Flexible risers used in the offshore industry for hydrocarbon production and transportation offer alternative options over steel risers for project consideration - they have higher structural flexibility, and therefore greater capacity to sustain dynamic loads from high motion floaters in hostile offshore environments where fatigue is a key design consideration. Tensile armor wires helically wound around the inner pipe layers provide the structural strength required to carry riser's weight and resist all dynamic tensile and bending loads over riser's service life. Current industry practice is to test un-corroded armor wire specimens to determine corrosion fatigue resistance. If the armor wires are prone to pitting corrosion and exposed to corrosive fluids in the riser annulus during service, then determining the fatigue resistance of pitted specimens becomes an important consideration for riser design and fitness for service evaluations.
The paper presents a methodology to evaluate fatigue lives of armor wires with pitting corrosion. Contained herein are the results from a series of small scale fatigue tests conducted on tensile armor wires on non-pitted and pre-pitted specimens, in air and sea water. This work is expected to influence the fatigue test design and qualification, and engineering criticality assessments of flexible risers.
Flexible risers are commonly used in the offshore Oil and Gas industry for hydrocarbon production and transportation. Flexibles offer many advantages over steel pipes, the most notable being their increased structural flexibility over conventional risers. Hence, they offer an improved ability to accommodate dynamic loads in hostile offshore environments under severe wave and swell conditions.
The external hydrostatic pressure is resisted by a stainless steel carcass, the production fluid is contained by an internal polymeric sheath and its pressure resisted primarily by a hoop-wound interlocking armor wire, all of which allow for enhanced bending flexibility over conventional steel risers. Two layers of armor wires laid in opposite directions helices over the length of the flexible pipe are intended to carry the weight and external axial loads acting on flexible riser and, thus, are subjected to fatigue loads during offshore operation. The multi-layered flexible pipe is a complex structure that creates a unique operating environment for the carbon steel armor wires in the annulus between the internal and external sheath. While the structure of flexible pipes is designed to prevent direct contact between the steel wires and internal or external corrosive fluids, under certain conditions, the ingress 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 gases from the inner bore. Subsequently, seawater in the annulus, when combined with corrosive gases such as Carbon Dioxide (CO2) and/or Hydrogen Sulphide (H2S), can lead to annulus corrosion which can manifest itself as general and/or pitting corrosion of the armor wires [see Figure 1]. Pitting corrosion can be particularly detrimental to service life of the flexible pipe since these corrosion pits, which can be as deep as 100 microns 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 flowlines under static load conditions, or fatigue cracks under dynamic load conditions in risers. This assumes more significance in light of the fact that thirty five percent of all flexible pipe damage incidents reported worldwide [2010 Sureflex JIP] are due to external sheath damage and annulus flooding.
