Experimental study on axial load transfer behavior of a coiled tubing stuck in a marine riser

Abstract

After years of using, the marine riser would need pigging operation to remove wax and other debris attached to its interior to recover production. Due to its low stiffness, coiled tubing would buckle when performing the marine riser pigging operation, which would make the injecting force not able to completely transfer into its end and finally damage coiled tubing and riser. Thus, in order to ensure the safety and reliability of the operation, this article reports an experimental study on this topic by building an indoor pipe-in-pipe platform. According to the experimental results, the inner pipe's axial force transfer efficiency is always less than 1. When injecting force is less than the inner pipe's critical helical buckling load, the inner pipe's axial load transfer efficiency is basically the same under "unfixed boundary" and "fixed boundary" at the same force-out. When injecting force is bigger than the inner pipe's critical helical buckling load, "unfixed boundary" would help decrease the inner pipe's axial load transfer efficiency; when the injecting force is three times the critical helical buckling load, the inner pipe's axial load transfer efficiency of "unfixed boundary" can be 3% smaller than that of "fixed boundary." As the outer pipe of "unfixed boundary" would elongate, its axial load transfer would be "delay" compared with "fixed boundary," which means injected displacement of "unfixed boundary" inner pipe would be bigger than those of "fixed boundary" at the same force-out. The research done above might provide important theoretical supports for the marine riser pigging operation.