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Flushing a longhole production hole with the HLX5T presents fundamentally different challenges from development drilling. In a 4-meter development hole, the flushing water travels 4 meters from the bit to the collar and returns in less than 3 seconds. In a 54-meter longhole at LKAB Kiruna, the flushing water must travel 54 meters to the bit face, lift the cuttings against gravity for uphole holes, and return through a 54-meter annular column. The pressure required to maintain adequate cuttings velocity in that long annulus is higher; the flushing circuit's response time to a bit-face cuttings blockage is slower; and the window between adequate flushing and flushing-seal-damaging over-pressure is narrower.
The HLX5T flushing device assembly includes a higher-capacity flushing pump than development drifter equivalents—designed for the sustained 20–25 bar required to maintain cuttings velocity in 115 mm diameter holes at 54-meter depth. The precision in this assembly's specification refers to the pressure regulating valve's accuracy at the upper end of the flushing range: at 22 bar supply pressure, a 2 bar regulation error is a 9% deviation—within the flushing seal's bypass risk zone for sustained operation.
Flushing Parameter Management for LKAB 54-Meter Production Holes
|
Parameter |
4 m Dev. Hole |
54 m HLX5T Longhole |
Consequence of Error |
|
Min flushing pressure |
10–12 bar |
20–25 bar |
Below minimum: cuttings pack at depth |
|
Pressure regulation accuracy |
±2 bar acceptable |
±1 bar required |
2 bar high: flushing seal bypass in longhole |
|
Check valve response |
<1 sec backflow |
<0.5 sec required at bit change |
Slow response: return flow contamination |
|
Flushing pump capacity |
15–20 L/min |
25–35 L/min |
Insufficient flow: poor cuttings velocity |
LKAB Kiruna's HLX5T fleet monitors flushing back-pressure—the gauge reading during active drilling—as the primary bit condition indicator. A sustained back-pressure above 8 bar above supply indicates cuttings packing at the bit face, which triggers a controlled rod withdrawal and re-advance to clear the blockage before the packed column damages the flushing seals. This 3-minute intervention procedure has eliminated the flushing device seal failures that previously occurred when packed columns were ignored and flushing pressure was increased instead. HOVOO supplies HLX5T flushing device assembly seal kits and valve components. References at hovooseal.com.
