Instability of Tensile Armour Layers of Flexible Pipes Under External Pressure

Abstract

During their installation and some phases of their service life, the armour wires of flexible pipes may be operated under high compressive forces. Under this kind of loading, there is a potential risk of damage on the tensile armour layers of those pipes along two main mechanisms: - Radial instability, usually referred to as “Bird caging”. - Lateral instability, usually referred to as “lateral buckling” when cyclic bending is applied to the pipe. This paper focuses on Lateral instability issues. It discusses a methodology for the prediction of lateral buckling based on extensive testing both onshore and offshore Brazil. In particular, the major influence of the armour annulus state (Dry or flooded) on the lateral buckling mechanism will be discussed. This approach of tensile armour instability has been recently confirmed by the successful testing of a 7” flexible riser in 2000 m water depth and of a 9” flexible riser in 1800 m water depth.