Tubulars: Drill pipe and collars are designed to satisfy certain operational requirements. In general, downhole tubulars must have the capability to withstand the maximum expected hookload, torque, bending stresses, internal pressure, and external collapse pressure. Operational capabilities of different sizes and grades of drill pipe and collars are tabulated in the API RP 7G to assist the drilling engineer in selection of pipe and collars for a given drilling situation. Other concerns, such as the presence of H2S, must also be considered in the selection process.
Drill Pipe Yield Strength and Tensile Strength: If drill pipe is stretched, it will initially go through a region of elastic deformation. In this region, if the stretching force is removed, the drill pipe will return to its original dimensions. The upper limit of this elastic deformation is called the Yield Strength, which can be measured in Psi.
Beyond this, there exists a region of plastic deformation. In this region, the drill pipe becomes permanently elongated, even when the stretching force is removed. The upper limit of plastic deformation is called the Tensile Strength. If the tensile strength is exceeded, the drill pipe will fail.
Tension failures generally occur while pulling on stuck drill pipe. As the pull exceeds the yield strength, the metal distorts with a characteristic thinning in the weakest area of the drill pipe (or the smallest cross sectional area). If the pull is increased and exceeds the tensile strength, the drillstring will part. Such failures will normally occur near the top of the drillstring, because the top of the string is subjected to the upward pulling force as well as the downward weight of the drillstring.
Grade E, composed of a lower grade of steel, is sometimes referred to as “mild” steel, because it has the lowest yield strength per unit area. As such, mild steel is generally defined as steel with a yield strength of less than 80,000 psi. As can be seen, Grade E drill pipe has a lower yield strength in psi than the high strength drill pipe grades, however once the yield strength is exceeded, it can withstand a greater percentage of stretch or “strain” prior to parting. Lower grades of steel such as Grade E are also more resistant to corrosion and cracking. Grade E has been utilized in medium depth wells (10,000 to 15,000 feet).
In the 1980′s, as horizontal drilling, high inclination extended reach wells and deep hole drilling applications increased, so has the demand for high strength drill pipe. It is common in deep hole applications for high strength drill pipe to be utilized in the upper portion of the string to keep the tension load within the capabilities of the steel. In high dogleg environments, such as those encountered in medium and short radius horizontal wells, high strength drill pipe can withstand the associated bending stresses. In high inclination and horizontal wells, high strength drill pipe is also commonly run in compression. One drawback of higher grades of steel is that they are generally less resistant to corrosion, like that caused by hydrogen sulfide (H2S). Limited availability also contributes to the higher cost.
The yield and tensile strengths are in “pounds per square inch of the cross sectional area” of the drill pipe. In order to calculate yield strength in pounds, this cross sectional area must be known. This leads to a discussion of drill pipe classes.
Drill Pipe Classification: Drill pipe class defines the physical condition of the drill pipe in terms of dimension, surface damage, and corrosion. Drill pipe class is indicated by paint bands on the drill pipe according to the following code:
Class 1 drill pipe is New and therefore the strongest. As pipe is used, the wall thickness will be gradually reduced. This reduction of the drill pipe cross sectional area results in a lower Total Yield Strength in pounds. This yield strength in pounds can be calculated using the following formula:
YIELD STRENGTH = Yield Strength x pi/4 (OD2 - ID2) – (in pounds)(in psi)