Welding residual stress has a significant impact on the fatigue life of a welding structure. Meanwhile, welding residual stress is not invariable, but will be redistributed with crack propagation. Therefore, the coupling study of welding residual stress redistribution and crack propagation is very important to predict the fatigue life of a welded structure accurately. Based on thermal elastic-plastic finite element method and extended finite element method (XFEM), a fatigue life analysis method considering the coupling of residual stress redistribution and crack propagation was proposed in this paper. Taking the tensile fatigue sample of TC4 titanium alloy as an example, the redistribution of the welding residual stress along with crack propagation was studied with the extended finite element method. The welding residual stress distribution in front of the crack tip during crack propagation and the fatigue crack propagation a-N curve under the redistribution of weld residual stress were calculated by cyclic iteration. The calculation results show that the welding residual stress at the crack tip increases firstly and then decreases with the crack propagation. Compared with the results based on constant value of the residual stress, the extended finite element fatigue life analysis method considering the redistribution of the residual stress is more accurate to predict the fatigue lives of welding structures.

With the continuous development and consumption of traditional land resources, the development and utilization of new water energy has become a new trend, and a large number of various floating structures have appeared. As the key to ensure the safe and stable operation of floating structures, mooring systems have always been the focus of the industry. In this paper, a large number of literature review and research were carried out on the existing floating structure mooring systems, the types of floating structures were summarized, the mooring system structures were analyzed from the aspects of the classification of the mooring systems, the way of chain distributions, the bottom anchorage foundation types and the new mooring systems, and the characteristics and advantages and disadvantages of various mooring cable materials were discussed. The static characteristics and dynamic response of the mooring systems were analyzed according to a large number of existing literatures, and the applicability evaluation and recommendation of various mooring methods were given through a comprehensive analysis of the water depth, seabed topography, geology, platform function, wind and wave conditions, economy and other aspects of the mooring engineering, and the shortcomings of the existing research were pointed out, and the current research direction still needs to be further developed.

In order to evaluate the fatigue performance and potential application prospects of a certain type of titanium alloy K-TIG welded joint for marine equipment, this study conducted high cycle fatigue tests and fatigue life prediction models research on titanium alloy welded joints with docking form based on the Basquin formula theoretical method and statistical P-S-N probability analysis method. Research results show that the yield and tensile strengths of the titanium alloy welded joint used in marine equipment are 941 MPa and 985 MPa, respectively, with a yield to strength ratio exceeding 0.95 and low plasticity. The high cycle fatigue strength of titanium alloy welded joints expressed as maximum stress under the condition of stress ratio R = 0.1 is 297 MPa, and the ratio to tensile strength (fatigue ratio) is only 0.3. Characterization of fatigue fracture characteristics using electron scanning microscopy indicates that fatigue cracks originate at the weld toe of the welded joint, fatigue striations appear in the fatigue crack propagation zone, with a large number of ductile dimples in the instantaneous fracture zone. The median curves of maximum stress, stress amplitude, maximum load, and fatigue life of welded joints were obtained based on the Basquin formula. Meanwhile, P-S-N fatigue assessment models were provided under different survival rate conditions. The research in this article has reference value for the service safety assessment of marine equipment structures.

In engineering applications, Very Large Floating Structures (VLFS) need to rely on mooring systems to achieve offshore positioning. However, due to their large length-to-width ratio, VLFS have low overall stiffness and are prone to significant elastic deformation under wave action. Based on the rigid body theory, the impact of such deformation on the mooring system cannot be taken into account, nor can the structural dynamic response of the floating structure in the moored state be easily predicted. To address this, this study established a coupled calculation model of the VLFS and the mooring system based on three-dimensional hydroelasticity theory and slender rod mooring dynamics theory. With this model, the structural response of the floating structure under mooring loads is calculable, and the dynamic response of the mooring system to the combined effects of the floating structure’s rigid body motion and elastic deformation is synchronously computable, thereby realizing the high-precision synchronous prediction of mooring tension and structural response. In this paper, this model is used to analyze the coupling performance of a three-module VLFS, and the floating structure’s load response of typical sections and the mooring line tensions are obtained. The results show that the elastic deformation of the floating structure significantly changes the displacement of the mooring points, leading to an increment in mooring line tension reaching the order of 10⁵ N, with the maximum increment being approximately 40% of its pretension.

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.

The coexistence of waves and broken ice is a prominent feature in marginal ice zones. Ships navigating in these areas are subjected to combined loads from both ice and wave actions. While current research has obtained substantial findings on ship ice resistance characteristics under either single wave or ice conditions, studies on coupled wave-broken ice interactions remain relatively scarce, resulting in insufficient understanding of the ship-ice-wave interaction mechanisms and their impacts on vessel navigation performance. To investigate ice resistance characteristics in broken ice fields under wave effects, this study developed a numerical model by adopting a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) for ship navigation in wave-affected broken ice environments. The numerical wave tank was first established using the Finite Volume Method (FVM), while the broken ice model was developed through the Discrete Element Method (DEM). The validity of the ship resistance model under either single wave or ice conditions was verified through comparisons between theoretical solutions and experimental data. Subsequently, the ice resistance characteristics of ships navigating in wave-affected broken ice fields were systematically analyzed, with particular focus on the influences of wave parameters and ice concentrations. The results demonstrated that wave action significantly alters the ship-ice interaction process, inducing distinct periodic fluctuations in ice resistance. Notably, the ice resistance under combined wave-ice conditions consistently exceeds that observed in still water conditions with equivalent ice concentrations. Furthermore, the influence of key parameters including wavelength, wave height, and ice concentration on ice resistance behavior was also elucidated.

The flexible hose connecting the deep-sea mining vehicle and the relay station is a key link in the deep-sea mining system and is suspended in a curved configuration above the mining vehicle. This paper extends the applicable conditions of the model from the vertical pipeline to the flexible hose based on the one-dimensional model (VHT_1D), by discretizing the long-distance curved pipelines into a series of inclined pipelines for force analysis. The extended model is fully compared with the key parameters calculated by the two-way coupling method of fluid dynamics and discrete elements (CFD-DEM), and this model shows high accuracy. At the same time, the influence of input parameters such as feed concentration and mixture transport velocity on the flow state in the pipeline is further explored. A reasonable range of input parameters is determined, and the conveying efficiency of the pipeline system is optimized. In addition, the hydraulic gradients in the single-peak and double-peak long-distance curved pipes are compared and analyzed, providing a basis for the design of the conveying pump. The extended model significantly improves computational efficiency while maintaining high accuracy, particularly for long-distance pipeline applications.

This paper presents an investigation on the target-guided coordinated control (TACC) of unmanned surface vehicles (USVs). In the scenario of tracking non-cooperative targets, the status information of the target can only be obtained by some USVs. In order to achieve semi-encirclement tracking of non-cooperative targets under maritime security conditions, a fixed-time tracking control method based on dynamic surface control (DSC) is proposed in this paper. Firstly, a novel TACC architecture with decoupled kinematic control law and decoupled kinetic control law was designed to reduce the complexity of control system design. Secondly, the proposed DSC-based target-guided kinematic control law including tracking points pre-allocation strategy and sigmoid artificial potential functions (SigAPFs) can avoid collisions during tracking process and optimize kinematic control output. Finally, a fixed-time TACC system was proposed to achieve fast convergence of kinematic and kinetics errors. The effectiveness of the proposed TACC approach in improving target tracking safety and reducing control output chattering was verified by simulation comparison results.

In this study, an integrated simulation method of floating wind turbines based on wind-wave joint probability model and Modelica coupling simulation framework is constructed for the development of renewable energy in the South China Sea islands and reefs. Taking the island and reef sea area of Sansha City as the research object, the joint probability distribution model of wind and wave is reconstructed by historical meteorological data, and the data of island and reef sea area of Sansha City are established by combining with ocean current data. Based on the Modelica platform, a fully coupled numerical model of aerodynamics-hydrodynamics-control-structure-mooring is developed to study the coupled dynamic response characteristics of Spar-type floating wind turbine in the island reef area of Sansha City. The results show that the peak pitch angle of the platform is 4.74° (<5 ° design threshold) under working conditions. The maximum tension safety factor of mooring system meets the requirements of China Classification Society. Under rated conditions, the turbine speed is stable at 11.98 rpm (rated value 12.1 rpm), and the power output reaches 4.60 MW (rated power 5 MW). The research verifies the engineering applicability of the proposed method system, and provides an innovative technical evaluation framework and scientific decision-making basis for the deployment of floating wind power projects in the South China Sea islands.

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.