When the frequency of external wave excitation or that of platform motion approaches the natural frequency of the moonpool structure, intense resonance phenomena occur within the moonpool. Under such conditions, both the water inside the moonpool and the platform motion exhibit strong nonlinear characteristics. Based on computational fluid dynamics (CFD) principles, this study conducts numerical simulations of a deep-draft cylindrical platform under regular wave action to investigate the influence mechanisms of incident wave frequency and environmental wave height on the coupled system of platform motion and moonpool water dynamics. The results indicate that increasing environmental wave height intensifies nonlinear phenomena in the moonpool water motion, characterized by enhanced resonance peaks in high-frequency bands and reduced piston-mode resonance peaks. A strong coupling relationship exists between platform motion and moonpool water dynamics, where both the moonpool resonance phenomena and platform motion responses demonstrate dual-peak characteristics. As the environmental wave height increases, the heave response amplitude operator (RAO) amplitude decreases, while the platform’s pitch RAO amplitude increases.

To address the practical requirements for enhancing the prediction methods of submarine maneuverability and seakeeping performance under complex sea conditions, overset mesh and sliding mesh techniques were employed to simulate the six-degree-of-freedom (6-DOF) coupled motion of the submarine, independent deflection of X-stern control surfaces, and rotation of the propeller. By integrating the Volume of Fluid (VOF) method to capture the free surface, a numerical simulation methodology for near-surface maneuvering motion of the free-running submarine model was developed. Through systematic investigations on the near-surface turning maneuvers of an X-rudder submarine under varying submergence depths in calm water and in regular waves of different wavelengths, critical challenges were resolved, including hydrodynamic coupling modeling of the hull-propeller-rudder system and the free surface, PD (Proportional-differential) control of the X-type autopilot, and PI (Proportional-integral) control of propeller rotational speed. The results demonstrated that when the submarine maneuvered excessively close to the free surface, the turning diameter, lateral advance, and maximum longitudinal advance of the X-rudder submarine increased drastically. This was primarily attributed to a significant degradation in rudder effectiveness, with secondary contributions from increased lateral forces on the propeller. Under specific wavelengths, vertical-plane motion attitude variations in the fourth quadrant during turning maneuvers induced substantial fluctuations in the normal force of the X-rudder, leading to pronounced adjustments in the drift angle and significant deviations in turning trajectories. These findings provide critical technical support for the prediction and evaluation of submarine maneuverability and seakeeping performance, as well as for the refinement of free-running submarine model experiments.

In the single-point mooring (SPM) system, the main bearing is a critical component that withstands mooring force, making its load-bearing performance under actual working conditions of great significance. However, research on this type of bearing remains limited, and existing studies have not considered the actual mooring force and installation structure. Therefore, this study focuses on the Soft Yoke Mooring System (SYMS) and proposes an overall finite element equivalent modeling method for the main bearing, by considering the mooring force and installation structure. In addition, to avoid extensive nonlinear contact problems, rollers and bolts are equivalently modeled using nonlinear springs and beam elements. After verifying the model's accuracy, we compare it with the traditional model under equivalent load and analyze the effects of upper load, bolt preload, and friction coefficient on the load-bearing performance of the main bearing. Finally, based on the maximum roller load obtained from both the overall and traditional models, a local contact model is established to calculate the safety factor of the main bearing. The results indicate that the overall equivalent model more accurately reflects the load distribution and deformation characteristics of the main bearing. Both the overall displacement and the bolt displacement decrease with increasing preload and friction coefficient, while contact pressure exhibits an increasing trend. The calculated safety factor of the main bearing in the overall model is 1.43, which is significantly lower than the safety factor of 2.51 obtained from the traditional model. This indicates that the traditional model overestimates the load-bearing capacity of the main bearing, while the overall model provides a more accurate assessment. Thus, the overall model provides a more reliable basis for the design of such bearings.

Model experiment is an essential approach for investigating ice-structure interaction. The model ice employed in different ice tank laboratories and ice indentation tests (including saline ice, urea ice, and EG/AD/S ice, etc.) exhibits variations, and studying the mechanical characteristics of various model ices is crucial for conducting model experiments. In this study, the aforementioned three types of model ice were prepared in a low-temperature laboratory, uniaxial compression tests were performed, and the macroscopic failure characteristics of the model ice under different strain rates were comparatively analyzed; based on the Digital Image Correlation (DIC), the full-field strain and crack propagation process of the model ice were analyzed in detail, and the differences in the mechanical properties of the model ice were ultimately examined. The results indicate that saline ice exhibits the most similar failure behavior to sea ice, urea ice presents layered failure at high strain rates, and EG/AD/S model ice demonstrates predominantly ductile failure characteristics; the three types of model ice exhibit distinct crack propagation characteristics, with saline ice and urea ice possessing ductile and brittle failure intervals comparable to those of sea ice.

This paper investigates the influence of initial geometric defects on the ultimate load capacity and failure mode of thin-walled titanium alloy ring-ribbed cylindrical shells. The study combines key geometric parameters, including the radius-to-thickness ratio (R/t), length-to-radius ratio (L/R), and rib spacing (u), to conduct structural design calculations and analyze failure modes, supported by experimental verification. A welded model (with initial defects) and a precision-machined model (without defects) were designed and fabricated. Theoretical analysis and numerical simulations were performed to obtain the structural stress distribution and critical buckling pressure at critical locations on both models. Subsequently, hydrostatic external pressure tests were conducted to evaluate the structural strength and ultimate load capacity, revealing the stress distribution characteristics, failure pressure, and failure modes of each model. By comparing the theoretical, numerical, and experimental results, the impact of initial shape defects on structural performance was assessed. This comparison provides a robust basis for the design, calculation, and evaluation of thin-walled titanium alloy ring-ribbed cylindrical shell structures.

To investigate the feasibility of determining submarine hydrodynamic derivatives through wind tunnel experiments, this study focuses on the SUBOFF AFF-8 model proposed by the David Taylor Model Basin (DTMB). Using the linear and rotary oscillation mechanisms equipped in the FL-10 wind tunnel of Harbin Aerodynamics Research Institute, AVIC, wind tunnel tests were conducted to obtain the hydrodynamic derivatives in the vertical plane of the SUBOFF AFF-8 model. A data processing method for evaluating submarine vertical-plane hydrodynamic derivatives based on wind tunnel testing is proposed. The validation results indicate that the hydrodynamic coefficients obtained from the wind tunnel tests exhibit trends consistent with those from the DTMB measurements. For the hydrodynamic coefficients used in vertical-plane maneuvering and stability predictions, the discrepancies are all within 10% except the pitch moment coefficient with respect to angular velocity. This demonstrates the feasibility of the experimental principle and methodology. The work lays a technical foundation for further studies on wind tunnel test methods for submarine hydrodynamic derivatives at large angles of attack.

In the multi-objective optimization of slender ring-stiffened cylindrical shells, it is difficult to balance the structural lightweight and critical pressure for overall instability, which restricts the improvement of the performance of Autonomous Underwater Vehicle. In this paper, a data-driven multi-objective optimization strategy is proposed, which can improve the critical pressure for overall instability and reduce the weight of the structure. Firstly, based on the error convergence criterion and accuracy requirement, the appropriate training set size is found in this strategy through iteration, and a low-cost and high-precision surrogate model is constructed. Then, the second-generation non-dominated sorting genetic algorithm is used to obtain the Pareto solution set. The non-inferior solution is screened by the minimum distance method based on maximum and minimum normalization, and the optimal design scheme with a well-balanced performance is obtained by combining the local accuracy enhancement strategy of the surrogate model. Using the optimization strategy proposed in this paper, the structural weight of the slender ring-stiffened cylindrical shell is reduced by 9.1%, and the critical pressure for the overall stability is increased by 13.4%. An effective design to improve the stability of the ring-stiffened cylindrical shell without increasing its weight is to increase the thickness of the cylindrical shell, increase the height of the stiffeners, and reduce their number.

Aiming at the limitation that the single-factor model ignores the combined effect of internal pressure and internal fluid weight in the traditional submarine pipeline buckling theory, this study proposed a multi-factor joint analytical solution optimization method based on secondary buckling analysis. Firstly, by combining the Coulomb friction law, Maltby formula and Hobbs and Taylor buckling theory, an analytical model for the secondary buckling deformation of the pipeline under the combined action of temperature-internal pressure-internal fluid weight was established, and the analytical solution of the cap-shaped buckling mode was derived. Secondly, through the thermal-solid coupled finite element model, the pipe-soil contact stiffness and boundary condition parameters were calibrated with experimental data to ensure the equivalence between the numerical model and the actual working conditions. On this basis, the empirical coefficient K₁ in the analytical solution is corrected by comparing the theoretical solution with the simulation results, so that the error in the buckling displacement prediction is reduced to less than 2%. The results show that the contribution rate of the internal fluid weight to the axial compression of the pipeline is 35.89%, a factor that significantly affects the critical buckling threshold. The prediction accuracy of the modified analytical solution is improved to more than 98% under the combined condition. The results provide a high-precision theoretical tool for the anti-buckling design of deep-sea pipelines, and lay a methodological foundation for the stability analysis of pipelines under the combined action of multiple physical fields.

In order to establish a comprehensive planar Real-Time Nearfield Acoustic Holography (RT-NAH) system and effectively avoid overdependence on the Laplace transform table when deriving impulse response functions, an alternative method based on the Rayleigh integral is proposed. Firstly, the pressure-pressure, normal velocity-pressure, normal acceleration-pressure, and normal displacement-pressure impulse response functions are systematically derived based on the Rayleigh integral. Then, a comprehensive planar RT-NAH system is constructed based on the obtained impulse response functions. Finally, simulation of a simply supported thin aluminum plate is performed to evaluate the correctness of the derived impulse response functions. The analysis of the reconstruction results demonstrates that high accuracy is achieved when reconstructing pressure and normal velocity, whereas relatively lower accuracy is observed for normal acceleration and displacement. The primary factors contributing to the lower accuracy when reconstructing the normal acceleration and displacement are further investigated.

Flow-Induced Vibration (FIV) energy harvesting is an effective approach for harnessing low-velocity ocean currents. Using a maglev support system to replace traditional metal springs enables better stiffness adjustment and improves underwater operation and maintenance performance. To enhance energy output and stability, dual-oscillator designs are widely used in vortex-induced vibration energy harvesters. However, the vibration characteristics of dual oscillators supported by maglev systems remain underexplored. This study establishes a coupled numerical model that integrates the FIV of rigid cylindrical oscillators with a maglev support system to investigate the influence of the spacing ratio (G/D) on their vibration responses. Results show that a smaller spacing ratio (G/D=2) intensifies hydrodynamic interactions, leading to a downstream oscillator's amplitude reaching over twice that of a single oscillator. The oscillation frequencies decrease monotonically with increasing spacing ratios, and distinct frequency variation patterns are observed between upstream and downstream oscillators. Vortex analysis reveals that the upstream wake reconstructs the flow field and strengthens the excitation forces on the downstream oscillator, thereby amplifying its vibration response. The maglev effect enhances the oscillators' responsiveness to flow field changes through nonlinear magnetic forces, further improving vibration performance and system stability. This study provides theoretical insights for optimizing dual-oscillator FIV energy harvesters with maglev support.