计及板厚腐蚀的半潜式平台疲劳裂纹扩展评估

Fatigue crack growth assessment of semisubmersibles based on plate thickness diminution

针对半潜式平台疲劳裂纹剩余扩展寿命的计算和预测问题,本文在计及板厚腐蚀的前提下,针对半潜式平台立柱与横撑连接部位的疲劳裂纹进行扩展评估。依照规范选取合适板厚减小系数,利用指数速率模型计算板厚20 a后的腐蚀余量,输入剩余板厚尺寸进行结构整体和局部的响应分析。利用波浪散布数据与传递函数获得离散化的随机疲劳载荷,应用裂纹扩展率单一曲线模型进行裂纹扩展计算,在扩展循环中加入失效评估图FAD进行安全评估,计算得出的裂纹临界尺寸与寿命均比完整板厚下的数值小,而应用单一恒定幅值载荷加载的裂纹扩展寿命比离散化随机载荷加载的数值小。结果表明:在海洋平台结构响应分析中计入板厚腐蚀的影响,利用离散化方式处理随机疲劳载荷与应用FAD进行裂纹扩展安全评估,能提高评估过程及计算结果的合理性。

This work focuses on the calculation and prediction of fatigue crack growth life. In this work, the fatigue assessment of existing cracks in the connection between the columns and braces of semisubmersibles is performed by considering plate thickness diminution. The appropriate coefficient of plate thickness diminution is selected in accordance with the code. Then, the exponential rate model is applied to calculate the remaining plate thickness after 20 years. The size of the remaining plate thickness is input to analyze the overall and local responses of the structure. The wave dispersion data and transfer function are used to obtain the discretized random fatigue load. Crack growth is calculated by applying a single curve model of crack growth rate. The failure assessment diagram(FAD) is added to the propagation cycle for safety evaluation. The calculated critical size and life of the crack are smaller than those of a plate with complete thickness, and the crack propagation life of a load with a single constant amplitude is smaller than that of a discrete random load. Results show that the rationality of the evaluation and calculation can be improved by accounting for plate thickness diminution in the analysis of the structural response of semisubmersibles and by using the discretized mode to address random fatigue load and FAD to evaluate crack growth.