Application of Computer Simulation and Full-Scale Testing in Research of Premium Tread Connection Tubing and Casing

The premium thread connection is a newly developed product aiming at worsening oil extraction condition where API thread connection cannot meet the requirement. Compared with API thread connection, premium thread connection has great advantages in joint strength, seal performance, thread glue, anti-corrosive of stress, etc. and meets technical request in special process of oil extraction. Structure design is essential to assure these fine properties.

The design characteristics are displayed following:

1. Design of thread shape: premium thread connection generally adopts buttress thread of high efficiency but makes suitable changes in shape. For example, load flank angle ranges 10-45 and stab flank angle ranges 15-3. The major aim of the changes is to improve thread connection ability of resisting complex load. Though some thread connections still adopts buttress thread of API, the adjustments are made in machining tolerance, thread height, taper and pitch etc. to reduce interference quantity of thread, to improve pressure distributing of thread connection and to reduce stress of peak value.

2. Design of seal structure: seal structure between metals is added and anti-leak ability is improved greatly. Pressure of gas seal reaches, even exceeds yield pressure inside pipe.

3. Design of torque shoulder: torque shoulder surmounts the side effect of make-up position and wide torque range of API thread connection on its performance, and improves the ability of thread connection in anti-glue and resisting over torque. In addition, torque shoulder can control interference quantity of thread and seal, assuring metal seal interference quantity under control. Torque shoulder itself can assist seal action.

4. Design of smoothness: smooth transition between couple and inside diameter of pipe body can eliminate turbulent fluid and improve connection’s anti-corrosion performance.

In our country, with the development of west oil and gas field and the applying of second oil extraction technology, the deep well, superdeep well, steam injection well, high pressure gas well, directional well (horizontal well, multiple well), high corrosive well will unceasingly increase. The demand of the premium thread connection will increase year after year. Present situation shows that over half of 26 oil fields have applied tubing and casing premium thread connection, including LiaoHe, SiChuan, ChangQing, TaLiMu, TuHa, DaGang, ZhongYuan, BuHai and NanHai etc.

Steel Tube company is an important production base of domestic oil country tubular goods. In order to satisfy requests from oil field users, a series of premium thread connection products have been developed and granted patents. Tubing with premium thread connection of BG series is successfully running. In the development process of premium thread connection products, the most advanced methods in the world, such as the computer simulation and full-scale testing, are adopted. With these methods, the developing cycle is shortened, expenses are reduced, and design parameter adjustments are visualized.

Every pipe body is connected to form string of tubing and casing by the connections of tubing and casing, and works chronically under worsening condition. During running tubing and casing, string of tubing and casing bears complex loads, including axial loads, pressure loads and bend loads etc. In the past, scholars and engineers analyzed mechanics characteristics of string of tubing and casing mainly by analytic method, numerical value method, experimental method (photoelasticity and photoplasticity). At present, with the quickly development of computer technology, contribution of mathematics and mechanics in analysis and application of engineering numerical value and with the advancement of computer graphics technology, analysis of numerical value simulation of finite element become an important method in design, development and research of premium thread connection.

2.1 Establishing of finite element model and realizing of simulation calculation

The comprehensive analysis process of finite element method includes three parts: preprocessing of analysis model, solutions to different questions, postprocessing of model of calculated results.

The preprocessing of model includes: establishment of finite element model, mesh partition, accession of boundary condition, confirmation of material performance, definition of geometry condition, setup of contacts and implement of project etc.

Establishment of finite element model is that the object, ready for analysis, is predigested to physics model, then processed by mesh partition of finite element. For tubing and casing, outside load of physics model is axisymmetric and thread spiral angle of connection is very small (less than 1). Therefore, finite element model of thread connection can be treated as axisymmetric question, which can completely satisfy the request of actual engineering computation precision.

The postprocessing of model is expressing calculated results by diversified distributing charts.

During developing premium thread connection of BG series, two finite element analysis softwares, I-DEAS and MARC are used, and simulation analysis calculating is processed on computer workstation of SGI INDIGO2 IMPACT R1000. Entitative model and mesh partition of finite element are processed in I-DEAS. Other work is completed by MARC software, mainly including model boundary condition, material characteristic, definition of initial condition, setup of contact body, definition of work status, selection and definition of postprocessing results.

Finite element model of tubing and casing thread connection formed by I-DEAS software is shown in Figure 1. In order to accurately calculate contact pressure of metal interference seal position, to improve condition of convergence satisfaction, and to make calculation smooth, element thin partition is conducted on all contact points and much precise partition specially on metal seal parts. Considering connection symmetry, half of it is taken. Considering calculation precision, enough length of pipe body is taken. Isoparametric element of axisymmetric entity of four node of quadrangle is adopted in the model.

Fig.1 Model of finite element analysis

After establishing the model of finite element, during the simulation analysis, definition and setup of solution condition are needed and completed by MARC software. Confinement of axial displacement on node of couple symmetry plane is added, and rigid body displacement of model is eliminated. Other displacement boundary condition is decided by simulation request, and outside load boundary condition is decided by actual simulation load value.

The model of premium thread connection includes two parts of contact: thread contact and metal seal contact. The contact is a big difficulty in finite element analysis. MARC software has strong function on contact analysis. Contact definition is simple, and the definition of special contact node between contact bodies is not required. Only contact bodies and their contact relations are needed to be defined, which is the innovation of conventional methods.After establishing the model of finite element, during the simulation analysis, definition and setup of solution condition are needed and completed by MARC software. Confinement of axial displacement on node of couple symmetry plane is added, and rigid body displacement of model is eliminated. Other displacement boundary condition is decided by simulation request, and outside load boundary condition is decided by actual simulation load value.

2.2 Result analysis of simulation calculation

The figure 2, 3 and 4 indicate finite element simulation calculation value of Von Mises equivalent stress and seal contact pressure distribution after the makeup of premium thread connection-


Fig.2 Von Mises stress of connection sealing under makeup		Fig.3 Von Mises stress of surface under makeup

Fig.4 Contact pressure distributionsealingunder of sealing surface under makeup		Fig.5 Von Mises stress of connectionunder makeup and internal pressure

Fig.6 Von Mises stress of sealing distribution surface under makeup and internal pressure		Fig.7 Contact pressureof sealing surface under makeup and internal pressure

casing. From the numerical value of stress and contact pressure obtained from these simulation makeup work status, interference quantity of joint thread and seal face of premium thread connection can be valuated. Through changing geometry parameter and structure of seal face of thread connection, the designed thread connection can achieve the requirement of makeup integrity. After casing makeup’s accomplishment and running, the casting bears multi-loads including: inside pressure, outside pressure, axial load, heat stress etc. Therefore structure integrity and sealing integrity of premium thread connection are needed under complex loads. Figure 5, 6 and 7 indicate finite element simulation calculation value of inside pressure status Von Mises equivalent stress and seal contact pressure distribution after the makeup of premium thread connection casing. From these simulation results, changes of connection stress and contact pressure of sealing face, after the connection bears high inside pressure, can be observed distinctly. Other load condition and complex load condition may also be simulated. In this way, at the beginning of development, the performance of products can be understood roundly.

2.3 Simulation effect

In the development of tubing and casing with premium thread connection, applying simulation of finite element, following effects are achieved：(1) Mechanical performance of the analyzed object can be reflected truly and visually, providing data for understanding properties of new products and parameter optimization. (2) Developing cycle of products is shortened. (3) Repeated experiments are reduced, and cost is saved.

3. Full-scale testing

After completing simulation and parameter design of product structure and processing products, which satisfy the design requirement, by numerical control machine tool, full-scale performance testing should be applied. The experiments include makeup performance, thread off performance and sealing performance. Experiment equipment is makeup and breakout experiment machine of USA H.O.MOHR Engineering Design Research Company, tension experiment machine of Japan SHIMADZU company and high gas pressure pump of USA, respectively.

3.1 Experiment analysis

Figure 8 indicates the appearance of casing connection after four makeup and four breakout on makeup and breakout experiment machine. Figure 9 indicates the relationship among turns, torque, and rotate speed during makeup. If makeup and breakout integrity of product can be satisfied, maximal interference value on sealing face and control value of makeup and breakout torque will be confirmed by experiment. Then, the experiment of sealing integrity is carried out. Satisfying the request of sealing performance under high gas pressure is the one of primary properties of premium thread connection. The minimal interference quantity on sealing face is confirmed by gas sealing experiment. The gas sealing experiment pressure must achieve 90 percent of inside pipe yield strength. Finally, experiment of structure integrity is completed. Figure 10 shows tension failure. From failure position in the picture we know that failure is rupture breakage but not thread-off. Moreover, in this experiment, the connection tension failure strength exceeds 22.7 percent of API recommended, which verifies structure integrity of thread connection.


Fig.8 Form of casing sealing Surface for the fourth time breakout		Fig.9 Turns and torque charton makeup

Fig.10 Failure form under axial tension condition

3.2 Experiment effect

The full-scale testing of makeup and breakout performance, sealing performance and thread off performance of premium thread connection is valuable for premium thread connection development：(1) Validating the reliability of structure design. (2) Validating processing performance, and coating performance of thread and sealing face because special cutting tool and measure tool are needed, and the precision request is high during the processing of premium thread connection. (3) Establishing the experiment foundation for evaluation experiment aiming at requests from oil fields.

4. Conclusion

Research on tubing and casing with premium thread connection by computer simulation and full-scale testing is conducted in Steel Tube Subcompany. BG premium thread connection tubing and casing products are successfully developed and prove the practical value of computer simulation and full-scale testing. Figure 11 is the development flow chart of computer simulation and full-scale testing. In all, the combination of modern simulation technology and full-scale testing technology is very efficient in new product development, improving development efficiency and optimizing experiment costs.