摘要
针对我国东海海域的粉土海床工况,基于模型试验,研究了循环荷载下吸力基础的荷载-水平转角响应规律,分析了吸力基础水平刚度与水平转角的相关特性,并建立了海上风力发电机使用寿命中吸力基础累积转角与循环次数的关系式。结果表明:循环荷载下吸力基础周围粉土的变形呈增加趋势,刚度随循环次数的增加而减小;加载或卸载过程中,吸力基础转动中心均呈明显的上移趋势,这将加速吸力基础的倾覆;卸载刚度与循环次数遵循对数增加关系。
较之陆上风电,海上风能资源丰富,而且沿海地区电网容量大、风电接入条件好,因此海上风电更具优势。据估计,在海上风电场项目中风力发电机基础的建设成本约占总成本的25%~35
吸力基础在服役期间受到风浪流等循环荷载的作用,而长期的桩-土循环荷载容易导致吸力基础水平承载性能劣化。对于风力发电机基础,应着重考虑低竖向力和高水平荷载的共同作
针对我国东海海域粉土进行了一系列的1g模型试验,研究了循环荷载下吸力基础水平承载特性,并对试验结果进行了量纲一处理。通过吸力基础的循环荷载-转角响应,得到了吸力基础水平刚度与水平转角之间的关系。基于试验中吸力基础的累积转角,分析了其与循环次数之间的关系,并预测了吸力基础服役期累积转角的变化。最后,探究了循环荷载下吸力基础内部的压强变化,并重新构建卸载刚度,以进行对比分析。
试验在砖-混模型箱内完成。模型箱内部尺寸为1.72 m×0.76 m×0.72 m(长×宽×高),壁厚为17.20 cm。模型箱内底部铺设的约5 cm厚的碎石垫层用作排水通道,土体与碎石垫层之间采用土工布进行分隔。模型箱外设水箱系统,用于反渗饱和土
图1 试验装置示意图
Fig.1 Schematic diagram of test set-up
吸力基础由不锈钢制成,桶壁和顶板厚度分别为2 mm和10 mm,长径比L/D=1.5,具体尺寸如
图2 吸力基础模型 (单位:mm)
Fig.2 Suction caisson model (unit:mm)
图3 水平荷载加载示意图
Fig.3 Schematic diagram of horizontal loading
试验用粉土(见
图4 试验用土及其颗粒级配曲线
Fig.4 Silt in tests and its particle grading curve
图5 单桥静力触探试验曲线
Fig.5 Curves of single bridge cone penetration tests
LeBlanc
(1) |
(2) |
式中:Mmax、Mmin分别为一个周期内作用于基础上弯矩的最大值与最小值;和分别为一个周期内作用于基础上水平作用力的最大值与最小值;Mult和分别为相同加载速率的单调荷载下基础所能承受(容许)的极限弯矩和极限水平荷载。
(3) |
式中:θmax,s为静力加载时吸力基础最大循环荷载对应的水平转角,θmax,s=θmax,1。
图6 累积转角和卸载刚度定义
Fig.6 Definition of accumulated rotation and unloading stiffness
若在服役期间海上风机基础刚度发生改变,则结构的一阶固有频率也发生变化,从而改变了结构的固有频率。Zhu
粉土试件制备完成后,以0.1 mm·
图7 单调荷载作用下水平荷载与转角关系
Fig.7 Relationship between lateral load and rotation under monotonic loading
图
图8 循环荷载-水平转角响应
Fig.8 Cyclic loading-lateral rotation response
图9 水平刚度-水平转角响应
Fig.9 Lateral stiffness-lateral rotation response
图10 累积转角和循环次数关系
Fig.10 Relationship between accumulated rotation and number of loading cycles
通过大量试验对单桩和吸力基础累积转角与循环次数的关系进行研究,Zhu
(4) |
式中:α和β为量纲一变量。
(5) |
图11 β与ξb的关系
Fig.11 Variation of β with ξb
海上风力发电机15年服役期(循环次数约4×1
为研究加载过程中桶内压强对吸力基础周围粉土的影响,参考沉贯过程归一化速度
(6) |
式中:v为加载速率;cv为垂直固结系数,对于黏土
cv=2.6
图12 循环加载下吸力基础与土相互作用示意图
Fig.12 Schematic diagram of interaction between suction caisson and soil under cyclic loading
图13 桶内压强与水平转角关系
Fig.13 Relationship between pressure in suction caisson and lateral rotation
假定转动中心在吸力基础的中轴线上,如
(7) |
式中:h1和h2为位移传感器距顶板高度;x1和x2为位移传感器得到的位移值。
图14 转动中心与水平转角关系
Fig.14 Relationship between location of rotation center and lateral rotation
Arany
图15 卸载刚度与循环次数关系
Fig.15 Relationship between unloading stiffness and number of loading cycles
ξc=0时B4C的k/((γ′pa
(1)吸力基础周围粉土变形显著,致使吸力基础水平位移增大,并且其最大水平荷载FH,max对应的水平刚度随着循环次数的增加而减小。非密封状态下吸力基础的首次卸载残余转角较大,而密封状态下加载过程中吸力基础顶板与土体分离使得桶内形成负压,负压有利于吸力基础的“复位”,因此密封状态下首次卸载残余转角较小。
(2)在初始阶段吸力基础水平转角快速积累,并随着循环次数的增大而增量逐渐减缓。当ξc=0时,Δθmax/θmax,s增长速率随着ξb的增加而增大。当桶内土体处于部分排水状态时,容易导致循环初始阶段吸力基础水平刚度较大。随着循环次数的增大顶板与土体之间出现存积海水现象,海水的不可压缩性与土体较小的渗透性使得土体卸载不彻底,继而增大累积转角。
(3)无论是加载还是卸载,吸力基础转动点均呈明显的上移趋势,此现象加速了吸力基础的倾覆。卸载刚度随着循环次数的增大而增大并遵循对数关系。随着土体固结和致密,卸载刚度逐渐趋于稳定。
作者贡献声明
马士力:试验设计,试验实施,结果分析,论文撰写与修改。
谢立全:方案讨论与优化,论文修改。
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