Drillpipe Fatigue Life Prediction Model Based On Critical Plane Approaches
Authors: Plácido J.C.R. , Azar J.J. , Sorem J.R. , Kessler Franz , Tipton S.M. 
Published: October 2008

ABSTRACT
The applicability of critical plane approaches to predict fatigue life expectancy of drillpipe is investigated. These approaches consist of physically-based damage parameters that are capable of handling complex axial, bending and torsional load histories. Two damage parameters are considered, one based on normal strain amplitude and the other on shear strain amplitude. Comparison of full scale results and data from polished coupons indicate a strong influence of microscopic surface discontinuities. Approaches are presented for dealing with these effects analytically, along with a discussion on effects of realistic drillpipe geometry and loading histories. Fatigue notch factors are estimated and used with the damage parameters. Results are correlated with data from full scale test facilities using Grade E drillpipe.

INTRODUCTION
Drillpipe failure due to fatigue is a very costly problem in the oil and gas industry. Although, many investigators1–19 have previously addressed this problem, its frequency of occurrence is still excessive.
Based on the work of Lubinski and Hansford1, the APl-RP7G20 provides the petroleum industry with a simplified procedure to calculate cumulative fatigue damage in drillpipe. However, this approach is strictly empirical and lacks the physical basis necessary to consider multiaxial effects (e.g. due to torsion). The most recent work concerning this problem was presented by Howard, et al19, describing a systematic method of tracking cumulative fatigue damage of each joint of pipe while drilling a well. However, his approach is essentially that developed by Lubinski and Hansford1.
It has been documented21 that fatigue cracks tend to initiate and propagate along particular planes. The so-called "critical plane approaches" account for this problem by considering the history of strain and/or stress components acting on individual planes to analyze fatigue damage. For example, the amplitude of normal or shear strain acting on a particular plane is considered to cause damage. However, damage mechanisms also are influenced by the presence of a mean stress acting on the same plane. The development of such physically-based parameters has received considerable attention21–24.
In this paper, the application of critical plane approaches to predict drillpipe life expectancy is presented and discussed. Two damage parameters are considered, one based on normal strain amplitude and the other on shear strain amplitude. They are evaluated by comparison of baseline fatigue data and full scale drillpipe test data. Results demonstrate a significant influence from surface microdiscontinuities and drillpipe geometry, Analytical techniques are presented for including these effects.
Full scale drillpipe fatigue facilities based on new design concept were built at the University of Tulsa and at the University of Leoben. The design allows the application of load histories that are similar to those experienced in service.

CRITICAL PLANE APPROACHES
Fatigue cracks tend to initiate on certain planes. Cracks have been observed to occur on planes experiencing the maximum normal strain fluctuation or the maximum shear strain fluctuation21–22. Which of these planes control fatigue strength depends on numerous factors which include : material (isotropic or anisotropic), load amplitude (high or low cycle fatigue) and material discontinuities (notches or upsets).