As global warming accelerates the melting of sea ice, the Arctic region witnesses an increase in ship navigation. The brash ice area, composed of brash ice of various sizes and shapes, is a common operational scenario for polar ships. Understanding the ice load characteristics of polar ships during oblique navigation in brash ice regions is crucial. This can enhance ship navigation safety in the complex polar marine environment, provide a reference for polar navigation route planning, and fill the gap in the current research that mainly focuses on straight-sailing conditions.

This study selects a specific type of polar ship as the research object and utilizes the discrete element method (DEM) to predict the ice loads on the ship during oblique navigation through brash ice regions. First, a numerical model of the target ship is established. The model parameters include a ship model with a scale ratio of 60, a total length of 2.04 m, a beam of 0.37 m, and a design draft of 0.13 m. The ice particles have a density of 917.0 kg/m³, a Poisson's ratio of 0.3, and other specific properties. The accuracy of the model is verified by comparing it with the experimental results from the literature under the straight-sailing condition. Then, different oblique-sailing angles (0° −15°), speeds (0.6, 0.7 m/s), and ice thicknesses (0.011 67, 0.014 97 m) are set. The ice-load calculation is carried out based on the momentum conservation equation, angular momentum conservation equation, and the linear spring contact force model in the DEM.

The results show that as the drift angle increases, the ice-breaking resistance and lateral force on the ship increase non-linearly. For example, at a speed of 0.6 m/s, an ice concentration of 70%, and an ice thickness of 0.014 97 m, when the drift angle is 15°, the ice-breaking resistance and lateral force increase by 4.25 times and 6.04 times respectively, compared to the straight-sailing condition. In terms of speed, when the drift angle is between 0° and 10°, the ice-breaking resistance increases slowly, but when it exceeds 10°, it increases significantly. The lateral force also increases non-linearly, and the influence of speed on the lateral force is more significant than whether the ship is on the ice-facing side. Regarding the influence of ice thickness, when the drift angle is greater than 10°, the ice-breaking resistance and lateral force increase significantly as the ice thickness increases.

In conclusion, this research provides reliable data support for the safety assessment of ships during oblique navigation in polar brash ice regions. It offers a valuable reference for predicting and studying ice loads on polar ships under such conditions. Ship operators should be cautious when increasing speed or entering thicker ice areas, especially when the drift angle is greater than 10°. This is to avoid potential risks caused by sudden changes in ice-breaking resistance and lateral force, ensuring the safe and stable navigation of polar ships in complex ice-covered waters.

To address the problem of inadequate path-following accuracy and stability in unmanned surface vehicles (USVs) operating in complex environments (characterized by uncertainties such as fluctuating wind speeds and initial position deviations), a guidance method called time-varying sideslip compensated adaptive line-of-sight (TSC-ALOS) is proposed.

First, a time-varying sideslip compensation mechanism is introduced based on real-time measurements of wind speed and direction, which forms the foundation of the improved TSC-ALOS algorithm. This mechanism dynamically compensates for sideslip angle variations induced by environmental disturbances, thereby optimizing the desired heading output of the USV. Subsequently, a proportional-derivative (PD)-based heading controller is designed. This controller translates the desired heading generated by the TSC-ALOS algorithm into actual rudder angle commands, enabling the USV to rapidly and stably track the target heading. This also establishes an effective connection between high-level navigation strategies and low-level control execution. Finally, numerical simulations emulating real marine environments are conducted. The performance of TSC-ALOS algorithm is compared with that of adaptive LOS (ALOS) and traditional LOS algorithms under three operational conditions: no wind, steady wind, and variable wind. Key metrics such as cross-track error and heading stability are specifically analyzed.

Simulation results demonstrate that under no-wind conditions, both TSC-ALOS and ALOS algorithms achieve higher path-following accuracy than traditional LOS algorithm, particularly in handling turning segments. Under steady wind (wind speed: 8.37 m/s) and variable wind (wind speed: 16.73 m/s) conditions, TSC-ALOS significantly reduces the cross-track error, showcasing stronger resilience to environmental disturbances. In scenarios with initial position deviations, the average cross-track error of TSC-ALOS is reduced by 24.6% and 36.8% compared to ALOS and LOS algorithms, respectively.

The TSC-ALOS algorithm demonstrates superior guidance performance across various complex environments, with particularly notable advantages in addressing environmental disturbances and initial position deviations. It offers essential technical support for the development of autonomous navigation systems for USVs and provides insights into future research directions for algorithm optimization.

To address the challenge of simultaneously maintaining formation integrity and enabling flexible obstacle avoidance for multi-unmanned underwater vehicle (multi-UUV) formations in complex underwater environments, this paper proposes a global path planning method that supports adaptive formation reshaping.

The proposed method is built upon an affine transformation framework that maps the cooperative path planning problem of the multi-UUV system into a two-dimensional affine parameter space. First, a front-end path search is conducted using an improved rapidly-exploring random tree* (RRT*) algorithm. By integrating fast exploration and iterative optimization phases, a weighted k-dimensional (KD) tree, a hybrid sampling mechanism, and adaptive tuning of sampling parameters, this algorithm efficiently generates an initial sequence of affine states. Subsequently, a B-spline-based back-end optimizer employs a gradient descent method to minimize a comprehensive objective function that accounts for trajectory smoothness, UUV kinematic feasibility, environmental collision safety, and the cost associated with adaptive formation scaling. The optimization process yields a continuous and smooth trajectory of affine parameters that satisfies multiple constraints.

Lake experiments demonstrate that the proposed planning method can generate safe and feasible formation paths. It successfully guided the multi-UUV formation through a simulated narrow obstacle region, while the actual velocities and accelerations of the UUVs remained within the predefined feasibility constraints.

The proposed global planning method, based on affine transformation, effectively generates safe and feasible paths for multi-UUV formations navigating complex obstacle environments by enabling adaptive formation reshaping. This method significantly enhances the autonomy and environmental adaptability of marine unmanned vehicles, and holds great value for advancing the development and practical application of marine unmanned systems technology.

To address the challenges posed by high-intensity noise and the structural characteristics of large obstacle targets in underwater sonar imaging, as well as the stringent requirements for lightweight deployment and high inference efficiency of perception algorithms in real-time underwater obstacle avoidance tasks, a semantic segmentation algorithm for sonar images with low computational cost and short inference time is proposed. The method aims to resolve the trade-off between the computational complexity of perception algorithms and the real-time response requirements in obstacle avoidance applications.

Based on an encoder-decoder network architecture, lightweight convolution operations were introduced to significantly reduce computational complexity. In addition, a large-kernel separable attention mechanism was incorporated into the skip connections to enhance feature fusion for obstacle avoidance scenarios. A dataset of 6936 sonar images collected and manually annotated from real environments was used for training and comparative experiments. Furthermore, the obstacle avoidance strategy based on the proposed perception algorithm was validated on the Gazebo simulation platform.

The improved algorithm specifically enhances the segmentation accuracy of large targets. Compared with the baseline model, the FLOP and the number of parameters are reduced by 69.2% and 83%, respectively. At the same time, the inference time is shortened by 22.6%, while perception accuracy improves by 10.8%. In addition, simulation experiments verify the effectiveness of the perception algorithm during the obstacle avoidance process, demonstrating that it fully satisfies the requirements of real-time perception tasks in underwater obstacle avoidance scenarios based on forward-looking sonar.

The proposed sonar-image-based perception algorithm can effectively meet the obstacle avoidance requirements of unmanned underwater vehicles in onboard operating scenarios and shows promising potential for engineering applications.

To investigate the acoustic target strength (TS) characteristics of extra-large unmanned underwater vehicles (XLUUVs), this study conducts a systematic analysis of the TS characteristics of the Orca XLUUV in the 1–10 kHz frequency band.

Based on the Orca model, the finite element method is applied to calculate its TS in the 1–3 kHz frequency band, while the planar element method is employed for the 3–10 kHz band. The results are compared with those obtained from the Benchmark model. To provide a more comprehensive evaluation of unmanned underwater vehicle (UUV) stealth performance, the concept of angular detection probability is introduced. Additionally, a scaled model experiment is conducted in a water tank, and a correction method is proposed for the experimental TS measurements.

The TS characteristics of the Orca model are first analyzed. Compared with the Benchmark model, the Orca model exhibits superior stealth performance in the azimuthal direction, along with additional advantages in the circumferential direction at higher frequencies effects that become more pronounced as frequency increases. For experimental cases in which the distance between the hydrophone and transducer does not meet the far-field conditions, the measured TS values are corrected, yielding improved consistency with the simulation results. This validates the accuracy of the numerical simulation results.

The findings of this study provide a theoretical foundation for optimizing underwater detection systems and enhancing the stealth design of UUVs.

Deep-sea pressure hulls are at risk of implosion when subjected to extreme hydrostatic pressures that exceed their ultimate bearing capacities. Therefore, it is essential to investigate the failure mechanisms and shock response characteristics of titanium alloy cylindrical shells under implosion conditions.

First, an independent deep-sea implosion experimental platform was developed, and underwater experiments were conducted on the titanium alloy cylindrical shell in a deep-sea high-pressure environment. A compressible multiphase flow module was then developed to simulate the high-speed motion of the flow field during the underwater implosion. The explicit nonlinear finite element method was employed to analyze the dynamic response associated with the collapse and failure of the titanium alloy cylindrical shell. Finally, the characteristics of the titanium alloy cylindrical shell implosion were investigated, focusing on the fluid-structure interaction mechanism, the evolution of asymmetric shock waves in the multiphase medium, the nonlinear dynamic response of the structure, and the energy balance relationships.

The results showed that the titanium alloy cylindrical shell, with a length-to-diameter ratio of 2, collapsed in the first-order instability mode, and the implosion center formed twice successively. As hydrostatic pressure increased, a pronounced migration effect of the first implosion center was observed. Meanwhile, the failure mechanism of the shell transitioned progressively from inward extrusion to inward curling, and the rupture morphology evolved from an arcuate shape to an M-shaped configuration.

This study reveals the failure mechanism and shock response characteristics of the titanium alloy cylindrical shell implosion, providing valuable insights for the implosion assessment and protection of deep-sea pressure hulls.

To address the inherent trade-off between large-scale exploration and high-precision manipulation in existing underwater vehicles, a novel morphable underwater intervention robot is developed. Designed for operations at depths of up to 1000 m, the robot integrates low-drag cruising with dual-arm collaborative capabilities, meeting the stringent inspection and maintenance requirements of offshore wind farms and subsea oil and gas platforms.

The overall design specifications were first established, followed by the optimization of the integrated design workflow. The configuration of the robot's pressure-resistant hulls and equipment layout were finalized, with the development of key components, including the morphing mechanism (lead screw lifting mechanism) and pressure-resistant hulls. Strength verification of key components was performed using finite element analysis (FEA) under a 12 MPa hydrostatic load, simulating a depth of 1000 m. Subsequently, the endurance and maneuverability during cruising mode, as well as the manipulator workspace and stability during manipulating mode, were systematically evaluated. Finally, hydrodynamic drag characteristics were verified through CFD simulations, and a coupled vehicle-manipulator dynamic model was developed in Matlab to validate the robot's self-recovery, disturbance rejection, and coupling suppression performance.

The results indicate that the internal layout is rational, with critical components meeting the operational requirements for 1000 m deep-sea environments. The maximum stress within the pressure hulls remains below the yield strength of the selected materials. In cruising mode, the robot achieves a maximum endurance of 7 h, and the configured propulsion system ensures high underwater maneuverability. At a cruise speed of 6 kn, the longitudinal drag is recorded at only 725.06 N, significantly lower than that in manipulating mode, demonstrating superior low-drag characteristics. In manipulating mode, the central buoyancy module is raised by 270 mm, increasing the vertical distance between the center of gravity and the center of buoyancy by 0.054 m. As a result, the maximum restoring moment increases by 202.1% compared to cruising mode, significantly enhancing operational stability. The heeling self-recovery time is reduced from 180 s to 60 s, alongside improved anti-disturbance capabilities. Furthermore, the dual-arm workspace effectively covers the lateral, forward, and downward regions of the vehicle, ensuring an efficient and collaborative operational envelope.

By utilizing autonomous configuration switching, an overall design scheme for a morphable underwater intervention robot with multi-task execution capability was proposed. This design effectively combines low-resistance detection in cruising mode with high-stability operation in manipulating mode, offering an innovative solution for underwater operations in complex deep-sea scenarios.

This study aims to develop a dynamic model of the longitudinal profile motion of wave gliders by modeling the umbilical cable as multiple hinged rigid rods, and to investigate the effects of environmental and umbilical cable parameters on the longitudinal motion characteristics.

Based on reasonable assumptions and simplifications, the umbilical cable was modeled as a series of homogeneous, multi-segment rigid rods connected by hinges. The Lagrangian method was employed to construct a multi-rigid-body dynamic model of the wave glider in the longitudinal profile. Incorporating calculation methods for wave force, fluid resistance, and hydrofoil external forces, a simulation program was developed on the MATLAB/Simulink platform to solve the model. The model's validity was verified by comparing its results with those of existing studies. Finally, a sensitivity analysis was conducted to examine the influence of environmental and umbilical cable parameters on the system response.

The results indicate that the longitudinal motion response increases with wave height; specifically, when the wave height rises from 0.2 m to 0.4 m, the longitudinal response increases by 78.20%. Under a current disturbance of 0.07 m/s, the longitudinal displacement within 60 s in the downstream condition increases from 1.53 m to 9.11 m compared with the upstream condition. Shorter umbilical cables amplify the longitudinal motion response; when the umbilical cable length decreases from 5 m to 2 m, the longitudinal response increases by 31.97%. Conversely, excessively small wave periods reduce the longitudinal response due to rigid impacts between the multi-segment hinged rigid rods. Changes in umbilical cable density, however, exert only a minor influence on the longitudinal motion response.

The findings of this study provide theoretical support for the structural optimization and motion control strategies of wave gliders.

To improve the overall operational capability of autonomous underwater vehicles (AUVs) and address the critical issue of collision risks during the dynamic docking process with towed recovery docks (TRDs), this study conducts a systematic investigation into the collision mechanisms and control strategies of the docking system. Reliable docking and recovery technology is essential for extending AUV operational endurance, enhancing data transmission efficiency, and enabling long-term underwater deployment. However, in real marine environments, limitations in sensor accuracy, external disturbances, and the dynamic response of the docking system often lead to unavoidable contact or collision between AUVs and TRDs, which may result in mission failure or structural damage to the equipment. Therefore, this study aims to clarify the influence of key initial operating conditions on docking-induced collisions and to propose an effective control strategy for optimizing the dynamic docking process, thereby providing theoretical and technical support for the engineering application of AUV towed recovery systems.

Based on dynamic analysis, a simulation model incorporating contact and collision dynamics was developed using the ADAMS-MATLAB co-simulation platform. First, rigid body dynamic models of AUV and TRD were constructed. The AUV model accounts for gravity, buoyancy, viscous hydrodynamic drag, inertial hydrodynamic drag, thrust, and environmental disturbances. The TRD model adopts a frame-cage structure with a bell-mouth guiding cover, and a discrete flexible body method is used to model the towing cable. Subsequently, a nonlinear contact model based on Hertz theory was employed to calculate the collision forces between AUV and TRD, which more accurately captures the transient impact characteristics of the collision process compared with the linear contact model. On this basis, the effects of initial operating conditions including eccentric angle, eccentric distance, relative initial velocity, and mother vessel acceleration on docking collisions were systematically analyzed using the control variable method. To mitigate attitude disturbances induced by collisions, a multi-stage coordinated control strategy based on PID control was proposed, which realizes active attitude adjustment of AUV by switching control modes across different docking phases.

The simulation results indicate that increases in eccentric angle and eccentric distance primarily prolong the docking time while exerting only a limited influence on the peak collision force, which remains within the range of 1 000–2 000 N under most working conditions. In contrast, increasing the relative initial velocity can shorten the docking time but significantly amplifies the peak collision force, showing a positive correlation between them. Further analysis of mother vessel acceleration reveals the complex, non-monotonic relationship between collision force and docking efficiency. As the mother vessel's acceleration increases, the amplitude of the TRD attitude variations intensifies, leading to greater uncertainty in the collision position, and the peak collision force reaches its maximum value when the acceleration is 0.2 m/s². Moreover, the proposed multi-stage coordinated control strategy enables effective post-collision attitude adjustment of the AUV. In the case of uniform motion of the mother vessel, the strategy reduces the peak collision force by up to 74.5% and shortens the docking time from 7.56 s to 5.93 s. Even under the complex working condition of uniform acceleration of the mother vessel, the peak collision force is reduced by 19.6%, and the docking time is shortened by 16.7%, effectively optimizing the dynamic docking process and ensuring both docking safety and efficiency.

This study systematically clarifies the effects of key initial operating conditions on the docking collision between AUV and TRD. The research findings indicate that controlling the initial eccentric angle and eccentric distance can improve docking efficiency, whereas adjustments to the relative initial velocity and mother vessel acceleration require a careful balance between collision risk and docking speed. The proposed multi-stage coordinated control strategy can significantly reduce the peak collision force while maintaining docking efficiency, achieving reductions of 14%–74.5% under different working conditions. This strategy exhibits superior robustness and stability compared with the traditional position tracking control strategy, effectively addressing the limitations of passive control methods that rely solely on the dock structure. Overall, this study provides a reliable simulation basis and design reference for the design and stability control of AUV towed recovery systems. In addition, the research framework and methods provide guidance for the collision analysis and control in similar underwater docking systems.