ln the event wellhead integrity is lost, the uncontrolled flow of hydrocarbons could risk life, property and the environment. To prevent this risk subsurface safety valves can be used to control the well below the wellhead. Conventional surface controlled subsurface safety valves (SCSSV) are used to control the production tubing. However, often the annular space between the tubing and casing contains large quantities of gas directly below the wellhead. An annular control (AC) safety valve is a surface controlled subsurface safety valve which allows annular flow when open but provides a barrier below the wellhead when closed. When the AC safety valve is used with a conventional SCSSV it can provide overall well safety.
The scope of this paper is to identify, review the design and summarize testing of the components that comprise an annular control safety system.
Annular control safety systems have been used since the early 1970s. Initial applications utilized wireline retrievable valves packed off through shallow-set production packers. As tubing retrievable safety valve installations increased, tubing retrievable annular control (TRAC) safety valves were also developed. Similar to the wireline retrievable annular control safety valve, the TRAC valve packed-off through a shallow-set production packer. Typical applications included: gas lift, gas storage and electrical submersible pump installations. The majority of these installations involved small tubing sizes which did not present any significant design concerns such as casing deformation.
However, after the Piper Alpha disaster, the philosophy of isolating all hydrocarbons, tubing and annular, from the surface was adopted. Consequently, this meant a broader application for annular control technology, Utilizing annular control in installations with large diameter tubing presented new concerns and the need for new design criteria. Annular control installations usually require a shallow setting depth. Because the casing is unsupported by cement at this location, maximizing the weight hanging capability of the hanger packer while minimizing the possibility of permanent casing deformation is the primary concern. Development of an annular control (AC) system that satisfies this concern has been accomplished by carefully evaluating operator needs and developing an effective design.
The basic AC system, whether a single or dual completion, is designed around a shallow set packer which supports or hangs the tubing string. In a single (or concentric) completion, the TRAC safety valve is controlled from the surface hydraulically and allows the annulus to be isolated when closed. An annular control nipple is installed directly below the packer to provide a seal bore for the lower pack off on the TRAC safety valve to bypass the packer element.
A majority of annular control installations will be at shailow setting depths and will use the annulus as an injection path for gas lift operations. With this in mind, the following design parameters were used to develop the annular control safety system:
Minimize casing deformation/stress Reduce the number of seals between tubing and annulus Maximize tubing and annular flow areas Design for failsafe operation Provide installation flexibility Utilize field proven technology.