Design Methodologies for Arctic Gas Pipelines

Abstract

The need to install gas pipelines in more technically difficult locations, coupled with very tight competitive economics necessitates increased efforts to consider more refined design methodologies that provide higher levels of certainty and result in more economical designs. The design of onshore gas pipelines in arctic environment must also address a number of Geohazards which will impose various external loads on the pipeline in addition to the internal pressure and other stress inducing loads that non-arctic pipelines typically experience. These geohazards include frost heave, thaw settlement, and soil movement and will tend to deform the pipeline and induce longitudinal strain at levels well above the linear elastic limit. The different design methods must account for these loads in order to ensure a rational and fit for purpose design. Currently, a design engineer can rely on one of three distinct design methods for onshore gas pipelines: Allowable Stress Design (ASD), Load and Resistance Factor Design (LRFD) or Reliability Based Design Assessment (RBDA). Each of the methods has advantages as well as disadvantages that could limit their applicability to a specific project. Clear understanding of the loads, the operational requirements, the environmental conditions and regulatory framework are all key factors in selecting the appropriate design method. Allowable Stress Design has been the traditional design method for onshore pipelines in the US and while the method leads to safe pipelines, the degree of safety and the inherent level of conservatism incorporated can often be improved. Gas pipelines can also be designed using probabilistic methods that require the consideration of credible modes of failure (limit states) and the calculation of the probability that these limits will be exceeded. Since the design engineer must evaluate each applicable mode of failure individually, the degree of conservatism and safety can be applied where needed the most. Probabilistic design methods can be used to achieve consistency and to provide a higher degree of certainty that pipelines would perform as designed [2]. Since probabilistic methodologies are typically applied only by small group of specialized consultants, a clear understanding of their strengths and limitations is required by all key personnel involved in the design and engineering decision making process. This paper will present a definition of the design methods as well as a direct comparison of all major components associated with each. Furthermore, it will provide a definition of commonly used terminologies associated with reliability and strain based design application in order to enhance the practical knowledge of the basis for each approach.