Elements of Thread Design
By Colin Rice
For a thread to fail it is necessary that the flanks of the threads are deformed to such an extent that the box and pin are able to move apart.
This is the second article of Part 2 of our Technical Series on Drill Rod Safety and we previously investigated how deep we can drill with steel rill rods. Click here for an outline of the entire Technical Series on Drill Rod Safety.
Thread design
Figure 1 shows some of the important elements of the pin thread of a simple square threaded connection. When the thread is made up, the crest of the pin engages the root of the box thread so that the tensile load is carried by the flank of the threads. For the thread to fail it is necessary that the flanks of the threads are deformed to such an extent that the box and pin move apart.
Figure 1: Elements of a pin thread
Threads used on wireline drill rod, rotary percussion and dual-tube reverse circulation drillpipe vary significantly in terms of their design.
Figure 2 shows a cross-section through an API thread, commonly used on rotary percussion drillpipe and a wireline thread common on diamond core drilling rods.
Figure 2: Cross-sections through an API regular pin and box connection (left) and a “Q” series wireline connection (right).
There are several very important differences:
The wall thickness of the API threaded connection is much greater than that of the wireline drill rod and this allows API threads to be steeply tapered. The wireline connections are only very slightly tapered because of the very small wall thickness of the tube.
The shoulders of the API pin and box threads are very wide and when the joint is fully made up they provide a sealing surface to prevent the escape of drilling fluid through the joint. It is important to note that when the joint is fully made up the base of the pin does not “bottom-out” in the box – the box is always cut deeper than the length of the pin and so this thread has only one sealing surface.
The very wide shoulder of the API thread allows the connection to remain stable when under high compressive or bending loads.
Wireline threads are designed so that when the joint is fully made up there is a double seal in the joint. This serves two purposes, firstly it acts as a fluid seal to prevent leakage through the joint and secondly it allows the joint to carry bending loads when rotating through a bend in the borehole.
In the next article in this series, we look at the development of wireline drill rod threads.