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Core Barrels: Do You Know Your Inner Tube?

posted by the Epiroc Exploration & OreBody Solutions team

April 3, 2018

In order to be able to get good drilling performance, diamond drillers should have a thorough understanding of their drilling equipment. They should be familiar with the individual pieces, their function and how they work together as part of the drilling process. The core barrel plays an important role as this is what you use to recover core. Today, most diamond drillers use wireline core barrels. A core barrel typically consists of a double tube system consisting of an inner tube and outer tube. You should understand the role of each piece in the coring system which we will describe below.

Head assembly:

This is the most complex part of the core barrel and it serves several purposes including landing indication, providing circulation of drilling fluids to the core bit and latching the inner tube assembly in place. The latch head mates with the outer tube assembly, and it spins along with the rods. While rotating, the bearings in the head assembly allow the inner tube to stay stationary, to better accept the in-coming core sample. There are several options when choosing latching systems such as the traditional spring latch or the L-latch. 

Inner tube:

This is the part that receives the core and stores it until it is retrieved. It is available in 5 feet and 10 feet lengths but extensions are also available.

Fordia Coring System 2018

Stop ring:

The stop ring is one of three integral parts that accept, hold and break the core sample off from the host rock. The stop ring provides a hardened surface for the core lifter spring to bear against. It is locked into a machined groove in the core lifter case. When the core sample is being recovered, it keeps the core lifter in place, preventing it from being pushed up into the inner tube. In broken ground, it keeps core lifter spring square to the core.

Core Lifter Spring:

The core lifter spring is one of the most important parts for core recovery. It allows the core sample to move into the inner tube as the bit advances in the formation.  When advancing, the core lifter spring slides up against the stop ring. In this position the spring is open and allows the core sample to enter unobstructed.  Once the tube is full, the driller will pull back on the rod string, this forces the tapered sections of the core lifter spring and core case together, gripping the core firmly. 

The inside profile of the core lifter spring comes in two styles, slotted or broached, to ensure a good grip regardless of the ground conditions. A core lifter that is slotted is good for competent rock, whereas a fluted core lifter can provide good recovery in fractured ground.

Core lifter case:

The core lifter case houses both the core lifter spring and the stop ring and is adjusted in relation to the bevel on the inside of the core bit. This adjustment will allow water to flow past the tube to flush the bit.  The bevel on the inside of the bit matches the bevel on the core lifter case exactly.  When the inner tube is full, the driller will pull back the drill string and this forces the core lifter spring to grip the core as it moves down the taper within the core case.  The compression spring in the head assembly allows for the gap between the bit and core case to close, giving support to the inner tube to help break the core.

Of course, there are options when choosing inner tubes and these address specific issues and deliver different performance. For example, you can buy a chrome-plated inner tube that avoids rust and allows the core to enter the tube smoothly and easily. To find out about options you can always contact a member of our technical field support team who will be happy to explain how to choose. Our goal is make your life easier and help improve your drilling performance.