Sep 7 2005
For Boart Longyear’s Environmental Drilling Division (EDD), taking its MINISONIC drill rig from the USA to international markets has been an important goal – and a contract on the Indian Ocean island of Madagascar recently provided an opportunity to test its offshore calibre in a uniquely challenging way.
Not only did the MINISONIC prove its capability to the satisfaction of the EDD team, but the excellent coring samples it yielded were critical in enabling customer Rio Tinto to take a positive decision on pursuing a project in Madagascar’s Mandena mineral deposit.
The MINISONIC’s job was to conduct sonic sampling that would validate drilling results already obtained from 1 500 vibra-core holes to estimate reserves in Mandena, near the village of Fort Dauphin on the south-east corner of the island. Aside from the tough formation the MINISONIC had to contend with, the EDD crew had to factor in torrential rains, washed out roads and some unexpectedly strange and curious onlookers.
Vibra-core drilling is a conventional and simple way to advance rods through unconsolidated material but, even with 1 500 holes drilled, Rio Tinto questioned the reliability of this sampling method and felt that uncertainty still existed as to the true deposit characteristics.
The Principle of Sonic Drilling:
Enter the MINISONIC. The principle of sonic drilling comes from a high-speed vibration within the drill head. Since the head is attached directly to the drill rod, the vibration is passed down through the drill bit, which causes the rock it encounters to displace and fracture. It’s a technique that works effectively in sand, clay, gravel and any unconsolidated material – including the beach sand containing ilmenite (titanium oxide) and zircon that characterises the Mandena deposit.
Another key capability, in this specific application, is the MINISONIC’s ability to extract a more pristine continuous sand core sample than is usually possible with traditional coring tool techniques. This is achieved by placing a plastic bag over the core barrel to house the contained “cuttings” with the result that critical information – which would normally be lost - is retained in the sample bag for analysis.
Rio Tinto determined that the MINISONIC was the ideal rig to drill a series of holes that would “parallel” the results obtained by the existing vibra-core data and, furthermore, to obtain a 15 tonne representative bulk sample for metallurgical testing.
Getting To Work:
Shipping the rig from Minnesota to the Malagasy capital of Antananarivo was the first task and it was on arrival in the southern town of Fort Dauphin that the adventure really began.
Due to heavy rains, the drill could not be hauled the 13 km to the drill site as planned and it was decided that, rather than waste any time waiting for the weather to clear, the team would “walk” (or tram) the MINISONIC there. It took two days to reach the first hole location, during which time they were accompanied by an audience of curious locals. Once there, drilling began immediately and the next challenge presented itself: an unusually high water table, just one metre below surface.
This necessitated advancing casing to the bottom of the hole after each core run and in many cases the sand heaved and sloughed into the hole but the crews were able to flush it back into the formation. There were, however, some difficulties in advancing through the highly washed out, indurated sand layer towards the bottom of the deposit and extra time was needed to remove the heave before continuing to extract the sample.
In total, 30 definition holes were drilled to depths between 15 and 30 metres at an average rate of 35 metres per 12 hour shift (which included the 2 – 3 hour journey from town to the site each day).
Standard ‘4-by-6’ in-hole tools produced 90 mm diameter samples inside a 150mm casing and these were logged by the site geologist then bagged and sent off for analysis. Overall, sample recovery was excellent, enabling the customer to log detailed information not previously documented.
Solving A Sticky Situation:
Phase two of the contract involved extracting a 15 tonne bulk sample in order to obtain a representative cross section for metallurgical testing. There were concerns that a sticky clay layer at the base of the deposit could complicate dredging operations and present too much of a challenge to spiral classifiers during extraction. Although a pit had been dug previously for a bulk sample, insufficient data was recovered and the MINISONIC had the capability to obtain the necessary samples at a much higher productivity rate (56 metres per shift) and dump the material directly into drums for processing.
Though it was the first time the MINISONIC had been used outside the USA, and a number of curved balls were encountered along the way, the EDD is satisfied that the technology has proven its capability in extracting undisturbed samples in ilmenite sand and Rio Tinto has been able to positively identify a potentially lucrative project site.
Next stop for the MINISONIC is Australia. Having successfully completed the job it set out to do in Madagascar, it will now cross the Indian Ocean to give Australian customers an opportunity to see its capability at first hand.
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