Modeling Approaches for Anisotropic Material Properties of High Strength Steel Pipelines and the Effect on Differential Settlement

High strength steel (HSS) pipelines exhibit anisotropic behavior; the yield stress in the circumferential direction is higher than the yield stress in the longitudinal direction. In addition, the shape of the stress vs. strain curve is distinctly different. The circumferential stress vs. strain curve has a sharp yield point, while there is no distinct yield point in the longitudinal direction. Most of the research done in the past on the behavior of high strength steel was based on the isotropy assumption. The material behavior of high strength steel pipelines cannot be satisfactorily modeled based on this assumption. Different material models are available which can take into account this plastic anisotropy of high strength steel. They can be grouped into two categories. First, there are models which treat material as intrinsically anisotropic [15]. And, there are other models which can take into account plastic anisotropy as being caused by the load history. In this paper, the plastic anisotropy is modeled using the second approach. Freezing and thawing of the discontinuous permafrost in the northern regions of Canada causes differential settlement of pipes. This induces significant longitudinal stress in addition to the circumferential stress due to internal pressure. It is very important to accurately model the differential settlement of the pipe and the stresses caused by it. In this paper the differential settlement is modeled using beam elements in Abaqus. The behavior of the pipeline under differential settlement loads is investigated using three different material models. The first two are assuming that the material behaves according to the traditional isotropic plasticity model, once with the longitudinal and another time using the circumferential stress strain curve as basis for the model. The third one is using an analytical virgin material stress strain curve based on the kinematic hardening plasticity model which predicts the appropriate behavior in each direction. The displacement versus the reaction force of the pipe is obtained for pipes without internal pressure and for pipes subjected to internal pressure causing a circumferential stress that is 80% of the specified minimum yield strength of the material. It is found that the response of the pipe is different for different material models. The response based on the analytical virgin material stress strain curve is closer to the response based on the longitudinal stress strain curve when the pipe is not subjected to internal pressure. But, when the pipe is subjected to internal pressure, the response using the analytical virgin material curve is closer to the circumferential stress strain curve.