A new look at the phenomenon of offshore pile plugging

Abstract

Open‐pipe piles are widely used for offshore structures. During the initial stage of installation, soil enters the pile at a rate equal to the pile penetration. As penetration continues, the inner soil cylinder may develop sufficient frictional resistance to prevent further soil intrusion, causing the pile to become plugged. The open‐ended pile then assumes the penetration characteristics of a closed‐ended pile. The mode of pile penetration significantly alters the soil‐pile interaction during and after installation. This affects the ultimate static bearing capacity (mainly in granular materials), the time‐dependent pile capacity (in clays), and the dynamic behavior and analysis of the piles.

Following a summary demonstrating the effects of pile plugging, a review of the common view of offshore pile plugging is undertaken. The interpretation of plugging by referring to the average plug length has led to the erroneous conclusion that in most piles significant plugging action does not occur.

Establishment of an analogy between soil samplers and open‐ended piles enabled correct identification of plugging by referring to the incremental changes in plug length. Examination of case histories of plugging of offshore piles revealed that beyond a certain penetration depth‐to‐diameter ratio, most piles are plugged.     

Self-expanded piles: A new approach to unconventional piles development

Abstract

In onshore and offshore fields of ocean engineering, piles are used as foundation systems for various structures. Piles are classified into different types depending on their materials, geometries, and particularly, installation methods, which have advantages or limitations. Companies and engineers have developed a new group of piles, because of necessity to improve their performance in terms of increasing the bearing capacity, reducing impacts of traditional installation procedures, implementing by low- torque power equipment, and utilizing them in widely different ground conditions, including in a marine environment. In the present study, three different models of a new pile with an expander body are introduced to increase the shaft and pile-toe diameters and its self-expansion in the embedment depth under the titles of the Bubble pile (BP), Self-Expanded pile (SEP) and Wing pile (WP). The main subject of this research is to achieve increased bearing capacity, reduced installation effects, and decreased required installation torque. The frustum-confining vessel of Amirkabir University of Technology (FCV-AUT) was employed for this purpose. Up to 14 axial compressive and tensile load tests were carried out on different model piles on sand collected from Anzali shore located on the northern coast of Caspian Sea in Iran, with relative densities of 45% to 50% within FCV-AUT. Comparing the performance of introduced pile with traditional pile corresponding to the same characteristics, the results indicated a significant increase in the axial bearing capacity and reduced disturbance effect of the pile. Also, a lower installation torque of the SE pile was required compared to the helical pile. The test results also demonstrated that the new pile could resist considerable compressive and uplift loads, and could be a possible alternative to traditional piles in the onshore sector.     

一种用于海上打圆桩过程监测的新型测量方法

针对打桩定位的测量方法,探讨了一种用于海上打圆桩过程监测的新型测量方法,使用全站仪无棱镜模式对圆桩横切面进行数据采集,应用最小二乘原理迭代拟合横切面,得到拟合圆心的坐标及圆半径,进而解算桩顶位置和高程、桩倾斜度及多桩控制,实现打桩过程动态监测。经过某项目打桩案例实践,验证了该测量方法的可行性和正确性。     

A Model Test on the Behavior of a Static Drill Rooted Nodular Pile Under Compression

A static drill rooted nodular pile is a new type of composite pile foundation with high bearing capacity, and mud emissions can be largely reduced using the static drill rooted method. This report presents a model test on the behavior of this composite pile in a test box. The load-displacement response, axial force, skin friction, and mobilized base load are discussed in the report; in particular, the force in the cemented soil was investigated based on the measured data. Moreover, the finite element software ABAQUS was used to help investigate this behavior more thoroughly. It was determined that the function of the cemented soil around the pile shaft was different from that at the enlarged pile base; the stress in the cemented soil around the shaft increased suddenly when nearing the pile base; the ultimate skin friction obtained in the model test was larger than that estimated in the field test; and the relative displacement between the precast nodular pile and the cemented soil could be ignored during the loading process, which corresponded to the result of the field test and demonstrated that the nodular pile and cemented soil act as one entity during the loading process.     

A Numerical Investigation of the Reduction of Solitary Wave Runup by A Row of Vertical Slotted Piles

To improve the current understanding of the reduction of tsunami-like solitary wave runup by the pile breakwater on a sloping beach, we developed a 3D numerical wave tank based on the CFD tool OpenFOAM in this study. The Navier Stokes equations were applied to solve the two-phase incompressible flow, combined with an LES model to solve the turbulence and a VOF method to capture the free surface. The adopted model was firstly validated with existing empirical formulas for solitary wave runup on the slope without the pile structure. It is then validated using our new laboratory observations of the free surface elevation, the velocity and the pressure around a row of vertical slotted piles subjected to solitary waves, as well as the wave runup on the slope behind the piles. Subsequently, a set of numerical simulations were implemented to analyze the wave reflection, the wave transmission, and the shoreline runup with various offshore wave heights, offshore water depths, adjacent pile spaces and beach slopes. Finally, an improved empirical equation accounting for the maximum wave runup on the slope was proposed by taking the presence of the pile breakwater into consideration.     

Scour effects in piled structures—A sensitivity analysis

A sensitivity analysis is presented in this paper to judge the importance of several parameters influencing scour effects on piles of offshore foundations. The decrease of the pile reliability due to scour is also studied by applying the first-order reliability method. Suggestions for lateral pile resistance in design are included.     

海洋平台隔水套管群桩性状的研究

海洋平台的隔水套管群桩与土共同作用研究是一个很复杂的课题,目前国内外研究资料甚少,因此在平台导管架设计中,一般不考虑隔水套管群桩承受水平力作用,这与实际不相符合。本文结合工程课题,在调查研究和模型试验的基础上,对隔水套管群桩在水平力作用下的工作性状与破坏机理、群桩的水平力及其主要影响因素、单桩与群桩情况下应力应变关系等方面进行较深入的研究;并提出砂土地基隔水套管群桩效应经验公式,弥补了现行计算方法的缺陷和不足。研究成果可供工程设计参考使用。     

A wave-flume study of scour at a pile breakwater: Solitary waves

Understanding the sediment transport and the resulting scour around coastal structures such as pile breakwaters under local extreme wave conditions is important for the foundation safety of various coastal structures. This study reports a wave-flume experiment investigating the scour induced by solitary waves at a pile breakwater, which consists of a row of closely spaced large piles. A wave blacking gate with a simple operation procedure in the experiment was designed to eliminate possible multiple reflections of the solitary wave inside the flume. An underwater laser scanner and a point probe were used in combination to provide high-resolution data of the bed profile around the pile breakwater. Effects of incident wave height and local water depth on the maximum scour depth, the maximum deposition height and the total scour and deposition volumes were examined. An existing empirical formula describing the evolution of the scour at a single pile in current or waves was extended to describe the scour at the pile breakwater under the action of multiple solitary waves, and new empirical coefficients were obtained by fitting the formula to the new experimental data to estimate the equilibrium scour depth. It appears that the maximum scour depth and the total scour volume are two reliable quantities for validation of numerical models developed for the scour around pile breakwaters under highly nonlinear wave conditions.     

Hydroelastic Investigation on A Pile Breakwater Integrated with A Flexible Tail for Long-Wave Attenuation

A novel concept of wave attenuator is proposed for the defense of long waves, through integrating a flexible tail to the lee-side surface of a pile breakwater. The flexible tail works as a floating blanket made up of hinged blocks, whose scale and stiffness can be easily adjusted. A two-phase-flow numerical model is established based on the open-source computational fluid dynamics (CFD) code OpenFOAM to investigate its wave attenuation performance. Incompressible Navier—Stokes equations are solved in the fluid domain, where an additional computational solid mechanics (CSM) solver is embedded to describe the elastic deformation of the floating tail. The coupling of fluid dynamics and structural mechanics is solved in a full manner to allow assess of wave variation along the deforming body. The accuracy of the numerical model is validated through comparison with experimental data. Effects of the flexible tail on performance of the pile breakwater are investigated systematically. Dynamic behaviours of the tail are examined, and characteristics of its natural frequency are identified. For safety reasons, the wave loads impacting on the main body of the pile breakwater and the stress distribution over the tail are specially examined. It is found that both the length and stiffness of the tail can affect the wave-attenuation performance of the breakwater. A proper choice of the length and stiffness of the tail can greatly improve the long-wave defending capability of the pile breakwater. The maximum stress over the flexible tail can be restrained through optimising the deformation and stiffness of the tail.

     

A simplified instrumentation for measuring scour in silty clay around a vertical pile

The obstruction to flow around a pile placed in an erodible seabed causes scour leading to changes in the bed elevation in the vicinity of the pile. In the present investigation, scour around piles induced by the seabed current has been studied in a wave flume using model piles of diameters 50, 90 and 110 mm embedded in a silty clay soil bed. The particulate movement due to scour is time dependent and in case of sediments with particle sizes ranging upto clay fractions, the measurement of scour becomes extremely difficult. This paper presents a simplified experimental technique for the measurement of scour depth with time around a pile foundation in a silty clay soil.     

A simplified calculation method for axial cyclic degradation of offshore wind turbine foundations in clay

Abstract

Pile foundation is the most popular option for the foundation of offshore wind turbines. The degradation of stiffness and bearing capacity of pile foundation induced by cyclic loading will be harmful for structure safety. In this article, a modified undrained elastic–plastic model considering the cyclic degradation of clay soil is proposed, and a simplified calculation method (SCM) based on shear displacement method is presented to calculate the axial degradated capacity of a single pile foundation for offshore wind turbines resisting cyclic loadings. The conception of plastic zone thickness R_p is introduced to obtain the function between accumulated plastic strain and displacement of soil around pile side. The axial ultimate capacity of single piles under axial cyclic loading calculated by this simplified analysis have a good consistency with the results from the finite element analysis, which verifies the accuracy and reliability of this method. As an instance, the behavior of pile foundation of an offshore wind farm under cyclic load is studied using the proposed numerical method and SCM. This simplified method may provide valuable reference for engineering design.     

桩靴贯入黏土层时邻近群桩相互作用分析

为研究钻井船插桩对邻近平台群桩相互作用的影响,采用耦合欧拉拉格朗日(CEL)方法对桩靴贯入黏土层时邻近群桩中各桩荷载分担比、桩头附加位移及两桩相互作用系数进行了分析。首先通过对缩尺模型试验的数值分析,验证了CEL方法的可行性;然后进一步分析了桩靴贯入黏土层时对邻近群桩相互作用的影响;最后探讨了净间距、桩间距对群桩相互作用的影响。结果表明,在桩靴贯入中,前桩的荷载分担比大于后桩,且桩靴贯入至一定深度后,当净间距越小或桩间距越大时,前桩的荷载分担比越大、后桩的荷载分担比越小,但各桩的荷载分担比随桩靴贯入深度增加时的变化规律不变;净间距越大,桩头附加位移及相互作用系数越小;在桩靴贯入时,由于受群桩遮拦效应的影响,桩头附加位移及相互作用系数随桩间距的变化规律同插桩前有所不同,当桩间距大于3倍桩径时,随桩间距的增加而减小,当桩间距小于3倍桩径时,随桩间距的增加而增大。     

潮间带钢管斜桩吊打导向装置与沉桩参数研究

钢管斜桩是潮间带风电的常用基础形式,常采用吊打沉桩的方式。文章针对潮间带吊打沉桩方式的难点开发吊打导向装置,并总结沉桩参数的确定方法。该导向装置包括定位桩、定位架和调向限位架,总体平面布局为六边形,可一次定位并完成整个风电基础的基桩施工,既能可靠保证桩的施工精度,又能极大地提高施工效率和降低安全风险;在应用该施工关键装备时,采用GRLWEAP打桩分析功能确定桩顶动荷载,采用有限元软件建立导向装置有限元模型,对导向装置和沉桩参数进行分析,从而实现斜桩顺利沉桩的目的。该装置和方法已成功应用于越南朔庄一期海上风电场项目,为潮间带风电钢管斜桩吊打施工提供沉桩装备和方法经验。     

Experimental Evaluation of Recycled Aggregate Porous Concrete Piles for Soft Ground Improvement

The use of sand compaction pile or gravel compaction pile is nowadays a common approach for soft ground improvement. In this article, a recycled aggregate porous concrete pile has been developed by replacing natural aggregates with recycled aggregates to overcome issues related to bulging failure or reduced section geometries. Such issues may arise during installation and during the early stages of operation. In addition, the proposed approach utilizes recycled aggregates instead of natural materials. To investigate the applicability of the recycled aggregate porous concrete pile method as a ground improvement technique, a series of laboratory model consolidation tests was performed on soft clay soil reinforced with sand compaction pile, gravel compaction pile, and recycled aggregate porous concrete pile, respectively. The results indicated that the settlement reduction effect of recycled aggregate porous concrete pile was significantly higher than the sand compaction pile and gravel compaction pile methods. The stress sharing ratio from the experimental program showed good agreement with those calculated by elasticity theory. Comparative analyses of the recycled aggregate porous concrete pile versus sand compaction pile and gravel compaction pile approaches, under the same replacement area ratio and surcharge pressure, showed significantly improved consolidation time, settlement reduction, and stress sharing effect.     

Longitudinal dynamic impedance of a static drill rooted nodular pile embedded in layered soil

The static drill rooted nodular (SDRN) pile is a new type of precast pipe pile with equally spaced nodes distributed along the shaft and wrapped by the surrounding cemented soil. In this paper, the longitudinal dynamic response of the SDRN pile embedded in layered soil is investigated with respect to the complexity of the pile body structure and the pile–soil contact condition. First, the shear complex stiffness transfer model is used to simulate the radial inhomogeneity of the surrounding soil. Then, the governing Equations of the pile–soil system subjected to longitudinal dynamic loading are established. The analytical solution for the dynamic response at the pile head is obtained by the shear complex stiffness transfer method and the impedance function transfer method. The degenerate case of the present solution is compared with the published solution to verify its reliability, and the complex impedance of the SDRN pile is compared with that of the precast pipe pile and the bored pile. Finally, a parametric study is conducted to investigate the influence of pile–soil parameters on the complex impedance at the pile head within the low frequency range concerned in the design of the dynamic foundation.     

Experimental Study on the Interaction Mechanism of Cap—Pile Group—Soil

Static load tests on pile group with prototype size were carried out in order to study the behavior and the working properties of the cap—pile group—soil interaction in the pile group foundation. The soil resistance under the cap, the pile shaft resistance and the tip resistance were measured by installing various measuring gauges. Based on these test results, the cap—pile group—soil interaction characteristics were analyzed. The regulations of the soil reaction on the cap, the shaft resistance and the tip resistance of pile, the mechanism of load transfer have been discussed with comparison to the result of the single pile tests. The bearing capacity of pile group is greater than the sum of the bearing capacity of the single pile obtained from testing in the same site in pile group foundation in the case presented here.     

海底防沉板—桩复合基础承载特性数值分析

为了研究桩长对海底防沉板—桩复合基础在水平、弯矩和扭转荷载作用下承载特性的影响,以我国南海水深200 m的某工程实例为研究对象,利用Flac3D有限差分仿真软件建立了计算模型。研究了桩长为4 m、6 m和8 m的防沉板—桩复合基础在水平、弯矩和扭转荷载作用下的极限承载力和荷载传递机理。结果表明,桩长超过6 m时复合基础的水平承载力显著增长,在水平加载过程中,防沉板总是先达到极限状态而破坏,桩基础的贡献在加载后期体现,且桩长为4 m、8 m时,桩基础与防沉板的连接处弯矩最大;随着桩长的增加,复合基础的抗弯承载力大幅提高,桩基础对复合基础的抗弯承载力贡献增大,当桩长超过8 m,桩长的增加对提高复合基础的抗弯承载力意义不大;在弯矩加载过程中,桩长对于防沉板、桩基础的荷载分配有显著影响,桩长为4 m时,外荷载主要由防沉板承担,当桩长超过4 m时,外荷载主要由桩基础承担;当扭转荷载不超过2 100 k N·m时,防沉板承担主要荷载,直至防沉板达到极限状态而发生旋转,随后桩基础的承载力逐渐发挥;对于桩长为6 m、8 m的复合基础,其极限状态根据防沉板适用性准则确定。     

Vertical dynamic impedance of pile considering the dynamic stress diffusion effect of pile end soil

A new analytical model is presented to analyze the dynamic stress diffusion effect of pile end soil on the vertical dynamic impedance of the pile. The surrounding soil of the pile is modeled by using the plane strain model and the pile is simulated by using one-dimensional elastic theory. Finite soil layers below the pile end are modeled as conical fictitious soil pile with stress diffusion angle which reflects the dynamic stress diffusion effect of pile end soil. By means of the Laplace transform and impedance function transfer method, the analytical solution of the vertical dynamic impedance at the pile head in frequency domain is yielded. Then, a comparison with other models is performed to verify the conical fictitious soil pile model. Finally, based on the proposed solution, the selected numerical results are compared to analyze the influence of dynamic stress diffusion effect for different design parameters of the soil-pile system on the vertical dynamic impedance at the pile head.     

波流耦合下桩周珊瑚砂冲刷机理研究

针对我国南海某岛礁珊瑚砂地基上的圆形桩基础,采用N-S方程k-ε模型、双向耦合方式跟踪流场中颗粒运动轨迹的方法,对桩周珊瑚砂的冲刷规律进行了求解,分析了桩体周围流体的速度场以及桩体表面剪应力场的分布规律,同时对桩周珊瑚砂冲刷坑的形成过程进行了模拟。计算结果表明,在桩体周围形成的马蹄形漩涡和桩柱后方的尾涡作用下,桩周土体出现了较为明显的冲刷现象,涡旋的释放显著地影响着珊瑚砂地基上桩基的冲刷坑形状;而且,由于珊瑚砂颗粒密度较石英砂小,水动力作用下桩周冲刷坑更容易形成,所以实际工程中需要考虑有效的防护措施。