ABSTRACT

Strain-Based Design (SBD) is used to complement conventional allowable stress methods to manage large ground motions and resulting longitudinal strains, which may be experienced by a pipeline in challenging environments. This is particularly true for pipelines crossing regions of discontinuous permafrost, where phenomena such as frost heave and thaw settlement may induce longitudinal strains in excess of 0.5%. A suitable SBD approach is necessary to achieve safe and economic pipeline design as well as robust and reliable integrity management for these regions.

This paper describes a comprehensive SBD methodology, which spans from the initial design to the integrity management of the pipeline. This approach has recently been applied with success to the conceptual design of an arctic pipeline project in North America. The application of a tensile strain capacity (TSC) model developed by ExxonMobil was integral to this methodology. This paper describes use of this TSC prediction model for the two key components of the methodology:

  1. the development of material specifications and flaw acceptance criteria to achieve the desired design strain capacity and

  2. the development of an integrity management framework to manage longitudinal strains during operations.

The TSC model has been previously validated through full-scale testing, and the scatter between predicted and measured TSC has been used to develop an appropriate safety factor for TSC. The use of the safety factor in design and during integrity management is also discussed.

INTRODUCTION

The need to develop remote energy resources has led to consideration of pipelines crossing challenging environments with the potential for large ground movements, which may result in significant longitudinal strains. Particularly, arctic regions with discontinuous permafrost may require pipelines able to withstand plastic deformations due to phenomena such as frost heave and thaw settlement. In these environments, a Strain-Based Design (SBD) approach is required to complement conventional allowable stress methods to ensure a reliable and cost-effective pipeline design. SBD is a limit state design approach that uses pipeline longitudinal strain capacity, in addition to yield strength in the transverse direction, to characterize the design margin of safety for displacement-controlled loading conditions. Thus, SBD allows for a more effective use the pipe performance in the longitudinal direction, while preserving the conventional level of reliability for the pressure containment capacity limit state (Fig. 1).

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