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Forces and stresses in drill rods

DrillSafe Articles

Safety information and collaboration forum for the exploration drilling industry in Southern Africa.

Forces and stresses in drill rods

Drill Safe

By Colin Rice

 

What forces and stresses are exerted on drill rods? Here's an explanation of the mechanical concepts.

 

This is the first article of Part 1 of our Technical Series on Drill Rod Safety. Click here for an outline of the entire Technical Series on Drill Rod Safety.

Every drilling operation requires that lengths of drill rod (or drillpipe) are connected one to another as the borehole is drilled deeper into the Earth’s crust, drill rods (and drillpipe) are therefore an essential element of every drilling operation. Similarly, all downhole tools; corebarrels, stabilisers and adaptors, for example, are equally as essential and so these downhole components must be carefully designed and manufactured to ensure that they deliver a long, trouble free service life. Frequently exploration drilling contractors use drill rod close to or beyond their safe working limit and this significantly increases the risk of catastrophic failures that can result in serious injury.

Drill rods vary substantially in terms of their dimensions and design, the materials used in their manufacture and their method of manufacture but all share the same essential functions:

  • They transmit rotary torque and rotational speed (power) to the drill bit.
  • They transmit the flushing medium (air or water or mud) to the bit.
  • They act as a link between the drill bit and the thrust control system of the drill rig.
  • They allow removal of worn drill bits and re-insertion of new drill bits.

The drillstring is subjected to a number of different forces as a borehole is drilled - the weight of the drillstring will tend to either stretch or compress the drillstring, rotational torque applied by the drill rig will tend to twist the drillstring and changes in direction of the borehole will tend to bend the drillstring. In addition, the drillstring will be subjected to abrasive wear as it is rotated in the borehole and its surface will be gouged by the jaws of wrenches or chucks and clamps used to make-up and break-out the joints.

Stress

When a force acts on a body it creates a stress in the body which tends to change the shape and dimensions of the body; a compressive force, for example, will tend to shorten and thicken the body while a tensile force will tend to stretch the body make it thinner.

Stress is defined as the force per unit area applied to a body and so we can determine the stress at any point in a body as follows:

$$stress = {force \over cross\,sectional\,area}$$
If force is measured in \(Newtons\) and if the \(cross\,sectional\,area\) is measured in \(mm^2\) then, \(stress\) will be measured in MPa (megapascals) – \(stress\) therefore has the same units as pressure.
Let’s look at an example to illustrate this; let’s assume that we have a drillstring that is \(1000m\) long hanging freely in a vertical borehole from a hoist plug. Let us also assume that the mass of the drill rods is \(8kg\,per\,meter\) and the cross-sectional area of the drill rod is \(800mm^2\).

We want to determine the stress in the top-most drill rod. The top-most drill rod is carrying the entire drillstring and so the force acting on the top-most drill rod is equal to the total weight of the drillstring.

If we assume that \(g = 9,8m/sec^{-2}\) then, we can say:
$$weight\;of\;the\;drillstring = {1000\times8\times9,8\;Newtons} \\ weight\;of\;the\;drillstring = 78400\;Newtons$$ And so we can say: $$stress\;in\;the\;topmost\;drill\;rod={78400 \over 800} \;Newtons\;per\;square\;millimeter$$ $$stress\;in\;the\;topmost\;drill\;rod=98\;MPa$$

Obviously, this will be a tensile (stretching) stress.

It is important to recognise that as any borehole gets deeper, the stress and strain on the top-most drill rod will increase and at some point the stress will be so great that the drill rod will fail. It is extremely important that we know at what depth this failure will most likely occur.

In the next article in this series, we look at how the tubular steel used for drill rods is manufactured, and the mechanical properties of this material.

Other articles in the Drill Rods Manufacturing Series