摘要
海洋立管是海洋油气开发中的重要设备之一，是连接海底井口与海面浮式结构物的最重要媒介。在深海条件下，海洋立管的结构形态为细长体，其所处的工作环境十分复杂和恶劣，风、浪、流等多种载荷都会影响到立管的结构安全。而在对深海立管结构安全的评价中，由于大量不可测量和预知的影响因素的存在，导致立管结构响应出现随机性。目前，借助专用有限元软件，可以计算出立管在特定的环境下的强度情况，但是在复杂的环境载荷作用下，很难能够定量的确定立管结构在多大程度上是安全的。所以，充分考虑各种随机性的影响，对深海立管结构进行可靠性分析，是十分必要和有意义的。
由于立管的结构响应的功能函数都是随机变量的隐性表达式，无法直接应用传统方法进行可靠性分析。因此，本文引入了响应面方法和最弱失效模式组理论，结合试验设计、有限元方法、一阶二次矩方法和蒙特卡洛法等方法，进行了极限风暴载荷及波流联合作用下的钻井立管结构可靠性研究，和流载荷作用下钻井立管涡激振动疲劳可靠性研究。本文首先应用通用有限元软件对深海立管结构进行了整体分析，掌握了顶端张力式立管在操作及悬挂工况下的整体响应特征。然后，在对立管进行试验设计和专用有限元建模的基础上，筛选出影响立管在各种工况下结构响应的主要因素，采用多项式响应面方法建立结构响应与随机输入变量之间的近似解析表达式，在此基础上确定立管在不同工况下的不同位置的结构响应功能函数，继而进行了立管在极限风暴载荷下的结构可靠性分析。在这一方法的基础上，结合疲劳累积损伤理论，进行了钻井立管在涡激振动作用下的疲劳可靠性分析，并分别应用基于重要抽样法的蒙特卡洛方法对以上可靠性分析结果进行了验证。最后，通过广泛的调研，针对可靠性分析的研究成果，给出了深海立管完整性监测系统设计方案。
本文所提出的方法基于响应面方法的可靠性分析方法可以直接使用现有的确定性有限元分析软件，并可以为有限元方法和蒙特卡洛法建立了应用的桥梁，解决了立管结构响应隐性功能函数的问题，降低的立管可靠性计算的工作量，具有一定的理论和工程应用价值。
Riser is one of the most important equipment of the development ofthe ocean oil and gas. It is the most important medium between the wellhead at the sea bottom and the floating structure at the sea surface. Indeep sea condition, the geometric shape of the riser is a slender column incomplex and extreme working environment. All kinds of load, like storm,wave and current would affect the structural safety of the risers. At thesame time, because of the considerable unmeasured and unforeseeablefactors in the estimation of the safety of the deep sea risers, the responseof the riser results in randomness. At the present time, the strength of theriser under specific conditions can be calculated with the help of thespecific finite element software.However, it is extraordinary difficult toquantitatively determine how much is safe of the riser under complicatedenvironment loads. Thus,considering all kinds of stochastic influence, thereliability analysis of riser is necessary and significant.
As the limite state function of the riser response cannot be expressedas an analytical form in terms of basic random variables, the traditionalmethod cannot be applied to procee reliability analysis. Therefore, anapproach combines design of experiment (DOE), finite element method(FEM), response surface method (RSM), and first order second momentmethod (FORM) is put forward to calculate the riser’s structural strengthreliability under extreme storm condition and the riser’s fatigue reliabilityunder wave load or vortex-induced-vibration, take top-tensioned riser(TTR) as an example. In this thesis, the general finite element software isapplied to proceed the global analysis of the deep sea riser to master theglobal response characters of the TTR under operation and hang-offworking conditions. And then, on the basis of design of experiment andspecific finite element software modeling, the main factors which affectthe structural response under different load conditions are selected. Usingthe polynomial response surface method, the approximate analyticalexpression between the structural response and random input variables is built. And under this basic procedure, the riser’s structural performancefunctions of different load conditions and different locations aredetermined. Afterwards, the riser’s structural strength reliability underextreme storm condition and the riser’s fatigue reliability under waveload or vortex-induced-vibration are finished. Furthermore, the results arevalidated by the Monte Carlo method based on selective sampling method.At last, an integrity monitoring system application plan is put forwardbased on the reliability analysis results.
The proposed method based on RSM can directly use thedeterministic FEM software, and build a bridge between the finiteelement method and the first order second moment method or MonteCarlo method, which can solve the reliability analysis of riser withimplicit performance function and reduce the word load. It has goodtheoretical and application value in practical engineering applications.
引文
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