Analytical estimation on the number of bending cycles to initiate armour wires lateral buckling in flexible pipes

Abstract

The flexible pipe industry has been confronted with the potential threat of armour wires lateral buckling for over two decades. Cyclic bending is recognized to play a significant role in the lateral buckling mechanism since it facilitates the armour wire sliding against the frictional resistance. If a flexible pipe is exposed to axial compression larger than the critical load for a certain period, the risk of lateral buckling may be controlled by limiting the number of bending cycles. Thus, the estimation of the bending cycles number to initiate buckling is favorable to prevent buckling failure and enlarge the potential application scope of flexible pipes. The present paper presents an analytical model for an approximate estimation of the number of bending cycles required to initiate the lateral buckling in wet annulus conditions. The analytical results are compared with the available test data in the literature and relatively good correlations are observed on the cycle numbers to buckle. Based on this analytical model, parameter studies are performed and the effects of axial compression, bending curvatures, and friction coefficient on the number of bending cycles to initiate buckling, as well as the stress states at the onset of buckling, are discussed.