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Underground Testing Perfected Big-Hole Drilling
Technology
On July 26, 1957, a safety experiment called "Pascal A" was detonated
in an unstemmed hole. Although the test was not spectacular, it does
hold the distinction of being the first nuclear test in the world to be
detonated underground.
From 1957 to 1992, 533 contained tests and nine unstemmed tests were
conducted at the Nevada Test Site (NTS).
If the depths of all the 36-inch diameter holes drilled for nuclear
tests were added together since 1961, they would total about 280 miles.
Big-hole drilling (holes larger than 36 inches in diameter) so essential
to the nation's testing program, was not invented at the NTS. As early
as 1850, 30-inch water wells were drilled in the eastern and midwestern
United States by rotary rigs. In 1927, the Dutch were drilling 216-inch
diameter holes to construct levees to reclaim land from the sea.
The need to drill large-diameter holes at NTS resulted from the Limited
Test Ban Treaty (TTBT), signed in Moscow on August 5, 1963. The TTBT
prohibited testing nuclear weapons in the atmosphere, underwater, and in
outer space. As a result, scientists from the DOE weapons laboratories
had to place all their test packages underground.
To accomplish this, employees of Reynolds Electrical & Engineering Co.,
Inc. (REECo) developed innovative drilling techniques to overcome the
problems of a penetration rate that was too slow and therefore too
costly, terrain that was too porous, the need for straight line-of-sight
holes, and the need to have a hole wide enough to accommodate the test
package.
Today, REECo's drilling techniques and hardware are used throughout the
world to construct bridge piers and make platforms for ocean drill rigs.
When REECo began drilling at the NTS in 1959, the biggest problem was
the time it took to drill into the desert floor. A 36-inch diameter
hole, 1,000 feet deep, could take up to 60 days. The initial method was
to drill in three successive passes, each one larger. This
time-consuming process was reduced somewhat when REECo mounted all three
bits together, in a tri-stage simultaneous drilling setup. Even with
this innovation, the drillers worked 30 days to complete a hole.
Successive modifications stacked the bits closer together. Eventually
the tri-stage gave way to the flat bottom bit, with 12 to 24 cutters
chewing up the rock as the entire unit rotated.
Still needing to speed up the process, drillers increased the weight of
the bit using split donut weights stacked on mandrels. The initial
weight of this assembly was about 300,000 pounds. This meant the workers
could drill a 1,000-foot hole in 20 days.
Today the 90-inch weights weigh 27,500 pounds each. Usually, drilling
takes place with 8 to 10 weights stacked on top of each other, for a
total weight of 450,000 pounds.
For every foot drilled with a 96-inch bit, more than 50 cubic feet of
cuttings have to be removed from the hole. The problem of getting rubble
chewed up by the cutters from working level to the surface while the
porous ground is absorbing the moisture injected to carry those
materials, was solved by the dual string-airlift reverse circulation
system.
In simple terms, dual string-airlift reverse circulation requires a
13-3/8 inch pipe into which a smaller 7-inch pipe has been inserted.
Water is pumped down between the inner and outer pipes, circulates over
the cutters, collects the ground-up rubble and then sucks it to the
surface through the 7-inch pipe using airlift pumping.
A line-of-sight drill hole is essential to prevent the diagnostic
package with the test experiments from binding or catching on the sides
of the hole. With a drilling assembly weight of 250,000 pounds, 100,000
pounds is supported by the bit and 150,000 pounds is suspended by the
drill pipe to keep the hole straight and plumb.
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