An Innovative Approach To Optimizing Design of HP/HT Tubular Strings
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 134550, ’An Innovative Approach To Optimizing Design of HP/HT Well Tubular Strings,’ by Guijun Deng, SPE, and Goang-Ding Shyu, Baker Hughes, originally prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed.
The full-length paper details a new approach to well-completion design that integrates design of experiments (DOE) and stochastic study into the design process. It explicitly considers the variability and uncertainty of each design variable in probabilistic terms and allows the assessment of probability of failure of the design; thus, it is able to help achieve better designs and higher-reliability completion tools.
Introduction
Oil and gas wells are becoming more challenging, and well-completion engineers are faced with increasing design pressure. Deep hydrocarbon targets lead to high requirements for completion-tubing strings to resist significant collapse under external pressure while high temperature, internal pressure, and axial tension or compression may exist at the same time.
Well-completion tubing traditionally has been designed using a deterministic stress-limits approach that is based on arbitrary multipliers such as safety factors. First, design engineers develop a conceptual design of the tubing string on the basis of specifications from the customer that include downhole environment and loading and operating conditions. Second, at the virtual test stage, maximum stress of this tubing string is calculated under different loadings such as collapse, burst, axial tension, compression, or their combination using formulas and von Mises criterion. Then, the calculated maximum stress is compared to the strength of the material. If the maximum stress is less than the strength of the material, the conceptual design is considered viable, and the design process can move to the next stages. The less the calculated maximum stress under the worst condition is, the safer the tubing design is. The purpose of introducing safety factor is to account for inherent uncertainties in the design variables or parameters such as imperfections in the tubular geometry and material variation attributable to manufacturing, transportation, or long-term operation. This deterministic stress-limits approach is an iterative and, therefore, time-consuming process and has evolved mainly on the basis of historical cases.
Process Methodology
The process flow of the new proposed process integrates DOE and stochastic study into the design process. A design-study tool (DST) is used to integrate processes from multiple disciplines such as mechanical elasticity, statistics, and probability. Another tool is used to mesh a 3D computer-aided-design model, apply boundary conditions, and loading. Morphing is used to parameterize the geometrical imperfections.
The objectives of this method are to help design engineers substantially reduce time to market of new completion products, provide them with a tool to balance cost and risk, and enable them to design a tubular completion tool with targeted probability of success, which cannot be achieved by using the current deterministic stress-limits method.