Effect of Measurement Procedures on Estimating Geometrical Parameters of Pipes
Authors: Assanelli Andrea P. , Turconi Gustavo Lopez 
Published: October 2008

The collapse of tubular members under external pressure has been the subject of intense research during the last years. Currently the industry is analyzing several collapse prediction procedures that include the effects of, not only the basic geometrical parameters (nominal or average dimensions), but also of the amplitudes of deviations from the theoretical cylindrical shape. However, different measurement procedures currently used in the estimation of the collapse properties of pipes may yield different estimations of the pipe imperfection, for the same pipe. In order to establish a new collapseprediction formula, a reliable measurement procedure must be assured.

Introduction

Since the work of Timoshenko1 it has been acknowledged that the most important geometrical parameters affecting the collapse resistance of a circular ring (or a pipe) are the slenderness (OD/t) and the lack of circularity of the section. The current work on the collapse prediction of OCTG pipe uses the same geometrical parameters. Since an important part of the prediction is still based on regressions (an complete discussion on this issue can be found in Ref.2), it is important to properly define these variables. The definition of slenderness of the pipe is straightforward, and there has been almost no problems with its interpretation3. For the lack of circularity of the section, Timoshenko considered in his analyses a perfectly oval initial defect, what we are going to call "mode 2". The following definition (generally normalized by the average or nominal diameter) has normally been used in the collapse prediction formulas4:

(mathematical equation)(available in full paper)

The above expression gives a complete description of the geometry for a perfectly oval pipe. However Yeh and Kyriakides5 showed that also higher order contributions are needed for a complete geometrical description of a "real" pipe. In the present paper we will show the effect of different representations of the lack of circularity of a pipe on the collapse prediction and its relation to different measurement procedures.

"Mode 2" approach to collapse prediction

During 1997, a research program was started at FUDETEC on the improvement of finite element predictions for the collapse resistance of OCTG pipes6. The previous work was performed using nonlinear elasto-plastic 2D finite element models, which showed which variables had the greater effect. However, when comparing actual collapse data with the predicted values, it was observed that FEA results tended to underestimate the collapse resistance. The observed underestimation (less than 10%), while acceptable on an "academic basis", was insufficient for practical applications. It is important to note that a similar "bias" was observed when comparing data from our database with the Tamano formula. Following this observations it was proposed that higher order 2D effects (mode 3, 4 etc.) in the 2D representation of the geometry, as well as 3D effects could be responsible of the observed discrepancies6,7. In order to measure the full geometric representation an Imperfection Measurement System (IMS) was developed7, following the original ideas of Arbocz et al.8 and Yeh et al.5.