Surface Controlled Subsurface Safety Valves (SCSSVS) are normally closed devices that provide control of wellbore fluids, and w-c required in virtually all offshore wells world wide. This paper describes self-equalizing SCSSVS, discusses the evolution of the technology, reviews current design issues, and offers a recent technological advance that places an equalizing passageway in a unique configuration. Procedures required to open non-equalizing SCSSVS are discussed, as well as methods of providing an internal equalizing passageway in the valve, and the inherent advantages and disadvantages of each.
Self-equalizing SCSSVS featuring a 'through-the-hard-seat' equalizing passageway will have wide apply in low to medium pressure offshore wells where operational difficulties may rule out a traditional surface equalizing system. The economics of most producing fields will be enhanced by the lower cost, ease, the simplicity and reliability of this development.
Performance of the valve in cycle testing, sand slurry testing, flow erosion testing and temperature testing is discussed in summary
The most common SCSSVS on the market today have flapper closures, and are biased closed by an internal spring. A small diameter control line conveys hydraulic fluid under pressure from a surface pump to a piston internal to the valve, which compresses the spring and holds the valve in the open position, Loss of control line pressure for any reason causes closure of the valve. Pressure builds below the closed SCSSV, while the pressure above bleeds off, and a pressure differential across the flapper is established.
For several reasons, the pressure in all SCSSVS upstream and downstream of the closure must be nearly equal before opening. When the valve is in the closed position, well fluid pressure acting upon the relatively large surface area of the flapper makes opening difficult because the hydraulic area of the operating piston is smaller. This difficulty in opening cannot be easily overcome simply by increasing hydraulic control pressure, The mechanical force exerted against the flapper by the relatively small cross-sectional area of the opening piston and cylinder assembly is simply inadequate to open the valve at normal hydraulic pressures. Increasing the control fluid pressure in an effort to overcome closing forces may burst the control line carrying the hydraulic fluid and/or damage critical internal parts. Even if the flapper could be opened under a differential, the initial flow of well fluid would erode the primary sealing surface of the flapper and/or seat, Any damage to this primary sealing surface is extremely critical because it must remain intact to prevent uncontrolled flow of well fluids when the valve is closed in an emergency, To overcome these difficulties, mechanisms have been developed to allow the pressure above and below the flapper to equalize prior to the complete opening of the flapper.