From stress to strain-based design

To meet the rapidly growing demand for energy, fossil fuels are now being extracted in more hostile and remote locations. These out-of-the-way environments can be under permafrost or they can be susceptible to landslides, ground settlements, and even earthquakes.

Due to the extreme loading conditions in these remote areas, pipelines transporting fuel can be subjected to deformation beyond the elastic range of steel. Under such conditions, in addition to strength and toughness properties, the strain capacities of pipe and weld metal are also crucial.

Stress or strain-based design

More demanding environments require correspondingly demanding pipeline design rules. The design should ensure not only the safety and reliability of the pipeline but its economical and efficient operation as well.

Most of the current pipeline design codes are based on limiting stress criteria, which is considered acceptable when stress levels remain within the elastic range. But stress in pipelines may exceed the yield limit under loads originating from earthquakes, landslides, or simply during the laying of submarine pipelines. In these cases, the design criteria for strength based on stress are no longer practical.

For spiral-welded line pipe sections, for example, the helical seam weld and anisotropic material properties pose real challenges to pipeline designers. To guarantee the structural integrity of the pipeline for strain-based design application, line pipe sections should be able to deform beyond the elastic range without failure.

We investigated the tensile strain capacity and defect tolerance of high-strength high-toughness spiral pipes. To obtain both qualitative and quantitative insights, we studied the thermal and mechanical operations (forming, welding, expansion …) performed during manufacturing, because they affect the local or global strength, toughness and ductility properties of the pipe metal.

ULCF test set-up

Pipe reeling is a fast and efficient method for laying offshore pipelines. When reeling, a pipe is typically submitted to a low number of cycles of with high plastic deformation. In the framework of an RFCS project, OCAS developed a custom test set-up for Ultra-Low Cycle Fatigue (ULCF) in pipes. The set-up has been used for ULCF tests on pipes between 8″ and 16″ outer diameter. The cyclic bending in the plastic range was continued through local buckling until a through-thickness crack occurred. During the test bending moment, displacement and local strains were monitored through an advanced data acquisition system. Thanks to the obtained test results the modelling work that has been performed over the last few years, could be validated.

“Our custom test set-up provides us with valuable information for fine-tuning our numerical modelling. This is critical for predicting and ensuring the safe and reliable operation of pipelines, both onshore and offshore.”

Jeroen Van Wittenberghe, Senior Team leader large scale testing, Applications & Solutions department, OCAS