Effect Of Makeup And Internal Pressure On The Structural Strength Of Api Buttress And 8 Round Threaded Connections
Authors: Lewis D.B. , Chappell J.F. , Katsounas A.T. 
Published: October 2008

The purpose of the study was to define the variables that effect the limit states of API 8 Round and Buttress connectors This work will be incorporated into Load and Resistance Factor Design (LRFD) equations. Connectors are an integral part of any well design program. Therefore, it is vital that they be included in the LRFD design approach. Variables that determine the structural capacity were identified and evaluated, within the API tolerances, using finite element analysis. The indications are that taper, lead, and friction have no significant effect on the structural capacity of API connections. The effect of internal pressure on the tensile strength of API round thread accounted by API Bul. 5C3 [1] was verified. In addition, internal pressure was shown to have a beneficial effect on the capacity of buttress connection. The API joint strength equation, which in its present form, accounts for makeup only through the engaged thread length. However, the true joint strength was found to also be function of makeup interference. Improved values for material and friction constants (a, b, and ø) introduced by Clinedinst, could be improved by regression analysis on data with variations in makeup. On the other hand, makeup variation within two turns from nominal had no effect on buttress capacity. This work identified additional variables that effect the joint strength of API connectors. Further analytical and experimental work would improve the degree of accuracy, with which the various resistances can be determined. However, the errors and inaccuracies in the data and in the limit state function are incorporated in the LRFD calibration process, as part of the probabilistic determination of reliability, and are thereby accounted for.

INTRODUCTION

The purpose of this work was to determine and to evaluate the variables that effect the limit states of 8 round and buttress connectors. The limit states have been defined to be leakage, thread jump out, thread shear, and tensile failure. The models were made-up to a prescribed preload and were subsequently subjected to various loading scenarios. The loading was a combination of internal and external pressure, tension and compression.

The variables that could possibly effect the limit states were first identified and then studied as to their importance. Initial comparisons were made to the limit states defined by the API equations found in API Bul. 5C3. If necessary, new recommendations could be determined based on theoretical equations, empirical relationships, algorithms, etc., to relate the important variables to failure loads. These relationships will subsequently be used as part of the LRFD development not covered herein. Errors and inaccuracies in the data and the limit state functions are accounted for, since they are incorporated in the LRFD calibration process.

The literature survey was confined to work done by API on the capacity of API connectors with size range of 8 5/8" through 13 3/8" for buttress, and 5" through 13 3/8" for 8 round threads. In 1964, API adopted the 8 round joint strength formulas developed by W.O. Clinedinst.