Research On Riser Structrual Reliability Based On Response Surface Method

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.