Modern engineering analysis software and computer-aided manufacturing have been employed to create completion equipment using larger tubing for a selected casing program. Early utilization of engineering analysis allows electronic testing to shorten the development cycle. Predicted loads can be applied and a mechanism cycled while the equipment exists only in electronic form. A new series of subsurface safety valves, packers and completion accessories has been developed using these techniques. Extensive testing has been performed to qualify the modeling and confirm the performance. New design of all components allows the system to be optimized for large bores and extremely high flow rates.
Over the past 30 years, the size of tubing used in the typical producing well has increased. Today, we are seeing the first completions using 9 5/8-in. production tubing as a standard. This increase from the now-commonplace 7-in. completion offers advantages to the operator. The design of a 9 5/8-in. completion presents new engineering challenges to the designers of completion equipment along with an opportunity to optimize a new completion system.
The first subsurface safety valves were usually slickline retrievable poppet-type valves. Little consideration was given to obstructing the bore of the tubing.
With the movement away from legislated production restrictions, high flow rates became the norm. Tubing retrievable surface controlled subsurface valves providing an unrestricted bore were required to allow high flow rates with less chance of damage from erosion. Ball type closure and flapper type closure valves were the standard subsurface valves in the mid 1970s.
The ball valve and the flapper valve designs each allowed a similar through bore inside a given housing diameter. As markets developed, curved flapper valves were developed to provide a larger inside diameter for a given housing diameter. At the same time, flat flapper valves were designed with an offset internal bore to reduce the diametric penalty imposed by the flat flapper as compared with a curved flapper.
However, even modern curved flapper safety valves have room for further optimization. The efficiency of the use of space in the flapper region can be improved. Opposite the flapper hinge area, the annular region is not used for valve operation. In the upper portion of the typical suburface flapper valve, spring and piston mechanisms occupy the available space. The relative cross-section differences are shown in Figure 1.
A more efficient design has been developed. The flapper mechanism is moved off the centerline of the outer housing. This produces a larger inside diameter for a given outer housing diameter. To take advantage of this change, the piston and spring mechanism must also be moved. Modern flapper safety valves typically use a single rod piston, which is located between the inner and outer housings. The change to an eccentric design allows more room for the piston mechanism. Most conventional subsurface flapper valves use a spring concentric to the valve OD and ID to provide closing force. With the change to an eccentric design, the room for an axial spring is reduced.