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DrillSafe Articles

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

Inner tube assembly and overshot innovations

Drill Safe

By Colin Rice
Colin Rice Exploration Drilling Advisory -


In this article we discuss some of the innovations that have been introduced by equipment manufacturers to eliminate some of the hazards associated with inner-tube head assemblies and overshots


This is the first article in Part 2 of our Technical Series on Wireline Retrieval Operations. Click here for an outline of the entire Technical Series on Wireline Retrieval Operations.

Some of the more innovative improvements in wireline equipment are examined below:

1. Easier releasing latch systems

Jammed latches are a common occurrence and commonly occurs when the inner-tube is overfilled. The process of trying to release the latches involves jarring with the overshot which will accelerate wear of critical components and can bring the wireline cable to its yield point or even its failure point. If the wireline cable snaps then a series of events are triggered that introduce many other hazards to the operation and so it is considered essential that incidences of jammed inner-tubes are eliminated completely.

In a “standard” inner-tube assembly, the problem is exacerbated by the path that the latches follow when the latch retracting case moves upwards. In these inner-tube head assemblies, as the latch retracting case slides upwards, the latches move slightly upwards before they move inwards, this effectively increases the contact load between the latches and the bottom edge of the locking coupling and further inhibits release of the inner-tube.

To overcome the problem of jamming at the latches, a manufacturer developed an inner-tube head assembly that uses a different latch configuration – commonly called the “link-latch” system. These latches are not “retracted” by a latch retracting case, they are released when the spearpoint id pulled upwards by the overshot. The latches move slightly downwards and then inwards when the spearpoint is pulled upwards and so reducing the load on the latches and the base of the locking coupling. This latch system effectively eliminates the incidence of stuck inner-tubes.

More recently, one manufacture has released a “Roller Latch” system for locking the inner-tube in position. This system is a significant improvement on previous latching systems.

The consequences of a stuck inner-tube are potentially serious and so it I believe that preventing inner-tube jamming is a critical element of the risk mitigation process, I therefore believe that link-latch systems should be mandatory on all diamond core drilling operations.

2. Safety overshots

De-latching of the inner-tube assembly from the overshot can have serious consequences and so all of the major drilling equipment manufacturers (MBI Drilling Products, Epiroc, Sandvik and Boart Longyear) have released what are commonly referred to as “safety overshots” – these are overshots that positively lock the overshot lifting dogs onto the spearpoint of the inner-tube head assembly.

The MBI type and the Longyear type safety overshots can be used with and standard or link latch type inner-tube head assemblies but the Epiroc and Sandvik types are completely new inner-tube and overshot assemblies.

The main feature of the Longyear Ezy-lock safety overshot is the locking sleeve that can be rotated into a position where the lifting dogs are locked in the engaged position onto the spearhead assembly.

Th MBI safety overshot works in a different way. In this overshot the lifting dogs are able to slide longitudinally inside the overshot body. When the overshot is dropped onto the spearpoint, the lifting dogs engage the spearpoint and as the overshot is pulled upwards the overshot body slides upwards relative to the lifting dogs and the bottom of the lifting dogs are forced inwards onto the spearpoint by a taper cut into the inside of the slots for the lifting dogs. The greater the upward pull on the overshot the more that the lifting dogs are forced inwards. Once engaged, the spearpoint cannot de-latch when under axial load.

This is a very simple overshot with few moving parts and so maintenance is relatively easy.


3. Lifting dog inspection slots

Lifting dog inspection slots milled into the body of the overshot.

Lifting dog inspection slots milled into the body of the overshot.

In all overshot assemblies, the lifting dogs and the lifting dog spring are housed inside the overshot body and so it is impossible to visually inspect the lifting dogs or spring without dis-assembling the overshot. As a consequence, the lifting dogs and spring are not inspected on a regular basis and so wear and tear of these critical components is often undetected. 

Manufacturers have addressed this issue by milling four slots into the overshot head to provide visual access to the lifting dogs. It is recommended that only overshots that incorporate inspection slots or some other way of regularly inspecting lifting dogs for wear are used.

4. Wireline rope attachment to the cable swivel assembly

Several types and diameters of wireline rope are used in exploration operations and so there is no standard method of terminating the wire rope at the cable swivel eye. Some contractors use rope clips to terminate, some roughly splice the dead end of the rope into the live end and some merely tie the dead end to the live end with light gauge wire. Frequently, the method of termination is determined by individual drillers and so within a contracting company one frequently sees multiple methods of termination.

This is not necessarily a problem; however, it is obviously necessary that the termination is secure and that the method of termination does not create any sharp edges or other hazards that could cause hand or other injuries during the retrieval process.

Cable swivel assemblies -note the machined eye in the assembly in the right-hand photograph.

Cable swivel assemblies -note the machined eye in the assembly in the right-hand photograph.

In the past, manufacturers supplied a rope thimble when they supplied an overshot to protect the rope for abrasive wear. In recent times however, this is no longer done and so wireline rope is potentially exposed to abrasive wear where it contacts the eye of the cable swivel assembly.

In poorer quality overshot assemblies, the eye is roughly machined resulting in sharp edges which can rapidly wear the wireline rope. In better quality assemblies the eye is milled so that its surface resembles a thimble.

It is recommended that contractors use a standardised method of terminating the wireline cable at the cable swivel assembly and also that that the termination incorporates a thimble to prevent cable wear. Alternatively, a cable eye with a milled surface may be used.

The next article in this series focuses on secondary safety devices to reduce risks..