Jökull - 31.12.2001, Page 30

Thorsteinsson et al.
Figure 2a. The drill camp at
the summit of Hofsjökull.
The drill tent was placed
within a 1 m deep trench
dug with the Pisten Bully
(left). Removing the core
barrel from the outer tube
(right). - Borbúðir á há-
bungu Hofsjökuls. Bortjald-
inu var komið fyrir í 1 m
djúpri gryfju, sem grafin
var með tönn snjóbílsins. A
myndinni til hœgri sést þeg-
ar kjarnarörið er tekið úr
ytri hólknum.
force (in kp) required to break the core off hole bot-
tom it would be necessary to subtract the weight of
the drill and cable from the recorded values, but this
seems pointless in view of several complicating fac-
tors. For example, it was noticed that the loadmeter
readings during core breaks seemed to depend on how
fast the cable was being pulled upwards. The occa-
sional high peaks are probably due to packing of chips
around the drill head and/or along the outer tube.
Figure 2b. Operating the drill. - Við stjórnvölinn.
Below we outline the main difficulties encountered in
drilling below the water table:
1. The AWI drill was not designed for operation in
a fluid-filled borehole. No valves or fixed openings
allow the water to flow readily through the drill, se-
riously limiting the lowering and hoisting speed. At
100 m depth, a typical 20 min run can be divided into
the following periods: 30 s lowering at 1.1 m/s to wa-
ter table at 35 m; 8 min lowering with a speed of 0.1-
0.2 m/s to 100 m; 2 min drilling the core; 7 min hoist-
ing up to 35 m; 30 s hoisting from 35 m to surface and
2 min surface time. Thus if the drill could travel up
and down the water column with a speed of 1 m/s, the
runtime at lOOm depth could be reduced to 7.5 min,
resulting in 2.7 times higher drilling and core-retrieval
productivity at that depth.
2. The motor section was not watertight and water
leaked into the drill motor below 35 m. Below the wa-
ter table, the motor current increased from 3 A to >4 A
during drilling. The motor was then running near its
assigned current limit most of the time, above which
it automatically stopped. Unsuccessful attempts were
rnade to seal the motor and associated components
with silicone grease, rubber plastic, and other ma-
28 JÖKULLNo. 51