Numerical Analysis of Stress/Strain Fluctuations in Coiled Tubing During Deepwater Deployment

Abstract

Conventional coiled tubing experiences a set of bending and straightening cycles during deployment into and out of a well bore. The severe bending strains that must be imposed on the tubing can be up to 3 % and are therefore well above the yield strength of the material. This leads to fatigue failure at extremely short lives.

However, during deep water well interventions coiled tubing is also subjected to high cycle fatigue. After exiting the sheave, the tubing is deployed through open water with a clump weight and suspended vertically at water depths up to 3800 m. This happens while fluid is being pumped at pressures that can reach 68 MPa. The ocean waves impose pitch and roll on the vessel. This causes wrapping and unwrapping motion of the tubing on and off the sheave, which induces stress/strain fluctuations at the tangent point and can lead to high cycle fatigue damage accumulation.

To understand the stress state at the critical location, detailed finite element analyses were conducted using sophisticated incremental plasticity and contact elements to quantify the influence of the angular displacement magnitude, the axial force, and the internal pressure on the stress range at the critical location. Four cases were investigated: low pressure/medium force, high pressure/medium force, low pressure/high force, and high pressure/high force. When comparing the maximum principal strain ranges at small pitching angle (≤2°), it has been found that at a fixed internal pressure, the same strain strange is depicted for both axial forces. At higher pitching angles, however, the strain range increases with increasing axial force. In addition, axial forces tend to have a greater effect on the maximum principal strain range at a smaller pressure.