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CASE STUDY LAFARGE – JOHNSONS WHARFE SHIP UNLOADING SYSTEM, GREENHITHE, KENT
Located on the south bank of the River Thames within sight of the Dartford Crossing Bridge.
The Sand & Aggregates Processing Plant was originally supplied by two separate 1000 t/hr offshore ship unloading conveyor systems delivering onshore to two separate static stockpiles. Materials from the stockpiles were picked up by loading shovel and delivered to a hopper for feeding by inclined conveyor to the processing plant.
The existing conveyors were some 30 years old and were heavily corroded.
A decision was made by Lafarge to replace the two conveyor systems with one 2000 t/hr fully galvanised single line conveyor system, incorporating a 25,000 tonne travelling tripper stockpile and an underground stockpile recovery conveyor system.In April 2004 Continental Conveyor Ltd were awarded a single contract for: –
- Removal and disposal of all of the existing offshore and onshore equipment.
- Design, manufacture, installation & commissioning, of a new 2000 tonnes/hour ship unloading system delivering onshore to a travelling twin discharge tripper stockpile.
- Design, manufacture, installation & commissioning of a 350 tonne/hour underground stockpile recovery system.
System Operation:
The incoming “delivery ships†are equipped with integral discharge conveyors which deliver product directly into a large reception hopper mounted on the tail end of conveyor 1. This initial gantry section of conveyor 1 is hinged and suspended from a winch by a multiple rope sheave which allows both the hopper and section of conveyor to be raised or lowered to accommodate tidal movement.
Conveyor 1 is totally over water. The drive and belt tensioning system along with the tail end/receiving section of conveyor 2 are located on a large pontoon in the river to facilitate a change of direction of the system.
Conveyor 2 is partly over water and starts to rise as it comes onshore. The drive and gravity tensioning system are located in an access tower. The drive unit is mounted at ground level for ease of access and maintenance. The tower allows stairway access up to the conveyor gantries and to the tripper gantry.
The 42m span travelling tripper gantry is located on 16m high tubular support legs to provide the site with a 25,000 tonne capacity stockpile.
Unloading System specification:
Materials: Dredged Sand & Gravel @ -75mm
Capacity: 2000 t/hr
Belt Width: 1200mm
Belt Speed: 2.75 m/s
Conveyor 1: 87m long with 5m lift – totally offshore – from mooring dolphin to transfer pontoon. 1 x 110kw drive
Conveyor 2: 190m long with 16m lift – offshore to onshore – from pontoon to stockpile. 2 x 110kw drive
A shutdown period of only 11 weeks was allowed:
- To remove all of the existing two conveyor systems both offshore and onshore including disposal.
- Remove the existing pontoon platform and fit a new platform to the existing piles.
- Install, test and commission the new conveyor system.
The system was ready to unload ships well within the allotted time.
Stockpile Recovery System
Located underground beneath the stockpile are 2 cross conveyors each running from the extreme ends of the stockpile to the centre where they load to an inclined conveyor. This conveyor loads to the Processing Plant.Product is controlled onto each cross conveyor by two vibro feeder units mounted in the tunnel roof.
System Specification
Capacity: 350 t/hr
Belt Width: 800mm
Belt Speed: 1.25 m/s
Cross Conveyors: 15m long level, 7.5kw drive each
Inclined Plant Conveyor: 33m long with 6m lift, 15 kw drive