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Risks of using wrenches in exploration drilling operations

DrillSafe Articles

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

Risks of using wrenches in exploration drilling operations

Drill Safe

By Colin Rice

Colin Rice Exploration and Training (Pty) Ltd. -


We were recently made aware of a fatality involving the use of a pipe wrench. This article analyses the use of wrenches in drilling operations.


This is the first article of our Technical Series on the Use and Abuse of Wrenches. Click here for an outline of the entire the Use and Abuse of Wrenches Series.

A number of injuries involving the use of wrenches have occurred on drill sites and most recently, a drill rig assistant was fatally injured when he was struck on the head by a heavy-duty pipe wrench. As a result of the number of injuries that have occurred, many mining companies have “banned” the use of pipe wrenches. In many cases they have given preference to purpose designed wrenches assuming that these devices would eliminate the risks associated with use of wrenches. I do not necessarily agree with this point of view, I believe that wrenches do not injure people, it is the people who misuse the wrench that injures people. 

When used correctly and within specification, all of the wrenches used in drilling operations will deliver safe and trouble-free service. However, these tools are frequently used beyond their design limits and for purposes for which they were not designed. It is these situations that are responsible for most wrench related injuries.  Management of the risk must therefore focus on the correct selection and use of wrenches. 

Wrenches of various types are used to make-up (tighten) or back-off (loosen) threaded connections between tubular components. In drilling operations, the threaded connections for which wrenches are commonly used are:

  1. Drill rod or drillpipe or casing connections,
  2. Connections between a drill rod or drillpipe and an adaptor or sub,
  3. Connections between a drill rod or drillpipe and another tubular component – a corebarrel, a stabiliser, a downhole hammer or a reaming shell and drill bit for example,
  4. Connections between components of a corebarrel outer-tube assembly,
  5. Connections between components of a corebarrel inner-tube assembly.

The purpose of this article is to provide an analysis of the risks associated with the use of wrenches in drilling operations and in order to do this it is necessary that we consider all of the many variables that will affect the risk. These factors include; 

  • the type of wrench used, 
  • the type of drill rig, 
  • the inclination of the borehole,
  • the drilling method and 
  • the design and characteristics of the drill rod or drillpipe being used. 

Types of wrench used

Several types of wrench are used in the drilling industry and the selection of wrench depends essentially on the torque required to make-up or break-out a joint. The following types of wrench are commonly used in drilling operations:

Stilson (bobbejaan) wrenches

Stilson wrenches are used in “high torque” applications (type 1, 2, 3 and 4 connections above).

Stilson wrenches have two hardened, serrated steel jaws that are designed to “bite” into the components being torqued. These wrenches provide two-point contact and so, as torque on the wrench is increased the load at the two points of contact increases and the depth of the “bite” of the jaws increases. Even at low torque, the jaws can cause severe longitudinal deformations on the surface of the components.

The depth of the deformations created by the jaws effectively reduces the wall thickness of the component and so creates stress risers which, under certain conditions, can lead to premature fatigue failure of the component. This is commonly seen in the industry. In addition, the sharp edges of the deformations can cause severe lacerations.

Stilson wrenches must never be used on thin walled tubulars because the t-o point contact can cause the tube to deform and become distorted (squashed). Similarly, they should never be used on reaming shells or diamond bits because of the potential distortion of the component. Stilson wrenches are available in several sizes, the most common sizes used in drilling applications are: 24”, 36” and 48”. As size increases so too does the mass of the wrench, a 36” wrench for example has a mass of 9 kg while a 48” wrench has a mass of 16 kg. 

In the heavy-duty versions, the handles of the wrenches are cast while an aluminium handle version of the 48” (8,5kg) wrench is commonly used in drilling operations.

Rod wrenches (full circle wrenches)

Rod wrenches are sized for a specific diameter of drill rod and incorporate multiple sets of inserts that spread the load over a greater surface area as torque is applied to the tubular component. This reduces any distortion induced by the wrench. 

The “jaws” of a full circle wrench normally consists of two hinged semi-circular sections that lock onto the handle of the wrench – as torque is increased the contact pressure between the jaws and the component increases but because contact is essentially over the entire circumference of the component little or no distortion occurs. In order to increase the torque that the wrench can provide before slipping, the jaws are typically very wide.

These wrenches typically tend to be very heavy and they require frequent maintenance to ensure that the jaw inserts remain sharp enough that they do not slip. Typically, these wrenches are much more costly than Stilson wrenches and so contractors tend not to use them.

Outer tube wrenches

Wireline and conventional corebarrel outer-tube components are relatively thin walled and so are very susceptible to crushing or distortion if a Stilson is used. 

Torque requirements are generally low and so connections are typically made and broken with medium duty full circle wrenches. As with a rod wrench, tungsten carbide inserts are used to provide the friction surfaces in the wrench. These wrenches can only be used in relatively low torque applications - these wrenches are more than adequate if in a good condition and correctly used.

Inner-tube wrenches

Wireline and conventional corebarrel inner-tube components are extremely thin walled and so are very susceptible to crushing or distortion. 

Torque requirements are very low and so connections are typically made and broken with light duty full circle wrenches. The entire contact surface of the wrench is coated with a tungsten carbide grit that provides a friction surface for the wrench. 

Read the next article in this series which details the use and abuse of wrenches.

Other articles in THE Use and Abuse of Wrenches SERIES