The Effect of Surface Enhancement on Improving the Fatigue and Sour Service Performance of Downhole Tubular Components

Jeremy E. Scheel and Douglas J. Hornbach

Abstract

Sulfide stress cracking (SSC) and hydrogen embrittlement (HE) prevent the use of high strength carbon steel alloys in the recovery of fossil fuels in H2S containing ‘sour’ environments commonly experienced in deep well fossil fuel recovery efforts. Couplings are a common weak point in casing strings as high magnitude mean tensile stresses are generated by connection interferences created during power make-up of downhole tubular components. When subject to service loads both mean and alternating stresses are increased further providing the high tensile stresses necessary for SSC initiation. Since high strength carbon steel alloys are not typically suited for sour service environments, the current solution is to use or develop much more expensive alloys with increased corrosion-cracking resistance, or limit their use to significantly weaker sour environments, or higher operating temperatures.

Failure due to fatigue is another major problem in downhole tubular components. Likelihood of fatigue failure is further exacerbated in corrosive environments (such as H2S and NaCl), commonly encountered in service. The cost for detecting the impending failure before final separation is dramatic at a factor 10X. A cost effective method of mitigating failure from SSC and corrosion fatigue would greatly reduce operational costs and extend component life.

Introduction of stable, high magnitude compressive residual stresses into less expensive carbon steel alloys alleviates the tensile stresses, and mitigates SSC, while also improving fatigue performance. Low plasticity burnishing (LPB) is an advanced surface enhancement process providing a means of introducing compressive residual stresses into metallic components for enhanced fatigue, damage tolerance, and SCC performance.

The effects of LPB on high cycle fatigue (HCF) and SSC were evaluated on quench and tempered API P110 grade steel. LPB processed specimens had an increase in fatigue life greater than an order of magnitude over untreated specimens. LPB was successful in completely mitigating SSC in all test specimens at tensile stresses up to 90% specified minimum yield strength (SMYS). The initial results indicate that LPB processing of P110 steel provides an economical means of SSC mitigation and fatigue strength improvement in sour environments.

Keywords: Low plasticity burnishing (LPB), sulfide stress cracking (SCC), fatigue, residual stress, sour service

Introduction

Surface enhancement of metals, inducing a layer of surface compressive residual stresses in metallic components, has long been recognized1-4 to enhance fatigue performance and mitigate stress corrosion cracking (SCC). The fatigue performance of many engineering components is often improved by surface enhancement methods including rolling or shot peening. Modern surface enhancement treatments such as low plasticity burnishing (LPB), 5 laser shock peening (LSP),6 and ultrasonic peening,7 have emerged that in varying degrees benefit fatigue and stress corrosion prone components. Maximum benefits are obtained when deep compression is achieved with minimal cold working of the surface.

Environmentally assisted cracking (EAC) in the form of Sulfide Stress Cracking (SSC), SCC, and Hydrogen Embrittlement (HE) prevent the use of less expensive high strength carbon steel alloys in the recovery of fossil fuels in corrosive-cracking environments commonly seen in offshore and deep well recovery efforts. Tensile residual stresses generated from straightening, machining and connection make-up, when added to applied stresses during downhole operations in high-pressure environments, are significant contributors to EAC and fatigue failure. The current solution is to develop new alloys with increased corrosion-cracking resistance in an attempt to mitigate the problems or limit their use to significantly weaker sour environments. This solution can be costly and can result in only a small benefit.

Introducing compressive residual stresses into less expensive carbon steel alloys can dramatically reduce the risk of failure, mitigate SSC, HE and SCC, and improve fatigue performance.8 This would enable the use of less expensive alloys in harsh environments where they currently are unable to be used. Low plasticity burnishing (LPB) is highly effective, reliable and reproducible method of producing deep compressive residual stresses in complex geometric components. With advanced CNC control systems LPB can be applied using a closed loop feedback surface enhancement method capable of introducing a customized compressive residual stress field specifically tailored for each application. LPB so applied produces a very smooth surface finish, which aids in nondestructive inspection and examination. LPB tooling can be integrated with existing equipment used for manufacture and repair of downhole tubular components.

HCF and SSC susceptibility in high strength API P110 grade tubular products prevents their use in 100% H2S sour environments. 9,10 As more deep wells and offshore resources are probed and recovered it is imperative to mitigate the problem of EAC and HCF failure in a cost effective manner. HCF and SSC tests conducted in a corrosive environment and with mechanical damage11 has shown that the LPB process can provide a substantial increase in HCF life and mitigate SSC.12, 13 In this work the LPB technology is being evaluated to determine if it can play a role in creating more reliable and efficient fossil fuel recovery systems that are capable of safely and reliably operating in aggressive environments. LPB processing has successfully been used to mitigate EAC in high strength steel, stainless steels, Ni based alloys and aluminums spanning many industries.13 LPB technology was developed in conjunction with NASA’s SBIR program and is currently being used to treat components in production for the aerospace, medical, and nuclear industries.14-17

Experimental Technique

Material

C- ring specimens used for fatigue and SSC testing were taken from API P110 grade quench and tempered coupling stock with 4.5 in. (114.3 mm) ID and 0.25 in. (6.4 mm) wall thickness. Cring samples were made per NACE TM0177-Method C. SSC tests were also conducted on full size coupling stock. An example of the coupling stock is shown in Figure 1. The chemistry and mechanical properties were verified and are shown in Tables I and II, respectively.

FIG 1

TABLE I: MATERIAL CHEMISTRY.

TABLE II: MATERIAL MECHANICAL PROPERTIES.

Specimen Processing

To simulate surface damage from any source (handling, tong marks, corrosion pitting or erosion), a surface notch with a depth of 0.02 in. (0.51 mm) was introduced in the center of the gage region of the HCF specimens by electrical discharge machining (EDM) following the LPB process. EDM leaves a cracked recast layer in residual tension at the bottom of the notch, producing a large fatigue debit. EDM notching is widely used for reproducible laboratory simulation of high kf damage.

FIGURE 2: API-P110 COUPLING BEING LPB PROCESSED ON A CNC LATHE

Residual Stress Evaluation

X-ray diffraction residual stress measurements were made at the surface and below the surface to determine the depth and magnitude of residual stress distributions of the untreated material and that produced by LPB. Measurements were employing a sin2 ψ technique and the diffraction of chromium Kα radiation from the (211) planes of P110 steel. The lattice spacing was first verified to be a linear function of sin2 ψ as required for the plane stress linear elastic residual stress model.18-20

Material was removed electrolytically for subsurface measurement in order to minimize possible alteration of the subsurface residual stress distribution as a result of material removal. The residual stress measurements were corrected for both the penetration of the radiation into the subsurface stress gradient21 and for stress relaxation caused by layer removal.22

The value of the x-ray elastic constants required to calculate the macroscopic residual stress from the strain normal to the (211) planes of 410 stainless steel were determined in accordance with ASTM E1426-9. Systematic errors were monitored per ASTM specification E915.

High Cycle Fatigue Testing

HCF tests were performed under constant amplitude loading on a Sonntag SF-1U fatigue machine. A photo of the fatigue setup is shown in Figure 3. Specimens were tested in the untreated and LPB conditions with and without mechanical EDM or corrosive damage. Fatigue testing was conducted at ambient temperature (\~72°F / 22°C) in four-point bending. The cyclic frequency and stress ratio, R (σmin/σmax), were 30 Hz and 0.1 respectively. Tests were conducted to specimen fracture or until “run-out” at 1 x 107 cycles. Specimens were subsequently broken open for optical and SEM fractographic analysis.

FIGURE 3: HIGH CYCLE FATIGUE TEST SET UP

Active corrosion (AC) fatigue testing was performed in a neutral 3.5% wt. NaCl solution prepared with de-ionized water. Filter papers were soaked with the solution, wrapped around the gage section of the fatigue test specimens, and sealed with a plastic film to avoid evaporation for the duration of the testing. Several fatigue samples were tested with prior exposure to a NaCl solution to determine the effect on the subsequent fatigue life. Sample exposure was conducted according to ASTM Standard G44-99. Specimens were exposed to 3.5% wt. NaCl solution by alternate immersion (10 minutes exposed and 50 minutes unexposed per hourly cycle). Specimens were exposed for 100 hours. Samples were then removed, cleaned with water, and tested in HCF.

Fractography

Following fatigue testing, each specimen fracture face was examined optically at magnifications up to 60x to identify fatigue origins relative to the specimen geometry. Photographs were taken through a Nikon Stereoscopic microscope at 15x. A representative photograph of a typical failure for each specimen group was obtained. A few selected specimens were also examined via SEM.

SSC Testing

SSC tests were conducted on untreated and LPB treated samples. All testing was performed in 100% NACE TM0177-Solution A at 1 bar H2S at 25° C. The pH was monitored continuously throughout testing to ensure conformance to NACE TM0177.

C-ring specimens were initially tested at a tensile stress of 45% specified minimum yield strength (SMYS). After exposure to at least 720 hours the stress was increased to 80%, 85% and 90% of SMYS. Stress was monitored using strain gage rosettes placed on the inner diameter opposite the exposed location of maximum applied tension. The entire specimen, except the outer gage region, was coated in a polymer based stop-off coating prior to immersion in solution. Figure 4 shows a fully prepared C-ring specimen.

FIGURE 4: C-RING SPECIMEN PRIOR TO SSC TESTING.

Testing of full size coupling blanks was performed using a custom made holding fixture connected to a pressurizing test station. The full sized coupling blanks were internally pressurized hydraulically to impart the desired amount of applied hoop stress. Test solution was monitored for pH and refreshed as needed to conform to the NACE TM0177 standard. Testing was conducted until specimen failure or a run out life of 720 hours (30 days) or more was achieved per NACE TM0177 standard. Specimens were tested at 45%, 80%, and 85% of SMYS. The 85% SMYS stress level is regarded as an aggressive performance test for metal that is in direct contact with a 100% H2S saturated environment. Pressure was monitored continuously throughout the test and the time to failure recorded. Figure 5 shows a coupling placed in the pressurized test apparatus.

FIGURE 5: FULL SIZE COUPLING BLANK PLACED IN PRESSURIZED TEST APPARATUS.

Surface Roughness

The surface roughness was measured for an LPB and untreated coupling. Surface roughness values were obtained using a Mitutoyo SJ-201 Tester. The arithmetic mean surface roughness, Ra, was determined over a 0.5 in. (12.7 mm) evaluation length in the axial direction. All reported values are the average of three measurements.

Experimental Results

Residual Stress

The residual stress distributions, measured as functions of depth, are presented graphically in Figure 6. Compressive stresses are shown as negative values, tensile as positive, in units of ksi (103 psi) and MPa (106 N/m2 ). LPB produces max compression of -105 ksi (\~ -724 MPa), and gradually decreases to zero at a depth greater than 0.040 in. (\~1 mm).

FIGURE 6: RESIDUAL STRESS PROFILES