Project

• Trial period : dredging of ± 20 million tons per year of material from the low OK Tedi River and securing the dredged material in an inland reclamation/storage area.
• Dredging and pumping ashore of about 10 to 12 mio m³ minetailings per year.

Location : Papua New Guinea
Client : Ok Tedi Mining (OTML)
Contractor : Dredeco (Asia Pacific Dredging Group)
Period : Trial period : May 1997 – June 2001
Contract : July 2001 – July 2006
Extension : till December 2012

Scope of work

Infill Dredging
Reclamation

General Information

The contract was awarded in 1997 by Ok Tedi Mining (OTML), operator of one of the world’s largest gold and copper mines.

The mine situated in the western Province of PNG, is 1.600 m above sea level in the Star Mountains, a remote region 1.000 km northwest of Port Moresby. This area suffers huge annual rainfall (over 8 m), geological instability, steep terrain and seismic activity. These conditions result in frequent landslides preventing the safe construction of dams to retain mine tailings and deliver huge natural sediment levels to the river system. The government had therefore approved the discharge of mine waste into the river system in association with an extensive environmental monitoring programme.
The mine waste and other sediments had caused the riverbed level to rise by 4 m at Tabubil and by 6 m at Bige. This has led to navigational difficulties on the Fly River, to which the Ok Tedi is a tributary.

Description of the works

The solution developed by Dredeco and OTML was to excavate a “sand trap” or slot in the riverbed near Bige. Continuous dredging of the slot would remove the sand from the river.

After dredging a shelter on the East Bank and an access channel to the selected trap site, the team began to construct the sand trap in the riverbed.

Excavation of this slot took 15 months, involving the removal of more than 1.5 million m³ of gravel, cobbles and boulders. 7.3 million m³ of sand carried down by the river has been pumped from the Lower Ok Tedi river and deposited in a reclamation area carved out in the jungle.

During this period the river was studied to calculate the optimum balance between rates of infill and dredge capacity.

Together with the Client, we worked on a strategy to incrementally enlarge the slot and through survey of infill rates, an optimum slot was developed that balanced economics and dredging efficiency. The final slot dimensions are 800 m long by 220 m wide with variable depths from 6 m to 12 m).

The Cap Martin has been extracting sand from the slot at a rate of 250.000 m³ per week, reaching a total of more than 45 million m³ so far.

Main Equipment

Cutter suction dredger CAP MARTIN