Buried Steel Pipeline Design: External Load Methodologies and Strain Limits

Abstract

Buried steel pipelines are one of the most efficient means of transporting oil and gas from their resource deposits to their markets. The pipeline industry is experiencing an increased demand for larger diameter pipelines along with the implementation of thinner walls and higher operating pressures. In these cases, the external pipeline loads have significant effect on the pipeline stresses and deformations, thus influencing wall thickness and associated cost effectiveness. The external loads are made up of the weight of the backfill material combined with live and impact loads due to traffic. For the designs of buried pipelines, API RP1102, CSA Z662, American Lifelines Alliance (ALA) “Guidelines for the Design of Buried Pipelines”, ASCE “Design and Installation of Buried Pipes” and AWWA Manual M11 are commonly used to calculate external loads on buried pipelines. For the calculation of backfill loads, these methods are based mainly on the same theory, i.e., Marston load theory, while the calculation of live load, due to traffic loads, is based on different approaches. Depending on the design methodology selected, there is a large variation in the calculated external loads due to both backfill and live loads. In this paper, the experimental results of a field monitoring program will be compared with the calculated results from the various methods. An alternative approach for calculating external loads is presented and verified to field studies. In addition, for the design of onshore pipelines the industry uses design criteria which are based on allowable stress and ovalization deformation limits. Pipe stresses and deformations resulting from the external loads are commonly based on the Spangler stress and the Iowa equations. The parameters in the formulas include pipe and soil properties, pipe-soil stiffness and the geometric relation of a pipe section during deformation. As pipe materials, pipe sizes, operating means and pipe coating techniques change over time, the allowable design criteria shall be re-examined, especially for the ovalization deformation limits. In this paper the allowable strain and the corresponding ovalization deformation limits are re-examined by reviewing experimental results and industrial requirements.