High strength armoring wire utilized in flexible piping systems for offshore applications is subject to harsh service conditions including serve stress loadings and corrosive environments. Thus, the wire materials must exhibit optimum strength and fatigue properties as well as resistance to environmental cracking. This paper presents a testing program that evaluated the corrosion fatigue behavior of several different high strength tensile armor wire materials. The testing regime consisted of a novel corrosion fatigue test that was based on a simple cantilever beam principle for three point bending. The test was developed as a means to rapidly compare and assess final product form in different environments. The S-N curves for several different conditions are presented. The test conditions included seawater and stimulated annulus conditions at ambient temperature. Variables such as wireline size, strength, solution, gaseous environment, presence of cathodic protection and material condition were analyzed and compared to different production armor wire form. It was determined that all of these parameters can be effectively evaluated utilizing the corrosion fatigue test described herein.
Flexible piping systems are comprised of several layers of different materials and material configurations to provide the needed strength and flexibility for deepwater subsea environments (Figure 1). The two main constituents are the pressure armor and tensile armor layers as seen in Figure 2. The pressure armor layer provides the primary support for the pressure retaining polymer layer in the pipe and also helps resist the hydrostatic collapse of the pipe. The tensile armor provides the piping system with tensile strength for installation in downhole applications and resists end cap loading in the pipe due to the internal pressure. The tensile armor wire has two flattened sides which act to optimize the fraction fill of the wires around the circumference of the pipe. Multiple layers of the wires are wound around the pipe to balance torsional reactions1.
The tensile armor wire must thus meet very complex in-service criteria to be suitable materials for deepwater applications. The material must withstand static and dynamic loadings with high alternating stresses. In addition, these materials may behave differently when exposed to different environments under these dynamic loading (fatigue) conditions. Many other authors explored fatigue issues in downhole environments2-7. This paper presents the results of an engineering test program including a comparison of fatigue performance of commercially produced armor wires to a similar set of test conditions. Consequently, it was not intended to be a scientific investigation covering all aspects of fatigue performance, but focused on the comparative evaluation of armor wire under the time constraints of manufacturing and delivery. It evaluated the corrosion fatigue behavior of several different high strength tensile armor wire materials. An additional variable included in this study was the test environment that involved use of seawater and simulated annulus conditions at ambient temperature. Other variables included the use of cathodic protection as well as comparison of base materials to weld materials.