考虑冲刷影响的筒型基础竖向极限承载力研究

大直径宽浅式筒型基础,阻水宽度大,在位工作期间受波浪海流作用,其周围土体易被冲刷。为研究单侧地基土体受冲刷后筒型基础的竖向极限承载力变化,通过引进冲刷率的概念,采用有限元方法研究了不同冲刷率下筒型基础的竖向极限承载力;并基于Meyerhof理论建立了计算不同冲刷率下筒型基础竖向极限承载力的极限平衡方法。研究结果表明,随着冲刷率增大,筒型基础的极限承载力出现不同程度的下降,当冲刷率为0.8时,即筒型基础单侧土体冲刷深度达6.4 m时,筒型基础的竖向极限承载力折减率为3.28%。建立的极限平衡算法可准确计算冲刷条件下筒型基础的竖向极限承载力。     

V-M荷载作用下双层黏土地基破坏包络线研究

地基破坏包络线的研究是复合加载模式下地基承载力设计的关键。根据建立的数值计算模型,用Abaqus商业软件分析了V-M荷载作用下,位于双层黏土地基上条形基础的破坏包络线随上层土厚度比、土层强度比的变化规律,以及地基破坏模式随荷载偏心率的变化趋势。用有效面积概念将传统竖向极限承载力换算为V-M荷载平面的破坏包络线,与数值计算结果进行对比。结果表明,公式换算方法适用于均质黏土地基,而对双层黏土地基偏于保守。     

海床表层软土对宽浅式筒型基础承载特性影响研究

近海海床表层多为软黏土或淤泥质土,为探究海床表层软土对海上风电宽浅式筒型基础承载特性的影响,以中国广东某海域风电场为背景,通过有限元分析的方法,研究竖向、水平、弯矩荷载作用下软土层厚度和土体强度对基础极限承载力、破坏模式以及筒基土压力分布的影响。研究结果表明:当软土层厚度小于H/2(H为筒裙高度)时,单向荷载作用下宽浅式筒型基础极限承载力随软土层厚度的增加呈线性减小的趋势;当软土层厚度大于H/2后,承载力降低速率逐渐增大。表层软土的存在,使得塑性区范围缩小,软土层内土体塑性破坏更加明显。竖向荷载作用下,随软土层厚度的增大,筒顶承载先减小后增大,筒内侧摩阻力先增大后减小;水平荷载和弯矩作用下,筒侧被动土压力的降低是引起软土覆盖地基中基础承载能力降低的主要因素。     

上软下硬黏土中四筒基础抗倾覆承载特性研究

随着海上风电向深远海发展,四筒基础发展潜力巨大。中国大多数海域存在较厚的上覆软土层,对基础抗倾覆承载能力有着重大影响。采用有限元软件ABAQUS建立上软下硬分层黏土模型,对四筒基础在单向水平荷载和弯矩荷载作用下的承载力特性进行研究。研究结果表明：四筒基础在水平、弯矩荷载作用下主要运动模式为转动,对边加载时,转动轴靠近受压筒且随着软土层厚度增加不断靠近四筒基础平面中心,但变化幅度较小;水平和弯矩极限承载力对于不同的筒间距和长径比具有相同的变化趋势,当软土层厚度h/筒高L≤3/4时,水平和弯矩承载力随着软土层厚度的增加近似线性降低,当h/L＞3/4后,承载力降低速率明显减小。     

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。     

Spar平台吸力式基础极限承载特性数值分析

以国外某深海Spar平台工程为背景,针对其所采用的细长型吸力式基础的抗拔承载特性进行三维有限元数值分析.分析中充分考虑土体强度、加载位置和加载角度对吸力式基础极限抗拔承载力的影响,本构模型中钢筒基础采用弹塑性模型.分析结果表明,吸力式基础的极限抗拨承载力随着土体强度的增大而增大,倾斜加载时在基础插入土体部分中点左右加载可取得最大的极限承载力,极限抗拔承载力还随着加载角度的增大而增大.吸力式基础存在倾斜加载时桶基础与桶内外土体的共同塑性屈服破坏和垂直加载时桶外土体的局部剪切破坏等两种不同的破坏模式.     

砂土中四筒导管架风机基础抗弯承载力研究

针对一种四筒导管架海上风机基础,基于有限元数值分析,通过建立砂土中不同筒径和筒高的四筒导管架基础有限元模型,研究砂土中单调弯矩荷载的作用下,筒径与筒高对导管架基础抗弯承载力的影响。分析结果表明:四筒导管架风机基础的抗弯承载力随着筒高或筒径的增加呈明显的增长趋势,相比于筒径的增加,筒高的增加对提高基础抗弯承载力更为有效;在极限弯矩荷载的作用下,基础旋转中心水平向位置受筒高的影响较大,但竖向位置受筒高和筒径的影响很小。     

单向荷载作用下海上风机多桶基础承载特性数值分析

作为一种新型的深水海上基础型式,桶形基础在海上风机设施设计与建设中逐步得到了发展和应用。以三桶基础为例,采用大型有限元软件ABAQUS对海上风机多桶基础的承载特性进行了三维有限元数值分析。根据海上风机的荷载受力特点,分别探讨了三桶基础在竖向荷载、水平荷载和力矩作用下的极限承载力,得出荷载作用方向及桶间距对极限承载力的影响程度,研究成果为复合加载模式下海上风机多桶基础的承载特性分析奠定了基础。     

复合加载下考虑接触作用的地基承载特性数值分析

港口、海洋工程结构物基础一般处于复合加载状态,其极限承载力通常采用近来引入的极限荷载图进行评价.对位于地基表面的重力式海洋基础,需要考虑基础与地基间的接触特性对极限承载力的影响.以大型通用有限元软件ABAQUS为计算平台,建立了复合加载模式的地基极限承载力数值分析方法;针对饱和黏土地基上的表面基础,利用在ABAQUS平台上开发的接触计算模块,模拟基础与地基间竖向可分离、切向完全粘结的接触作用;进而基于建立的分析方法,进行系统的有限元计算,分析地基的破坏模式随荷载条件的变化,给出地基的极限荷载包络图,并与经典承载力计算公式结果进行对比.研究结果表明,经典承载力计算公式低估了三维荷载条件下的地基极限承载力,有限元计算模型及数值分析方法,可以较好地分析研究地基的失稳机理及承载力特性,并可考虑基础与地基不同的接触条件对破坏模式及组合极限承载力的影响.     

组合荷载作用下伞式吸力锚基础承载特性数值分析

针对用于海上风力发电机的伞式吸力锚基础(USAF)实际条件下的受力特点,采用数值模拟方法,基于大型通用有限元软件ABAQUS构建数值计算模型,对伞式吸力锚基础在H-V、H-T、V-T荷载平面内以及H-V-T非共面复合加载模式下的承载特性进行分析,进而推导其破坏包络面数学表达式。分析中采用固定位移比加载法进行复合加载,并将桶顶位移作为失效破坏标准。结果表明：(1)拟定的应力归一化复合加载破坏包络面椭圆曲线方程可以较好地模拟不同主筒长径比USAF在不同荷载空间内的破坏包络面形式;(2)H-T空间内USAF复合承载性能随主筒长径比(L/D)的增大而提高,而H-V、V-T空间内变化不明显;(3)绘制了H-V-T空间内USAF三维破坏包络面,可根据实际受荷状态与包络面之间的相对位置关系,评价实际工况下伞式吸力锚基础的稳定性。     

Scour effects on bearing capacity of composite bucket foundation for offshore wind turbines

Abstract

Composite bucket foundation (CBF) is a wide-shallow foundation for offshore wind turbines, which can be transported and installed with the turbine as one unit at a one-step operation. Compared with deep pile foundations, its structural stability is more sensitive to the scouring by waves and currents. In this paper, a three-dimensional finite element model with CBF and surrounding soil is established to estimate the failure mode at different given soil scour conditions. The loading on CBF for offshore wind turbines is characterized by relatively small vertical loading V, larger horizontal loading H, and bending moment M, and the effect of erosion on bearing capacity of CBF is determined by using the fixed displacement ratio method. In addition, the failure envelopes of the CBF applied in H–M and V–H–M loading modes are obtained. Results indicate that the bearing capacity of CBF under horizontal loading and bending moment will be significantly reduced by the decrease in the embedded depth of CBF due to the scouring depth and extent, as well as the H–M, and V–H–M failure envelopes. The structural stability safety factor of CBF under different scouring conditions can be obtained through the three-dimensional envelope surface with respect to scouring depth and extent.     

波浪作用下桶形基础冲刷特性试验研究

在设计结构和考虑有效的海洋工程基础的冲刷防护工程时,了解和预测海洋结构周围的冲刷发展机理是非常重要的。通过波流水槽研究规则波作用下桶形基础的冲刷特性,试验主要考虑波高与结构尺寸对桶形基础周围局部冲刷的影响,并分析波浪冲刷过程中模型周围的时程地形、波浪作用后的冲刷坑性状、冲刷影响要素以及不同结构间的冲刷差异。结果表明:桶形基础的冲刷过程是震荡加深发展,最大冲刷深度与冲刷宽度随波高增大而增加,随上部结构直径增大而增加,最大冲刷深度位置随波高与结构尺寸变化而变化;不同结构间的冲刷坑形态与冲刷后的地形不同,最大冲刷深度相差可高达4倍;桶形基础结构直径比范围为0.2～0.5时,结构自身具有防冲刷能力。     

Effect of embedment on the vertical capacity of bucket foundation in loose saturated sand: Physical modeling

ABSTRACT

Bucket foundations have been widely used for a variety of offshore applications. The effects of skirt length on ultimate bearing capacity of bucket foundation have been studied and reported in published scientific papers. However, few studies have addressed the behavior of bucket foundations in loose saturated sand. In this paper, a series of experimental investigations were performed to determine the bearing capacity of bucket foundation under uniaxial loading. The experiments were conducted on small-scale foundations under vertical loading in loose saturated sand. It was found that increasing the skirt length would enhance the bearing capacity of bucket foundation. As reflected in the present study, bearing strength might be enhanced more than 5 times in loose saturated sand in comparison to surface footing with equivalent diameter. Based on the experimental investigation, a depth factor was proposed to approximate bearing capacity of bucket foundations in terms of those for surface footing and embedment ratio. Moreover, the corresponding settlement of foundation at the failure load was found to increase with skirt length.     

Rotation Center and Horizontal Bearing Capacity of the Bucket Foundation in Soft Ground

In the last 20 years, the bucket foundation has been developed as a new type of offshore platform structure. Because of its short period of application to engineering practices, the theoretical decision on the rotation center and horizontal bearing capacity of the bucket foundation has yet to be agreed. A limit analysis method is used to determine the updated rotation center position and horizontal bearing capacity to evaluate the failure mechanisms of the bucket foundations. The results are compared with numerical simulation and experiments, and also with other theoretical methods. The proposed method can satisfactorily consider the engineering conditions and the result is accurate in determining the rotation center and horizontal bearing capacity.     

Lateral Bearing Capacity of Multi-Bucket Foundation in Soft Ground

The bucket foundation is a new type of foundation for offshore application to intermediate-depth waters. It has advantages over conventional ones. However, there is no consensus in the analysis and design of this type of foundation. In this paper, the lateral bearing capacity and the failure mechanism of multi-bucket foundations are studied with different connection stiffness and bucket spacing by use of a three-dimensional finite element method. Based on the numerical analysis results, a limit analysis method of plasticity for evaluating the lateral bearing capacity of large-spacing multi-bucket foundation with rigid connection in soft ground is proposed. This method provides a simple procedure that gives results comparable to those from the finite element analyses.     

复合加载下桶形基础循环承载性能数值分析

作为一种新型基础形式,吸力式桶形基础除了承受海洋平台结构及自身重量等竖向荷载的长期作用之外,往往还遭受波浪等所产生的水平荷载及其力矩等其它荷载分量的瞬时或循环作用。对复合加载模式下软土地基中桶形基础及其结构的循环承载性能尚缺乏合理的分析与计算方法。应用Andersen等对重力式平台基础及地基所建议的分析方法,基于软黏土的循环强度概念,在大型通用有限元分析软件ABAQUS平台上,通过二次开发,将软土的循环强度与Mises屈服准则结合,针对吸力式桶形基础,基于拟静力分析建立了复合加载模式下循环承载性能的计算模型,并与复合加载作用下极限承载性能进行了对比。由此表明,与极限承载力相比,桶形基础的循环承载力显著降低。     

Load-bearing behavior of suction bucket foundations in sand

Suction buckets are a promising foundation solution for offshore wind energy systems. The bearing behavior of monopod buckets under drained monotonic loading in very dense and medium dense sand is investigated in this study by means of numerical simulation with the finite element method. Special focus is given to the ultimate capacity and the initial stiffness of the bucket-soil foundation system. The numerical model is validated by comparison with field test results. The bearing behavior of the structure is explained through an evaluation of a reference system. It is shown that the bucket experiences a heave during horizontal loading, which leads to the formation of a gap between the bucket lid and the soil with increasing load. At large loads and rotations close to failure of the system there is no contact between lid and soil, and the whole load is transferred to the soil via the bucket skirt. A parametric study shows how the ultimate capacity and initial stiffness of the system depend on the bucket dimensions and loading conditions, i.e. load eccentricity. Normalized equations for ultimate capacity and initial stiffness are derived from the numerical simulation results, which can be used in the scope of a preliminary design for buckets in sand.