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
Figure 1 - 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 (see Figure 2).
Strain (ε) = ∆L/L
Figure 2 – Strain Relationship
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)
Where;
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.
Figure 3 – Stress-Strain Curve
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.
Figure 4- Young’s Modulus in The Elastic Zone
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”.
Table 1 – API Drill Pipe Grade US customary unit
Table 2 – API Drill Pipe Grade SI unit
Drillpipe Tensile Capacity
Tensile capacity of drill pipe is maximum tension applied before the elastic limit is reached and the formula is below;
Tensile Capacity = Cross Sectional Area x Yield Strength
In this article, we refer to US customary unit therefore the units for calculations are as follows;
Tensile Capacity is in lb.
Cross Sectional Area is in square inch.
Yield Strength is in psi.
Example
API 5”, S-135, NC50, Class New (100% Wall Thickness)
OD = 5 inch
Nominal ID = 4.276 inch
Minimum yield strength = 135,000 psi
What is the tensile capacity of this new pipe (100% wall thickness)?
Figure 5 – Diagram of new pipe (100% wall thickness)
Wall thickness = (5-4.276) ÷2 = 0.362 inch
Figure 6 – Wall Thickness of New Pipe
Cross Sectional Area of New Pipe = π x (OD2 – ID2) ÷ 4 = π x (5.02 – 4.2762) ÷ 4
Cross Sectional Area of New Pipe = 5.275 square inch
Tensile Capacity = Cross Sectional Area x Yield Strength
Tensile Capacity = 5.275 x 135,000 = 712,070 lb.
What is the tensile capacity of the premium class (80% wall thickness)?
Premium class is defined as the minimum of wall thickness is 80% of new pipe. We will discuss a little more about class of pipe later.
Wall thickness of new pipe = 0.362 inch
Wall thickness of premium class pipe = 0.8 x 0.362 = 0.290 inch therefore the OD is 4.856”.
Figure 7 – Wall Thickness of Premium Class Pipe
Cross Sectional Area of New Pipe = π x (OD2 – ID2) ÷ 4 = π x (4.8562 – 4.2762) ÷ 4
Cross Sectional Area of New Pipe = 4.154 square inch
Tensile Capacity = Cross Sectional Area x Yield Strength
Tensile Capacity = 4.154 x 135,000 = 560,764 lb.
As you see from the calculation, premium class drill pipe has approximately 79% of tensile of new pipe. This figure is very important for engineering design and drilling operation. For instant, if you get stuck with the premium class pipe, you must apply tension less than the tensile capacity of 560 Klb in order to ensure that you will not deform or damage your drillstring.
Please always remember that in our drilling operation, we must operate the drillstring within the tensile limit.
Reference - Applied Drilling Engineering Book