An experimental study of mechanical behavior of coiled tubing in pipelines

•Experiments with terminal constraints of free end, fixed end and loading end have been carried out.•Results of an experimental study on buckling of coiled tubing inside pipeline are presented.•Effects of radial gap and load jumps on the course of coiled tubing helical buckling and lock-up are investigated.•The decision criterion of helix lock-up in the field operation of coiled tubing inside the pipeline is proposed.

Coiled tubing operating system can improve the efficiency of offshore oil and gas exploration with a wide prospect of application. Coiled tubing is commonly subjected to complex loading inside the pipeline or wellbore. Through experimental research, we can derive the experimental explanations of actual pipe depth the coiled tubing goes into, buckling, lock-up, release, etc. In this paper, the method of combined numerical calculation and experimental validation is used to study coiled tubing buckling inside the pipeline or wellbore. The geometric parameters of the experimental system are presented. A test bench including a vertical segment, a bending segment, a horizontal segment and an end loading unit is designed. We carry out experiments using rubber rods and steel wires with three different diameters, bending segment of two kinds of lengths, and terminal constraints of the free end, fixed end and loading end. Experimental phenomena such as sinusoidal buckling, helical buckling, lock-up, pitch reversal, yield of head and load jumps in end loading experiments are analyzed. The experimental results are in good agreement with the numerical results. The load jumps in end loading experiments and radial gap affect helical buckling and lock-up process. Moreover, the decision criterion of helix lock-up in the field operation is proposed, which can provide a guide for engineering practice.