Abstract
For special high dynamic loading applications, the structural integrity of the girth welds shall withstand stress levels that might be on the limits of the permissible defect tolerances for current production welding standards for plain pipe ends. In addition, unexpected loading conditions might take the stress limits out of safe operation, which can compromise the entire line. As a solution, the cross section of the girth weld may be increased for ensuring the strength and fatigue resistance under any loading circumstances, including strain cycles of reeling installation technique. The employment of pipes with upset ends is an excellent option for those cases. To propose this option as an alternative to current offshore solution for a Major O&G company, Tenaris developed a long upset pipe end with enhanced fatigue life. The challenges of this work included the manufacturing of very long upset ends from a medium wall thickness pipe, very tight mechanical properties difference between pipe and upset material properties, and finally a welding qualification program. The improvement of the fatigue strength of this product was highly expected.
In order to achieve all requirements, especial arrangements were performed on the upsetter machine for achieving the target upset geometry; which was previously obtained by a design of experiments technique. Then the heat treatment of the pipes was designed for obtaining the tight mechanical properties difference between pipe body and upset sections. The main outcomes of the whole development are described within this paper; which include key information of how to overcome issues that might arise during the development and production stages of upsetted line pipes. The upset ends undertake a cylindrical machining; this process provides the advantage of achieving tight dimensional tolerances in the high-low girth welding alignment. The fatigue endurance data after full scale reeling experimental test are included, as well as the numerical comparison between the strain fields of plain pipe and upset girth weld unions. The welding procedure qualified during this work is described. The results of the whole development were very satisfactory and, as expected, the fatigue strength of upset ends was higher than the plain pipe.
