Research On Structural Integrity For Deepwater Subsea Separator

The exploration activities of offshore oil and gas gradually move to deep water,subsea production system has become an effective mode of deepwater oil and gas exploration.Subsea separator acts as the key equipment of subsea production system.Deepwater subsea separator faces huge risk in the deep-water environment and once a major accident happens,serious economic loss and environmental damage will be caused.This dissertation is supported by "Research on Structural Analysis and Testing for Pressure Bearing Structure under High Alternating Pressure Load and Research on Subsea Separator Security Assurance Technology",which is the subtopic of the National High Technology Research and Development Program(863 program)"Research on Key Techniques of Subsea Separator".For meeting the deepwater production operation demand of the South China Sea,systematic research on structural integrity of subsea separator at 2000-m water depth has been carried out.The research covers structural suitability assessment,structural strength and stability experiment study,full life cycle risk analysis and control method,leakage evolution mechanism and countermeasures,which acquires major progress.Furthermore,the structural integrity management methods for deepwater subsea separator are formed.The present study could provide scientific basis for the safety operation of deepwater subsea separator in the South China Sea.The main research progress of this paper is summarized as follows: 1.Research on structural suitability assessment for deepwater subsea separatorConsidering the material nonlinearity and geometrical nonlinearity of deepwater subsea separator,a structural suitability evaluation method consisting of "stress classification + 5% strain + consistent imperfection buckling" limit analysis is put forward for the installation condition and the normal operating condition.Taking a 2000-m subsea separator prototype as an example,the structural failure limit load is computed.A fine three-dimensional numerical model of subsea separator containing embedded nozzle is established based on the submodeling technology.The structural stress analysis method for embedded nozzle is formed to explore the stress distribution along saddle-shape weld toe.Furthermore,the impact of geometric parameters such as cutting hole radius,weld bead angle and weld toe radius on the stress distribution along weld toe is analyzed.2.Structural strength and stability experiment study for deepwater subsea separatorHyperbaric chamber strength experiment of 2000-m subsea separator prototype and hyperbaric chamber crushing experiment of 2000-m subsea separator scale prototype are designed and the key problems in hyperbaric chamber experiment including ACFM detection before the experiment,establishing the layout plan of measuring points,formulating the loading program are solved to form a structural testing method for ultra deepwater subsea separator.Limit analysis results of the scale prototype and the prototype are compared to verify the feasibility of using the scale prototype for crushing experiment instead of the prototype based on the similarity theory.According to the crushing experiment results,buckling development process of the scale prototype is explored.Strain data of measuring points are extracted and compared with the numerical results to verify the accuracy of the numerical model.3.Full life cycle risk management for deepwater subsea separatorThe security assurance technology framework of full life cycle for deepwater subsea separator is established,which is based on the life indicator system of full life cycle including design stage,manufacture stage,installation stage and operation stage.Risk control is done by means of inspection and early warning before the accident as well as emergency response after the accident.The security assurance technology provides guiding outline for risk management of deepwater subsea separator.The pendulous installation method is recommended to install deepwater subsea separator considering some aspects such as adapted condition,marine resources,economy and so on.Meanwhile,the process risk of pendulous installation is analyzed on the basis of HAZOP and JSA analysis.Cause-consequence diagram and Bayesian network for the clog accident during the operation stage of deepwater subsea separator are established and the posterior probability characteristic is studied.The most probable path result in the clog accident and the sensitivity of the influence factors are explored.Furthermore,the dynamic risk of the clog accident is analyzed based on safety barrier failure precursor data.4.Research on leakage evolution mechanism and countermeasures for deepwater subsea separatorAny failure form in the operation stage of deepwater subsea separator will result in leakage accident eventually.Three-dimensional structured mesh of inner flow field of deepwater subsea separator is established using O-Block mapping technology of ICEM.On this foundation,the initial flow field is acquired by introducing Realizable ?~? turbulence model and Mixture multiphase model.The initial flow field of deepwater subsea separator is taken as initial conditions for hole-caused leakage approximately.Using VOF model to track the free interface,CFD analysis model of hole-caused leakage is established and the influence of different leakage position and water depth on hole-caused leakage is analyzed.In terms of deepwater oil spill accident caused by subsea separator rupture leakage,the evolution mechanism of deepwater subsea separator rupture leakage is explored considering the influence of deepwater internal waves.In view of the leakage risk during the operation stage,leakage inspection and control strategy for deepwater subsea separator are proposed.5.Structural integrity management for deepwater subsea separatorStructural integrity assessment,structural reliability assessment and risk-based inspection and maintenance planning for deepwater subsea separator are studied systematically and the structural integrity management framework of deepwater subsea separator is perfected.The influence of corrosion,dent and ovality defects on the bearing capacity of deepwater subsea separator is analyzed.Given the weld feature,the structural integrity assessment technology of deepwater subsea separator is proposed.Based on the reliability analysis theory of series system,the structural reliability limit state equation of the deepwater subsea separator is proposed.Dynamic reliability evaluation method for deepwater subsea separator structure is formed by taking dynamic corrosion characteristics into consideration.The cost analysis and probability analysis of inspection and maintenance for deepwater subsea separator are carried out on the basis of the "minimum total cost" principle.The risk-based inspection and maintenance optimization model of deepwater subsea separator is established and the optimal inspection and maintenance scheme is proposed.