Application of Increased Make-up Torque on API Rotary-Shouldered Connections: Goodman Diagram vs. Strain Life Model
Authors: McCarthy John P. , Everage S. DeWayne , Lee Kang 
Published: December 2007

This paper details the evaluation of rotary shouldered connection (RSC) fatigue performance by comparing the widely used Modified Goodman Diagram (MGD) to a more advanced and accurate elastic-plastic Strain Life Model (SLM). The technical approach defined in this paper challenges the application of the MGD for made-up API rotary shouldered connections. Likewise, the approach challenges the general assumption that the make-up torque (MUT) values defined in API RP7G are set in stone. These make-up torque values do, however, have a consistent basis. Understanding this basis, the state of stress in the connection and how a rotary shouldered connection functions, allows us to use the mechanics of the connection to our advantage. Such an assessment recently led to a justification for increasing the make-up torque on an API NC-50 connection for a major operator while drilling a challenging directional well in the Gulf of Mexico. This example is reviewed in light of both the MGD and the SLM. A concerted effort to understand and evaluate the connection performance allowed application of increased MUT for reaching TD without having to lay down drill pipe and incur the cost associated with switching to a high-torque rental string. Any operator or service provider can utilize the evaluation techniques presented in this paper to assess the impact a controlled increase in make-up torque may have on a rotary shouldered connection.

Introduction

Current drilling prospects continue to require operators, both large and small, to extend the reach, depth, and complexity of the wellbore to achieve the target objectives. This often translates into higher mechanical loads - both torque and tension - acting on the drill string. As a result, a shift in the market is reflected by the introduction of several new technologies and enhanced-feature tools to meet the increased demands [1][2]. High-torque drill strings, torque reducing subs, and rotary steerable systems all aim at increasing the available capacity under given geometric design constraints[3]. Practically speaking, one of the first load conditions that pose a design challenge in deviated well trajectories is torque. Certainly, there are situations where specialty drillstrings with proprietary connections and rotary steerable assemblies are justified, and in some cases required to mitigate, among other things, increased operating torque. However, there also exists those opportunities when simply using the available torsional capacity of the standard (and readily available) API rotary shouldered connection is a technically founded and cost effective option. However, some continue to use the Modified Goodman Diagram (MGD) as a means to show that the fatigue life of a RSC is reduced simply by increasing MUT beyond that specified by API. This paper provides an accurate explanation for interpreting the Modified Goodman Diagram, a proposed explanation as to why the MGD does not apply to RSC's in the made up condition, and an introduction to a more accurate fatigue life estimation tool - the Strain Life Model (SLM). This model will also be used to show that MUT has little effect on the fatigue life of rotary shouldered connections, in most situations encountered in the industry.

Make-up Torque (MUT)

A basic understanding of an API RSC is required before it is possible to justifiably extend beyond the sound basis of the API's recommendations regarding make-up torque. Comprehensive discussions on the basis for tool joint make-up torque and the resulting states of stress at critical locations in the connection geometry have been explored in detail in both References 4 and 5 as well as in API RP7G and Standard DS-1™. Here we will repeat the important points for convenience.

Make-up torque for rotary shouldered connections serves several purposes:Connect joints of drill pipeContain pressure (internal and external)Maintain connection integrity during bendingTransmit torque to the bitMaintain tensile integrity while pulling on the pipe