# Presentation: 466289

## The Icecube Drillhead

**Drill Body - 11 ft. long**

**Weight Stack**
**44 ft. long**

**Nozzle Tip**

**Load Cell**

**Hose and Cable**

- _The Icecube Drillhead_

**Notes:**
- I am going to say a little about what appears to be going on when the drill goes into this oscillating mode above about 4 ft/min.

- The drillhead consists of an 11 ft. drill body and a 44 ft. long weight stack.
Mention calipers and pressure sensor.

-As this is lowered water from tip melts ice ahead of tip.  Water flowing up the hole opens it up so the drillhead can get through.  Long weight stack gives a delay between passing of tip and drill body.

- Notice the load cell here.  It carries all the weight of the drill head and weight stack and sends that information up to the surface.

- When drilling slower than about 4 ft/min it just reads out the static weight of the drillhead.  It stays pretty constant.

-This changes above 4 ft/min...

## Slide 2

**Notes:**
- Flat part of load (green) is still the static weight of drillhead on load cell.

- Spikes show a sudden brief decrease in load.

- Pressure sensor shows variation in pressure that could be variation in depth OR variation in ambient hole pressure.

- We can calculate the approximate natural frequency of vertical oscillation for the drillhead on the hose and cable.  The observed frequency is about the same as that calculated.  This supports the idea of a variation in depth i.e. vertical oscillation.

- Calculated and observed frequencies are also consistent at 4300 ft and 1900 ft where calculated natural frequency is faster.  Show next slide and return.  (Same time scale.)

-Note spikes occur at highest pressure depth - deepest excursion of drillhead.
		However...

## Slide 3

**Notes:**
- At 4300 ft the spikes occur at the lowest pressure - what would correspond with the highest excursion of the drillhead.

- This weighs in against the somewhat intuitive idea that the spikes are caused by the drill somehow banging against the bottom.

- If whatever is causing the spikes is giving the drill a shove up and driving the oscillations you would expect to see the spike occur at about the point in the drillheads up and down cycle - independent of depth. The phase relationship at 4300 and 7900 ft. are about 180 degrees apart!

- This suggests that the spikes are a by-product of the oscillations rather than the root cause of them.  

Perhaps omit this.  (It may also suggests that there is some delay between the cause of the spike and the spike itself. )

## Consider the region around the tip.

**Notes:**
- I believe the oscillation can be explained by considering the tip of the drill.

- If the drill descends too far towards the narrow bottom of the hole the flow of water up the hole, between the hole wall and the weight stack, is restricted - slowing the flow of water.

- This slowing of the water column in the hose causes a rise in pressure in the region below the tip. This is the “water hammer” effect heard in pipes when a valve is closed quickly.  This higher pressure zone would push the drill back up. 

- When the drill is pushed up the restriction opens, the pressure decreases, and the drill drops back down.

- This can drive the oscillation without the drill.   The drill may never hit the bottom of the hole.

-Restricting the flow at the tip will cause the pumps to raise the pressure in the entire hose.  This would not happen instantly over the whole 11,000 feet of hose.  It would take a few seconds to fill in from the top down.  The spikes could be caused by the delayed arrival at the tip of this pressure increase.  This question is still being debated.

## Heat loss through weight stack robs heat from the tip.

**Notes:**
So... what is the solution.

- Also under current investigation.  

- Current suspicion is that it is caused by heat flow through the side of the weight stack into the surrounding water.

- Note this is not lost heat - it still opens the hole up.  But if it bypasses the tip the hole at the tip is slender causing the oscillation just described.

-If, however, more heat makes it to the tip...

## More heat at the tip means a larger diameter hole at the tip.

- Hole diameter above weight stack is unchanged.

**Notes:**
- ...a bigger hole ahead of the tip.  This allows a higher speed.