Basic of Drillpipe Tensile Capacity and Its Calculation

This article demonstrates basic knowledge understanding of tensile capacity of the drill pipe and some calculation. First of all we need to know basic of material strength and for our case is strength of metal

There are few simple terminologies which we would like to explain

(Stress (σ

Stress (σ) equals to force divided by cross sectional area of the material (F/A). For our case, we will discuss about only stress in tensile because the drill pipe is almost always designed to work in a tensile condition

Stress (σ) = F/A

(Strain (ε

Strain (ε) is a change of material per an original length. From the definition, it equals to ∆L/L

Strain (ε) = ∆L/L

(Young’s Modulus (Modulus of Elasticity

Young’s modulus (the tensile modulus or elastic modulus) is a ratio of stress and strain along the axis and we can write into the following equation

(Young’s modulus = Stress (σ) ÷ Strain (ε) = (F x L) ÷ (∆L x A

F is pulling force.
L is an original length of pipe.
∆L is an amount by which the length of the pipe changes
A is a cross sectional area of object
The Young’s Modulus of material represents the factor of proportional in Hook’s Law therefore it will valid under the elastic zone.  There are several units for Young’s Modulus as N/m2 (Newton), Maga Pascal (N/mm2) and Pound per Square Inch psi

Stress-Strain Curve

A stress-strain curve is a graph derived from Stress (σ) versus Strain (ε) for a sample of a material. The nature of the curve varies from material to material. The following curve shows a behavior of metal

Yield Point or Yield strength, is defined as the stress at which a material begins to plastically deform. Before the yield point the material will deform elastically and it will return to its original shape when the stress is released. If the tension applied is over the yield point, the deformation will be permanent and non-reversible
Ultimate strength is the maximum stress applied before the material is completely parted
Young’s Modulus (modulus of elasticity) is the slope of the Stress-Strain curve within the elastic limit (see Figure 4). It means that once tensile is less than Yield Point, the Young’s Modulus is valid for the calculation
Young’s Modulus of steel is 30 x 106 psi

In drilling operation, we must operate within Yield point because the metal will become the original shape. For example, if you get stuck, the maximum tension applied to free the stuck drillstring must be always under yield point with a designed safety factor for the operation

Drill Pipe Body Grade

API RP7G classifies a grade of drill pipe body according to yield strength and tensile requirement (see Table 1 and Table 2). Four grades of drill pipe are “E”, “X”, “G” and “S”