Effects of defects and reverse bending on tensile properties of tensile armour wires

Abstract

Flexible pipes are used for risers and flowlines in the offshore oil and gas industry and in many other applications. As part of the construction of these pipes, tensile armour wires are incorporated to resist longitudinal stresses which arise during installation and service. Tensile armour wires also resist hoop stresses for pipes without a designated pressure armour layer. The flexible pipeline manufacturing industry desires a better understanding of the tensile armour wire fracture mechanism, and especially the effects of defects with dimensions less than 0.2mm. Reverse bending operations (which arise due to the wire moving through paired rollers on unreeling during pipe manufacture) also affect the tensile properties of the tensile armour wires. Customarily, engineers estimate the safe load carrying capacity of defective wires solely by multiplying the ultimate strength obtained from a tension test by the original nominal area of the wire without any consideration for the fracture mechanisms of the wire. This approach may overestimate the strength of the wire. Recent research considering the fracture mechanisms of wires has employed a classical fracture mechanics approach, mainly using Linear Elastic Fracture Mechanics (LEFM) and/or Net Section Theory (NST). Obtaining parameters for fracture mechanics analyses requires large/thick standard fracture mechanics test specimens which cannot be made out of tensile armour wires due to their small size. Also fracture mechanics analyses based on these parameters including the elastic plastic crack opening displacement (COD) and J-integral parameters are largely size and geometry dependent making transferability of the results obtained from full size specimens to actual structures questionable. Laboratory tensile testing and tensile testing finite element simulations with mechanism-based fracture mechanics carried out on the as-received tensile armour wire and tensile armour wires with engineered defects reveal that the tensile armour wires fail by a shear mechanism. They also reveal that flat bottom scratches, pointed end scratches and dents identified from the Scanning Electron Microscope images of the as-received wire surface reduce the ultimate load and extension at fracture of the wires. In addition, denting was found to increase the wires yield load while scratching reduced the wire‟s yield load. The reduction in the tensile/ mechanical properties of tensile armour wires were found to depend largely on defect dimensions rather than defect locations with defects less than 0.2mm in any of its dimensions causing less than 0.072%, 0.238% and 10.946% reduction the yield load, the ultimate load and the displacement at fracture of tensile armour wires respectively. Laboratory and finite element simulations of reverse bending, straightening and tensile testing of the reverse bent tensile armour wires reveal that reverse bending and straightening operations reduce the ultimate load and fracture displacement of the wires. This work also reveals that the reverse bending process can only reveal near surface laminations as wires with mid depth laminations or with scratches less than 1mm deep would pass through the reverse bending process without fracturing.