The remotely operated electromechanical control device (ECD) is coupled to an open-close valve that would normally require hydraulic lines from surface. With each function of the valve, an intervention is eliminated, which increases operational efficiency and saves rig time. As these devices are permanently installed in severe downhole environments, design-in reliability is paramount. Reliability demonstration test (RDT) is a key element of the design-for-reliability process to verify that the product satisfies the system reliability target. This paper presents a structured RDT approach to demonstrate the reliability of a remotely operated ECD in a cost- and time-efficient manner.
The reliability target of the ECD was established based on the tasks requirements (open-close functions), well conditions, and mission life. Key subassemblies of the ECD were identified, and a system reliability target was allocated to the subsystem level using a weight factor-based approach. A test-to-success methodology was used to design the RDT of individual subassemblies by identifying the underlying failure mechanism, applicable test stresses, and acceleration factors. A parametric cumulative binomial test design model was used to optimize the test parameters, such as sample size, test time, and number of valve open-close functions.
Conducting a system reliability test is often cost prohibitive. Therefore, performing a reliability test at the subsystem level is an alternative approach of verifying system reliability. Reliability allocation weight factors are determined based on the cost, time, and relative difficulty in testing the design feature. Aging parameters were found to be the number of valve open-close functions based on the underlying tasks, operating time, and well environment (temperature).
This paper highlights the structured methodology and application requirements of RDT to meet the mission reliability target of a remotely operated ECD. A comprehensive reliability target was established based on the underlying tasks, operating time, and well environment. A combination of overstress (temperature) and use-rate accelerations was used in the test design. An optimum value assessment of test design parameters was performed for developing a cost-effective test design. The approach and benefits of structured reliability test design are discussed in the paper.