Surface Roughness in Internally Coated Pipes (OCTG)
Authors: Farshad F. , Pesacret T.C. , Bikki S.R. , Davis R.H. 
Published: October 2008

Proper selection, design and installation of the tubing string (OCTG) is a critical part of the completion program. The tubing must be selected so that production operation can be carried out efficiently; it must be designed against failure from tensile forces, internal and external pressure and corrosive actions. In addition, it must be designed in such a way that maximum production rate and total optimization of the production system is achieved.

One frequently encountered fluid flow problem is the determination of the absolute surface roughness, ?, and relative roughness, ?/d, which is the ratio of absolute surface roughness to internal diameter of the pipe, d, a dimensionless ratio. Relative surface roughness is needed in estimating friction factors to be used in computing the friction pressure losses in the tubing. In 1944, Moody1 furnished the practicing engineers with a correlation for relative roughness for any size pipe of a given type of surface. Moody's1 relative roughness correlation was based on experiments on pipes artificially roughened with sand grains. Since then, a large number of experimental and theoretical studies2–6 have been performed on friction pressure losses of fluid in smooth and rough pipes. However, relatively little work has been published on the standard surface roughness values in internally coated pipes and its effect on the production rate and hydrodynamics in pipes. In particular, reports on research of physical measurements, mathematical modeling studies and statistical analysis of surface roughness characterization7 are still scarce.

The objective of study was to develop a reliable method for the measurement of pipe surface roughness. Surface roughness of coated pipe was measured by the Dektak3ST Surface Profiler®, and the Hommel T 1000®. Arithmetic roughness (Ra), root mean square roughness (Rq), and mean peak to valley height (RZD), were statistically analyzed. Each instrument yielded reliable results. The ability of RZD to ignore intermediate ZD height data while focusing on extreme height data made it a useful parameter for detecting differences in height that would be the most likely to affect turbulent flow.

Introduction

Internally coated pipes are currently being utilized for completion of oil and gas wells worldwide. However, relatively little work has been published on either the measured absolute surface roughness or the relative roughness of coated pipes and their effect on production rate and hydrodynamics in pipes. In particular, reports on research of physical measurements, mathematical modeling dies and statistical analysis of surface roughness in coated pipes are still scarce.

The seriousness of corrosion problems has since been recognized in most phases of the oil industry. The use of internally coated oil field tubular goods began in 1943, when the oil industry first became concerned with corrosion in gas condensate wells. An increasing concern with such problems has also occurred in the chemical industry. The primary use of coatings has been to extend the life of pipe by mitigating any interaction with corrosive fluids. To a much lesser extent, coatings have been used to decrease or even eliminate problems associated with paraffin and scale deposition in pipe.