Local scour significantly threatens the life-cycle safety of offshore monopiles. While extensive research has been conducted on the bearing capacity of scoured piles, the lateral response mechanisms and their evolution across piles with different stiffness levels (flexible, semi-rigid, and rigid) remain insufficiently understood. This paper systematically investigates the effects of local scour on the lateral response of piles under both serviceability limit state and ultimate limit state, using finite element analysis. The results show that scour depth considerably reduces lateral capacity, with rigid piles being the most susceptible. Scour also shifts the maximum bending moment downward and alters the distribution of soil resistance along the pile. The contributions of different soil reaction components, including lateral soil reaction, distributed moment, base shear, and base moment, are quantified, with their load-sharing ratios varying systematically with pile stiffness. A multi-spring modeling framework integrating all soil reaction components is formulated using hyperbolic tangent functions to capture these stiffness-dependent responses. Validation against experimental and numerical results confirms the model's accuracy in simulating the lateral response of monopiles under scour.
Helical piles are a common foundation type for offshore wind turbines (OWT), widely utilized because of their low construction costs and straightforward design. However, the response of helical piles subject to combined loading is highly complex, and related research is relatively limited. In this paper, the coupled Eulerian-Lagrangian (CEL) analysis is employed to model the ultimate limit states of a typical HELICAL PILES subject to combined loading and quantify its load-bearing capacity. A series of CEL analyses of the helical piles was employed to provide insight into the impacts of the embedment depth ratio, helix diameter ratio, and soil strength ratio on the loading behavior of helical piles subject to combined loading. Based on the parametric study results, a set of closed-form expressions was used to estimate the uniaxial bearing capacity coefficient, V-H, V-M, H-M, and V-H-M failure envelopes of the helical piles were established, and an example problem was presented to illustrate the applicability of the developed design procedure. The findings from this study are highly valuable for understanding the load characteristics of helical piles subject to uniaxial and combined loading, contributing to improving the safety design level of the OWT foundation.
This study presents a comprehensive numerical investigation of the bearing behavior of a novel high-pile cap foundation for offshore wind turbines. The foundation consists of a central monopile surrounded by six inclined piles and is subjected to complex loading conditions. Using Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D) with advanced liner elements and interface modeling, the analysis incorporates realistic geological stratification and load combinations. Firstly, the load response and the bearing characteristics of pile foundation under a single working condition are examined. Then, six distinct working conditions are compared. The results show that horizontal and moment loads dominate the system response. The inclined piles act effectively as tension anchors, significantly reducing shear forces and bending moments in the monopile and improving the overall horizontal resistance. The concrete cap plays a critical role in stabilizing vertical displacements and redistributing moments. The maximum Mises stresses in the steel pile (214.18 MPa) remains well below the material yield strength, confirming the structural adequacy of the designed foundation system. The study provides validated numerical insights and practical design equations for optimizing similar high-pile cap foundations in offshore environments.
Submarine metal mining is associated with a high risk of seawater inrush, making the rational design of crown pillar thickness essential for isolating seawater intrusion. This study develops a theoretical framework for determining the optimal protective layer thickness in submarine crown pillar design under wave loads. An analytical model for seabed dynamic response under wave loading is developed to quantify seabed shear failure depth, and a Winkler-based elastic beam mechanical model is established to calculate the required protective layer thickness. The proposed framework is applied to the Sanshandao submarine gold mine to investigate the effects of extreme wave parameters and seabed geological conditions on seabed stability. The research findings reveal that under extreme wave conditions, the seabed shear failure depth in the Sanshandao mining area reaches 13.7 m, requiring a protective layer thickness range of 10.7-11.6 m and a minimum total crown pillar thickness of 52 m to enable safe extraction of ore bodies below the elevation of -97 m. Comparative analysis revealed that the current 120-m-thick crown pillar design and the -165 m mining elevation in the Sanshandao mining area are overly conservative.
Local scour beneath submarine and piggyback pipeline systems remains a major challenge in offshore and hydraulic engineering due to its potential to induce free-span formation, structural instability, and pipeline failure. Although numerous scour protection techniques have been proposed, the application of polymer-assisted sediment stabilization for mitigating scour around piggyback pipeline configurations has received limited attention. This study experimentally investigates the effectiveness of cationic polyacrylamide (CPAM) treatment in suppressing local scour beneath single and piggyback pipelines installed on non-cohesive sandy beds under steady-current conditions. A series of movable-bed experiments was conducted in a recirculating flume under both clear-water and live-bed regimes (U/Uc = 0.92-2.0)). The influences of relative flow velocity, diameter ratio, and gap ratio on scour development were systematically evaluated for untreated and CPAM-treated sediment beds. The results showed that untreated piggyback configurations generated substantially deeper scour than corresponding single-pipeline cases, particularly at small gap ratios and high flow intensities due to intensified local flow contraction and near-bed sediment entrainment. In contrast, incorporation of 8% CPAM completely suppressed measurable scour for all investigated hydraulic and geometric conditions, including severe live-bed cases after 72 h of continuous flow exposure. Regression and sensitivity analyses identified relative flow velocity as the dominant parameter governing scour development, followed by diameter ratio and gap spacing. An empirical relationship for predicting normalized equilibrium scour depth was also proposed and showed good agreement with the experimental data (R 2 = 0.94). The findings demonstrate the strong potential of CPAM treatment as an effective scour mitigation strategy for submarine and piggyback pipeline systems installed in coarse non-cohesive sediments.
In the northwestern South China Sea (SCS), highly coupled wave-current dynamics create complex sediment transport patterns that challenge marine engineering. This study integrates multi-source datasets of waves, tides, and sediments with transport modeling to analyze sedimentary responses and multiphase transport under monsoon-tide interaction. Results reveal a source-dynamics coupling mechanism: the northern silt zone shows strong positive skewness, suggesting coarse particle retention, while the central-southern sandy transition zone features a high-low-high sorting pattern, indicative of non-steady sorting under variable energy. Bedload transport exhibits a tidal energy hierarchy (spring > mean > neap), with flood-dominant transport in the fine-sand zone (critical shear stress: 0.283 N/m(2)) and episodic spring-tide transport in the coarse-sand zone (>2.2 N/m(2)), demonstrating that the spatial variability of critical shear stress provides a useful indicator of seabed stability and scour sensitivity. Suspended sediment stratifies during spring tides but homogenizes during neap tides. Transport direction shifts from southwestward to southeastward across tidal regimes. A counterclockwise residual circulation drives bidirectional sediment migration, with nearshore suspended loads converging and offshore bedload diverging. Wind-driven currents during neap tides promote offshore sediment export. In the bedload-suspended sediment multiphase transport, suspended sediment transport plays a dominant role and is the primary source of sediment spatial transport in the study area. This study quantitatively reveals multiphase sediment transport mechanisms, advancing understanding of continental shelf sedimentary-morphodynamic processes and seabed stability.
Natural gas hydrate (NGH), a vast, low-carbon, clean energy resource, is significant for national energy security and "dual carbon" strategic goals. However, hydrate-bearing sediments (HBS) feature poor permeability and low productivity, and are prone to sand production during exploitation, restricting commercial development. Water jet (WJ) technology, offering good controllability, operational simplicity, low cost, and environmental friendliness, holds great potential for HBS production enhancement. This paper reviews research progress on applying WJ to NGH development across four aspects: WJ erosion characteristics, high permeability channel construction using WJ, WJ-assisted solid fluidization, and WJ application in trial productions. Specifically, WJ erosion mechanisms and operating condition influences were analyzed. Furthermore, WJ perforation in HBS was simulated, HBS disturbance from WJ slotting was analyzed, and single-well productivity enhancement via WJ slotting and radial WJ drilling was evaluated. WJ breaking tools for solid fluidization were developed. Additionally, WJ adoption in two HBS trial production projects demonstrates its value. This review provides a reference for WJ research and application in HBS exploitation.
