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Hazards associated with inner-tube head assemblies

DrillSafe Articles

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

Hazards associated with inner-tube head assemblies

Drill Safe

By Colin Rice
Colin Rice Exploration Drilling Advisory -


In this article we explore two of the most significant hazards related to inner-tube head assemblies.


This is the third article in our Technical Series on Wireline Retrieval Operations. Click here for an outline of the entire Technical Series on Wireline Retrieval Operations


A number of things can go wrong in the process of retrieving and deploying a wireline inner-tube assembly. This article and the following article examine some of the most significant of these hazards.

The inner-tube can get stuck in the corebarrel or in the drillstring

The inner-tube assembly can become solidly stuck in the corebarrel or inside the drillstring for various reasons:

  • If the driller “overfills” the inner-tube, the inner-tube head assembly is put under an added axial load which forces the inner-tube upwards inside the corebarrel and in turn forces the latches hard against the base of the locking coupling. The increased axial load inhibits movement of the latches and can prevent them from retracting when the latch retracting case is pulled upwards.

  • In some drilling conditions, the shut-off valves can compress to such an extent that they jam inside the corebarrel and prevent release.

  • If the drillstring is over-torqued or if the drill rods are very worn or poorly manufactured, it is possible that the base of the pin thread can “pinch” and so reduce the effective internal diameter of the drillstring. In severe cases, this may be enough to prevent passage of the inner-tube assembly through the drillstring and the assembly will become stuck at the pinched pin connection.

In these situations, the driller will try to free the stuck inner-tube through one or more procedures:

  1. The driller may increase the pulling force of the winch. 

  2. The driller can attempt to “jar” the inner-tube free by repeatedly lowering and then raising the overshot. The jar staff travels inside the jar tube and when the jar tube reaches the limit of its upward travel it strikes the head of the jar staff and transfers an impulse to the inner-tube assembly.

  3. If the above two procedures are not successful in releasing the stuck inner-tube assembly, the driller will drop a dry-release device down the open drill rods. The dry-release impacts the top of the lifting dogs forcing them to open thereby releasing the overshot from the spearhead point. The overshot can then be retrieved to surface and the drill rods tripped from the hole. The stuck inner-tube assembly can then be freed from the corebarrel when on surface

In the first two procedures, the wireline rope will be subjected to increased stress and impulses which can bring the rope close to or beyond its yield point and in some cases its ultimate tensile strength. 

In the second procedure, in addition to the load on the wireline rope, the jarring action of the jar tube on the top of the jar staff places significant load on both the top of the jar staff and the base of the jar tube. Over time this will lead to fatigue failure of either component.

If the load on the rope exceeds its ultimate tensile strength it will snap and, depending upon where the rope fails, this may require that the drillstring is tripped with wire rope inside the drillstring. Once the rope is retrieved to surface it will have to be spliced (joined together) and then reeved through the sheave wheel system in the mast. This introduces a number of risks.

The inner-tube and the overshot can become de-latched 

Once the inner-tube assembly has been latched and retrieved to surface, it is necessary that the inner-tube assembly and overshot are raised clear of the open drillstring and then lowered into a horizontal position to allow the core to be removed from the inner-tube. 

Once the core has been extracted, the inner-tube assembly is reassembled, it is then raised back into position and then lowered into the open drillstring so that it can be dropped down the open drill rods. 

These operations require that one of the drill rig assistants (offsiders) handle the suspended inner-tube assembly to guide it into position. If the inner-tube becomes de-latched from the overshot at any time then the inner-tube will fall in an uncontrolled way. 

Typical lifting dog from an overshot assembly.

Typical lifting dog from an overshot assembly.

The overshot latches onto the spearhead using two, spring loaded lifting dogs which slide over and engage the base of the point of the spearhead, pictured to the right.

There are several possible reasons why the lifting dogs could disengage from the spearpoint:

  1. the shoulder of the lifting dogs can become worn over time which could allow the spearpoint to slide out of the lifting dogs or,

  2. the lifting dog spring can become worn and so lose its’ ability to hold the lifting dogs in position which could allow the lifting dogs to slip off of the spearpoint or, 

  3. the overshot can bump the mast which could cause one lifting dog to disengage from the spearpoint. If the remaining lifting dog is worn or the spring is not strong enough, it is possible that the lifting dog could slip off the spearpoint.

To eliminate the possibility of the inner-tube de-latching from the overshot, manufacturers have come up with several innovative solutions to positively lock the lifting dogs onto the spearpoint. These systems are very effective and so it is considered essential that only overshots with the ability to positively lock the lifting dogs in the engaged position are used.

Look out for our December edition, where more information will be published on these innovations.

The next article explores the hazards associated with wireline overshots.