Timothy Galle

Validating numerically predicted make-up of threaded connections using digital image correlation and infrared monitoring

To ensure a reliable connection between two pipe sections, an initial make-up is applied to the threaded connections to induce a favorable stress state. Using finite element analysis techniques, it is possible to predict the internal strains and stresses of the connection when torque is applied. This article presents the outline of an experimental setup, which allows to directly validate the occurring strains together with the torque versus turn diagram and indirectly the contact pressures. The strains are measured by means of digital image correlation and strain gages. Both methods provide similar results and comply with the predicted finite element analysis strains when taper mismatch is taken into account. In an effort to qualitatively validate the simulated contact pressures, the temperature of the box is measured during make-up by means of infrared monitoring. The maximum temperature increase occurs near the vanishing threads where contact pressures are larger. Despite promising results, no decisive validation for the contact pressures could be obtained.

Effect of Make-Up on the Structural Performance of Standard Buttress Connections Subjected to Tensile Loading

When establishing oil wells, pipe sections are connected by means of threaded couplings. In an effort to minimize the possibility of failure by jumpout, standard buttress connections were introduced. Part of their strength is directly acquired as a result of radial interference during make-up. This paper discusses the results of a numerical study evaluating the effect of make-up on the performance of a standard 4.5 inch API buttress connection when axial tensile force is applied. In order to characterize the structural performance, the load distribution along the coupling length is evaluated, combined with a parameter defining thread separation. The latter is indicative for jumpout and the tendency of creating a leak path throughout the thread helix. From the results it is clear that relative axial displacement within the coupling occurs, even when made up, because of an initial clearance among the load and stab flanks. This clearance may cause a connection to leak through the thread helix when available thread compound cannot heal this leak path. Despite undesirable effects on the sealability and rigidness of this joint, such a clearance is required to decrease frictional forces during make-up while maintaining the desired radial interference.

Evaluation of a numerical model for tapered threaded connections subjected to combined loading using enhanced experimental measurement techniques

Threaded connections used in the oil and gas industry have to withstand ever-increasing axial and pressure loads. The structural behavior of these joints can be predicted using numerical models, provided these are soundly validated. Within this article, a finite element model of tapered, shoulderless connections containing a trapezoidal thread type is experimentally validated conducting a test load envelope experiment and a tensile failure test. The evaluation is performed using both conventional and advanced measurement techniques. Surface strains are validated by comparing digital image correlation and strain gage results with the numerical output. An almost perfect quantitative match between the numerically predicted axial strains and the experimentally measured strain gage data is obtained. In addition, a qualitative match can also be observed when considering the axial strains obtained by digital image correlation. Furthermore, infrared thermography measurements are compared to the numerical deformation energy, assuming that temperature is an additional, but indirect parameter to evaluate the validity of the model. From the results, the proposed model has proven to be reliable when a combination of axial tension and internal pressure is applied. While the model is originally designed to predict the connection’s structural behavior for loads up to the yield strength, it is possible to estimate this behavior for higher loads by modifying the contact area.

Enhancing Trapezoidal Threads Using a Parametric Numerical Approach

A parametric program designed for modeling tapered, trapezoidal threaded connections is used in combination with Abaqus to investigate the behavior of couplings subjected to static load combinations containing make-up, axial tension and internal pressure. Three criteria are defined and used to quantify the performance of the connection: load distribution, helical gap size between the threads and the amount of global plasticity. From these parameters, the load distribution provides valuable information about the effectiveness of the load bearing characteristics of the thread and can be used to detect possible overstressing of the connection. The helical gap size is used to estimate its tendency to provide a leak tight thread seal, while the global plasticity reflects the total amount of plastic deformation within the connection. During the investigation, the effects of taper angle, load flank angle, wall thickness, size of threads and initial thread clearance are considered. The presented modeling approach consists of three parts. First, the optimal make-up position for a trapezoidal threaded 4.5 inch TN80 connection is calculated using the plasticity criterion. Next, the results for the three performance parameters as a function of both axial tension and internal pressure are discussed. Finally, after investigating the various isolated effects induced by geometric changes, a newly defined, enhanced threaded geometry is suggested and compared with the standardized API-buttress thread.

OPTIMAL MAKE-UP TORQUE FOR TRAPEZOIDAL THREADED CONNECTIONS SUBJECTED TO COMBINED AXIAL TENSION AND INTERNAL PRESSURE LOADING

When assembling tubing strings in the oilfields, threaded connections are used to connect the pipes with each other. The possibility to reuse these connections is often required and a certain degree of leak tightness is required, even without the use of a sealing surface or shoulder. For this reason, the total plasticity within the connection should be limited and relative movement between pin and box ought to be restricted. Within this publication, a finite elements analysis is conducted using a 4.5 inch buttress threaded connection as defined in API 5B together with a connection using the enhanced SR23 buttress thread. In addition, an experimental validation of the make-up stage is conducted by comparing the strains generated during make-up using Digital Image Correlation and infrared monitoring. In order to determine the optimal make-up, values found in literature are compared with a developed method using the magnitude and size of the plastically deformed zones during make-up. In addition, the effect of external axial and pressure loading is examined to identify the effects and critical areas. As a result, it is observed that pressure loading and make-up tend to have similar effects and in order to determine the optimal make-up torque, the pressure ratings of the assembly should be taken into account to prevent overtorqueing the connection. For the case of axial loading, a critical zone is visible near the last engaged thread and excessive loading of this thread can cause premature failure within this zone. Overall, the SR23 connection shows limited, yet visible, advantages over the standard BTC connection as described in literature.