ABSTRACT
Flexible pipes are frequently used both as flowlines and risers in the oil and gas industry. A flexible pipe has a complex structure consisting of layers of polymer and metallic materials. The armor wire layers — shielded with polymer materials from seawater on the outside and well fluid on the inside — are the load and pressure bearing parts. Due to diffusion from the well fluid and/or damage of the outer polymer layer, the annulus can be water-filled, and armor wire can corrode. In this work, the susceptibility to hydrogen embrittlement (HE) with the presence of atomic hydrogen due to cathodic polarization has been investigated for six different tensile armor wire materials. Samples were exposed to Slow Strain Rate testing (SSRT) in 3.5% NaCl solution and cathodic polarization to −1.1 and −1.4 VAg/AgCl at room temperature. Reference samples without hydrogen charging were tested in air for comparison. Stress-strain curves, reduction in area (RA) and the microstructure of the fracture surfaces were investigated. The HE susceptibility tended to increase with the carbon content, strength and hardness and the materials tended to be more brittle when charged to −1.4 VAg/AgCl than −1.1VAg/AgCl.
Flexible pipes have been widely used in the oil and gas industry since the 1970's.1–4 Their popularity is partly attributed to fast and easy production, high durability and recoverability.2 Flexible pipes consist of layers with specific purposes that combined give a very versatile pipeline. The structure of a flexible pipe is displayed in Figure 1. The number and types of layers are determined according to the specific conditions that the pipe is expected to meet in its lifetime, however, the following description is the basis that most flexible pipes have in common. The innermost layer is a carcass made of a corrosion resistant alloy with interlocked parts. Outside the carcass, a polymer layer is placed to reduce the permeation of species from the bore to the outer layers. Layers of pressure armor are placed outside the polymer layer to resist internal pressure loads. Anti-wear tape is placed in between layers for protection. A new plastic layer is placed between the pressure armor and the tensile armor. The tensile armor consists of high strength carbon steel wires wound in a 25–55° angle to the longitudinal direction of the pipe with a second layer of wires wound the other way around to achieve symmetry.5 Finally, a polymer layer is positioned around the tensile armor to protect the pipe from seawater. This polymer layer is the only barrier between the tensile armor and the seawater outside. When flexible pipes were first used it was assumed that the annulus environment in a pipe would stay dry as long as the external layer was intact.6 This proved not to be the case as water vapor would permeate through the pressure layer and condense to a water phase in the annulus. Also, the external polymer layer was often damaged, causing ingress of air and/or seawater into the annulus and possibly cathodic protection of the wires. The cathodic protection reduces the corrosion rates of the wires and can have a beneficial effect on corrosion fatigue but can also cause hydrogen embrittlement.5
