Behavior of Tensile Wires in Unbonded Flexible Pipe Under Compression and Design Optimization for Prevention

In ultra deepwaters, helical tensile armour wires under certain loading conditions may exhibit localized deflection in either the radial (out-of-plane) or lateral (in-plane) direction when subjected to significant high axial compression and bending. This phenomenon is often referred to as birdcage buckling. This paper presents the development of a total strain energy approach for modeling the buckling and post-buckling behaviour of these wires and for illustrating the characteristics of such behaviors. The paper presents the summary of the full scale offshore (Deepwater Immersion) DIP tests performed by Wellstream to date, all of which have been successful in resisting the failure modes. In addition, the results of pressure chamber tests are also presented and discussed, especially some of the key test constraints which can have a significant influence on the final result as observed on a recent 10-inch structure chamber test. The presented model provides a tool to define and implement the design criterions against such failure modes. The further validation of the model through comprehensive tests is planned, which is critical to ensure the ability to economically design and optimize flexible pipe structures to prevent this phenomenon, and to facilitate more cost effective products that meet the ultra deepwater design challenges.