WASHOUTS IN DRILLSTRINGS

Tool joint failure is one of the main causes of fishing jobs in the drilling industry. This failure is due entirely to the tool joint threads not holding or not being made properly. The make up torque puts the pin in tension and the box in compression, see Figure below. If the pin and box are not properly torqued, then the seals may separate under downhole conditions allowing the a leak path for the mud.

Each drillpipe joint has a pin and a box. Hence for a length of 1000 ft of drillstring there are 66 separate pins and boxes that need to mad up and broken regularly. The threads of the tooljoints seal at the shoulder area only. This feature requires that enough torque is applied during makeup to reduce the risk of having a loose connection in the drillstring. Leak paths within the tool joints develop if the seal is broken or if improper torque is applied. The leak path will lead to tool joint erosion by the drilling mud and if this erosion is severe enough that causes the surface of the pipe to be broken, the pipe is said to have a "washout".

Washouts can also develop due to cracks developing within the drillpipe due to severe drilling vibrations or cyclic loading, Figure below This is especially true in drillstrings rotating at RPM’s matching the drillstring natural (harmonic) frequencies. Washouts are usually detected by a decrease in the standpipe pressure, between 100 -300 psi over 5-15 minutes. This is easily distinguished from sudden drops in pump pressure which could be due to a lost jet nozzle or some surface leak. If a decrease in pump pressure is seen at surface, drilling should stop, pumping resumed. If the pump pressure is still less than before and a bit jet is not suspected to have been lost or no surface leaks detected, then the drillstring should be pulled out of a hole and the defective drillpipe joint should be replaced. Drilling records show that in some cases when a washout is suspected and the pipe was POH (pull out of hole), no cracks or washouts could be visibly seen on any of the drillpipe joints. The reason that the cracks could not been seen at surface is that under pumping and drilling conditions, the cracks open letting mud out and reduce the pump pressure requirements. When at surface, however, the drillpipe is under zero tension and the cracks therefore close escaping detection by the observer on the rig floor.

The author has come across several such situations when a washout is detected and when the drillstring POH, no defective pipe is seen. When the drillstring is re-run in hole, the pump pressure was still lower than before and in some cases it continued to drop. If drilling was resumed a twist off usually occurs resulting in a fishing job. To overcome this, it has been found useful in practice to pump a soft plastic line prior to POH. The soft line wedges itself into the crack(s) while the pipe is still down hole and whilst pumping, Figure below When the pipe is POH to surface, the defective drillpipe joint is easily noticed by the presence of the soft plastic line in the cracks.