In the Safety Note “Use and abuse of pressure relief valves on positive displacement pumps” published previously in DrillSafe, I discussed some common problems encountered with the incorrect installation and operation of pressure relief valves on drilling fluid circulation systems. In this article, I would like to share a very unfortunate accident that occurred in about 1997. The accident dramatically illustrates the significant force that is created when a high-pressure fluid acts on a large surface area. I use this example in some of my training programs to illustrate the effect of pressure.

The photograph below shows a very large truck mounted rotary percussion drill rig laying on its’ side with several lengths of Schedule 80, 4 ½” drillpipe protruding from the collar of the borehole but still attached to the rotation head of the drill rig.

What happened?

In this accident, the contractor was drilling a 10” borehole using a downhole hammer. 

At some point, the borehole packed off above the hammer and the drillstring became stuck in the borehole. The driller tried unsuccessfully to free the stuck drillstring and eventually decided to connect the booster compressor to blow the packed cuttings out of the annulus.

The driller anticipated that when the cuttings were blown out of the borehole, they would come out at very high speed, and so he positioned himself a safe distance away from the rig.

He obviously did not pay too much attention to what was happening because when he again looked at the drill rig, he saw what we can see in the photograph!!

It seems that the boosted air could not blow the cuttings from the annulus but instead the high-pressure air acted upwards on the full face of the drill bit and literally pushed the drillstring upwards and out of the borehole. I am not sure how the driller explained the accident to his manager but I assume that the exchange would have included some unprintable words!

This incredible accident teaches us a very important lesson about pressure – Pressure is Force divided by Area and so, Force is Pressure multiplied by Area and so, even a small pressure, acting over a very large area can produce a large force.

In this case, the bit diameter was 10” (254 mm) and the booster delivered air at approximately 40 bar (4MPa)

A 254 mm drill bit has a surface area of 50 674 mm2 and so the upward force exerted on the face of the drill bit can be calculated as follows:

A pressure of 40 bar (4 MPa) acting on a 254 mm drill bit therefore creates a force that will lift a load of over 20 MT. It is easy to understand therefore how the drill rig could have been lifted off the ground and tipped over as shown in the photograph – a very costly lesson in simple physics !!