Vortex-induced vibration (VIV) of a marine riser is a great threat to its service safety. From the perspective of energy, the hydrodynamic force on the riser undergoing vortex-induced vibration was divided into three components, i.e. vortex-induced force acting as energy input, drag force acting as energy dissipation and added mass force acting a neutral role in energy. The energy competition between the first two components determines the final energy effect of the fluid on the structure. Furthermore, the identification method of hydrodynamic coefficients based on the flexible riser model experiment was derived for the new hydrodynamic force model in detail. Through the towing experiment of the flexible riser, the vortex-induced vibration response, and coefficients distribution characteristics under different flow velocities were identified. The results show that the vortex induced vibration response of the flexible riser under uniform flow has multi-mode participation characteristic, which leads to the "jump" phenomenon of hydrodynamic coefficients. The vortex-induced force coefficients and drag coefficients behave significant correlation with the amplitude of VIV. Based on the measured values between hydrodynamic coefficients and response amplitude, an empirical model for hydrodynamic coefficients under energy competition force model was preliminarily established. The research in this paper provides a valuable reference for the development of fast empirical prediction methods of marine risers in the future.

The precise determination of low-frequency wave loads on ship hulls is an indispensable cornerstone and core challenge in hull structural design. Low-cost ship model testing is often employed in engineering to forecast wave loads. However, test data frequently suffer from deficiencies or abnormalities due to various reasons. Consequently, predicting wave loads from data with defects or anomalies, remains a major engineering challenge. This paper presented an efficient method for accurately determining the wave design loads on ship hulls, specifically tailored to handle deficient or abnormal test data. By integrating 5,400 sets of wave load data calculated using two-dimensional strip theory, a machine learning transfer network was constructed. To address deficient data, we innovatively introduced a fine-tuning network layer, and designed a novel loss function that ignores zero terms, thereby enhancing the network's adaptability. This method achieved rapid wave load forecasting by transferring simulation results to ship model tests, with an accuracy better than 90%. This technique enhances design efficiency, reduces labor costs, and maximizes data utilization, providing a reliable and efficient solution for wave load prediction in hull structural design.

In recent years, shipowners have become increasingly concerned about the actual performance in the real marine environment. At present, ship design optimization is mainly conducted based on performance in still water which has a certain difference from real-sea performance. The factors influencing the powering performance of ships in wind and waves were investigated taking series oil tankers, bulk carriers, and container ships as the research subjects. The theory of two-dimensional strip was used to calculate the added resistance under typical sea states BF6 and BF8. The propeller was redesigned using graph method to research the influence of light running margin in the wind and waves. The results show that the principal dimensions have a significant impact on the added resistance of oil tankers and bulk carriers, and have a relatively small impact on container ships. Therefore, it is necessary to consider the ship type separately when optimizing the powering performance in wind and waves. In addition, with the increase of the light running margin, the reserved space for power becomes increasingly sufficient.

The generation of internal wave wakes by submerged objects in density-stratified environments is closely linked to the navigation speed. This study develops a technique of step-layer injection in a wide-scale density-stratified simulation tank and proposes a high-precision multi-array conductivity detection method. Experimental investigations on the excitation of internal wave wakes by an underwater sphere driven by cyclic towing were conducted. Using probability density statistics, root mean square analysis of wave amplitudes, and other analytical methods, this research delved into key issues such as spatiotemporal probability distribution density of Froude number correlated with internal wave, transition zone delineation, and vertical displacement field characteristics, etc. The findings demonstrate that the experimental system and techniques employed can accurately capture the fluctuation information within the stratified flow field and precisely determine the relationship between the characteristics of internal wave wakes and the Froude number. The probability distribution density of the internal-wave-correlated Froude number reveals the transition process of internal wave wakes and clearly identifies the transition zone as approximately 1.6 ⩽ Fr < 3.3. It is discerned that, after the transition, the dominant internal wave correlated Froude numbers within the wake wave system approximately fall within the ranges of [0.3, 0.4] and [2, 2.8] for the inner and outer layers, respectively. Additionally, Lee wave correlated velocity can still be detected in the outer layer region.

As offshore floating structures continue to grow larger, the rain load of large offshore floating structures under extreme conditions has become one of the focal points of concern for designers. Based on the discrete particle model and the rain load calculation formula, this paper completed the rain load calculation for different wind field and raindrop spectrum combination states of the offshore platform, and the study shows that the rain load caused by fluctuating wind is much more discrete than the raindrop spectrum; the rain load of the offshore platform under the action of fluctuating wind follows Gamma distribution; and the rain load variation caused by time-varying rain field follows normal distribution; when the rainfall intensity R is 800 mm/h, the exceedance probability is 95%, the rain load accounts for 4.65%, and the maximum rain load accounts for 8.07%. The research results help to reveal the influencing factors of rain load on offshore platforms and can provide data support for designers to select rain load reasonably.

Fishnet is a complicated flexible structure, and the drag force coefficient is important for the cage netting load calculation and deformation in the numerical simulation. Firstly, the actual netting samples were obtained through the netting patch hanging test in the open sea area, then they were studied in the laboratory to determine the drag coefficient under different marine growth conditions. The equivalent method of the netting in Morrison model was introduced in the numerical simulation to analyze the hydrodynamic load of the fishnet on the 50,000-cubic meter huge aquaculture cage. The results show that the cage netting load under large amounts of marine growth (solidity=0.69) can be 60% higher than that of the netting under small amounts of marine growth (solidity=0.28), and that the design size of the net rope can be reduced by optimizing the layout of the netting in the cage design.

The study of vortex induced vibration under high Reynolds number is of great importance. While flow sensors are generally fixed to structures, it is of great practical importance to analyze and simulate the measurement information under vibration conditions. In this study, the subcritical Reynolds number three-dimensional flow and vortex-induced vibration of a cylinder are simulated and analyzed using the translational moving reference coordinate method together with a self-developed fluid computation program. The results show that the method accurately simulates the vortex-induced vibration phenomenon.It is found that the phase difference between the lift and the displacement produced a jump within the VIV lock-in region, and the vortex broke up during the vibration, showing strong three-dimensional characteristics. Time-averaged physical quantities near the wall and the wake region were obtained by statistical calculation of the grid in the translational reference frame. The simulation method based on the translational reference system is similar to the observation method of sensors attached to the structure in experiment and on-site testing, which can provide references to model tests and field measurements.

Based on the classic honeycomb sandwich structure, an enhanced honeycomb lattice sandwich structure was proposed in this paper, and its load-bearing performance was analyzed and optimized. Firstly, unit cell specimens of both classic and enhanced honeycomb structures were fabricated using 3D printing technology. Quasi-static compression tests and numerical simulations were conducted to verify that the enhanced honeycomb unit cells have superior load-bearing performance compared to the classic honeycomb unit cells. Secondly, five key parameters of the enhanced honeycomb were selected as design variables to create a Kriging surrogate model for structural mass and a radial basis function neural network model for energy absorption. Multi-objective genetic algorithms were used to invoke the surrogate models for optimizing load-bearing performance. The optimal structural parameters of the enhanced honeycomb unit cells were obtained and validated through experiments and simulations. The results show that, under the same mass conditions, the optimized configuration improves energy absorption performance by 24.25% compared to the initial configuration and by 35.9% compared to the classic honeycomb sandwich structure.

Aiming at the practical application of I-core metal sandwich structures in the main hull section, the four-point bending ultimate bearing capacity test of a I-core metal sandwich composite cabin model was carried out. The failure mode and ultimate bending moment of the I-core metal sandwich composite cabin model were obtained by test, and the test results were in good agreement with the nonlinear finite element calculation results. Test results show that the failure of I-core metal sandwich structure is dominated by the overall buckling while the local buckling is secondary. Meanwhile, I-core metal sandwich structure has a high load bearing capacity and can replace the deck and side stiffened plates in the hull structure. In addition, the metal sandwich composite cabin model is successfully manufactured, which verifies the feasibility of the application of metal sandwich structure in hull structure.

The stability of thin plates plays an important role in ship design and strength check. To investigate the shear stability of ship's thin plates, a picture frame fixture was designed to conduct an in-plane shear buckling test on square thin plates. In the test, 3D Digital Image Correlation (3D-DIC) was used to obtain mechanical response information such as load-end elongation curve, full-field displacement/strain, etc. The load-end elongation relationship reveals the load-bearing characteristics of the thin plate under in-plane shear condition and determines the critical load for buckling instability. According to the displacement and strain field information of the thin plate at typical moments, it is found that the normal deformation of the thin plate increases with the increase of in-plane shear load. After the instability of the thin plate, there are three half-waves symmetrically distributed along the vertical diagonal, and the internal wave amplitude is greater than the external wave amplitude. The strain waveform and contour plot tend to stabilize during the post-buckling process. By analyzing the response curves of the normal displacement and Mises strain at the key points of the thin plate with time, a new method for identifying the buckling instability of in-plane sheared thin plates was proposed and verified. This study provides a useful reference for experimental research and mechanical behavior analysis of in-plane shear stability for ship’s thin plates.

The extreme polar marine environment is harsh and the ice conditions are complex, requiring sufficient structural strength for navigation safety of ships. Existing design specifications and monitoring guidelines for polar ships senerally only consider the ultimate strength within the elastic phase of material under single loading conditions, resulting in overly conservative strength design. The ultimate bearing capacity of polar ship structures were analyzed based on the Combined Theory of Strength and Stability (CTSS) and plastic failure models, and a calculation formula for the ultimate strength of typical structures was derived in ship-ice collision areas under plastic deformation conditions. By using the finite element analysis method, a numerical analysis model for the ultimate strength of structures was established. By comparing the theoretical and numerical results, it was found that the theoretical calculation formula is highly accurate and can be used as a quick verification method for the ultimate bearing capacity of polar ship structures. Besides, with reference to the design requirements of specifications, the article provides recommendations for the arrangement of rib space in the ice belt areas of various ice-class ships based on the theory of ultimate strength, providing reference for the design of the ultimate strength of polar ship structures.

In this paper, a theoretical formula of plastic deformation and impact force of rectangular lightweight sandwich plate struck by a rectangular frustum impactor was derived based on the approximate yield criterion and rigid-perfectly plastic theory. Numerical simulations were conducted to verify the accuracy of the analytical prediction model. In addition, the influences of load distribution, side length ratio and location of impact loads on the dynamic response of a rectangular sandwich plate were theoretically analyzed. Results show that the theoretical predictions for PVC foam core sandwich plate struck by a rectangular frustum impactor show good agreement with numerical results. With the increase of load distribution coefficient, the impact strength per unit area decreases, which leads to the decrease in plastic deformation; when the impact position is closer to the boundary, the length of plastic hinges increases, resulting in a significant decrease in deformation; the plastic deformation achieves its most significant value under impact when the aspect ratio of foam sandwich plate is 1.

Some ship equipment with weak anti-shock properties has the flowing characteristics such as small space proportion, variable and large shock loads, dynamic load changes, rigid-flexible state transitions and passive operation. Howerver, these needs cannot be met by traditional vibration isolation devices. Therefore, this paper proposed a novel integrated bi-directional vibration isolation device and conducted the corresponding structural design. Then, the dynamic model of the vibration isolation device was established to predict the dynamic response under complex loading and to explore the effects of frequency and damping ratio on the anti-shock properties of the device. Finally, the test bench for the vibration isolation device was built to verify the validity of the structural design and theoretical analysis. The results show that the direction of the impact load and dynamic sway load affects the anti-shock properties of the vibration isolation device, When the loads act in the same direction, the anti-shock performance will be improved, whereas when the performance will be decreased if the loads act in opposite directions. As the frequency or damping ratio increases, the anti-shock properties of the vibration isolation device gradually decrease, thus requires optimization based on key performance indicators. The test results of the vibration isolation device have smooth curves without distortion, and the overall trend is basically the same as that of the theoretical calculation results, which can verify the validity of the structural design and theoretical analysis. The results of the study can provide useful guidance for the design of vibration isolation and anti-shock for weak ship equipment.

The acoustic analogy theory is currently the most significant theoretical framework in the field of flow acoustics, with widespread engineering applications. Thus, this paper focuses on the development history and engineering applications of the acoustic analogy theory, concentrating on the three core issues: sound sources, acoustic variables, and wave operators. It then analyzes and prospects the future research of the acoustic analogy theory, considering the major needs in China and adapting to the era of artificial intelligence. The purpose is to provide some useful references for scholars through the analysis and summary of the acoustic analogy theory, and also to offer possible solutions for some engineering problems.