Radial instability of flexible pipes with defects in the high resistance bandage submitted to axial compressive loads

Abstract

Flexible risers comprise multiple structural layers, which combined lead to characteristics of resistance, air tightness and desired flexibility, both for their installation and operation. Regarding mechanical strength, flexible riser structure must withstand several kinds of loads acting together or isolatedly. Within this context, axial compression acting individually is responsible for radial instability of the flexible pipe. Radial instability occurs mainly when the flexible pipe presents damage on the outer layer, which is responsible for radial strain resistance. Radial instability occurs mainly when the flexible pipe presents damage on the outer layer, which is responsible for radial strain resistance. This damage on the external bandage occurs due to launching procedures, project or material failures, excessive loading or possible falling. Damages on the external bandage layer together with the axial compressive load may lead to catastrophic failures due to radial strains, also known as birdcaging, thereby leading to a complex local analysis in the search for solutions capable of predicting riser behavior. Therefore, this study intends to measure the size of the defects in the external bandage of flexible risers subjected to compressive loads that lead to birdcage formation which consequently reduces pipe stiffness. The measurements were performed using a finite element method developed in the Abaqus FEM software and the compression tests in full scale were performed using LAMEF facilities to validate the model.

Keywords: birdcaging, damage in the external layer of risers, finite elements, compressive loads.