变截面劲性水泥土桩承载特性室内模型试验研究

研究变截面劲性水泥土桩的几何特征对承载特性的影响,结果表明:具有1个扩大盘或2个扩大盘间距较大的变截面桩,盘下部的土体发生压缩和局部剪切破坏现象,上部的土体则发生梨形滑落;盘间距较小时,上下两盘之间的土体与两盘成为一体;变截面桩的桩侧荷载分担值均远大于桩端荷载分担值,盘的数量及间距对桩侧及桩端荷载分担值影响不大;1个盘时,其位置对承载力有一定的影响;2个等间距盘的变截面桩,盘位置越高承载力越高;盘间距对承载力影响不显著;3个盘的承载力大于2个盘的承载力,但结果相差不大;变截面桩的承载力得到显著提高,其承载力不小于与扩大盘直径相等的等截面桩;随着桩顶荷载的增大,盘承担的荷载增加显著,盘以下桩身的轴力因盘承担大部分而骤减,其降低幅度与盘的数量、位置及间距有关.     

An experimental facility for testing loaded structures in a high hydrostatic pressure environment and an application to axially loaded columns

An experimental facility is described for the testing of structures under a combined axial load and hydrostatic pressure load. This facility was used to determine the effects of hydrostatic pressure (up to 6000 psi) on the load-carrying capacity of slender metal columns. There appeared to be no significant effect in this case.     

Performance of small group of geosynthetic-reinforced granular piles

Granular piles are frequently used as a method of improving soft grounds as they provide increased bearing capacity and reduce foundation settlements. However, in very soft clayey soils, they may not derive their load-carrying capacity by low confining pressure provided by the surrounding soil. In such circumstances, granular piles may be reinforced with suitable geosynthetic to increase its load-carrying capacity and to reduce excessive bulging. In this study, the performance of small group of geosynthetic-reinforced granular piles (GRGPs) is examined in terms of load-carrying capacity, settlement, and modulus by laboratory model tests. The parameters investigated include modulus of reinforcement material, area replacement ratio (ARR) based on the column diameter and reinforcement length. The results indicated that increasing the modulus of the reinforcement and the ARR based on the column diameter enhances the overall performance of the GRGP group. It was also observed that reinforcement on top portion of the granular pile is sufficient to substantially increase the load-carrying capacity of granular pile group.     

Ultimate Strength of Ship Plating under Axial Compression

The failure of a ship hull girder is governed by buckling and plastic collapse of the deck, bottom and side shell steel stiffened plates. The stiffened steel plating in ships is generally subjected to both in-plane and out-of-plane loading and is more important to understand the characteristics of these panels under buckling. Tests are reported on the collapse load of stiffened plates with and without cutout and with reinforced cutout under uniaxial compression. A generalized computer program for the semi-analytical solutions proposed by various investigators based on strut approach and orthotropic plate approach, and a finite element analysis program based on orthotropic plate approach are developed. The panels are also analysed using the finite element analysis software ANSYS. An approximate method based on strut approach is proposed to calculate the collapse load of stiffened plates with cutouts and initial imperfections. The reduction in strength of the panels due to the presence of square cutout, rectangular cutout and increase in strength due to reinforcement around rectangular cutout are calculated based on the test results. Comparisons are made between the test results and predictions based on semi-analytical solutions and finite element analyses, and the uncertainty parameters calculated are discussed. Based on this study it is concluded that the cutout can be reinforced with a maximum increase in strength up to 19% for plate initiated failures.     

海上风电大直径加翼桩水平承载性能研究

为改善海上风电大直径钢管桩的水平承载性能,基于ABAQUS有限元软件对单桩改进形式的加翼桩结构进行了系统研究,计算分析了软黏土地基中加翼桩在水平荷载作用下桩身弯矩、应力、位移、桩身泥面处倾斜率和极限承载力,研究了加翼桩面积、形状、埋深和刚度等翼板参数对加翼桩水平承载性能的影响规律,根据加翼桩的桩-土作用机理,参考现行规范模式提出适用于软黏土地基大直径钢管桩的P-Y曲线。研究结果表明,加翼桩通过在泥面处设置翼板可降低桩基泥面处倾斜率50%、提高桩基极限承载力60%以上,加翼桩水平承载性能明显优于单桩。     

Dynamic Performance Analysis of the Tuned Heave Plate System for Semi-Submersible Platform

Logistical supply is costly for the deepwater oil and gas exploitation, thereby it is necessary to develop a novel power supply solution to improve the offshore structure’s self-holding capacity. The two-body point absorbers, as a renewable energy device, have achieved a rapid development. Heave plate is used to constrain the truss’s motion in the two-body point absorber, and the floater moves along the truss up and down. This two-body point absorber can be considered to be an essentially mass-spring-damper system. And it is well known that the heave plates have been widely used in the Spar platform to suppress the heave motions. So if the two-body point absorber can be modified to combine with offshore floating structures, this system can not only offer electric power to support operations or daily lives for the platform, but also control the large motions in the vertical plane. Following this concept, a novel tuned heave plate (THP) system is proposed for the conventional semi-submersible platform. In order to investigate the dynamic performances of the single THP, two experiments are conducted in this paper. First, the hydrodynamic coefficients of the heave plates are studied, and then the THP experiments are carried out to analyze its dynamic performance. It can be concluded that this THP is feasible and achieves the design objective.     

Local joint flexibility of CHS X-joints reinforced with collar plates in jacket structures subjected to axial load

This paper aims to investigate the local joint flexibility (LJF) of tubular X-joints with collar plates subjected to axial load. A total number of 136 FE models were generated and analyzed to investigate the effect of collar plate size and joint geometry (η, τ_c, β, γ, and τ) on the local joint flexibility factor (f_LJF). Also, the weld profiles and the contact between the chord wall and the collar plates were modeled in the numerical simulations. Results showed that the increase of the collar plate thickness and length results in the decrease of the f_LJF. Despite the difference between the f_LJF of unreinforced and collar plate reinforced X-joints, there has not been any study on the f_LJF of the X-joints reinforced with collar plate. Also, there is no design equation for determining the f_LJF of the collar plate reinforced X-joints. Thus, the results of the parametric study were used to develop a design formula, to determine the f_LJF of X-joints reinforced with collar plates under axial load.     

Numerical analysis of suction embedded plate anchors in structured clay

As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.     

Evaluation of the uplift behavior of plate anchor in structured marine clay

The uplift behavior of a plate anchor in a structured clay (soft Ariake clay) is investigated through a series of laboratory tests and method of finite element analysis. The tests are adopted to identify the factors influencing the behavior of the anchor, including the thixotropic nature of Ariake clay, consolidation time, and embedment ratio of the anchor. A finite element method (FEM) is used to analyze and predict the uplift behavior of the anchor plate well in the elastic region and the yield load. The results from both the laboratory tests and the FEM analysis suggest that the embedment ratio for a deep anchor in Ariake clay is close to 4. Further increase in embedment ratio improves the capacity to a lesser extent. FEM overestimates the failure load of the uplift anchor in soft Ariake clay by about 20%. This may be ascribed to the hypothesis in the FEM analysis that there is continuous contact between the clay and the anchor until failure. Vesic’s theory for deep anchors, which may be used to predict the ultimate pullout resistance of the plate anchor in reconstituted Ariake clay, is verified to be applicable. In this paper, the plastic flow zone around the anchor is discussed using FEM which makes the behavior of anchor more understandable during the design stage.     

Behavior of a helical anchor under vertical repetitive loading

Abstract

In the field of ocean engineering, anchors are used for several purposes. This article studies the behavior of a helical anchor embedded in soft marine clay under vertical repetitive loading. Helical anchors are simple steel shafts to which one or more helical plates are attached at regular intervals. The tests are conducted on a model helical anchor installed in a soft marine clay bed prepared in a test tank. Repetitive loading is applied using a pneumatic loading arrangement. Different cyclic load ratios and time periods are adopted. In each test, after the application of repetitive loading, poststatic‐pullout tests are conducted to observe the effect of repetitive loading on anchor behavior. From the test results, it is found that, up to a cyclic load ratio of 55%, there is no reduction in capacity. Instead, there seems to be a marginal increase in capacity and reduction in displacement. The reasons for this behavior are explained in terms of induced changes in strength and deformation behavior of marine clay under repetitive load. However, at higher cyclic load ratios, there seems to be reduction in pullout capacity of the anchor, and the reason for this is explained in terms of strain criteria. From this investigation, it can be concluded that the deep anchor is more suitable to a marine environment than a shallow anchor.     

Uplift of symmetrical anchor plates by using grid-fixed reinforced reinforcement in cohesionless soil

Uplift response of symmetrical anchor plates with and without grid fixed reinforced （GFR） reinforcement was evaluated in model tests and numerical simulations by Plaxis. Many variations of reinforcement layers were used to reinforce the sandy soil over symmetrical anchor plates. In the current research, different factors such as relative density of sand, embedment ratios, and various GFR parameters including size, number of layers, and the proximity of the layer to the symmetrical anchor plate were investigated in a scale model. The failure mechanism and the associated rupture surface were observed and evaluated. GFR, a tied up system made of fiber reinforcement polymer （FRP） strips and end balls, was connected to the geosynthetic material and anchored into the soil. Test results showed that using GFR reinforcement significantly improved the uplift capacity of anchor plates. It was found that the inclusion of one layer of GFR, which rested directly on the top of the anchor plate, was more effective in enhancing the anchor capacity itself than other methods. It was found that by including GFR the uplift response was improved by 29%. Multi layers of GFR proved more effective in enhancing the uplift capacity than a single GFR reinforcement. This is due to the additional anchorage provided by the GFR at each level of reinforcement. In general, the results show that the uplift capacity of symmetrical anchor plates in loose and dense sand can be significantly increased by the inclusion of GFR. It was also observed that the inclusion of GFR reduced the requirement for a large L/D ratio to achieve the required uplift capacity. The laboratory and numerical analysis results are found to be in agreement in terms of breakout factor and failure mechanism pattern.     

A Computational Investigation of the Group Effect of Pits on the Ultimate Strength of Steel Plates

In this work, we focus on assessing the group effect of localized corrosion on the ultimate strength of the marine structural plates and study the load-deformation behaviors of plates of various slenderness and uniaxial compression. Meanwhile, we investigate different corroded patterns from a single circular pit to 25 circular pits distributed over the plate and carry out hundreds of nonlinear finite element simulations by combining the number, depth, distribution of pits with imperfections and slenderness of plate. The distribution of multiple pits causes scattering of stress concentration on the plate, then the plastic section of plate changes with wider distribution of damage simultaneously. The ultimate strength arises when un-loading zone comprised of the yielding strips and holes extends across the plate. It can be concluded that the corroded condition defined as group effect of pits manipulates the deformation state and the loading capacity of plate at the ultimate strength mode that coincides with the proportion of effective loading area and section in the process of post-buckling. To validate the effect of pits group, we perform the numerical experiments of the post-buckling of steel plates containing pits in a row with different orientation.     

Linearised water wave problems involving submerged horizontal plates

In this paper a number of related linearised water wave problems all involving thin submerged horizontal plates are considered. An integral transform approach is adopted and used to formulate integral equations in terms of unknown functions related to the jump in pressure across the plate. A Galerkin method is applied to the solution of these integral equations leading to elegant expressions for quantities of interest and a rapidly convergent numerical scheme. The focus of the paper is to demonstrate the application of this method in a number of settings including both two-dimensional problems applied to infinitely-long plates of constant width and three-dimensional problems involving circular discs. In the process we present new results including, for example, for wave-free forced oscillations of plates.     

基于平行双板结构的圆管尾流抑制方法研究

通过数值模拟方法对基于平行双板的圆管尾流进行抑制研究。以二维不可压缩粘性流体方程为流动控制方程,并采用稳定化流体有限元方法求解。基于数值结果,详细探讨了平行双板的安装角度和板长等关键特征参数对尾流的抑制作用和影响。数值分析发现,平行双板结构对尾流的抑制作用明显优于传统单板控制。计算结果显示,基于平行双分隔板的尾流抑制最优位置为从圆管后驻点起向上游40°~50°处,并且与单板相比,在更短板长条件下,亦可完全抑制圆管的旋涡脱落,从而尾流从非定常涡旋流转入定常流动。当安装位置为θ≥80°时,被控圆管表面压力分布与无控圆管类似,说明此时平行双板控制无减阻效应。最后,基于数值模拟结果,对本方法的尾流高效控制效应,给出了相应的流动机制。     

Analytic Solution of Interaction Between Surface Waves and Floating Elastic Plates with Elastic End-Restraints

Based on the dynamic theories of water waves and Mindlin plates,the analytic solution of interaction between surface waves and two-dimensional floating elastic plates with edge-restraint is constructed by use of the Wiener-Hopf technique.Firstly,without regard for elastic edge restraint,the wave-induced responses of elastic floating plate analyzed by the present method are in good agreement with the results from literature and experimental results.Therefore,it can be shown that the present method is valid.Secondly,three end-restraint cases(i.e.,the left-end elastic restraints,the both-end elastic restraints,and the right-end elastic restraints)are proposed to reduce the vibration of floating plates,in which the spring is used to connect the sea bottom and the floating plate's left(or right)edge.The relations between the spring stiffness and the parameters of wave-induced responses of floating plates are discussed.Moreover,the effective method to reduce the vibration of floating elastic plates can be obtained through comparison.     

加筋板轮印载荷分布特性的试验与数值分析

轮印载荷的分布对于直升机甲板加筋板结构的设计至关重要。通过在橡胶块上加压的方式模拟轮印载荷,直接测量橡胶块与加筋板结构之间的接触压力,揭示轮印载荷分布特性,得到轮印载荷在加强筋和铺板之间的分布具有较强的不均匀性。利用ABAQUS软件进行轮印载荷分布的数值计算,结果表明,数值计算结果与试验结果吻合较好,试验工况下分布在加强筋上均布轮印载荷为接触面上均布轮印载荷的1.279倍。研究了加筋板面板初始缺陷、加强筋高度以及加筋板面板厚度对轮印载荷分布的影响。数值结果表明:考虑面板初始缺陷、降低面板厚度和增加加强筋的高度使轮印载荷分配到加强筋上的比例增大,但在相对刚度变化相同的情况下,降低面板厚度和增加加强筋高度对轮印载荷分布的影响不同。     

轴向受压矩形开孔板极限强度近似计算

针对受均匀轴向压力的矩形开孔板剩余极限强度的问题,考虑矩形板中间开圆孔和腰圆孔两种典型形状,四种边界条件下,以板的柔度系数、开孔率为主要影响参数通过大量非线性有限元数值计算分析,采用一种新的拟合方式提出了矩形开孔板轴向受压剩余极限强度的简化计算公式,新经验公式能够很好地反映开孔所导致板极限强度的衰减趋势。将所提出的经验公式计算结果分别与相应的试验结果、非线性有限元计算结果及其他经验公式计算结果进行对比分析。结果表明所提出的经验公式相比已有的经验公式计算更为准确,适用范围更广,可以用来预报不同边界条件下轴向均匀受压开孔矩形板的极限强度。     

Buckling of circular, annular plates of continuously variable thickness used as internal bulkheads in submersibles

This paper deals with the determination of the critical load in the buckling of isotropic, circular annular thin plates whose thickness varies slightly according to an exponential law.The outer edge of the plate is elastically restrained against rotation while the inner boundary is free. Classical small-deflection theory is used. An hidrostatic pressure acting in the middle plane is considered.The eigenvalues of the problem are obtained by means of an analytical approach based on the optimized Rayleigh-Ritz method. It is shοwnι that the results are in good agreement from an engineering viewpoint, with those available in the literature.     

Hydrodynamic Characteristics of Square Heaving Plates with Opening Under Forced Oscillation

The existence of the heaving plates can improve the heaving motion performance of an offshore structure significantly by providing both extra added mass and damping. In the current research, numerical investigation is carried out on the hydrodynamic characteristics of both isolated square heaving plate and double square heaving plates with opening by an immersed boundary-lattice Boltzmann method. The effects on hydrodynamic performance of plates due to Keulegan-Carpenter(KC) number, frequency number, opening ratio, opening distribution and spacing of plates are examined. It is found that the heaving plates with optimized opening ratio can provide additional damping compared with the plates without opening. Better hydrodynamic characteristics of double plates can be obtained with the increase of plate spacing.     

Modified eigenfunction expansion methods for interaction of water waves with a semi-infinite elastic plate

The eigenfunction expansion method (will be referred to as error function method in this paper) of analysing reflection and transmission of ocean waves at a semi-infinite thin elastic plate [J Geophys Res 95 (1990) 11629; Phil Trans R Soc Lond A 347 (1994) 185] is modified and extended to cases with simply supported and built-in edges. The form of the error function in the eigenfunction expansion method of Fox and Squire [J Geophys Res 95 (1990) 11629] has been modified based on a dimensional analysis and also extended to plates with either a simply supported or a built-in edge. The modified error function does not include the so-called Lagrange multipliers used in the original method and therefore the modified solution is independent of the selection of the Lagrange multipliers. It is demonstrated that the modified error function method satisfies the relation of energy conservation very well for the three edge conditions examined. The relation of energy conservation for plates is derived for elastic plates with simply supported and built-in edges and it is found that the relation of energy conservation for a free edge is also held for simply supported and built-in edges.In addition, a minor modification has also been made to the eigenfunction method (will be called inner product method in this study) of Sahoo et al. [Proceedings of 10th International Offshore and Polar Engineering Conference 3 (2000) 594]. It will be shown that the modified inner product method becomes mathematically well defined. The modified coefficient matrixes for different edge conditions are diagonal and thus the linear simultaneous equations can be solved very easily. It is also demonstrated with examples that the extended error function method and the modified inner product method give identical results.