Research On Influence Of Corrosion Pits To The Strength Of Coiled Tubing

The applications of coiled tubing in oil and gas industry are becoming more widely, almost 5 million meters coiled tubing are used every year. Its working condition is extremely bad, and corrosion defects are always found. Combined with tension and bending loads, cracking formed or fractured at the point of pit, this makes the coiled tubing failure. So in order to guarantee the service safety of coiled tubing, it is the most imperative problems to be solved to settle down strength evaluation method for corroded coiled tubing, and to put forward critical depth of corrosion pit.The research object is coiled tubing of QT900 grade and Ф38.1mm×5.16 mm, with corrosion pits. Theoretical analysis and simulation method are used to study its mechanical characters under internal pressure, tension and bending load with the guidance of API 579 guidelines. The influence of pits on remaining strength is made clear, critical pit depth and remaining strength are proposed, these provides theoretical foundation and evaluation items for the safety evaluation of coiled tubing working in this well block and provides reference for coiled tubing working in other blocks.The results show: the main damage phenomenon is corrosion pit for QT 900 coiled tubing working in KS well Block, and the remaining strength decreases with the increase of depth and density of pits. The evaluation results of Level 2 assessment are little more accurate than Level 1, and critical pit depth is 4.13 mm under 20 MPa internal pressure. With Level 3 assessment, the proposed critical pit depth is smaller than 0.005 mm under 439 MPa tension load. It’s 0.516 mm, 1.032 mm and 1.548 mm respectively with bending radius of 1219.2mm, 1257.3mm and 447.8mm. Further more, the most dangerous procedure appears when coiled tubing come down through bending Goose neck, where the critical pit depth is between 0.516 mm and 0.774 mm under 439 MPa tension load with 1828 mm bending radius.