james pile geotechnical engineer, engineering department 30 th july 2013

Longwall Shield Recovery, Using Phenolic Foam Injection for Gob Control as an Alternative to Recovery Mesh

James PileGeotechnical Engineer, Engineering Department30th July 2013

Introduction

James Pile, Geotechnical Engineer, Engineering Department, 30th July 2013 Slide 2

Introduction

• Following a methane ignition in the active panel of a longwall coal mine, the affected panel was successfully isolated from the rest of the mine.

• This allowed for the rest of the mine to return to regular operation.

• After the affected panel had remained isolated for a regulatorily mandated period of time, it was reopened in preparation for longwall face recovery.

• As it was not possible to install recovery mesh prior to shield recovery, an alternative method of gob stabilisation had to be found.

• The objective of the paper is to show how gob stabilisation was achieved.

Nitrogen City.

Nitrogen City at night.

Panel Sealed Off From Surface


Nitrogen Injection

• Within four hours of the occurrence, nitrogen was being injected directly into the area of the ignition, in quantity from the surface.

Gob vent boreholes.

Nitrogen injection on gob vent borehole.



Nitrogen Injection

• Full control of the situation was established within forty eight hours of the incident.

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ULE TB #31 - CO (ppm) ULE TB #31 - Hydrogen (ppm) ULE GC14 - Acetylene (ppm)ULE GC14 - Ethylene (ppm) Nitrogen Flow (mcf.)

Hours Following Gob Ignition Event

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Onset of Extinguishment

Extinguishment Nearly Complete

ResidualEffects



Materials Testing – Cementitious Products

• A test site was established to test the suitability of different materials to successfully isolate the affected longwall panel from the rest of the mine.

Test trench.

Lightweight foamed cementitious product.

“Flowable fill” cementitious product.



Materials Testing – Phenolic & Other Foams


Test trench with nitrogen inertisation.

Nitrogen inertisation manifold.

Interior, with resistive temperature devices (6), cameras (4), and oxygen sensors (2).



Materials Testing – Phenolic & Other Foams


Phenolic foam pump.

Phenolic foam mixing nozzle.

Start of phenolic foam testing.



Materials Testing – Phenolic Foam

• Nitrogen inertisation. Foam injection started once the oxygen level had dropped to ≤ 8%. Oxygen level at the end of the test was ≤ 1%. Ends of test form effectively isolated from one another – no communication to the opposing oxygen sensor when the first end of the form was breached. Intimate contact over full perimeter for considerable distance. Maximum temperature of cure recorded was 207° F.

Finish of phenolic foam testing.

Finish of phenolic foam testing. End result of phenolic foam testing.



Materials Testing – Seals• 50 psi mainline seals. Required thickness

for height and width determined, with both dimensions rounded up to the nearest foot, and then one foot added to both. Minimum of one additional foot of thickness then added on top. 3 or 6 ‘topping off’ wands per seal, depending on thickness.

Topping off 50 psi (0.34 MPa) test seal.

Front wall of 50 psi (0.34 MPa) test seal form with topping off manifold.

Seal building practice under apparatus.



Longwall Panel Sealing• Five in-mine ventilation control structures,

remotely placed from surface at depths of approximately 750 to 900 feet (230 to 275 metres). First two structures used two injection holes and two camera holes apiece. Last three used one injection hole and one camera hole each.

Night-time pumping.

Improved hose “winder”.Lowering hoses from surface.

Panel Reopened Underground


50 PSI Mainline Seals Breached

• To minimise the stress on the MRTs working under apparatus, the viability of breaching the 50psi (0.34MPa) mainline seals hydraulically with high pressure water was tested at the surface test site.

• The test was successful, and the procedure was used to create the first breaches through the seals underground.

Test breaching of 50 psi (0.34 MPa) seal.

Test breaching of 50 psi (0.34 MPa) seal.



Phenolic Foam In Situ Underground

• Headgate crosscut. Results achieved using a single borehole to pump down. Dimensions approximately 20 wide by 10 feet high. Intimate contact over full perimeter for considerable distance (East-West).

Foam in situ. South side of crosscut. Foam in situ. North side of crosscut (with skid steer).

Foam in situ. Centre of crosscut.



Phenolic Foam In Situ Underground

• Headgate crosscut. Results achieved using a single borehole to pump down. Dimensions approximately 20 wide by 10 feet high. Intimate contact over full perimeter for considerable distance (East-West).

Admiring our handiwork!

Didn’t quite make French fries!Close up of centre of crosscut, showing excellent conformation to the roof.

Gob Stabilisation


Gob Stabilisation

• As it was not possible to move the shields from their existing positions to facilitate the installation of recovery mesh prior to shield recovery, alternative methods were considered to stabilise the gob and minimise flushing, thereby improving operator safety.

• Decision was made to inject phenolic foam from between the shields, into the gob behind them.

• Technique used in Germany and Australia, but not knowingly to date in the United States.

• Same product, procedure, and equivalent injection hardware used to ensure that nothing was left to chance, and guarantee the continuing success of the project.

A. Weber SA, 2013Concept of gob stabilisation using Rocsil® foam (A. Weber SA, 2013).

Rocsil® foam.

Phenolic Foam and Injection Hardware


Phenolic Foam

• Rocsil® foam. Manufactured by A. Weber SA in Rouhling, France. Represented in the US by Jennchem.

• Two-part foam – resin and catalyst

• Mixing ratio: 4:1 (3.5-3.6:1)

• Expansion ratio: 20-30:1

• Density (lbs/ft3): 1.9-2.8 (30-45 kg/m3)

• Comp. strength: 14.5-29 psi (0.1-0.2 MPa) (at 10% deformation)

Injection Hardware

• Externally threaded hollow bar.

• 1-3/16” (30 mm) OD, 5/8” (16 mm) ID.

• 1-5/8” (42 mm) button bits (single use).

• Threaded couplers with ‘O’ rings.Rocsil® foam data sheet (A. Weber SA, 2013).

Drilling


Drilling

• Jacklegs, striker bars, threaded couplers.

• Water flushing to prevent sparking or heat.

• Two holes planned per shield.

– Upper one, towards the back of the canopy/caving beam at ± 45°.

– Lower one, at chest height straight back into the gob.

– Initially drilled to a depth of ± 6’ (1.8 m).

• Coupler flush with the back of the shield/gob.

• Mixer connected to end of drill string.

• After pumping, proximal end unwound out of coupler to clear obstruction.

• Distal end left in gob, helping to reinforce it as a ‘rib’ bolt.

Pumping and changing drill strings.

Drill strings in gob between shield pairs.

Foam Calculations


Foam Calculations

• Shields 5’ 9” wide (1.75 m).

• Set height 10’-12’ (3-3.7 m).

• Used width of 6’ 7” (2 m), height of 13’ (4 m).

• Desired gob consolidation (‘wall’) thickness of 6’ 7” (2 m).

• Assumed void space of 50%.

• Volume of ± 280 ft3 (8 m3) per shield, or ± 140 ft3 (4 m3) per drill string.

• At ratio of 3.5:1 this equated to ± 4 drums of resin at 2.5 lbs/ft3 (40 kg/m3), or ± 4½ drums of resin at 2.8 lbs/ft3 (45 kg/m3), per drill string.

• Started with 5 drums of resin per drill string.

Gob consolidation.

Gob consolidation.

First Four Shields


First Four Shields

• First four shields on tailgate end pumped with 108 drums of resin in 9 drill strings (± 12 drums each).

• No show of resin onto face or between shields.

• Shields pulled without incident.

• Unfortunately no pictures of this were taken, but the contact between the roof and the gob on the tailgate corner was a near perfect right-angle.

Line shields, cribs, props, and walkers.

Gob consolidation.

Refinement


Refinement

• With no show of resin on the first four shields, drill strings were tried at different depths to find the optimum

• Too far back may create better consolidation and a thicker ‘wall’ further back into the gob, reducing the possibility of major flushing or failure. At the same time it may create poorer consolidation immediately behind the shields, possibly increasing minor flushing and dribbling.

• Conversely, not far enough may have the opposite effect.

• In reality no appreciable difference was observed.

Pumping entry adjacent to headgate shield.

Pumping entry adjacent to headgate shield.

Steady State


Steady State

• After a short time steady state was achieved.

• A maximum of 15 drums of resin were pumped through each drill string, unless more was added to compensate for adjacent drill strings that could not be drilled, or did not take much resin.

• In a few cases when inundation of the shields started to occur before reaching the desired quantity, pumping was halted.

• The maximum number of drums of resin pumped into any one drill string was 50, with only a modicum of return into the shields.

• At the end of the project 3,305 drums of resin, and 904 drums of catalyst had been pumped, at a ratio of 3.7:1. Close to the expected 3.5-3.6:1.

Pumping crew getting suited up.

Foam between shields.

Face Widening


Face Widening

• After gob consolidation, the shearer, drives, and panline were removed.

• The face was then widened, bolted and dusted, to provide sufficient clearance for shield recovery.

• The face was widened one section at a time; TG to Chute 1, Chute 1 to Chute 2, Chute 2 to Chute 3, and Chute 3 to HG.

• To prevent damage to the shields, the left hand end of the miner drum was modified.

Miner tramming to take cut.

Cut taken.

Face Widening


Face Widening

• After gob consolidation, the shearer, drives, and panline were removed.

• The face was then widened, bolted and dusted, to provide sufficient clearance for shield recovery.

• The face was widened one section at a time; TG to Chute 1, Chute 1 to Chute 2, Chute 2 to Chute 3, and Chute 3 to HG.

• To prevent damage to the shields, the left hand end of the miner drum was modified.

Bolting.

Dusting.

Shield Recovery


Shield Recovery

• After the face had been widened, bolted and dusted, the shields were recovered.

• As with the face widening, shield recovery also took place one section at a time.

• Shield recovery was conducted as it would have been for a ‘normal’ recovery, with a mule and pair of walkers.

Walkers at Shield 116.

Minor flushing and excellent gob control.

Shield Recovery


Shield Recovery

• After the face had been widened, bolted and dusted, the shields were recovered.

• As with the face widening, shield recovery also took place one section at a time.

• Shield recovery was conducted as it would have been for a ‘normal’ recovery, with a mule and pair of walkers.

Walkers at Shield 59.

Minor flushing and excellent gob control.

Negative ‘Angle of Repose’?


Any Questions?

james pile geotechnical engineer, engineering department 30 th july 2013

Documents

affected panel

affected longwall panel

surfacejames pile

nitrogen injection

active panel

test trench

test form

phenolic foam injection