collision avoidance path of unmanned ships, and an S-T grid map of the collision avoidance path was established. The PRM-A* algorithm was used to plan the speed of unmanned ships on the S-T grid map, thus obtaining the collision avoidance trajectory of unmanned ships at the current time. Secondly, a dynamic collision avoidance model was established for unmanned ships based on the event triggering mechanism. An unmanned ship collision risk assessment model was established based on the TCPA and DCPA of unmanned ships, based on which event triggering condition was set. When the event was triggered, the unmanned ship collision avoidance trajectory was planned based on the current time window environmental situation. Finally, simulation experiments were conducted on the trajectory planning of unmanned ships in open and restricted waters. The results show that the algorithm can effectively make an unmanned ship avoid static and moving obstacles in waters, save computational resources, and improve computational performance.

Underwater target azimuth estimation is a critical technology in array signal processing, with wide applications in military operations, marine resource development, and environmental monitoring. A comprehensive review of the current development status of underwater target azimuth estimation methods is provided in this paper. Firstly, an introduction to the acoustic mathematical model based on an uniformly distributed sound pressure line array was given. Next, azimuth estimation methods are classified into four categories: classical beamforming, statistical, subspace, and AI-based Direction of Arrival (DOA) estimation methods. Key factors affecting azimuth estimation accuracy, such as array calibration errors, array geometry, signal processing techniques, and underwater acoustic channel characteristics, were also analyzed. Finally, the paper discussed the limitations of current azimuth estimation technologies and proposed future research directions, including multimodal data fusion, integration of deep learning with physical models, and the development of new array structures etc, to enhance the accuracy and robustness of underwater azimuth estimation.

Combining the resonance method and the direct wave extracting method, the dynamic mechanical parameters of viscoelastic materials in a continuous and wide frequency range under controllable water pressure were measured. A pressure chamber test system capable of realizing underwater pressurization was built, a short time broadband impulse generated by an electromagnetic shaker was employed to excite a bar-like sample attached inside the pressure vessel. On one hand, the resonance method was used to calculate the mechanical parameters at resonance frequencies while on the other hand, the direct wave signal was extracted, and the mechanical parameters in a wide frequency range were calculated by the wave velocity method. The broadband mechanical parameters (including the storage modulus and loss factor) under variable water pressure conditions (0.1-6 MPa) for two types of viscoelastic materials were obtained. The experimental results agree well with each other, proving the test method in this paper could directly determine the Young’s modulus at frequencies ranging from 500 Hz to 5000 Hz under variable water pressure. This method provides simplified processing for underwater dynamic mechanical parameter measurements.

The study of probability characteristics of ice loads can offer more precise input loads for the assessment of ship structural safety, taking into account the uncertainties involved in the interaction between polar ships and sea ice. By integrating the ice load estimation formula with a statistical characteristic analysis method, a Monte Carlo-based approach was proposed to investigate the probabilistic characteristics of ice loads on polar ships. According to the typical sea area and route, the scenarios of interaction between the midship and bow of the vessel with sea ice were considered separately. Based on the study of the probability distribution types and statistical parameters of the relevant variables, the ice loads at different positions was calculated by the method presented in this paper, and the corresponding probability density function was obtained. It can be found that the probability characteristics of ice force in the midship can be accurately modeled by a normal distribution, and the Weibull distribution can be used to describe the probability characteristics of both horizontal and vertical ice forces at the bow. Moreover, the horizontal force exerted on the ship due to ice crushing is significantly greater than that resulting from its bending failure.

In order to accurately calculate the stress intensity factor at the deepest point during the crack propagation process of high-pressure vessels, a method was proposed for fitting the stress data with high-order polynomials and then calculating the stress intensity factor. Taking the crack at the opening of a high-pressure vessel as an example, based on stress data collected with varying data volumes, polynomial fitting of varying degrees was employed to calculate the stress intensity factor at the deepest point of each crack depth. The influence of polynomial degree and data collection volume on the calculation results was analyzed, and the calculation results of this method were compared and validated against the linear interpolation method in the literature. The research results indicate that as the increase of polynomial degree, the characterization accuracy of the fitted curve improves, and the calculation results gradually converge and stablize. The relative error between the calculation results using low-order (third-order) and high-order polynomial fitting shows an "inverted S" trend, with a minimum relative error of about −20%; As the amount of data increases, the calculation results gradually converge, and the relative error of the calculation results under lower and higher data volumes shows a trend of oscillation attenuation, with a maximum relative error of about 7.1%; The calculation results based on high-order polynomial fitting and piecewise linear interpolation are basically consistent, indicating that this method has certain reliability and is suitable for calculating the stress intensity factor at the deepest point during crack propagation.

In view of the fact that it is difficult to obtain analytical solutions for vibration problems of combined shells and it is hard to solve strongly coupled acoustic and vibration control equations, a Ritz-Legendre spectral element method was proposed to discuss the vibration characteristics of underwater conical-cylindrical-spherical shells. Based on Reissner shell theory, virtual spring technology and the displacement angle relationship of adjacent subshells, the theoretical structural model of the combined shells was established. The Legendre spectral element method was introduced to avoid the problem of discontinuity of normal derivative and discretize the Kirchhoff-Helmholtz boundary integral equation, then the theoretical model of underwater external sound field was constructed. Based on Fourier transform and coupled surface Euler equation, the coupled vibration control equation of underwater combined shells was obtained. Compared with FEM simulation results, the convergence, reliability and correctness of this method were verified. This method can provide theoretical reference for engineering application in the design stage.

The welding displacement of marine medium plate caused by welding seriously affects the structural integrity. Butt-welded joint of AH36 steel with the thickness of 14 mm was fabricated using CO₂ welding process and the out-of-plane displacement was straightened using self-developed electromagnetic induction back-heating equipment. A series of experiments were conducted to measure the microstructure, mechanical performance, transverse residual stress and out-of-plane displacement. A welding-back-heating numerical simulation method was proposed, and applied to predict the heating conduction, stress, strain and out-of-plane displacement during the processes of butt welding and back heating. Based on the deformation theory, the transverse bending moment after butt welding and back heating was computed. Results show that the microstructure is mainly cementite and back heating almost has no influence on either the microstructure or mechanical performance. The peak value of transverse residual stress is mainly at the weld seam and becomes larger due to back heating. The back heating generates a larger transverse compressive plastic strain near the surface of AH36 plate, thereby obviously straightening the out-of-plane deformation through the transverse bending moment.

The storm model is a simplified model to describe the random wave loads borne by the ship structures in complex marine environments. This paper proposed an improved Newman model, namely the NPhi model, to consider the load order and interaction effects of storm models, which is based on the Newman model, considering the load interaction effects in Huang’s model and the improved McEvily model. An overload coefficient was introduced into the effective stress intensity factor range to characterize the hysteresis and acceleration phenomena caused by overload. Using the crack propagation program established in this paper, the correctness of the test results using the propagation from multi-level block loading and storm model was verified, and the influence of overload coefficient was studied. Based on the study by the Fatigue and Fracture Technical Committee of the International Ship and Offshore Structures Congress (ISSC), it is shown that the NPhi model has better prediction ability compared to other typical crack propagation models. Additionally, a reasonable overload coefficient value is crucial for the prediction results.

The volume fraction equation is an important control equation for multiphase flow such as cavitation, which is derived from the mass conservation equation in incompressible cases. However, there is currently no universally recognized descriptive form for compressible fluids. The paper uses the body-fixed coordinate system as a reference frame to describe the problem, focusing on local fluid units. Starting from the volume changes of multiphase fluids, the relationship between the body derivative of volume fraction and the local average velocity divergence of each phase fluid is derived. The article also discusses the relationship between the volume fraction equation and the mass conservation equation, as well as the influence of phase transition and compressibility on the evolution of volume fraction. The forms of the volume fraction equations under two-phase pressure equilibrium are provided.