LRFD Derived Performance of a Qualified API Connection Population
Authors: Schwind Brian E. , Curington Donnie W. , Miller Richard A. 
Published: October 2008

Tubular products are among the most critical items in a well, and the threaded connection can represent one of the more complicated and limiting mechanical features for tubular products. Previous tubular design methodology did not specifically address threaded connections, rather it appliedsafety factors to pipe body ratings. In contrast, Load and Resistance Factor Design (LRFD) methods specij7cally address connection performance. This work is a discussion of how the limit performance is being defined and quality systems are beingrevised for API connections in accordance with the objectives of the LRFD method for OCTG.

INTRODUCTION

Tubular products are among the most critical items in a well, and the threaded connection can represent one of the more complicated and limiting mechanical features for tubular products. Previous tubular design methodology did not specifically address threaded connections, rather it applied safety factors to pipe body ratings. In contrast, LRFD methods specifically address connection performance. Limit performance has been defined and quality systems are being revised for API connections in accordance with the objectives of the Load and Resistance Factor Design (LRFD) method for OCTG. The following presents a summary regarding the limit performancefor API connections, After briefly discussing the background regarding tubulars, the topics of API connection performance states are addressed. API connection performance will be shown to be highly dependent on quality requirements (dimensional control and buck on) during manufacturing and running.

Industry Background

Table 1 lists API connection leak resistance research. The first three studies identified makeup and tolerances (particularly taper) as being significant to leak resistance. The J&L Steel [4] workwas performed on J-55 casing, and provided an initial basis for makeup, however it was never used to revise product specifications or define performance. Furthermore, none of theseearly studies evaluated tolerance extremes. Pete Wiener and Humble Oil and Refining [7] performed testing in the 1960?s which provided the basis for the initial Exxon makeup procedure (the fourth item in the table). The Humble Oil and Refining work provided a method for calculating performance which did not consider the effects of tension.

In the mid 1980s interest in leak resistance of API 8-Round connections resulted in both an API funded project performed by Enertech [9] as well as the DEA-5 project performed by Stress Engineering Services [11] (both of Houston, Texas, USA). The API work provided a general basis for performance ratings which included factors such as makeup, connection tolerances, tension and pressure. The DEA project included testing of 7", 29 ppf P-1 10, and provided empirical data which generally supported the API effort. Although this work was performed roughly 10 years ago, the API has not acted on any of the issues. The following summarizes the results of these industry studies concerning API connection leak resistance.

Conclusions of Industry Studies:

8-ROUND LEAK RESISTANCE ESA FUNCTION OF TENSION AND MAKEUP EXTREMES IN API TOLERANCE CAN RESULT IN SIGNIFICANTLY DECREASED LEAK RESISTANCE LEAK RESISTANCE ISA FUNCTION OFDIAMETER, WEIGHT AND GRADE