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The ends of deep-sea submarine pipelines are usually connected to wellheads, suspended chain line risers, pipeline end manifolds, etc. , and the boundary conditions at the end of the pipelines are complicated. In order to investigate the influence of different pipeline boundary conditions on pipeline walking behaviour, the boundary conditions at the end of the submarine pipelines were simplified into three types: type Ⅰ is horizontal force at both ends, type Ⅱ is horizontal force at one end and inclined force at the other end, and Type Ⅲ is elastic boundary at one end and horizontal force at the other end; Numerical models of submarine pipeline walking were set up with different boundary conditions, and the behaviour of submarine pipeline walking was comparatively analyzed with the different types of boundary conditions. The results indicate that: compared with type Ⅰ, with the increase of pipeline end tension inclination angle, the pipeline axial tension decreases, and the amount of pipeline walking also decreases in type Ⅱ boundary conditions; With the pipeline tension inclination angle increased from 0° to 60°, the maximum axial tension decreases by 6.73%, and the pipeline walking rate decreases by 50%. This means that the change of the riser shape of the suspension chain line leads to the direction of the tension force acting on the submarine pipeline changes, and the amount of submarine pipeline walking also corresponding change. The comparison between type Ⅲ and type Ⅰ boundary conditions shows that the submarine pipeline walking rate under type Ⅰ boundary conditions is a constant value, with the number of heating and cooling cycles increase, the submarine pipeline walking rate is approximately linear growth; the submarine pipeline walking rate under type Ⅲ boundary conditions is influenced by the pipeline end manifolds elastic stiffness, the greater the elastic stiffness, the greater the walking rate of the pipe line in the first temperature cycle; with the number of heating and cooling cycles increase, the walking rate decreases rapidly until it reaches 0. The amount of pipeline walking under type Ⅲ boundary conditions is usually smaller than that of type Ⅰ. The walking behaviour of submarine pipelines varies greatly under the three types of boundary conditions, which provides some reference for analyzing the types of boundary conditions at the end of submarine pipelines and selecting appropriate boundary condition in actual project.

In order to predict the damage of submarine pipeline on saturated clay seabed by falling objects, a dynamic finite element model of interaction between saturated clay seabed and submarine pipeline under the impact of falling objects was established. The influence of six parameters, namely, energy of falling objects, diameter of the pipeline, wall thickness, grade of the steel material, internal pressure, and undrained shear strength of seabed soil on the damage of the submarine pipeline was analyzed by combining with the range of changes of the actual working conditions of the submarine pipeline. Taking the six parameters as input parameters, the damage of pipeline as output parameters, and the numerical simulation results as training samples, a BP neural network model for submarine pipeline damage prediction was constructed through learning and training. The results show that the larger the impact energy of the falling object is, the larger the pipeline damage is, and the pipeline damage growth rate tends to slow down with the increase in the impact energy of the falling object. The increase in pipeline diameter, wall thickness, internal pressure, and the pipeline yield strength leads to a decrease in the pipeline damage. The saturated soil undrained shear strength increases, while pipe damage increases. The established BP neural network prediction model for submarine pipe damage needs only six parameters, namely, falling object impact energy, pipe diameter, wall thickness, steel grade, internal pressure and undrained shear strength of seabed soil. It is simple and convenient, and can better predict the damage of submarine pipeline impacted by falling objects in saturated clay seabed. The numerical examples cover the working conditions of common saturated clay seabed submarine pipelines, so the model has good applicability, which provides new ideas for the prediction of submarine pipeline damage.

In order to predict the penetration depth of submarine pipeline on sandy seabed,a coupled Eulerian-Lagranginan finite element model of submarine pipeline penetration on sandy seabed was established under ABAQUS to simulate the process of submarine pipeline penetration into sandy seabed under the loads such as wind,wave,current and self-weight.The effect of the friction coefficient of the pipe-soil interface,the internal friction angle,the dilation angle,and the elastic modulus of sand on the penetration depth of the pipeline was studied.The results show that,during the penetration of the submarine pipeline,the seabed on both sides of the pipeline is uplifted,and the seabed soil forms a continuous sliding surface extending from the bottom of the pipeline to the surface of the seabed.The soil failure is in the form of general shear failure.The friction coefficient of the pipe-soil interface,the internal friction angle and the dilation angle of the sand all affect the penetration depth of submarine pipeline on sandy seabed.The internal friction angle of sand has the greatest impact on the penetration depth of the submarine pipeline,while the elastic modulus of sand has no significant effect on the penetration depth of the submarine pipeline.The penetration resistance of the submarine pipeline increases as a power function with the increase of the friction coefficient of the pipe-soil interface,the internal friction angle and the dilatation angle of the sand.The proposed prediction formula can accurately predict the penetration depth of pipelines on the sandy seabed by considering the effects of pipe-soil interaction and sand mechanical properties, and the calculation results provide basic data for in-situ stability evaluation of submarine pipelines on the sandy seabed.

Helical piles are utilized worldwide as foundations for onshore infrastructure, providing fast installation and high capacity. Their unique advantages have led to their potential as an alternative to monopile foundations for offshore wind turbines. One challenge in their service life is dealing with combined loading caused by wind, waves, current, and the weight of the structure itself. While research has focused on helical piles’ capacity for uniaxial tensile loads, there is limited knowledge regarding their performance under combined loads. This study used FEM analysis to investigate the impact of aspects such as helix-to-shaft diameter ratios and helix position on the capacity and failure mechanisms of hollow shaft single-plate helical piles in clay. With 561 analysis cases under both uniaxial and combined loading, failure envelopes were evaluated for various helix-to-shaft diameter ratios and positions. The study revealed a linear positive correlation between helix-to-shaft diameter aspect ratios and load-bearing ability, while the effect of helix positioning on failure envelopes was more complex and nonlinear. The outcomes from this comprehensive analysis enabled the development of a formula to predict the bearing capacity of helical anchors under combined loading.

Helical piles with small sizes are widely used in traditional civil engineering infrastructure application, especially in onshore electrical power industry. The floating offshore wind power industry has taken an interest in helical piles because of their rapid installation and ability to support immediate loading. Compared with onshore structures, wind power structures suffer larger external load and it becomes necessary to increase the overall size of helical piles, leading to considerable load capacity provided by the shaft, which is typically neglected for a light structure. In this paper, the undrained uplift capacity of helical piles was studied using finite-element limit analysis. The analysis explored various diameter ratios of the shaft to the plate and embedment depths, as well as different interface conditions. The local bearing capacity on the plate was also explored for attached and vented interface conditions. The failure mechanisms of a helical pile were presented to explain the mutual influence between plate anchor and shaft on the uplift capacity. Algebraic expressions for the uplift capacity of a helical pile were proposed. Engineers may use these expressions to improve the accuracy of their estimates for the uplift capacity of helical piles.

Helical anchors are widely used in engineering to resist tension, especially during offshore wind energy harvesting, and their uplift behavior in sand is influenced by many factors. Experimental studies are often used to investigate these anchors; however, scale effects are inevitable in 1× g model tests, soil conditions for in situ tests are challenging to control, and centrifuge tests are expensive and rare. To make full use of the limited valid data and to gain more knowledge about the uplift behaviors of helical anchors in sand, a prediction model integrating gradient-boosting decision trees (GBDT) and particle swarm optimization (PSO) was proposed in this study. Data obtained from a series of centrifuge tests formed the dataset of the prediction model. The relative density of soil, embedment ratio, helix spacing ratio, and the number of helices were used as input parameters, while the anchor mobilization distance and the ultimate monotonic uplift resistance were set as output parameters. A GBDT algorithm was used to construct the model, and a PSO algorithm was used for hyperparameter tuning. The results show that the optimal GBDT model accurately predicted the anchor mobilization distance and the ultimate monotonic uplift resistance of helical anchors in dense fine silica sand. By analyzing the relative importance of influencing variables, the embedment ratio was found to be the most significant variable in the model, while the relative density of the fine silica sand soil, the helix spacing ratio, and the number of helices had relatively minor influence. In particular, the helix spacing ratio was found to have no influence on the capacity of adjacent helices when S/D > 6.

Suction bucket foundation is a novel and cheaper foundation used in marine structures, such as offshore wind turbines, breakwater and oil platforms. Compared with a single bucket foundation, tetrapod bucket foundations can bear larger loads because of the group effects. However, the vertical, horizontal and moment capacity factors of tetrapod bucket foundations have not been presented in existing specifications. A series of three-dimensional finite-element analyses were conducted to investigate the group effects on uniaxial capacities and failure mechanisms of tetrapod bucket foundations in undrained clay considering various foundation separation distance ratios, embedment depth ratios, soil-strength heterogeneity indices and load direction angles. Generalized formulas for undrained uniaxial capacities of tetrapod bucket foundations were proposed in order to establish a bridge connecting the capacities of tetrapod bucket foundations and those of the single bucket foundation, which can provide a reference for industrial designs of capacities of tetrapod bucket foundations. The results show that the vertical group effect factor of tetrapod bucket foundations is basically not affected by the foundation separation distance ratio, embedment depth ratio, soil-strength heterogeneity index and load direction angle, which can adopted 0.9 based on a conservative estimation. The normalized horizontal and moment group effect factors of tetrapod bucket foundations are both affected by the separation distance ratio, embedment depth ratio and soil-strength heterogeneity index, but the moment group effect factor is also obviously affected by the load direction angle. The value of the horizontal and moment capacity factors of tetrapod bucket foundations are about 2.3 and 13.8 times that of a single bucket foundation, respectively, when the separation distance ratio is 3.5, embedment depth ratio is 1.0 and soil-strength heterogeneity index is 10, which have both been significantly enhanced. A value of 3.5 is suggested for the separation distance ratio to attain good capacities and a relatively high global stiffness for the tetrapod bucket foundations.

Subjected to continuous wave loading, the responses of pile foundations and seabed develop gradually, severely affecting the serviceability of piled structures. This paper presents the results of a series of flume experiments on pile foundations in fine sandy and silty seabed under regular wave loading. Pile-head displacement and pore water pressure were measured and the effects of pile diameter, cross-section, pile stiffness and wave height were investigated. The experimental results indicate that the pore pressure in fine sandy seabed varied only slightly even under 640 s of wave loading but showed an increase of 15.7–25.9% around a pile. In silty seabed with much lower permeability, pore pressure accumulated quickly due to piles and oscillated impressively at the depth of soil liquefaction. Based on the comparison between the calculated and measured pile-head displacement, we found that the response of smaller-diameter piles in lower-permeability seabed was much more easily magnified by the induced pore pressure. Increasing the pile diameter and attaching fins could lead to a smaller response of piles. Wave height was a major factor in the experiments that affected the development of response.

To predict the bearing capacity of plate anchors subjected to combined loads, based on the assumption that there is no separation between the plate and soil, the numerical model of the motion and transformation of the plate anchor subjected to combined loads including the normal force, tangential force and bending moment loads in ABAQUS software is set up. The numerical model is proved by comparing the calculated bearing capacity with that of analytical solution. The numerical model is used to calculate the ultimate bearing capacity of the plate anchor subjected to the combined loads including the normal force, the tangential force and bending moment. The results show that murff model is used to preferably fit the ultimate bearing capacity envelope of plate anchors subjected to combined loads.

Pipelines are the critical link between major offshore oil and gas developments and the mainland. Any inadequate on-bottom stability design could result in disruption and failure, having a devastating impact on the economy and environment. Predicting the stability behavior of offshore pipelines in hurricanes is therefore vital to the assessment of both new design and existing assets. The Gulf of Mexico has a very dense network of pipeline systems constructed on the seabed. During the last two decades, the Gulf of Mexico has experienced a series of strong hurricanes, which have destroyed, disrupted and destabilized many pipelines. This paper first reviews some of these engineering cases. Following that, three case studies are retrospectively simulated using an in-house developed program. The study utilizes the offshore pipeline and hurricane details to conduct a Dynamic Lateral Stability analysis, with the results providing evidence as to the accuracy of the modeling techniques developed.

The helical piles have been being treated as a kind of novel foundation for offshore wind turbines recently, due their fast installation, high uplift capacity, convenience for recycling, and other advantages. The recycling of the helical pile especially will reduce the cost significantly and protect the environment as much as possible. However, the research for this area is basically in infancy and there is no reference for predicting the recycling torque of a helical pile in sand. In order to predict the recycling torque of single-plate helical piles in dense sand: a theoretical model, which was inspired by the way to predict the installation torque of single-plate helical pile in sand, was developed, and a series of single gravity model tests were conducted to verify that theoretical model. The theoretical model can predict the recycling torque of single-plate helical pile considering the influences of the size of helix and the vertical force on the shaft. This model fills in the blank of predicting the recycling torque of a single-plate helical pile in sand and it is also useful guidance for the choice of suitable recycling equipment.