The Buckling Behavior Of Plug Removal By Using Coiled Tubing In Offshore Pipelines

Coiled tubing (CT) operating system can effectively improve the operating efficiency of offshore oil and gas exploration with a wide prospect of application, and removal of plugs in pipelines by CT has become a new direction in the field of the application. However, application research of CT on offshore oil fields is quite different from that on onshore oil fields, for the complex structure of offshore pipeline. If the theory analysis and laboratory experiments can be based on, to establish the relevant mechanical model and calculation model of actual running depth of the CT can provide a theoretical basis for removal of plugs in pipelines. It also can provide the calculation basis for relevant operational process of plug removal, which is of important theoretical and practical significance.This paper adopts the method of combining theoretical research with experimental verification to study the buckling of CT for removal of plugs in pipelines. First of all, the differential equations caused by helical buckling of CT in pipelines are set up, and the torsion effects are compared and analyzed. Based on this, helical reversal calculation model and modified calculation model of contact force have been established, and calculation method of axial force of CT in the bending section is also put forward. Secondly, the experimental research methods of force and buckling of CT in pipelines are established, the appropriate geometric parameters of experimental system are presented, and the test bench is designed. The test bench includes three sections:a vertical section, a bending section and a horizontal section. And CT simulation tests by rubber rod and steel wires with three different diameters have been respectively carried out, where in the bending section, and two kinds of radius of curvature have been used. At the ending point, three different constraints (free, fixed and loading forces) are respectively considered. The experimental phenomenon such as buckling, helical buckling, lock-up, helix reversal, yield of head and load jumps in end loading experiments are analyzed. Thirdly, helix reversal and model of contact force correction coefficient are analyzed emphatically combined with the experimental results. With the consideration of elastic effect, temperature effect and the effect of helical buckling, it puts forward the calculation model of actual running depth of CT in pipelines. The criterion of lock-up of CT in pipelines in the actual operation has been set up, and the alarm setting in accordance with load emergency of injection side of CT is also put forward. Finally, for structure of pipeline of three oilfields in the South China Sea, this paper discusses the feasibility of removal of plugs in pipelines by CT.The study shows that the torsion effect caused by helical buckling is small, which can be ignored. Based on this, the helical reversal calculation model and axial force calculation method for bending section based on above are accurate and practical. And the experimental data show that the experimental results are in good agreement with the calculation results, and the load jumps in end loading experiments and radial gap will affect helical buckling and lock-up process, but the injection rate has little impact on experimental mechanics behavior. And experiments and theoretical analysis show that the diameter of CT, radial gap between CT and pipeline, the coefficient between CT and pipeline and radius of curvature of the bending section are all of the greater effects on the buckling behavior of CT in pipelines. During the experiment, the assumption of head force is 10 times the magnitude of end force while end force is 3 to 5 times the magnitude of the critical buckling forces is adopted to judge whether the lock-up occurs. But in the field operation, the criterion that injection force is 30 to 50 times the magnitude of critical helical buckling force, which provides guidance for engineering practice, can be used for the judgment of helix lock-up.The feasibility calculation of removal of plugs in pipelines by CT of three oilfields in the South China Sea shows that plug removal can be effectively carried out by CT with any diameter within the scope of operation, but the drag reducing fluid must be used when the smaller diameter CT is applied.