Safety Assessment Research On Blowout Preventer Shell Crack Defects

The boundary dimension of blowout preventer shell varies since it is big and has a complex shape. When oil spilling happening around the wellhead, it is necessary to take urgent action by using blowout preventer to perform shut-in action. As hard shut-in technology becoming increasingly popular, it is more likely for the blowout preventer to suffer impacts of ultrahigh pressure oil and gas. The shell is the main pressure-bearing subassembly of blowout preventer. Stress concentration tends to form on the local area of the shell when under instantly impact of the super load, which will continue impacting the shell during its attenuation. The local area of stress concentration may generate crack defeciency or even lead to brittle rupture. Thus, it is crucial to cunduct safety assessment on blowout preventer shell.SolidWorks is used to build 3D models and SimulationXpress is used to conduct finite element analyses on blowout preventer FZ35-105 and FH35-105. Under the 157 MPa simulation of hydrostatic test, the simulation results showed that the maximum equivalent stress of the two types of preventer are 239.9MPa and 227.4MPa with maximum displacement of 0.039 mm and 0.005 mm. the finite element analyses verifies the veracity of the safety assessment results.Based on the characteristic of fault tree analysis and the specialty of shell crack on blowout preventer, four types of defects, which are heat crack, cold crack, stress-corrosion cracking and mechanical fatigue fracture, are studied. Then a falut tree is built to analysis the 22 elementary events which may lead to blowout preventer shell crack. FTAS softwear is used to carry out their structure importance, and the reason of these elementary event happens as well as the influences they have on the shell are also studied. Then boolean algebra method is used to figure out the minimum cutset. Considering the formation mechanism and current situation on the shell, a definition description value is given to the each of the 22 elementary events. Again, FTAS softwear is used to caculate the occurrence probability of the top event as well as the structure importance, probability importance and critical importance of elementary events. It is concluded that the probability of top event P(T) is 2.37×10-4. Lastly, though importance analysis, the criticality of elementary envents are confirmed, which provide the theoretical support of the safety assessment on the blowout preventer.Analytic hierarchy process(AHP) is used to conduct stratification analysis on the crack tending to occur areas of ram preventer and annular preventer. The influence factors which caused the crack happens is analyzed to determine criteria factors of AHP hierarchical structure. The dangerous areas are analyzed to make sure the AHP hierarchy alternatives. By studying the correlation between the criteria factors and alternatives, the AHP structure system is established. MCEAHP softwear is used to calculate weight of alternative options. Origin softwear is used to draw the coordinate graphs of rules and weight. Grading method is used to form the evaluation basic factors of the blowout preventer shell grey-fuzzy model. Evaluation matrixes and whitenization weight functions are established to sovle the gray coefficient of the crack tending to occur areas on the ram preventer and the annular preventer shell.