Study On Dynamic Risk Assessment Method Of Subsea Wellhead During Service Life

As a bridge between oil-gas wells and blowout preventers,subsea wellhead system undertakes the whole weight of blowout preventer and casing in connection operation mode.It provides a safety barrier for drilling operation and plays an important role in the process of offshore oil and gas development.Generally,the service life of subsea wellhead is no more than20 years.During its life cycle,the operations of drilling,completion,well intervention,plug and abandonment are required for many times.However,during the operation process,the subsea wellhead is suffered from cyclic dynamic load induced by ocean environment change.Under the dynamic load,varying degrees of fatigue damage of the subsea wellhead will be induced.With the accumulation of fatigue damage,fatigue degradation will occur,resulting in a significant increase in the risk of fatigue failure.Since,during the wellhead design stage,it is necessary to consider whether the wellhead can bear these loads in its whole life cycle.If the allowable stress of the load is exceeded,fatigue damage will lead to the failure of the subsea wellhead and eventually fracture.At present,most subsea wellheads are close to or already have exceeded their design life.To further improve the oil recovery,the overhauled subsea wellheads must undergo fatigue conditions and corresponding risk assessment to achieve the purpose of oil well life extension.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,and subsea wellhead research has been carried out.The main progress and results are summarized as follows.(1)Uncertainty analysis and risk analysis method selection of subsea wellhead fatigueThe subsea wellhead structure was introduced.According to characteristics of the subsea wellhead,uncertainty of subsea well was analyzed from four aspects: load,operating parameters and equipment,anti-fatigue capability and critical fatigue position of subsea wellhead.Some commonly used risk analysis methods were introduced,such as: fault tree(FT),event tree(ET),event sequence diagram(ESD),Bow-tie(BT),bayesian networks,and dynamic bayesian networks.Then their advantages and disadvantages were compared.In order to solve the three difficulties of subsea wellhead fatigue failure risk analysis,the matching point between the method and the object were found,and the dynamic bayesian network was finally chosen.(2)Identification and coupling analysis of risk factors for subsea wellhead fatigue failureThrough the description of subsea wellhead fatigue failure accidents and related literature research,comprehensive risk factor identification was carried out for the subsea wellhead failure mode from five aspects: environmental factors,human management factors,equipment factors,design factors and operational factors.Based on the identified risk factors,a comprehensive risk evaluation index system of subsea wellheads was established.Meanwhile,considering the subjective initiative and objective mechanics,the weighting method combining analytic hierarchy process and entropy weight method was adopted to analyze the weighting value of risk factors,and a simple ranking of risk factors importance was carried out.On the basis of the traditional four-factor risk coupling method,the coupled topological structure of the five major risk factors subsea wellhead was established,and then the risk factor coupling analysis was carried out.(3)Risk analysis of subsea wellhead during service lifeThe concept of the subsea wellhead whole life cycle was introduced.Most of the subsea wellhead is close to or has exceeded its design life,thus it is necessary to carry out the risk analysis to ensure the safety of subsea wellhead operation,.Aiming at the six stages of subsea wellhead operation,a static bayesian model for predicting fatigue failure of subsea wellhead was first constructed.Considering the dynamic development of fatigue accumulation over time,the static bayesian network was transformed into a dynamic bayesian network.Through parameter learning,the prior probability of basic events,conditional probability of intermediate nodes and transition probability of dynamic nodes were obtained.The data was imported into the model for the calculation of the lifetime cycle fatigue failure probability of the subsea wellhead and the reasoning of the posterior probability of basic events.The sensitivity analysis was then carried out to verify the accuracy of the model and put forward targeted risk control measures.