RATIONAL STRESS LIMITS AND LOAD FACTORS FOR FINITE ELEMENT ANALYSES IN PIPELINE APPLICATIONS PART III - ELASTIC-PLASTIC LOAD FACTOR DEVELOPMENT

摘要

In this paper, a methodology is presented to develop load factors for use in elastic-plastic assessments of pipelines and their components. The load factors are based on the pipe material properties and the ASME pipeline code's design margin for the service and location of the pipeline installation [1, 2]. These codes are recognized by 49 CFR 192 and 195 [3, 4]. Minimum required load factors for internal pressure loads can be derived analytically based on design equations from the ASME B31 piping codes and minimum material requirements for API 5L line pipe [6]. Once the load factor is established for a particular case, the elastic-plastic methodology may be used in the Finite Element Analysis (FEA) of pipelines and related components. This methodology is particularly useful in the assessment of existing systems when linear elastic numerical analysis shows that local stresses may exceed the elastic design limits. Two case studies are presented showing analyses performed with Abaqus [5], a commercial, general purpose FEA software package. The first case study provides an assessment of a large diameter elbow where the stress on the outer fibers of the intrados exceeded the longitudinal stress limits from B31.8. The second case study examines an assessment of a tee connection where the stresses on the ID exceeded the yield strength of the component. In addition to the case studies, the paper also presents the results of a full-scale test that demonstrated what margin was present when the numerical calculations were based on specified minimum properties. This paper is not intended to revise or replace any provision of B31.4 and/or B31.8 [1, 2]. Instead, it provides the means for calculating load factors that can be used with an elastic-plastic analysis approach in a manner that provides the same design margins as the ASME B31 codes. The approach described in this paper is intended for use in the detailed FEA of pipelines and their associated components.