Abstract

Hydraulic control lines are commonly used to actuate surface controlled sub-surface safety valves, and new applications include choke operation and the control of more complex "smartwell" completions. In general, control lines are not subject to routine failures. However, the analysis of world wide 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 where 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.

Background

In a recent study of world-wide down-hole completion failures utilizing an industry recognized completion reliability database, down-hole control lines were identified to be a critical component in the failure analysis. Current well scenarios and conditions are requiring greater use of hydraulic down-hole control systems connecting to intelligent completions and deep set SCSSV's. Well configurations with geometry changes from multilaterals and high deviations are putting greater stress on the lines. The reliability of the control line is paramount to the success of operation for any of the down-hole components to which it is connected, i.e. failure of an SCSSV to operate because of a 'leaking' control line could cause catastrophic well failure (loss of well - MM$ or worst loss of life).

Specific analysis based on cost of intervention for just the control lines revealed that control line failures alone could amount to approximately $150K USD/well-yr (average 4 lines per well).

  • Recorded Control line items - 4000

  • Recorded service years - 21000

  • Recorded failures - 64

  • Known failure causes - 10

  • Unknown failure causes - 54

MTTF = service time/no of failures; 21000/64=328.125 (Failure rate = 0.003)

Assuming a 12.5MMUSD intervention cost for fixing failed control lines Þ $38,000USD/line-year.

Assume average four lines per well Þ $150K USD/well-year. (This does not include the lost production.)

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