自升式海洋平台桩腿焊缝裂纹扩展模拟及寿命预测

发布时间：2018-07-09 18:16

本文选题：自升式海洋平台 + 桩腿环焊缝　；参考：《东北石油大学》2015年硕士论文

【摘要】：自升式海洋平台是海上石油开发的作业与生活场所,桩腿是自升式钻井平台的重要部位,支撑其整个结构在海上作业。平台桩腿结构由于制造工艺的限制,不可避免存在微小气泡、夹渣等缺陷,长期处于恶劣的海洋环境中桩腿不断遭受来自风、浪、流等交变载荷的作用,微小缺陷在这些交变载荷的影响下,容易形成裂纹并不断扩展,进而使桩腿的承载能力急剧下降,造成严重的后果。本文通过对桩腿环焊缝焊趾部位的裂纹进行扩展模拟,找到适用于海洋工作环境下结构裂纹扩展的规律,以达到发现结构出现裂纹时,能够及时采取相应的措施来控制裂纹的扩展,从而避免海洋平台结构发生重大的灾难,对海上的安全生产具有重要意义。首先介绍了自升式海洋钻井平台的主要结构和工作环境载荷,进而围绕平台桩腿这一核心结构,结合断裂力学理论,系统研究了含初始裂纹桩腿结构的裂纹扩展过程。具体来说,本论文着重在以下几个方面进行了探索和研究。首先在对海洋平台整体结构研究分析的基础上,建立了自升式海洋平台有限元模型,并适当简化了平台的一些次要结构。通过《海上固定平台入级与建造规范》计算得出平台所受的的环境载荷以及自身重力载荷,并通过有限元软件进行施加,经过计算得到风暴自存工况下整体海洋平台有限元分析结果,表明桩腿结构飞溅区比较薄弱,有必要对此部位进行重点研究,计算结果符合实际工程情况。建立了海洋平台含有环焊缝的桩腿模型,其中桩腿模型省略了竖焊缝以及加强板等结构,只保留了飞溅区部位的环焊缝。通过施加风、波浪荷载以及平台重力载荷,对桩腿结构进行了具体计算,得到桩腿有限元分析结果,其中海平面以上1.8m处桩腿受力比较大。为了减少不必要的计算量,从桩腿环焊缝处剖分出一块模型,成为子模型,利用专业三维裂纹扩展分析软件,在子模型上添加椭圆形初始裂纹,并对裂纹前缘应力强度因子进行了计算,结果表明裂纹尖端处应力强度因子数值最大,推动裂纹不断扩展。利用复合裂纹扩展准则中的最大周向应力准则来模拟裂纹扩展,通过设置裂纹扩展长度,并以应力强度因子为推动力模拟裂纹扩展,得到了裂纹扩展十步后的形貌图。模拟裂纹扩展的形貌图很好地表现了实际工程裂纹的走向,对工程检测具有一定的指导意义。通过得到的裂纹扩展长度与裂纹应力强度因子之间的关系,结合Paris裂纹扩展速率模型来计算裂纹尺寸和施加载荷循环次数之间的关系,通过计算得到了由初始裂纹开始扩展直至裂纹扩展结束后,总的应力循环次数,实现了桩腿疲劳裂纹扩展寿命的预测。
[Abstract]:The jack-up offshore platform is the working and living place of offshore oil development, and the pile leg is an important part of the jack-up drilling platform, which supports the whole structure of the platform in offshore operation. Because of the limitation of manufacturing technology, the structure of platform pile leg inevitably has some defects, such as micro bubble, slag inclusion and so on. The pile leg is constantly subjected to the action of alternating loads from wind, wave, current and so on in the bad marine environment for a long time. Under the influence of these alternating loads, the micro-defects are easy to form cracks and continue to expand, which makes the bearing capacity of the pile legs drop sharply, resulting in serious consequences. In this paper, by simulating the crack propagation in the weld toe of the girth weld of the pile leg, we find out the law of crack growth suitable for the offshore working environment, so as to find the crack in the structure. The corresponding measures can be taken in time to control the crack growth, thus avoiding the major disaster of offshore platform structure, which is of great significance to the safety of production at sea. Firstly, the main structure and working environment load of jack-up offshore drilling platform are introduced, and then the crack propagation process of pile-leg structure with initial crack is studied systematically around the core structure of platform pile leg and the theory of fracture mechanics. Specifically, this paper focuses on the following aspects of exploration and research. Firstly, based on the research and analysis of the whole structure of offshore platform, the finite element model of jack-up offshore platform is established, and some minor structures of the platform are simplified appropriately. The environmental load and its own gravity load of the platform are calculated by the Code of entry and Construction of fixed platform on the Sea, and applied by the finite element software. The finite element analysis results of the whole offshore platform under the condition of storm self-storage show that the spatter zone of the pile-leg structure is relatively weak, so it is necessary to focus on the research on this part, and the calculated results are in line with the actual engineering situation. A pile-leg model of offshore platform with girth welds is established in which the vertical weld and stiffening plate are omitted and only the circumferential weld in the splash zone is retained in the pile-leg model. By applying wind, wave load and platform gravity load, the pile leg structure is calculated, and the results of finite element analysis of pile leg are obtained, in which the pile leg force is relatively large at 1.8 m above sea level. In order to reduce the unnecessary calculation, a model is divided from the weld seam around the pile and leg to become a sub-model. The elliptical initial crack is added to the sub-model by using the professional three-dimensional crack growth analysis software. The results show that the stress intensity factor at the crack tip is the largest, which promotes the crack propagation. The maximum circumferential stress criterion of the composite crack growth criterion is used to simulate the crack growth. By setting the length of the crack growth and taking the stress intensity factor as the driving force to simulate the crack growth, the shape map of the crack propagation after 10 steps is obtained. The morphologies of simulated crack growth show the trend of actual engineering cracks well and have a certain guiding significance for engineering detection. Based on the relationship between crack growth length and crack stress intensity factor and Paris crack growth rate model, the relationship between crack size and the number of cycles under load is calculated. The total number of stress cycles from the beginning of the crack propagation to the end of the crack growth is calculated and the prediction of the fatigue crack propagation life of the pile leg is achieved.
【学位授予单位】：东北石油大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TE95

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