Mechanical behaviour of steel lined pipes under monotonic and cyclic bending

Abstract

Steel pipelines that transfer hydrocarbons may be exposed to several corrosive elements. In order to ensure the structural integrity of the pipeline against corrosion, double-walled pipes are produced, containing a thick-walled low-carbon steel (“outer pipe”) providing strength, and a thin layer (“liner pipe”) from a corrosion resistant alloy (CRA) material, which is fitted inside the outer pipe, resulting in a cost effective solution instead of producing pipelines from stainless steel or nickel alloy.

First, the structural response of double-walled pipes, also called “lined pipes”, under monotonic bending is investigated. In the first stage, two types of lined pipes are examined, with and without mechanical bonding between liner and outer pipe referred to as tight-fit pipe (TFP) and snug-fit pipe (SFP), respectively. It is shown that, upon applying monotonic bending, the liner pipe gradually detaches from the outer pipe, forming a uniform wrinkling at the compression zone leading to localized buckling with further increase of the curvature. Subsequently, the bending response of lined pipes under low or moderate levels of internal pressure is investigated, showcasing its beneficial effect on the bending performance of lined pipes. Furthermore, the influence of initial geometric imperfections on liner pipe buckling is examined, showing the imperfection sensitivity of internally pressurized and non-pressurized bi-metallic (double-walled) pipes.

Additionally, the effects of manufacturing process on the structural performance of mechanically lined pipes are investigated. Alternative manufacturing processes are considered, associated with either purely hydraulic or thermo-hydraulic expansion of the pipes. A three-dimensional model is developed, which simulates the manufacturing process in the first stage of the analysis, and subsequently, proceeds in the bending analysis of the lined pipe. This integrated two-stage approach constitutes an important contribution of this research to existing knowledge. Thermo-hydraulically expanded lined pipes are examined, with special emphasis on the case of partially heated liners, and reverse plastic loading in the liner pipe wall has been detected during depressurization. Furthermore, the numerical results show that the thermo-mechanical process results in higher mechanical bonding between the two pipes compared with the purely mechanical process, and that this bonding is significantly influenced by the liner pipe temperature level. It is also concluded that the value of initial gap between the two pipes before fabrication has a rather small effect on the value of liner buckling curvature. Numerical results on imperfection sensitivity are reported for different manufacturing processes, and the beneficial effect of internal pressure on liner bending response is verified.

Furthermore, the structural performance of a lined pipe under cyclic bending is investigated, motivated by offshore reeling installation. Five bending cycles are considered, representing the two installation cycles and three additional cycles of a failure/repair scenario. The loading cycles impose a bending curvature range corresponding to the strains developed during a typical reeling installation process. Different loading patterns are considered and their effect on liner performance is investigated. The results show that the application of reverse (negative) curvature during the loading cycles, representing the straightener, has a significant influence on the wrinkle size of the liner developed at the two critical generators and its rate of increase, compared with cyclic bending patterns with non-negative curvature. Numerical results on imperfection sensitivity are obtained, considering two types of imperfection of the liner pipe. In addition, the structural performance of liners with different thickness is examined, and the results show that there exists a minimum value of wall thickness, above which the liner does not exhibit local buckling at the end of the cyclic loading history. The beneficial effect of internal pressure on liner cyclic response is also verified, especially for thin-walled liners, preventing the development of wrinkles. The effect of manufacturing process is also examined, showing the superior structural performance of partially heated lined pipes, with respect to fully heated lined pipes, and to lined pipes manufactured by purely mechanical process.

Moreover, a three-dimensional numerical model of a mechanically bonded lined pipe is also developed, simulating its structural response during the reeling installation method. An installation failure/repair scenario is considered with five winding/ unwinding cycles, accounting for the straightening process. The cyclic deformation of a lined pipe is presented, monitoring ovalization, local curvature and liner detachment from the outer pipe during reeling. The influence of pipe contact with the reel and the effects of back tension on the structural response of the lined pipe are examined. The results indicate a residual curvature of the pipeline at the end of unspooling. Applying different levels of back tension, the local curvature on the pipe decreases, affecting the detachment of the liner pipe from the outer pipe. Numerical results on imperfection sensitivity demonstrate the significant influence of geometric imperfections on liner buckling. Comparison of the present results with those from a pure bending model shows that a lower liner detachment rate is predicted by the present model. Furthermore, the reeling performance of thicker liners is examined, showing the gradual reduction of liner detachment with increasing wall thickness. Finally, the reeling process in the presence of moderate levels of internal pressure is simulated, verifying its beneficial effect on structural performance, preventing local buckling.