Swivel joints play an important role in hydraulic fracturing (fracking) and flowback operations in the oil and natural gas industry. Swivel joints are manufactured from high quality 43XX steel alloys suitable for service in cementing, fracturing fluids, drill mud, crude oil, and other applications [1-3]. Swivels are used in the above operations to facilitate divergent flowline around fixed assets, where exact alignment and additional degree of freedoms are required, avoiding rigid make-up and possible damping of flowline vibration. The product is used in standard and sour service applications. The main failure mechanisms are erosion related metal loss along the long elbow radius and into the straight section just beyond the elbow. Additionally, fatigue cracks form behind the flange on the male sub-end common defect that pose significant threats to the operation. Swivel failures due to erosion and fatigue cracking are commonly detected during visual inspections and hydrotesting. Other flowline non-destructive testing methods are used including magnetic particle testing, ultrasonic thickness testing, magnetic flux leakage testing, and acoustic emission testing to detect these phenomena earlier.
Swivel Joint Erosion and Non-destructive Testing
Swivel joint erosion is dependent on the metal alloy used, characteristics of the solid particles that carry the pressurized fluid, and fluid velocity among other paramters. During the fracturing process, sand-laden fluid containing quartz sand and/or ceramsite based proppants are pumped through the flowline. Both contain oxide components that pose erosion threats to the swivel inside diameter surface at high particle velocities. Figure 2 shows an example fluid velocity distribution for a 2” swivel joint [3]. Notice that the maximum velocity occurs at the elbow intrados as expected. This is consistent with where maximum erosion and wear marks are typically observed. High velocity particles are observed just beyond the elbow extrados in the straight section where inside-diameter wear is also commonly observed.
The current NDT testing methods to detect and size swivel erosion include borescope visual inspection, visual inspections with LED mirrors, ultrasonic thickness testing and hydrostatic testing. With the exception of ultrasonic thickness testing, all methods require tear down prior to testing. Typically, any inside diameter wear grooves are cause for swivel retirement from service. This approach is recommended since erosion wear progresses at an unpredictable rate compared to generalized corrosion that may occur in downstream applications. Ultrasonic thickness testing is used to assure the remaining wall thickness exceeds the minimum wall thickness recommended by the manufacturer [5]. Thickness testing from the OD provides only a very small sampling of the overall ID condition. The likelihood that flowline inside diameter erosion is undetected with UTT can be significant. For this reason, it is recommended to perform a thorough visual inspection of flowline bores and to supplement ultrasonic thickness testing findings visual inspections.
Hydrotesting does not detect nor provide any indicators of swivel erosion since this phenomenon is largely independent of sealing surfaces. Similarly acoustic emission testing does not detect erosion unless corrosion fatigue or stress cracks have developed and propagate at the test pressures.
Swivel Joint Fatigue Cracking and Non-destructive Testing
During the fracturing and flowback operations the pressurized fluid traveling though the multi-component flowline incur a low amplitude high frequency vibration. This is amplified at the hammer union/flange/ACME threaded connections. The most common location for fatigue cracks to develop is directly behind the male sub-end. This location is shown in Figure 3 on 3” 1502 swivel joint. The male sub-end is connected to the mating component’s female ACME threaded end via a threaded hammer union. The low amplitude high frequency vibrations fatigue the male flange in the axial direction via a cyclic bending stress applied across the elevation of the flange.
The fatigue crack initiates at the corner between the swivel flowline and flange as shown in Figure 4. After crack initiation, the crack commonly propagates at a 45-degree angle towards the sealing surface on the inside diameter.
The crack shown in Figure 4 was detected during a hydrotest which is problematic and a very serious safety concern since it was detected at the latest possible inspection stage. The observation lends itself to a few alarming scenarios 1) The component was very close to failure during last operation posing significant personnel safety hazards 2) The crack existed previously and was missed by earlier non-destructive testing 3) The crack initiated during the last operation and propagated very quickly. The latter is the most unlikely, but all scenarios are alarming in terms of the quality of personnel providing flowline NDT, NDT flowline procedures, and the frequency at which flow iron NDT is performed.
The current testing methods to detect and size swivel fatigue cracks at problematic areas include visual inspection with proper lighting and viewing angles, magnetic particle testing, hydrotesting and acoustic emission testing. Excellent pre-cleaning of the inspected area is the most critical step for a successful inspection. After teardown, a quick visual inspection using proper lighting can detect large cracks at the flange – flowline corner, however, this approach will miss tightly closed cracks. Tightly closed cracks may be detected with dry, visual wet, and fluorescent wet magnetic particles testing. Magnetic particle testing the recognized as the primary NDT method to reliably detect fatigue cracks in flowline threaded components. Acoustic emission testing for fatigue cracks in swivel joints and other flowline permits screening the entire component for fatigue cracks during a hydrotest. Acoustic emission testing may reduce the frequency for visual and/or magnetic particle testing in the future as the technology matures for this application. Additionally acoustic emission may reduce labor and materials required to prepare surface for inspections since the technology can detect cracks in the presence of grease and other surface components.
Summary
Flowline swivel elbows are prone to wear along the long radius of the elbow and just beyond. Fatigue cracks form at connections due to low amplitude high frequency vibrations. While standard visual and ultrasonic testing techniques due routinely detect these phenomena, the quality of non-destructive testing personnel, procedures, and the frequency at which the inspections are performed are critical performance parameters. Acoustic emission testing has shown promise in many other steel pressure vessel testing applications but is largely untested in this market. Acoustic emission testing shows potential to reduce the frequency for visual and/or magnetic particle testing in the future as the technology matures for this application. Additionally acoustic emission may reduce labor and materials required to prepare surface for inspections since the technology can detect cracks in the presence of grease and other surface components.
References
1. SPM® Well Service Pumps & Flow Control Products Swivels (Standard & H2S Service) Operation Instruction and Service Manual
2. SPM® Flow Control Product Safety, Usage and Maintenance Guide
3. FMC FCD OPERATING AND MAINTENANCE MANUAL, TRIPLESTEP SWIVEL JOINT
4. Chuan Zhang . Zheng Liang . Yong Lin . Qiulin Tang . J. Zhang, “Failure Analysis and Structural Improvement for the Swivel Joint in High-Pressure Manifold”, J Fail. Anal. and Preven. (2018) 18:969–974.
FMC Technologies, REPORT, ENGINEERING BULLETIN, FOR CUSTOMER USE, FLOWLINE, WALL THICKNESS MEASURMENT CRITERIA FOR WECO AND CHIKSAN PRODUCTS, Doc No: RP