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.

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.

To address frequent actuator failures caused by complex underwater environments and the inherent characteristics of unmanned underwater vehicles (UUVs), this study investigates a prescribed performance path-following fault-tolerant control scheme for an underactuated UUV subject to ocean current disturbances, model uncertainties, and actuator faults. To ensure safe UUV navigation, a path-following fault-tolerant controller is designed by integrating an improved prescribed performance function with a barrier Lyapunov function, enabling full-state-constrained fault-tolerant control.

A novel predefined-time disturbance observer is developed to estimate the lumped disturbances arising in UUV path-following, including ocean currents, parameter perturbations, unmodeled dynamics, and thrust loss caused by actuator faults. The lumped uncertainties with actuator faults are incorporated into the prescribed performance fault-tolerant controller for compensation, ensuring that all path-following state errors remain within predefined bounds.

Simulation results demonstrate that the position error, attitude angle error, and angular velocity error converge rapidly while strictly satisfying the prescribed safety constraints, achieving a steady-state position error bound of 1 meter and an attitude angle error bound of 0.05 radians. When the actuators suffer up to 80% thrust loss, the disturbance observer rapidly estimates the lumped disturbances, and the controller compensates for the faults within 1 second without significant path-following deviation. The maximum transient error does not exceed 20% of the prescribed limit. These findings validate the strong robustness of the proposed method against actuator faults. By unifying disturbance observation with prescribed performance constraints, the fault-tolerant control structure is simplified, achieving both fast fault response and full-state safety guarantees.

This work provides a universal solution for high-performance UUV navigation in complex underwater environments.

This study aims to investigate the dynamic behavior and flow field characteristics of trans-medium submersibles during underwater straight-line navigation and turning maneuvers.

To this end, computational fluid dynamics simulations were employed, using the VOF multiphase flow model and the SST k–ω turbulence model to establish a numerical model of the underwater navigation of the trans-medium submersibles. The accuracy of the numerical method was validated by comparing the experimental total drag data for the DARPA Suboff submarine model at various speeds with the numerical calculation results. On this basis, numerical simulations and analyses of underwater straight-line navigation and turning maneuvers of the trans-medium submersible were conducted, focusing on the effects of ducted propeller rotation speed and tail fin deflection angle on the underwater straight-line navigation and turning performance of the submersible.

The research results indicate that during straight-line underwater navigation, the forward speed of the trans-medium submersible exhibits an approximately linear relationship with the propeller's rotational speed. For instance, as the propeller speed increases from 600 r/min to 4800 r/min, the forward speed rises from 1.1 m/s to 8.1 m/s. At the same time, the pitch moment becomes less negative with increasing propeller speed (from −0.35 N·m to −0.17 N·m), indicating that the submersible remains stable in pitch during high-speed navigation. The propeller speed has little effect on the surface pressure distribution and the structure of the surrounding flow field. During underwater turning, the turning radius is mainly determined by the tail fin deflection angle and is largely unaffected by the propeller speed. The turning radius decreases with increasing tail fin deflection angle (from 3.35 times the submersible's body length to 0.75 times), though the rate of decrease diminishes. In contrast, the turning speed is affected by both the propeller speed and the tail fin deflection angle. The thrust generated by both propellers increases with higher propeller speeds and larger tail fin deflection angles. During turning, the thrust of the outer propeller consistently exceeds that of the inner propeller, and the thrust difference increases with greater tail fin deflection. Furthermore, tail fin deflection during turning leads to a significantly asymmetric surface pressure distribution on the submersible. This asymmetry becomes more pronounced with increasing tail fin deflection and is closely associated with the asymmetric flow characteristics of the surrounding flow field.

This study provides a reference for the design and performance analysis of trans-medium submersible configurations.

To address the challenges in multi-AUV formation maneuvering, such as limited state perception and transmission capabilities, acoustic communication delays, data loss, and reduced observability due to the lack of position information exchange, this study proposes an event-triggered metrology−communication unified framework with a Lyapunov-based model predictive formation control method (ETMCU−LMPC). The proposed approach aims to enhance formation stability and tracking accuracy.

First, by integrating the formation communication topology with system states, an event-triggered mechanism based on state observation is established. This mechanism leverages relative measurements among AUVs to mitigate delays and data loss caused by acoustic communication failures, while improving system observability in the absence of position information exchange. Second, a distributed model predictive controller based on Lyapunov theory is designed. The controller employs backstepping to construct contractive constraints, ensuring recursive feasibility, and incorporates adaptive Kalman filtering (AKF) to compensate for measurement noise, thereby guaranteeing closed-loop stability.

Simulation results of the formation control for five AUVs (1 leader and 4 followers) show that, compared with the traditional LMPC, the proposed ETMCU−LMPC method reduces the convergence time from 8 s to 6 s, the maximum error from 1.12 m to 0.36 m, and the steady-state error from 0.57 m to 0.06 m. Additionally, the control input exhibits greater stability.

The proposed method can effectively cope with communication anomalies, improve the reliability of multi-AUV formations under scenarios with limited state perception and transmission, and thus possesses practical engineering significance.

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.