Chen Lei, Xiong Xuebing, He Maolin, Han Hongguang
(Sinopec Shengli Oilfield Branch, Dongying, Shandong 25723)
Abstract: In cementing-truck operations in the oilfield, 40 MPa swivel joints suffered liquid washout leakage, making it impossible to maintain the required operating pressure and flow rate, contaminating the wellsite and shortening swivel joint service life. Analysis showed that the leakage was caused by a defect in the sealing principle. The seal relies on rubber expansion, and the rubber expansion depends on fluid pressure. Before the rubber has expanded sufficiently, the expansion amount is inadequate, causing poor low-pressure sealing and washout leakage. By adding an elastic preloading element to the seal ring and increasing the preload, liquid washout leakage from the swivel joint can be completely eliminated.
The 40 MPa swivel joint is an essential component for connecting high-pressure manifolds during oil and water well operations. Through the swivel joint, the pup joint of the oil or water well can be connected to the pup joint of the cementing truck in any direction. For a long time, swivel joints have had problems such as poor sealing at the connection under low pressure, liquid washout leakage and wellsite contamination. At the same time, pressure-fluid leakage prevents the cementing truck from reaching the specified pressure and flow-rate requirements during operation. Therefore, it is necessary to completely solve low-pressure washout leakage in high-pressure elbows.
1. Washout Leakage in 40 MPa Swivel Joints
The 40 MPa swivel joint mainly connects the wellhead pup joint and the cementing-truck pup joint. During use, liquid washout leakage occurs at the swivel joint interface and is especially serious at low pressure. After liquid washout leakage occurs, the pressure and flow rate of the fluid injected into the well cannot meet process requirements, and the resulting wellsite contamination also fails to meet clean-production requirements.
Swivel joint washout leakage
Scrapped swivel joint
2. Structural Analysis of the 40 MPa Swivel Joint
Research shows that the 40 MPa swivel joint is mainly composed of the elbow body, the matching elbow sub and the Hammer Union. The elbow sub is placed in the bore of the elbow body. An expansion-type rubber seal ring is placed at the joint between the elbow body and the elbow sub. A spherical ball race is provided between the elbow sub and the elbow body, and axial positioning of the elbow body and elbow sub is achieved by loading steel balls into the spherical ball race.
Elbow body, expansion seal ring, steel ball and elbow sub
3. Cause Analysis of Low-Pressure Washout Leakage in 40 MPa Swivel Joints
Based on the structure of the 40 MPa swivel joint, after assembly the rubber seal ring is located between the end face of the elbow sub and the end face of the internal bore of the elbow body. Because the swivel joint must be able to rotate, the seal ring cannot be compressed tightly, so the seal ring and the two flat faces remain in a clearance-fit condition.
According to the design, the swivel joint uses an expansion-ring seal: the pressure of the high-pressure fluid expands the seal ring to achieve sealing. The expansion seal ring is a hollow rubber component. Under fluid pressure it expands and creates end-face sealing. However, the expansion deformation of the seal ring increases gradually as fluid pressure rises. Good sealing can be ensured only after the seal ring has deformed enough to press against the matching end faces. If the joint works under low pressure for a long time, the seal ring cannot maintain sealing and liquid washout leakage occurs.
Liquid washout leakage process in a high-pressure elbow
4. Technical Improvement Measures for 40 MPa Swivel Joint Sealing
According to the analysis, the key reason for poor low-pressure sealing in the 40 MPa swivel joint is a structural defect that cannot guarantee sealing under low-pressure conditions. To improve sealing performance at low pressure, a corrugated elastic retaining ring is added inside the swivel joint. The elastic force of the corrugated elastic retaining ring provides preload to the seal ring under low pressure, solving liquid washout leakage caused by insufficient seal-ring preload. At the same time, the elasticity of the retaining ring can effectively cushion wear of the spherical ball race and steel balls caused by hydraulic impact during operation.
5. Application Results
Since 2009, the technical improvement of 40 MPa swivel joint sealing has been successfully implemented in the special-vehicle brigade of the oil production plant, with 52 scrapped swivel joints repaired in total. Field tests completely solved liquid washout leakage under low-pressure conditions, improved onsite safety and environmental protection, and ensured that operating pressure and flow rate during construction were real and effective. The annual economic benefit reached RMB 60,000, with good results achieved.
Conclusion
The technical improvement of 40 MPa swivel joint sealing uses the principle of elastic preloading to solve the original low-pressure sealing defect of the swivel joint, achieving good sealing under both low-pressure and high-pressure conditions. Field application shows that this improvement is simple, practical, low in cost and effective, with good value for wider application.
