摘要
Spar平台是目前深海油气资源开发的主要装备之一,其中的系泊系统是深海平台的定位系统,当系泊的平台浮体发生大幅运动时,系泊系统会出现松弛-张紧状态的改变,引起系缆张力的迅速增大,可能危及深海平台的安全生产。本文针对深海系泊系统松弛-张紧过程的冲击张力进行了研究,主要工作和结论如下:
(1)在Mohammad Behbahani Negad研究工作的基础上,考虑缆绳内水平张力和位置,用泰勒级数展开,得到缆绳的曲率和张力的表达式,建立包含松弛和张紧两种情况的连续模型,采用应力波理论分析了缆绳内水平张力、波数、缆索坐标等参数对缆绳内应力波传播特性的影响,证明了本文考虑缆绳张力和曲率变化的合理性。
(2)根据应力波基本理论和非线性动力学基本理论,计算得到缆绳在松弛、松弛-张紧和张紧三种状态下缆绳的运动和张力,比较分析了缆绳三种运动状态的区别,发现三种状态下,缆索结构的运动特点、最大张力和对结构的破坏程度都不相同。在松弛-张紧状态,缆绳的横向运动出现跳跃,张力中出现突变的尖峰和倍频成分。松弛-张紧时产生的冲击张力是松弛情况的5倍,是张紧情况的2.4倍,容易引起缆绳的强度破坏,缩短疲劳寿命。
(3)根据冲击动力学理论,对系泊平台浮体-缆绳系统进行了耦合分析,给出系统的松弛区域和张紧区域的边界曲线,并对影响因素进行了分析。结果表明,缆绳-浮体本身的系统参数和外部载荷条件的改变,例如无量纲的位移比,或阻尼比发生改变,会引起系统在松弛-张紧区域间的转变,从而引起缆绳内的冲击张力。缆绳内的最大动态张力随波高、波浪频率、预张力的增大而增大,随缆绳长度的增大而减小。
(4)采用实验方法,对浸没在准静态流体中的绳索的冲击张力问题进行研究,观察到缆绳中的冲击张力,证明了冲击张力的存在性和此问题研究的必要性。同时,还研究了缆绳内最大张力的影响因素。结果表明,在相同的条件下,缆绳的最大张力随端点激励频率和激励幅值、缆绳刚度、缆绳直径、缆绳弹性模量的增大而增大,随缆绳长度的增大而减小。实验验证了理论计算的正确性。
(5)对空气中的冲击张力实验和水中的冲击张力实验进行比较分析,给出缆绳发生冲击载荷时缆绳内张力的特点:冲击载荷发生在激励端,冲击张力迅速变大,张力中出现高频成分,同时出现负张力。这些可以作为缆索中是否出现冲击载荷的判断依据。另外,冲击载荷发生在激励端,这就要求实际的设计中要加强系缆点附近缆绳的强度,以保证系泊系统的安全。
A Spar is considered as the excellent type for platforms in deep water and the mooring system of Spar is used to maintain the buoy’s position,relating to the security of platform. When the moored platform moves with large amplitude, the mooring line will transfer from taut to slack, going with the tension increasing sharply, which may endanger the security and production. In this paper, the snap tension in mooring line of deep water induced by taut-slack condition is studied, and the main work and conclusions obtained are as following:
(1) Based on the research of Mohammad Behbahani Negad, considering the arc coordinates and horizontal tension, the expressions of tension and radius of curvature are obtained with Taylor series expansion, and the continuum model including taut and slack state is established. The effects of tension in cable, wave number, coordinates of cable on the freely propagation characters of stress wave in cable are studied by stress wave theory and the rationality for the considered factors is testified.
(2) Based on the stress wave theory and nonlinear dynamic theory, the motions and tensions of line under three different states of taut, slack and taut-slack are calculated, and the differences between the three states are compared. The results show that, the motion characters of line, the maximum tensions and the threats to structure corresponding to the three states are different. When the system exists in taut-slack state, the motion in normal direction jumps from a steady state to the other, spike and higher frequency components appear in the tension. The maximum tension in taut-slack state is 5 times higher than that in slack, and 2.4 times higher than that in taut. The taut-slack state can easily cause the damage of the structure and shorten fatigue life.
(3) According to the impact theory, the analysis on Spar platform-mooring line coupled system is taken on, and the boundary curve of taut-slack region is given, and the influencing factors are discussed. The results show that, the alters of system parameters in moored Spar platform-mooring line coupled system, such as the dimensionless radio of displacement and damping, will cause the transition from the taut to slack state and cause the high tension in line. The maximum dynamic tension in line increases with the increasing of the wave height, wave frequency and pretension, decreases with the increasing of the length of line.
(4) With experimental method, the snap loading of lines immersed in a quiescent fluid medium is carried out and the snap tension is observed, which proved the presence of snap tension and necessity of this studying. The results show that, the maximum tension in line increases with the increasing of the excitation frequency, excitation amplitude, stiffness, diameter and modulus, decrease with the increasing of the length of line. The experimental results prove the validity of the theory study mentioned above.
(5) Comparative study on the experimental results in air and that in water are carried out, and the tension characters under snap loading are obtained as following: the snap loading appears at the exciting end, the snap tension increases sharply, harmonics of higher frequency components and negative tension appear in the tension. All of these can be regarded as the judgment basis for snap loading. In addition that, the snap tension appears at excitation end, which indicates that the mooring line near attaching point should be strengthen to ensure the security of platform.
引文
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