Injector Head is one of the six major components of coiled tubing unit, used to run or pull and hold the CT string into the wellbore.
Major parts of injector head
chain system, drive and brake system, chain tension system, chain traction system, weight measurement system.
The CT unit is identified by the rating of injector head. For example: 60K and 80K etc.
One benefit of CT as compared to other services (wireline or slick line) is that we can snub the CT pipe into the wellbore.
General information helpful for CT crew.
Maintenance of Chain Assembly
Wash and lubricate on periodic basis
Make sure that injector chain engages smoothly with the sprocket teeth.
Check for any abnormal noise.
Check that chain bearings are rotating smoothly. If got stuck then replace with new.
Check for chain elongation. Only 3% chain elongation is acceptable.
During travelling cover the chain assembly, to protect it from dust etc.
Drive and Brake System in Coiled Tubing Injector Head
The function of drive and brake system is to drive the chains assembly so that coiled tubing pipe could move in or out of the well. And if required provide the brake to stop the chain movement.
Overall components are similar in all the injector heads irrespective of the manufacturer. But time being we will understand the injector drive and brake system with the help of drive and brake system of 60K and 80K HPT injector head.
Chain Tension System in Coiled Tubing Injector Head
Function of Chain tension system is to avoid the slacking or gathering of injector chains and thus prevention of CT pipe from damage.
Parts of chain tension system and its working
Four hydraulic cylinders, two on each side of injector head are used to provide the required force to the idler sprockets which then keeps the chains in tension.
The chain tension idler sprockets are supported by take-up bearings which reside in bearing guides mounted to the injector side plates.
The hydraulic pressure supplied to the chain tensioning system can be varied from the control cabin.
Orientation of chain tension system is different in 60K and 80K injector head.
Important concepts regarding chain tension pressure
During pipe heavy load, chain tension pressure is normally set at a minimum fixed value depending upon the chains life.
However, in case of pipe light weight operation (snubbing) it is required to change the tension pressure depending upon the snubbing force required to RIH.
Following formula is used to calculate the snubbing force for a given value of well head pressure.
Chain Traction System in Coiled Tubing Injector Head
Chain traction system is used to provide the necessary force to chains to make a strong grip to CT pipe so that it could move in a controlled manner.
Traction pressure (force) need to increase as we go deeper into the well.
Ratio of traction force to pulling force may be as high as 5:1 depending upon different factors including gripper block condition, tubing surface condition, tubing wall thickness, running speed and the ovality of tubing.
For smooth working of traction cylinders, shock absorbing accumulators are installed in the injector head. The number of accumulators installed in an injector head varies depending upon the injector model.