Study On Fatigue Assessment And Countermeasure Of Subsea Wellhead In Deepwater

This dissertation is supported by China Offshore Oil Research Center project “Integrity technology and its engineering application for deepwater riser-subsea wellhead system in full life”,which is a subproject of national major science and technology projects in the 13 th Five-Year Plan.In accordance with the requirement of oil and gas exploration and development in the South China Sea,a research on assessment and countermeasures of subsea wellhead fatigue in deepwater is carried out firstly in domestic.Major progress has been achieved in local analysis of subsea wellhead,fatigue assessment method,the whole life cycle fatigue assessment and technical countermeasures.The study will provide technical support and guidance for the design and application of subsea wellhead system in the South China Sea,and it has good academic value and engineering application prospect.The main progress and results are summarized as follows.(1)Local analysis of subsea wellhead systemThe local finite element model of subsea wellhead was established with consideration of the interaction between soil and conductor.The local analysis method and process of subsea wellhead were established based on API RP 2A WSD and other norms.With systematic research on static characteristics,the study on factors of stress and the comparison of conventional model and local model were carried out.The equivalent model of the subsea wellhead was proposed,and the calculation method of its key parameters was introduced.At last,the equivalence model was compared with the local model to verify its rationality.(2)Fatigue assessment method of subsea wellhead systemBased on the local model and equivalent model,the semi-coupling assessment method and process of subsea wellhead wave-induced fatigue in the time domain were established with reference to DNV-RP-F204 and other recommended practices.The results of decoupling method,coupling method and the semi-coupling method were compared and the cyclic bending moment of the wellhead was extracted to study the essential differences between the three methods.On the basis of the study above,the application scope of the three methods was given.(3)Fatigue assessment of subsea wellhead in life cycleVIV fatigue assessment method of subsea wellhead was established firstly.And based on the investigation of the configuration of the riser-wellhead system in different operation modes,study on wave-induced and VIV fatigue damage of subsea wellhead under drilling,completion and intervention condition was carried out respectively.In addition,comprehensive fatigue assessment method of subsea wellhead in the whole life cycle was introduced and the occupancy ratio of fatigue damage under different operating modes was identified.(4)Sensitivity analysis of influential factors for subsea wellhead fatigueThe effects of soil strength,riser size and other factors on fatigue damage of subseawellhead were studied by the semi-coupling method,and some technical countermeasuresto improve the fatigue life of subsea wellhead were put forward.The intelligentoptimization method of buoyancy configuration for deepwater drilling riser system isproposed based on GA and VIV analysis model with buoyancy configurationcharacterized by the gene coding method.The intelligent optimization system ofbuoyancy configuration for deepwater drilling riser system is developed and enumerationmethod is applied for verification.At last,buoyancy configuration optimization designfor different water depth is carried out.