Tubular Goods -Service Conditions and Performance
Authors: Oxford W.F. 
Published: December 2007

Demands upon equipment performance created by deep high-pressure service for the past 10 years in both the Gulf cies of tubular goods. Wells in which high-performance materials are required are costly, and failures in casing or tubing can easily result in losses of hundreds of thousands of dollars. High-strength tubular goods have been used in high-pressure service for the past 10 years in both the Gulf Coast and West Texas areas, and use of such goods is increasing. However, problems associated with their use are not widely known. Problems (and possible solutions) encountered in the manufacture, procurement, inspection and performance of high-strength tubular goods are discussed in this article. Any material with a yield strength greater than 80,000 lb/sq in. is considered to be a high-strength material. Failures are normally caused by one of these factors: manufacturing defects, damage during handling or stress corrosion cracking. The importance of planned purchasing and stocking of required tubular goods is emphasizes. Also presented are new metallurgical techniques for production of high-strength tubular goods, and comments on a 150,000 lb/sq in. yield-strength material for use in deep wells.

Introduction

Present-day drilling depths and the high pressures encountered are creating an insistent demand for steels that can be used at higher levels of stress than can conventional materials. High-strength materials considered in this article are listed in Table 1 (API-graded materials) and Table 2 (non-API-graded materials); N-80 is included because its yield strength often exceeds the 80,000-psi minimum requirement of a high-strength material.

Problems in Manufacturing and Handling High-Strength Tubular Goods

Any sharp-edged notch or crack in the surface of a material is a point of stress concentration which can be likened to driving a wedge into the material. Low-strength materials are soft and ductile and will yield to relieve the stress. High-strength materials, by their nature, have low ductility and cannot yield to relieve the stress concentration. In this case, it is like driving a wedge into a brittle substance such as stone. Only a slight penetration causes the stone to split. In high-strength tubular products, a crack will propagate from the point of stress concentration and continue to the point of failure.

Manufacturing DefectsCracks and seams created in tubular products during the manufacturing process can cause failure. Usually, the most critical manufacturing defect is not visible to the naked eye. This defect is the fine crack or longitudinal seam which can be detected only by magnetic particle or electromagnetic inspection. Fig. 1 depicts typical defects in tubular goods and Figs. 2 through 4 further typify mill defects. It must be recognized that hardness is a function of the yield strength and tensile strength, and that limitations placed on any of these three properties automatically affect the other two. Some feel that extreme hardness is detrimental and that maximum hardness levels should be in the range of 28 to 30 Rc. This level can readily be maintained, provided the strength values are properly adjusted to conform with it.

JPT

P. 617ˆ