Monitoring the slip of helical wires in a flexible riser under combined tension and bending

When a flexible pipe is subjected to combined tension and bending, the helical wires in the external tension armour layer are most failure prone. Understanding the true relative slippage of helical wires is very important to accurately predict the mechanical behaviour of flexible risers under complex loads. This study proposes a monitoring method for the relative slippage of helical wires in flexible risers under combined tension and bending, using three-dimensional digital image correlation technology. For the flexible riser test specimen with an internal diameter of 8 inches, a four-point quasi-static test was carried out, and full-field displacement vectors of helical wires marked by a range of scattered spots were accurately monitored. Thereafter, the relative slip character of the helical wires is obtained by processing the measurement data. The results indicate that the measured relative slip vectors are essentially tangential to the surface of the internal cylinder, while the measured total slip distances and angles are compared with the results from the existing theoretical model and the good agreement is achieved, thus verifying the correctness of the measurement method. Additionally, the laws of slip responses, such as the total slip distance, slip direction, and slip components along the axial and lateral wire directions, of helical wires in flexible risers are investigated, and the slip responses of the helical wire under different axial tensions are discussed.