Corrosion fatigue behavior of high-strength steel wire

Peter Barnes

August 1, 2015

Applying materials and corrosion science to the study of corrosion fatigue in high-strength steel wire aids flexible riser performance understanding. Intertek’s Peter Barnes explains.

Example of a riser performing on a platform. Photo from Thinkstock.  

In the oil and gas industry, flexible pipework has been used for over 30 years to transport oil-based products from the seabed to floating production, storage and offloading (FPSO) vessels.

Their flexibility means that this type of pipework can withstand a greater wave loading and that it requires shorter lengths than rigid-bodied alternatives, making it cost less. It also has the advantage of reduced installation times and being better at adapting to change in field layout.

Flexible pipes are commonly used in dynamic and static risers, static flow lines, jumpers and expansion joints. Although suitable for all markets, their versatility and ability to adapt to the changing demands of the offshore oil and gas industry makes them ideally suited to deepwater, ultra-deepwater and the increasingly challenging environments of new field developments.

Flexible riser schematic. Source: Intertek.

The structure of an unbonded flexible pipe, consists of a carcass, an inner liner, pressure armor, tensile armor and an outer sheath. The carcass prevents the inner liner from collapsing while performing under high external pressures in deep oceans. It also provides protection against maintenance operations, such as the pigging tools that are used to inspect the pipeline. The pressure and tensile armor layers are both made from carbon steel wire with either a flat or interlocking profile.

 

The region between the inner liner and the outer sheath is termed the annular space. The environment within the annular space can determine the life of a flexible riser as it can contain corrosive gases, condensed water and seawater. There are acknowledged to be two main modes of failure of the steel wire. The first is damage to the outer sheath, due to the aging of the thermoplastic sheath or damage during installation, caused either by foreign objects rubbing against it or becoming embedded in the pipe sheath. This leads to ingress of seawater into the annular space, enabling the corrosion fatigue process to begin. The second mode of failure occurs when permeation of corrosive gases and condensed water through the inner polymer sheath leads to a corrosive environment within the annular space.

Both corrosive environments, combined with the wave loading from the sea, lead to corrosion fatigue failure of the steel wire within the tensile armor.

We tested the corrosion fatigue of high-strength steel wire in four different solutions. The purpose of this was to compare the effects of chloride and iron ion concentration on the corrosion fatigue behavior of the high-strength steel wire, which allowed us to simulate the different failure modes. The four solutions tested were modified (5%NaCl) ASTM D1141 synthetic seawater, iron supersaturated modified (5%NaCl) ASTM D1141 synthetic seawater, deionized water and iron supersaturated deionized water

FPSO in service. Photo courtesy of Thinkstock.

These tests explored the effects of the different environments on the corrosion fatigue behavior in terms of levels of iron (Fe2+) and sulfide (S2-) ions in solution and pH. The fracture surfaces that considered iron saturation and the seawater environment without iron saturation demonstrated signs of corrosion on the fracture surface and exhibited a ductile type failure. The fracture surface of the tensile armor tested in a deionized water environment without iron saturation exhibited a brittle type failure.

Iron saturation reduced the effects of hydrogen sulfide corrosion through the reaction of Fe2+ ions with the S2- ions, which produced an iron sulfide precipitate. This reduced the sulfide in the solution to zero after one and two days’ exposure for the seawater and deionized water environments, respectively. The iron ion saturation ultimately leads to the lowering of the corrosion rate. There are approximately twice as many Fe2+ ions in the solution for the seawater environment than for the deionized water environment, which would correspond with a lower corrosion rate.

The pH of solution of the two iron-saturated environments showed a reasonably stable pH throughout the fatigue test period of 6.2. The pH for the seawater and deionized water environments without iron saturation were lower, with values on average of 5.4 and 5.1, respectively.

Using a materials and corrosion science approach to study the corrosion fatigue of high-strength steel wire in various environments aids understanding of flexible riser performance thresholds for industry suppliers and operators. Putting this knowledge into action allows operators to engage in better planning for prevention of corrosion fatigue, with a more diligent approach saving them time and meeting health and safety regulations more effectively. This approach also saves costs, improves safety and reliability and enhances the life of the riser by ensuring that risks to its integrity are mitigated insofar as possible.


Dr. Peter Barnes
is a senior engineer at Intertek Production and Integrity Assurance, part of the company’s Exploration and Production business. He has 13 years’ experience in the water, nuclear and oil and gas industries working with clients on asset integrity management, corrosion fatigue, design, construction and water treatment projects, among many others. Barnes has a PhD in corrosion fatigue from the University of Manchester.

*Reproduced with permission from NACE International, Houston. All rights reserved. Presented at NACE CORROSION 2015 in Dallas, Texas, USA, 2015. Co-author Tom McLaughlin, Senior Engineer at Intertek Production and Integrity Assurance.