Life Extension of Degraded Subsurface Safety Valve Based on Numerical Analysis of Vibration and Erosion

Abstract Subsurface Safety Valves (SSSV) hydraulically surface-controlled are fail‑safe valves that remain open under applied control pressure. Most of the SSSV mechanisms rely on an inner shroud called flow tube to push the flapper mechanics to open position. Evidence gathered in Brazilian fields suggests that some models of SSSVs were experiencing a fail-to-open failure mode, in which the flow tube could not complete its entire course and would remain in uppermost position, instead of moving down. Without the covering and support provided by the flow tube, injection can still be performed, since flapper allows for downward flow. However, the flapper would be directly exposed to downward turbulent flow, which could cause vibration (chattering effect) and can incur cumulative damage. A series of studies were conducted to evaluate risks of flapper falling downhole or of compromising well safety barrier. These studies started with a Computational Fluid Dynamics (CFD) analysis, to characterize the turbulent flow pattern surrounding the exposed flapper and calculate the fluid forces arising during injection at high flowrates. A structural numerical model was then built, using a Finite Element Analysis (FEA), incorporating both the fluid forces and structural interactions of the components. History of stress was obtained for the components deemed more susceptible to fail: flapper, hinge pin and torsional spring. Erosion analyses were also performed, to determine if the unstable behavior of the flapper body, during injection, could enhance impacting velocities to inner walls of the SSSV, which could produce higher erosion rates and a shorter productive life. Safety factors were found to be sufficient for fatigue and for erosion, considering a mission of 27 years. This result indicates that, if flow tube is unable to move, the increased vibration in flapper system should not produce significant damage, which would risk in rupture of any subcomponent or loss of sealing capability. The decision to perform a maintenance intervention must be made by operator, considering the encountered low risks of leaving the degraded state as is, as opposed to incurring various risks present in a Hard-workover operation, such as releasing of packers and pulling completion accessories out of hole through the blow-out-preventer (BOP).