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  Title A reviwe of strain capacity assessment methodsfor transmission piplelines
  Author(s) Dr Robert Andrews et al.  
  Abstract For many years strain based design concepts have been used in subsea pipelines for both installation and service. During installation, reeling strain levels may approach 3%, whilst in service lateral buckling requires consideration of both static strain limits and high strain fatigue. In contrast, most onshore transmission pipelines have been designed on a stress basis. Some onshore pipelines have been designed for a limiting axial strain generated by causes such as seismic activity, frost heave, discontinuous permafrost or landslides. Models have been developed to predict the axial strain capacity in both tension (usually limited by the girth welds) and compression (where the limit is local buckling of the pipe wall). The strain based approach places onerous demands on the linepipe material and the girth welds, but for a new pipeline these requirements can be addressed during material specification, procurement and weld procedure qualification.

In service monitoring of a pipeline initially designed on a stress basis may reveal that strains approaching or exceeding the design level are occurring, or are predicted to occur. The monitoring may be by direct measurement, for example using strain gauges or measuring bending strains using an In-Line Inspection tool equipped with an Inertial Measurement Unit (IMU). It is also possible to model the response of the pipeline to predictions or measurements of ground movement. In all of these cases the pipeline operator will have to assess if the pipeline is fit for continued service.

The European Pipeline Research Group (EPRG) has initiated a programme to investigate methods of assessing the fitness for purpose of a stress based design pipeline that is found to be experiencing high axial strains. As the first part of this programme methods for predicting the compressive and tensile strain capacity of steel transmission pipelines were reviewed. This paper presents the results of this initial review.

The review has considered the range of validity (pipe diameter, thickness, grade etc.) of the models and the inputs required for their application, such as fracture toughness, uniform elongation, weld high – low and weld metal mismatch. The results are illustrated by two case studies, one for a typical onshore transmission pipeline and a second for a smaller diameter, thicker-walled configuration typical of many offshore applications.

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