Improved Model for Tubular Burst

Abstract

Modern casing design can reduce significant amounts of CO2 and considerable cost per well [1]. Collapse design was modernized by ISO/API Technical committee 67, Sub Committee 5, Work Group 2b (ISO/API TC67/SC5/WG2b). Modernization of burst design has so far not had the same focus and only minor changes have been made. A new burst design model has been developed to add to the collapse prediction for a complete environmental and cost effective well design tool. It is based on the theories of Lubinski [2] (1975) and presents designs using “exact von Mises ellipsis” together with the Klever and Stewart ductile rupture model [3]. This paper presents the model developed for burst design and the improvement compared to current industry practice. Inspired by the current most accurate collapse prediction model [4], the modified burst model (prototype) is the first to consider actual wall thickness to predict a more accurate internal yield of OCTG (Oil Country Tubular Goods). Investigations show that the standard 12.5% wall thickness reduction for manufacturing tolerances may be obsolete. ISO 10400 offers physical measurements and statistics of tubular properties. Following the principals by WG2b applied with the Klever & Tamano collapse prediction, there is a set of data to be used for a specific batch of tubulars or they are deducted through large quantum of measurements; ensemble Probability Density Function (PDF). The value proposed as “ensemble PDF” for wall thickness is based on more than 10 000 measurements of tubulars from 11 vendors distributed over Electrical Weld (EW) and quenched and tempered (Q&T) qualities of miscellaneous sizes and grades. The batch specific value proposed is based on more modest numbers of specimens from 4 sources but offers “minimum measured wall thickness” for all the samples. Adding to the confidence of the final design is the automated ductile burst calculation, which is one of the latest contributions to burst modeling in the industry [4] [5] [6]. It is a useful aid for the design engineer to know the potential failure mode and the limit before loss of integrity. However, burst is limited to yield because exceeding this limit may lead to loss of the pipe’s effective diameter and eventually loss of integrity. Therefore, the ductile burst prediction is proposed as a visual aid only.