To elucidate the influence of initial water content on the undrained cyclic shear behaviour of marine soft clay, reconstituted specimens with initial water contents of 1.4wL, 1.8wL and 2.2wL were prepared using the slurry-consolidation method, and a series of undrained monotonic triaxial compression tests and cyclic triaxial tests were performed. The results show that, as the initial water content increases, the water content and void ratio of the consolidated specimens increase, whereas the monotonic and cyclic shear strengths decrease. During cyclic loading, excess pore-water pressure accumulates more rapidly, the effective mean principal stress decreases more markedly, and the double-amplitude axial strain enters the rapid-growth stage earlier. When the cyclic stress amplitude is normalised by the corresponding monotonic shear strength, i.e., qcyc/qf, the cyclic strength data for specimens prepared at different initial water contents converge to a unified curve. This indicates that the influence of initial water content on cyclic strength can be characterised through the corresponding monotonic shear strength under the tested effective confining pressure and loading conditions. In addition, analysis of the evolution of the cyclic secant modulus (E) using a stiffness degradation model shows that the initial water content mainly affects the shape of the E curve.
Safety issues during design and installation of flexible offshore pipelines should be addressed. However, existing models neglecting axial-bending coupling or treating bending stiffness locally may over predict the stress of pipeline, leading to overdesign and resource waste. To address these issues, a nonlinear finite element model that accounts for the axial-bending coupling effect is proposed, and is utilized to examine the mechanical behaviors of flexible offshore pipelines during laying process. A systematic comparison is conducted between the proposed model and the classical catenary and boundary layer models to demonstrate the necessity of globally incorporating bending stiffness and axial-bending coupling, and its effectiveness and correctness is further validated against OrcaFlex software. Parametric studies examine the effects of seabed strength, vessel tension, water depth, and current velocity and direction. The results indicate that increasing water depth raises free span and damage risk of pipeline. Higher soil strength increases stress in the touchdown zone. Appropriate vessel tension reduces overall stress of pipeline. Ocean current loads impair pipeline safety more severely under upstream conditions than under downstream conditions. Changes in current direction induce simultaneous axial and lateral forces. This investigation provides a valuable guidance and theoretical basis for offshore pipeline design and installation.
Submarine stratum instability poses a potential threat to subsea tunnels and other marine infrastructure in geologically complex areas. This study develops an interpretable data-driven framework for assessing submarine stratum instability in the Miaodao Islands region of the Bohai Strait, China. A database of 120 borehole-derived and laboratory-tested submarine-soil samples was established using ten geological, physical, and mechanical indicators. GA-optimized K-means clustering was employed to extract instability-risk grades from the mechanical parameters and establish a four-level grading system. Based on the derived labels, BP, SVM, ELM, and PSO-ELM models were developed and compared. Their testing accuracies were 0.444, 0.500, 0.444, and 0.639, respectively, with PSO-ELM achieving the best overall performance and a macro-averaged F1-score of 0.641. SHAP analysis showed that submarine-stratum instability is jointly controlled by geological structure, burial conditions, physical state, and mechanical properties. In the PSO-ELM model, void ratio, depth, and elastic modulus contributed approximately 23.5%, 19.0%, and 18.7%, respectively, while the two fault-related variables together contributed approximately 26%. These results highlight the importance of fault-influenced, deeply buried, loose, and low-stiffness strata in subsea engineering. The proposed framework integrates data-driven risk-grade extraction, nonlinear prediction, and interpretable analysis, providing a quantitative basis for submarine-stratum assessment and risk-informed decision-making.
To address dynamic hazards in unlined subsea tunnels within fault fracture zones under high static loads and intense blasting impacts, this study investigates their nonlinear response and damage evolution. Taking the Qingdao Jiaozhou Bay Tunnel as a case study, we employed a self-developed shock simulation system for 1:60 scaled physical model tests and numerical simulations. Multi-level impact loads (0.2-0.8 MPa) were applied under a 0.1 MPa hydrostatic confining pressure. Results reveal that wave impedance mismatch in fault zones causes energy trapping and stress concentration, while stress wave phase inversion at the free surface induces shallow tensile failure. Damage evolution exhibits four stages: microcrack initiation, thin-layer spalling, fault-zone block detachment, and full-ring crushing, with 0.6 MPa identified as the critical threshold for global instability. Numerical simulations confirm severe stress concentrations and incompatible deformations at the arch waist and fault contacts, with damage propagating deeply along fault interfaces. Consequently, targeted reinforcement of the arch crown and fault "wedge zone", primary support toughening, and restricting peak blasting stress below 0.6 MPa (actual stress of 36 MPa in the engineering prototype) are recommended. This provides a robust theoretical foundation for blast-resistant subsea tunnel design.
To address the limitations of single suction anchors in deep-water environments, this study investigates mooring-loaded suction anchor groups as an innovative anchoring solution for high-power Floating Offshore Wind Turbines (FOWTs). The uplift capacity and failure mechanisms of suction anchor groups were systematically evaluated via 1 g physical model tests and 3D Finite Element Method (FEM) simulations. Results reveal a fundamental transition in the pullout resistance-displacement curves: unlike the strain-hardening behavior of single anchors, anchor groups exhibit pronounced strain-softening characteristics owing to the increased overall stiffness of the soil-anchor system. The bearing performance of the anchor group is further influenced by loading angle, while the effect of mooring-point position depends on the loading condition. With increasing loading angle, the dominant resistance mechanism gradually shifts from passive soil resistance to skirt friction-controlled vertical uplift. The sensitivity of bearing capacity to mooring position is governed by the dominant failure mechanism and decreases with increasing loading angle. At larger loading angles, the influence of mooring position becomes limited due to the transition toward a vertical uplift mode. These findings are expected to provide valuable guidance for optimizing the design of suction anchor group mooring systems under complex loading conditions.
