A Novel Approach for Determining, Evaluating, and Applying Stress-Concentration Factors for Rotary-Shouldered Connections

Abstract

Summary This paper presents a new analysis method for stress-concentration factor (SCF) in rotary-shouldered connections (RSCs). The proposed method uses finite-element analysis (FEA) as a primary tool to explore the maximum peak-stress behavior in RSCs and calculate SCF to evaluate the connection performance. Key findings include facts that the maximum peak stress and SCF value are functions of connector geometry and loading condition. The paper defines SCF in RSCs and provides a thorough discussion about the SCF-analysis method and its characteristics in evaluating drillstring-connection designs. The paper compares maximum peak-stress behaviors and SCF values among various RSC designs to demonstrate their role in selecting connections for drilling-, completion-, and intervention-riser applications. Introduction In recent years, drilling programs have become significantly more aggressive in both onshore and offshore operations. Harsh conditions in deepwater, extended-reach, and ultradeepwater drilling often place severe axial, lateral, torsional, and pressure loads on the drillstring and its connections. As a result, RSCs often experience exceptional elevated-stress conditions. In some cases, RSCs are being used for applications beyond conventional drilling, such as completion operations and riser intervention. Meanwhile, advanced RSC designs incorporate features such as multiple shoulders and metal-to-metal seal features, adding further complexity to the stress distribution in the connection design. The need to understand the connection-stress behavior fully, especially the maximum peak-stress behavior for the various applications, is critical for properly selecting and safely using the drillstring connections. (For a comparison of a premium casing connection and a proprietary RSC, see Fig. 1). SCF is a useful parameter in terms of evaluating the connection maximum peak stress in response to the operation loads. In the past, SCF has been employed to characterize design strength and fatigue performance of premium casing, tubing threads, and some riser threads under anticipated operation loads. However, this has been applied only to RSCs, mainly for two reasons:the traditional SCF-analysis method is not designed for RSCs with high makeup preload anda lack of standardization of the SCF-analysis method within the industry for RSCs. Because of increasing aggressiveness of drilling conditions and merging of new applications, SCF analysis to predict stress behavior becomes necessary. This paper presents a series of evaluations that will explore various aspects of SCF analysis to find a logical and conservative approach to evaluate the maximum peak-stress behavior in RSCs, in response to operation loads. This approach will help to better understand RSC-loading limit, stress distribution, and fatigue characteristics for various existing and new applications.