A single-degree-of-freedom model was established to describe the roll motion of ships under combined effect of wind and wave, then the accuracy of the numerical model was verified by model tests. The time history of roll motion responses in various sea conditions was calculated by the numerical method. The Gumbel method and average conditional exceedance rate (ACER) method were used to predict the extreme value of ship roll motion response. Then based on the extreme value prediction theory, an efficient method for evaluating the ship capsizing probabilities was established. The Monte Carlo method and Level 2 vulnerability criteria for dead ship stability were employed to validate the feasibility of the evaluation method. The research demonstrates the suitability of both the Gumbel method and the average conditional exceedance rate method in predicting the extreme value distribution of ship roll motion in various sea condition, while the average conditional exceedance rate method has a higher accuracy. Compared to the traditional Monte Carlo method, the ship capsizing probability evaluation method proposed in this paper could ensure the accuracy and improve the calculation efficiency. The research in this work could provide references for the dead ship stability evaluation as well as for the safety of ships at seas.

Anisotropic complex flows are prevalent in various engineering applications, with transition phenomena occurring at low to moderate Reynolds numbers. Turbulence models based on Reynolds stress anisotropy are employed to handle these anisotropic complex flows. However, such models are based on fully developed turbulence assumption and lack the capability to predict transition phenomena. In recent times, these models are modified with transition models, which still exhibit poor robustness and practical inconvenience. To address this, this paper combines γ transition model with the SST turbulence model, forming the ASST-γ transition model to effectively deal with the transition phenomena in complex flow fields. To comprehensively evaluate the predictive ability of the ASST-γ model for transition, classical transition cases are numerically calculated for three main types of transition: bypass transition, natural transition, and separation-induced transition. The results indicate that the numerical calculations of the ASST-γ model for the three types of transition are in good agreement with experimental results. In particular, it demonstrates better predictive accuracy for the type of separation-induced transition compared to the SST-γ model. ASST-γ model is capable of predicting these three types of transitions, offering a promising solution to transition issues in complex flow fields.

Surface piercing propellers are one of the preferred propulsion systems for high-speed crafts due to their favorable features such as no cavitation, low resistance, shallow draft and high efficiency. Since the propellers often operate in oblique flow with blade emergence, there will be vertical forces generated which will have an important impact on the hydrodynamic performance of the planing crafts. In this paper, the effects of vertical force generated by surface piercing propellers on hydrodynamic performance of a stepped planing craft through numerical and experimental methods were studied. And the cases for planing boat with different locations of center of gravity were also involved. The results show that the vertical force produced by surface piercing propellers has a positive effect on the resistance of the planing crafts and that the effect is related to the longitudinal position of the center of gravity of the planing boat. Consequently, the longitudinal position of the center of gravity should be adjusted to the stern for planing craft equipped with surface piercing propellers, and the vertical force of the propellers should not be too large.

Aiming at the problem that the ship's course is easily disturbed by the external environment during navigation, an improved non-singular terminal sliding mode control strategy based on Nonlinear Extended State Observer (NESO) is proposed. Firstly, a nonlinear mathematical model of ship's heading system is established, and a nonlinear expanded state observer based on the hyperbolic tangent function is designed to estimate the external disturbances encountered during the ship's navigation and perform feedforward compensation. Then, based on the improved non-singular terminal sliding mode surface and the double power reaching law, an improved non-singular terminal sliding mode control law is designed, which reduces the chattering of the sliding mode control law and improves the convergence speed. The nonlinear extended state observer and the improved non-singular terminal sliding mode control law are combined to form a composite control law. Finally, the stability of the control law is analyzed based on Lyapunov stability theory. Simulation results show the proposed control strategy can effectively improve the tracking performance and robustness of the ship heading control system.

Accurately identifying the far-field radiation hotspots of structures is of great significance for vibration and sound radiation control. In this paper, an open-source boundary element program is used to obtain the radiation impedance matrix of the cylindrical shell, and the normal vibration velocity of the shell is extracted. Based on the singular value decomposition, the non-negative intensity and radiated sound power of the underwater cylindrical shell are obtained, and the far-field acoustic radiation model is acquired, moreover, the method is verified by finite element software. On this basis, the influence of large stiffener at the stucture, where an excitation force is applied, on non-negative intensity is discussed. Furthermore, the mathematical model of active control is established with the aim of minimizing the acoustic radiation power. The influence of control force on structural sound radiation mode is analyzed, and its influence mechanism is revealed using the non-negative intensity distribution. The results show that the radiation mode of the structural surface must be changed to reduce the radiation acoustic power. And the essence of secondary force controlling structural vibration radiation is to change the structural vibration from strong radiation mode to weak mode, thus reducing the radiation efficiency of the structure.

Accurate motion prediction is crucial for the safe navigation of Autonomous Underwater Vehicles (AUVs). A fast Least Squares-Support Vector Machine (LS-SVM) motion prediction approach based on ridge regression algorithm is proposed in this paper. Firstly, the ridge regression analysis was incorporated into the traditional hydrodynamic model and the correlation analysis was conducted on the acceleration time-series input variables to identify the relatively important components. This step effectively reduces computational complexity while maintaining prediction accuracy. Subsequently, for the issue of high dimensionality and computational complexity in the LS-SVM algorithm's kernel function matrix, an improved Lagrange function was designed to eliminate redundant bias terms. This modification lightens the burden of calculating high-dimensional kernel matrices and further enhances the speed of maneuvering prediction. Finally, case studies based on the REMUS model demonstrate that the proposed strategy, compared to the standard LS-SVM prediction method that relies on traditional hydrodynamic models, reduces computational runtime by 29.8% while ensuring prediction accuracy.

In order to solve the problem of path redundancy and long algorithm execution time, this paper proposed a path planning method that combines the Northern Goshawk Optimization (NGO) algorithm with the improved rapidly-exploring random tree (RRT*). First, a fitness function with obstacle avoidance and goal orientation was designed to optimize the initial NGO population. Additionally, the adaptive sampling step size of the RRT* algorithm was designed according to the fitness function to improve the search efficiency in a large-scale map. Then, the optimal neighbor node sampling mechanism was designed to simulate behavior of the northern goshawk transmitting information to its nearest companions, while also the RRT* node sampling was constrained by considering the USV’s (unmanned surface vehicle) heading angle. Finally, in order to improve path smoothness, the Metropolis criterion was introduced and the smoothness and minimum rudder angle design fitness function were combined to select a more suitable parent node for dynamic rerouting. The experimental results show that compared with RRT*, Informed-RRT* and RRT*-smart algorithms, the improved algorithm reduces the path length by 19.36%, 3.36% and 5.98%, and decreases the search time by 49.33%, 57.01% and 59.16%, respectively. At the same time, the curvature of the path also decreases significantly.

Large-scale offshore floating platforms belons to special offshore structures. Their bearing pin shaft connection structures have good flexibility and applicability. In order to ensure the safety of the large-scale offshore floating platform connection system during service, it is necessary to analyze its fatigue life. Firstly, the large-scale offshore floating platform connection system with multiple sets of bearing pin shaft was studied. A nonlinear contact analysis method of various contact forms was adopted to investigate the overall strength of the structure under the design working condition load. Then, the fatigue life of the platform connection system was calculated and analyzed before and after the local structural improvement. The results show that, for large-scale offshore floating platform with multiple sets of bearing pinsnajt, the lowest fatigue life appears at the flat steel of the connection between the outermost bearing and the plate-frame structure of the loading end. In addition, the fatigue life could be effectively improved by increasing the overall plate thickness of the outermost bearing base plate-frame at the loading end. The thickness of the plate-frame and flat steel near the opening should also be increased. In this paper, a set of fatigue strength analysis methods for large offshore floating platform connection system with multiple sets of bearing pin shaft is finally formed.

The Boussinesq model is a kind of wave model widely used in near-shore engineering, and its computational accuracy mainly depends on the basic performance of the model, while the upper bound of nonlinear application of the model has always been the focus of attention. In recent years, the two-layer Boussinesq model has gained great progress regarding theoretical properties, numerical modeling and applications. However, the value of its nonlinear upper bound has not been given in any literature. So in this study, the stream-function of the two-layer Boussinesq model was solved using a combination of genetic algorithm and Newton's method to determine the upper bound value of the model, considering the highest spatial derivatives of order 3 and 5. In the same way, the stream-function solutions of the corresponding one-layer Boussinesq model were derived. The numerical results show that the nonlinear upper bounds of the two-layer Boussinesq model with the highest derivatives of order 3 and 5 are H/L = 0.137 and 0.138. Compared with the one-layer Boussinesq model, the two-layer model has a greater water depth of applicability regarding strong nonlinear characteristics. The combination of genetic algorithm and Newton's method proposed in this study can provide some references for solving the stream-function waves of the related Boussinesq models.

The safety of a mooring system is one of the key performance that require verification for floating offshore platforms. Based on the requirements of lightweight design, precise control of active breaking and avoidance of measurement interference, this paper developed an active broken simulation device that can be used in marine engineering model tests. The semi-submersible floating wind turbine platform was used to carry out an active broken model test of a mooring chain in an extreme environment. The results show that the device can be triggered based on tension threshold judgment and timing, and has stability, accuracy and real-time performance. It can effectively capture the whole process of dynamic response of floating structure and mooring system after mooring failure. After breaking occured near the maximum mooring tension, the tension of the adjacent mooring chain surges, the safety factor decreases sharply, the platform undergoes a long-distance offset, and experiences severe oscillation, and the safety and positioning ability of the mooring system are seriously reduced.