Cold formed bends are regularly used in onshore pipeline design to follow the profile and contours of the ground and to ensure any necessary bending is performed under controlled conditions. The cold forming process induces compressive and tensile plastic strains in the pipe wall which remain in the line pipe throughout the service life of the pipeline.
Strain based design of oil and gas pipelines is well established in the offshore environment, where high operating temperatures and pressures have pushed engineers to design more efficient structures through a better understanding of post yield material behaviour and loading mechanisms. The industry standard code DNV-OS-F101 covers the design and limit states for offshore pipeline environments.
There is less experience of strain based design in onshore pipelines, and some of the problems unique to the onshore environment have not been fully investigated.
Low lateral and vertical soil restraints can be observed on a pipeline operating in a desert environment, this in combination with high operational temperature and pressure loading can result in the potential for displacement of the bends. This displacement can induce significant strains in the pipeline. The compressive strains may be sufficient to develop local buckling. Consequently, the local buckling strain represents a key performance limit for the design of the bends.
There is currently no code guidance to include the additional strain component generated by the cold forming process for onshore strain based design. This results in overestimation of the allowable strain capacity of the bends.
This paper investigates how the fabrication of cold formed bends affects the local buckling strain limit and how this reduces the structural capacity of the bend.
Finite Element analysis has been used to assess how the additional strain components, induced during cold formed bending affects the operational capacity of the pipeline. Sensitivity analysis was carried out on a range of unpressurised line pipe, with a diameter to wall thickness ratio varying between 25 and 60, using a specific material model.
Sensitivity analysis indicated a strong correlation between the Finite Element results and those calculated from the local buckling strain limit equation within the industry standard code DNV OS-F-101. The results suggest that an increase in wall thickness, at the bends within a pipeline, could be a solution to offsetting the overestimation in allowable strain capacity due to the cold formed bending process.
Fully understanding the effect that cold form bending has on reducing the local buckling strain limit will promote a more robust design of onshore buried bends under high temperature loads.
