基于断裂力学的高温高压阀体疲劳评价

J. Sahoo, M. Campbell, M. Cerkovnik
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引用次数: 1

摘要

高温高压油田设备的发展通常导致了重壁设计的建设,其中额定工作压力的增加是通过增加截面厚度来适应的。然而,这种设计方式受到实际困难的限制,这些困难出现在制造、搬运/提升和通厚材料性能均匀性等方面。通过放松假设的设计因素和水压测试压力,可以设计出更有效的尺寸和重量,但这需要更严格的分析、验证和QA措施。特别是,设计人员必须解决高温高压设备的疲劳敏感性问题,即使在纯静态条件下,也可能在单独关井压力循环下发生故障。这些故障通常是由应力产生管引起的,如交叉井眼、阀座袋或内径变化,在内压作用下表现出复杂的应力状态。基于断裂力学(FM)对这些特征的分析对设计人员和分析人员来说是一个长期的挑战,因为目前还没有针对KI和σref的通用解决方案。因此,本文的目标是提供一种有用的方法,使用API 579-1/ASME FFS-1中提供的KI和σref解决方案进行基于fm的任意几何分析。该方法以实例研究的形式提出,描述了基于有限元法的阀体内阀座袋半径疲劳分析。本文采用三维有限元分析的方法,对一种假设的位于座椅袋半径处的半椭圆形表面断裂缺陷的I型行为进行了分析。这种方法一般包括两部分。第一种方法是开发类似于传统耐久性分析的3D有限元模型。从该模型中,沿预期断裂面提取应力,并结合权函数法从API 579-1/ASME FFS-1中提供的解中导出KI和σref。然后使用这些解来计算不稳定断裂的循环次数。第二部分涉及将裂纹直接纳入有限元模型。该方法得益于子建模技术,该技术减少了计算费用,并允许该方法用于复杂结构。该数值模型与传统的线弹性断裂力学假设相结合,可以推导出感兴趣几何形状的KI解。这些KI结果用于确认基于代码的解决方案的保守性,从而确认之前FM分析的保守性。本文中描述的方法允许设计人员在类似于传统S-N分析的时间框架内快速开发和执行基于fm的任意几何特征的疲劳分析。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Fracture Mechanics Based Fatigue Assessment of an HPHT Valve Body
The development of HPHT oilfield equipment has typically resulted in the construction of heavy-walled designs, where the increase in rated working pressure is accommodated by an increase in sectional thickness. This manner of design, however, is limited by practical difficulties which arise in the areas of manufacturing, handling/lifting, and uniformity of through-thickness material properties. Designs of more efficient size and weight may be developed by relaxing assumed design factors and hydrotest pressures, but this requires more rigorous analysis, validation, and QA measures. In particular, designers must address the fatigue susceptibility of HPHT equipment which, even in purely static conditions, may fail under cycles of shut-in pressure alone. These failures typically originate from stress risers such as cross-bores, seat pockets, or transitions in bore diameter, which exhibit complex stress states under the action of internal pressure. A fracture mechanics (FM) based analysis of such features has presented a longstanding challenge to designers and analysts as general solutions for their KI and σref are not presently available. It is therefore the objective of this paper to provide a useful methodology for conducting FM-based analysis of arbitrary geometry using the KI and σref solutions provided in API 579-1/ASME FFS-1. The method is presented in the form of a case study which describes the FM-based fatigue analysis of a seat pocket radius within a valve body. Here, the mode I behavior of a hypothetical surface-breaking, semi-elliptical flaw located at the seat pocket radius is evaluated by means of 3D finite element analysis. This method generally comprises two parts. The first involves the development of a 3D finite element model similar to what would be used in a conventional durability analysis. From this model, stresses are extracted along an anticipated fracture plane and used in conjunction with a weight function method to derive KI and σref from solutions provided in API 579-1/ASME FFS-1. These solutions are then used to compute the number of cycles to unstable fracture. The second part involves the direct incorporation of cracks into the finite element model. The approach benefits from a submodeling technique which reduces computational expense and allows the method to be used on complex structures. The numerical model is used in conjunction with conventional linear-elastic fracture mechanics assumptions to derive KI solutions for the geometry of interest. These KI results are used to confirm the conservatism of the code-based solutions and, thereby, the conservatism of the previous FM analysis. The method described in this paper allows designers to rapidly develop and execute FM-based fatigue analyses of arbitrary geometric features in timeframes similar to those associated with traditional S-N analysis.
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