Hydraulic control lines are commonly used to actuate surface- controlled subsurface safety valves (SCSSVs), and new applications include choke operation and the control of more-complex "smart-well" completions. In general, control lines are not subject to routine failures. However, the analysis of worldwide completion failures indicates control lines to be a critical component of failure. In fact, control lines and associated components, such as clamps and fittings, are not engineered with the same rigor as the rest of the well completion.
The first step in understanding control-line failure is predicting the loads and stresses in a control line strapped to the tubing. Tubing movement causes loads in the control line through stretching and bending. To a lesser degree, the tubing is loaded by the control lines. To determine this interaction, a calculation is performed in which the control line and the tubing are treated as a composite, with axial displacement constrained to be the same in both. This analysis provides the average stress state in the control line. Because the control line is fixed at only certain points along the tubing, the variation in stress from the average must be determined between clamps. This paper provides the technical details for both calculations.
Several example cases based on field data are presented that give insight into the potential problems that typical production scenarios create for control lines.