The successful operation of completion tools, especially intelligent completion tools, at high pressure/high temperature (HPHT) operating conditions is dependent on reliable design and robust production and operating procedures. Recent technological advancements in high-temperature electronics can lead to changes in the production processes of existing completion products. Possible reasons for change include component obsolescence, consolidation of production processes, or leveraging technological advancements from one product to another. Verification of changes made in production processes is critical to maintain high operational reliability of the product. This paper presents a methodical test design approach to verify changes made in production processes in a cost- and time-efficient manner.
The purpose of the Production Acceptance Test (PAT) is to provide assurance that the reliability of standard production items meets the reliability specifications. PAT usually involves testing of a sample of items drawn from a production batch. The results obtained from testing these samples enables an informed decision regarding the reliability of the entire production population. The concept and philosophy of PAT stems from the military handbook, MIL-HDBK-781, based on non-parametric distribution utilizing Mean-Time-Between-Failures (MTBF) as the performance criteria. To overcome the limitations of MTBF and non-parametric distribution, a modified test design methodology includes a comprehensive reliability statement of the product along with the use of parametric cumulative binomial distribution. Elements of PAT design (Discrimination Ratio (DR), producer's risk, consumer's risk, etc.) are assessed for developing high-temperature electronics for HPHT environments.
A key prerequisite of employing PAT is to assess product reliability through either reliability test data or field operational data. A reliability specification of the product is an upfront requirement of designing PAT. The test design methodology presented utilizes comprehensive reliability statements in terms of % probability of success, % confidence, lifetime, and operating conditions. DR is another key element of PAT design and typically ranges from 1.5-3.0. However, for manufacturers with robust production processes and well planned management-of-change, DR lies somewhere in the range of 2.5-3.0. By selecting optimum values of test design parameters, an effective test plan can be developed which can result in time and cost savings compared to a standard reliability test.
This paper discusses the methodology and application requirements to enhance PAT as a verification test to validate any changes in production processes. Test design methodologies are upgraded to include a comprehensive reliability statement along with the use of a parametric cumulative binomial algorithm to overcome the limitations of original PAT methodology highlighted in MIL-HDBK-781. Optimum value assessment of PAT design parameters was performed for developing high-temperature electronics used in HPHT environments. A case history of a downhole electronics module is presented to describe the enhanced approach and benefits of PAT.