Session: 12-12-01: Modeling of the Fracture, Failure, and Fatigue in Solids

Paper Number: 140035

140035 - Numerical Modeling and Life Prediction of Double Shoulder Connections for Drill Stem Elements 

Threaded connections are critical elements for rotary drill stem to both transfer mechanical load and seal downhole fluid. The API specified rotary shouldered connection (RSC) has been widely used on drill stem elements. RSCs are usually the most stressed elements of the drill string and subjected to make-up torque, pressure, axial load, and bending. The most common RSC failure is fatigue due to rotating and bending of the drill string. Once started, a fatigue crack can grow and ultimately lead to twist-off of the drill string, interruption of the job, and costly fishing work. Different from RSC which has one shoulder, double shoulder connection (DSC) has two shoulders, a primary external shoulder, and a secondary internal shoulder. The primary external shoulder serves as the sealing surface, while the secondary shoulder serves as the mechanical stop for the connection. Comparing to the traditional RSC, DSC carries special advantages such as higher torque capacity which benefits from the secondary shoulder, increased hydraulic efficiency from a flush ID, and extended wear allowance due to higher torque capacity. DSCs are often used for challenging drilling job conditions such as extended and deep drilling, and for horizontal and slim hole wells. These benefits are primarily from the secondary shoulder in DSC as an added design variable; however, special attention is needed in designing DSC due to the same reason of the added design variable. Since DSC is facing the same harsh conditions and risk of fatigue as RSC downhole, evaluating the performance of DSC and understanding the effect of its increased design variables are important.

In this study, we develop a finite element analysis-based approach for modeling and evaluating DSC and use it to understand effect of critical design variables. The high-fidelity finite element model of DSC considers the make-up torque, pressure, and a bending load. We use the strain-life approach and results obtained from the finite element model to predict fatigue life of the connection. The following criteria are used to evaluate performance of the DSC design as a substitute of RSC: 1) good sealability on the primary shoulder which shall be comparable to the RSC counterpart; 2) no excessive yielding in both the pin and box; and 3) comparable fatigue life to the RSC counterpart. First the predicted fatigue life of an existing DSC is compared with available test data. A favorable agreement is obtained and serves as a validation of the method. The method is then used to evaluate performance of new DSC designs for both structural integrity and fatigue life. Sensitivity studies are conducted to understand effects of critical design variables such as the lengths of the pin and box, total make-up torque, and distribution of the make-up torque on the primary and secondary shoulders. The development approach can be used to evaluate performance of an existing DSC design and to further optimize the design based on specific design requirements.

Presenting Author: Haitao Zhang SLB

Presenting Author Biography: Haitao Zhang received his PhD in Solid Mechanics in 2005 from the Johns Hopkins University. He joined Schlumberger in 2009 and is currently the FEA Domain Manger in the Modeling and Simulation CPE based at Enabling Technology Development in Sugar Land. His expertise is on solid mechanics and finite element modeling.

Authors:

Haitao Zhang SLB
Amandine Battentier SLB
David Oliver SLB