Buried pipelines can be locally buckled (wrinkled) by a load combination of axial deformations and rotations. Previous test results show that those wrinkled pipes do not lose their safety and integrity if they possess sufficient ductility. However, if those wrinkled pipes are going to continue operating under the condition of cyclic loading, their low cycle fatigue (LCF) behaviours have to be thoroughly investigated. This paper presents LCF tests for two full-scale pipes and the relevant results. Those two pipes were tested under a complicated loading procedure. The entire loading consisted of two stages: the monotonic loading stage and the cyclic loading stage. The monotonic loading was designed to form an enclosed wrinkle around the pipe under a locked curvature, and the cyclic loading was planned to fracture the wrinkled pipe. Firstly, the loading procedure was demonstrated by viewing the spectra of MTS load, MTS stroke, jack load, internal pressure, as well as pipe end average rotation. Secondly, the global behaviour was investigated by examining the relation between bending moment and global curvature, the relation between pivot axial load and relative axial deformation (RAD) between pivots, and the relation between internal pressure and RAD between pivots. Thirdly, the phenomenon of ‘deformation localization’ was studied by investigating the relation between the global RAD and the local RAD, and it was found that the global deformation was totally localized into the wrinkle area. Fourthly, the failure modes were discussed and it was found that the seam weld was more liable to be fractured under the condition of cyclic axial deformation. At last, the failure mechanism was investigated by macroscopically examining fracture surfaces. It found out that the cracks initiated from multi-locations on surfaces, then those cracks propagated from surfaces into the interior and finally fractured the pipe wall. Moreover, it found out that more damage was generated from the inside surface than from the outside surface.

Corroded pipes for oil transportation can eventually experience low cycle fatigue failure after some years of operation. The evaluation of the defects caused by corrosion in these pipes is important when deciding for the repair of the line or continuity in operation. Under normal operational conditions, these pipes are subject to constant internal pressure and cyclic load due to bending and/or tension. Under such loading conditions, the region in the pipes with thickness reduction due to corrosion could experience the phenomenon known as ratcheting. The objective of this paper is to present a revision of the available numerical models to treat the ratcheting phenomenon. Experimental tests were developed allowing the evaluation of occurrence of ratcheting in corroded pipes under typical operational load conditions as well as small-scale cyclic tests to obtain the material parameters. Numerical and experimental tests results are compared.

During pipe laying at large water depth using S-lay over a stinger, the pipe may be subjected to reversed plastic strains which could lead to low cycle fatigue failure. X65 pipeline girth welds with wall thickness 22mm were tested in cyclic strain control. Undermatched and overmatched welds were tested. Two types of cyclic strain programs were applied. In the first test series the welds were subjected to a tensile-tensile cyclic strain, simulating pipe going over the stinger, to validate that a pipe going over a stinger would not violate criteria for low cycle fatigue design. In the second test series, the specimens were initially pre-strained to 2% and subsequently fatigue-cycled with a maximum strain of 2% to obtain design criteria for a pipe laying stationary over the stinger for a period of time, subjected to cyclic loads due to waves. The results are presented in terms of Δ ε–N curves, with proposed design curves for the two types of welds.

Modern high strength steels need accurate control of microstructure in order to meet the steadily increasing requirements on mechanical properties, toughness or corrosion resistance. An appropriate steel making with inclusion control and a sophisticated thermo-mechanical controlled processing are the key factors, but we have to recognize that the process parameters have to be refined continuously to meet the targets. The present paper is going to demonstrate this progress using the examples of two recent development activities at Arcelor. At the same time it is shown that the development is often accompanied by the installation of new testing equipment for a more accurate and target-oriented material characterization. The first example deals with the development of an X70 for a 36” OD spiral welded pipe with 20.6mm wall thickness, a challenge for a hot strip mill when a good toughness is required at this thickness. We will briefly describe the sheet and pipe production and compare the mechanical properties on sheet and on pipe. The toughness performance of the new steel has been characterized in different orientations with respect to the rolling direction. In an attempt to quantify the crack growth propagation during the Batelle test, we equipped the existing testing device with a load cell so that we are now capable to document the energy absorption during crack propagation. The second example concerns the development of a coiled tubing grade 90 with HIC resistance for downhole application in corrosive environment. The challenging combination of high strength level, excellent low cycle fatigue (LCF) performance and corrosion resistance gave rise to the launch of a metallurgical study with the aim to develop a new steel grade. For a more complete material characterization we took advantage of this opportunity to install new CO 2 corrosion test facilities (atmospheric pressure and high pressure). The LCF performance of the steel sheets has been characterized under constant strain amplitude conditions.