The Feasibility Of Mini Coiled Tubing To Cleanout A Plugged Umbilical

ABSTRACT
The purpose of this paper is to present an evaluation and assess the feasibility of various Coiled Tubing (CT) sizes to reach and remove a wax plug in a capillary in an umbilical. The wax plugs are located at a prescribed distance along the length of the umbilical.

The equations governing the deformation of CT have been identified and modified. A field example is considered with the associated choices of mini coiled tubing that can be used to potentially achieve the objectives of the job.

The main factors that influence the ability of the CT to reach a given location in the umbilical are:

• Friction between the surfaces of the capillary in the umbilical and the CT
• Radial clearance between the capillary and the CT
• Weight of the CT per unit length
• Bending stiffness of the CT
• Inclination of the umbilical
• Curvature of the path of the umbilical
• The helical pitch of the capillary in the umbilical.

These parameters affect the following:

• The normal force between CT and umbilical
• The friction force between CT and umbilical
• The axial compression (or tension) on the CT
• The critical helical buckling load.

When the compressive force on CT is greater than the critical helical buckling load, the CT will buckle into a helix, which leads to lockup. The equations that govern the normal force and the frictional force on the coiled tubing have been modified to account for the curvature of the umbilical and the helical pitch of the capillary.

Previously, there has been no attempt to model the affect of the helical pitch from a capillary or even a tubing string. The exclusion of the pitch effect may have shown why previous simulations may have been successful, whereas the actual field job unsuccessful due to premature lockup.

The inclusion of the helical pitch leads to additional normal forces which restrict the length/depth at which the tubing can be deployed. The premature lockup can now be accounted for by the additional normal/frictional forces induced by the helical pitch.

Bhalla, K. and Gong, Lixin, “The Feasibility Of Mini Coiled Tubing To Cleanout A Plugged Umbilical,” OMAE2010-20567 Proceedings of the 29th International Conference on Ocean, Offshore and Arctic Engineering, OMAE 2010, Shanghai, CHINA, June 6 – 11, 2010.