Current industry practice in flexible pipe tensile armor wire fatigue testing involves use of un-corroded specimens; however, if the armor wires in a flexible are prone to pitting corrosion during service, this adds a new dimension to the fatigue life consideration which is the focus of this paper. The paper presents a methodology to consistently evaluate fatigue lives of armor wires with pitting corrosion. Described herein is a methodology to create corrosion pits on armor wires and the results from a fatigue test program involving Non-Pitted and Pre-Pitted specimens.
Flexible risers, used in the offshore Oil and Gas industry for hydrocarbon production and transportation, offer many advantages over steel pipes- the key one being their higher structural flexibility, and hence greater ability to accommodate dynamic loads in hostile offshore environments, especially those induced by vessel motions. A typical flexible riser consists of two sets of tensile armor wires helically wound around the inner pipe layers to provide structural strength for weight and dynamic loads. Fatigue of tensile armor wires used in flexible risers is often a consideration, from a design perspective as well as a remnant-life-assessment perspective.
The multi-layered flexible pipe cross section is a complex structure that can create unique operating conditions for the carbon steel armor wires in the annulus between the internal and external sheath. The annulus, which houses the armor wires, comprises the volume between the pressure sheath and the external sheath. The structure of flexible pipes is designed to prevent direct contact between the steel wires and external sea water, and between the steel wires and the internal produced fluid. Presence of liquid water or seawater in the annulus together with corrosive gases such as Carbon Dioxide (CO2) and/or Hydrogen Sulphide (H2S) can lead to wire corrosion; and under certain conditions, the presence of corrosive fluids in the annulus is possible either due to accidental damage of the outer sheath allowing seawater to enter the annulus, or due to condensation of diffused fluids from the inner bore. The corrosion in the annulus can manifest itself as general and pitting corrosion of the armor wires if the wires are not designed to resist such corrosive conditions in the annulus. Pitting corrosion is of particular interest since the pits, which can be as deep as 0.1 mm with aspect ratios in the range of 10 to 50, have the potential to initiate Stress Corrosion Cracking (SCC), Sulfide Stress Cracking (SSC) or Hydrogen Induced Cracking (HIC) in flexible flowlines under static load conditions, or fatigue cracks under dynamic load conditions in flexible risers. This assumes more importance when one considers the fact that thirty five percent of all flexible pipe damage incidents reported worldwide, according to the 2010 Sureflex JIP, is due to external sheath damage and annulus flooding.
