The Effect of Increased Make-up Torque on Performance of Rotary Shouldered Connections
Authors: Breihan Jim , Muradov Andrei , Shepard Jeff 
Published: December 2007

In recent years, drilling programs have become significantly more aggressive and challenging. Rotary shouldered connections (RSC) in extended reach and ultra-deep wells face unprecedented loads. Torque is one of the constraints imposed by modern drilling applications on standard RSC. In order to prevent uncontrolled downhole make-up in severe drilling environments, standard RSC are, in some cases, made-up above recommended torque values.

This paper discusses the effect of increased make-up torque on the fatigue performance of RSC. It presents the results of full-scale fatigue testing on API NC connections and discusses the correlation with various analytical modeling methods currently in use to determine the suitability of increased make-up torque in specific drilling applications. Make-up torque values ranging from 30% to 90% of the connection torsional strength are compared to determine the full impact of make-up torque on fatigue life.

The paper provides drilling engineers additional information to assess the potential risks associated with varying make-up torque in relation to drilling parameters.

Introduction

Higher drilling torque was made possible by the increase in torque output of Top Drives as compared to rotary tables. In turn, this created the opportunity to drill more highly deviated wells, and extended reach horizontal wells to improve finding costs as well as reservoir productivity. Utilization of double-shouldered connections allows full advantage of the torque output of Top Drives beyond what standard RSC can offer. However, higher drilling torque requirements have also created the need to increase make-up torque (MUT) on standard RSC above API recommended maximum levels. One of the earlier discussions concerning how this could be accomplished to minimize impairment of the performance of RSC was covered in an ASME technical paper[1], which identified the following options:Tool Joint Stress-BalancingHigher Friction Thread CompoundsDouble-Shouldered ConnectionsHigher Strength Tool Joint MaterialsReduced torque through the use of Raised, Low-Friction, Tool Joint Hard-bandingLighter Weight Drill Pipe to Reduce Torque and Drag in ERD Wells (Aluminum and Titanium)

One or more of these options can be used to meet drilling torque needs when it exceeds the maximum recommended make-up torque of the connection in use. A more recent technical paper[2] presented an elastic-plastic Strain Life Model as an alternative to the application of the Modified Goodman Diagram (MGD) when assessing the effects of higher MUT on RSC. The MGD is an analytical model used to determine how the combination of mean stresses and cyclic stresses influence the fatigue life of RSC[3].

This paper presents conclusions based on data gathered from full scale fatigue testing of API NC38 connections that were made-up at different MUT values yielding a range of preload stress at the last engaged thread (LET) of the pin up to and including 90% of the minimum yield of the material.

Test Setup

Resonant fatigue machines are becoming increasingly more utilized primarily due to their superiority in cyclic rates, which are typically set at 25 to 30 hertz (cycles per second). This results in fatigue cycles accumulating at a rate of about 100,000 cycles per hour or 2.5 million cycles per 24-hour day[4]. Mean tension, if necessary, can be applied to the sample by means of internal pressure (due to the pressure end loads). The fast cyclic rate plus the low power requirements and low test cost make the resonant bending fatigue machine very efficient at developing high-cycle fatigue test results for full-scale pipe and connection samples.