Mapping Reliability of DIACS to that of SCSSVs Over Three Decades
Authors: Robison Clark E. 
Published: October 2008

Reliability of surface controlled sub-surface safety valves (SCSSVs) was examined in the 1970's on a component level basis by analyzing failed devices. This examination broadened in the 1980s using a system approach via traditional FMEA and Fault Tree analyses. These methods were applied to determine areas of concern in design, material selection, manufacturing processes, installation practices and operation. Many contingencies have been identified and mitigated for SCSSVs, through experience, performance, and reliability testing.

In the last decade new reliability analysis tools have been developed1 that are useful in predicting failures prior to installation. Furthermore, test programs have evolved into robust life cycle evaluation methods. All of these techniques are essential when consideration is given to fail safe closure devices. And they are transferable to other downhole devicesthat share structural similarities or system configurations. This paper addresses the similarities between SCSSVs and downhole instrumentation and control systems (DIACS), and relates the applicability of the reliability analysis currentlyaccepted for SCSSVs to the DIACS.

Background

In 1997, the first installation of a downhole instrumentation and control system changed forever the landscape for completions2. These early installations included single point pressure and temperature sensors for each flowing interval as well as hydraulically operable sliding sleeve flow controls. For the first time an operator could view the performance of a specific flowing interval and by pressing a few buttons on the surface, reconfigure the flow profile.

Hydraulic fluid, pressurized from the surface, provided the impetus to manipulate the flow controls, and the fluid was routed to the desired flow control with a system of electrically operated solenoids. Combining electrically addressable controls with hydraulics seems like a logical extension of proven technology, however, many valuable lessons were learned from these early installations.

Today there is a strong emphasis on using simple hydraulic control systems to manipulate the flow controls and configuring the sensor systems in a modular fashion to separate the functions of the two systems. Problems resulting from integration difficulties are then minimized. Clearly, this is a move in the right direction.

The reliability history for approximately 1,000 downhole permanently installed pressure and temperature sensor systems was previously analyzed3 and the investigators concluded that cable and electrical connector issues were the most significant contributors to poor reliability. When sensors are integrated in the flow controls the possibility for premature failures is increased significantly. Additionally, as the structural and installation similarities between SCSSVs and DIACSs are mapped the presence of the integrated sensors may negate the comparative analysis.

Although emplacing sensors and remotely operated flow controls downhole enables real-time optimization of the production process and enhanced reservoir management, the system complexity and attendant reduction in reliability may overshadow the benefits. Therefore the reliability analysis of the DIACS becomes a critical path issue in the completion design decision process.