OK Tedi Mining Ltd. (OTML) is a major producer of copper concentrate and operates an open-pit copper, gold and silver mine located in the Star Mountains of the Western Province, Papua New Guinea (PNG). The Mt Fubilan mine is located at Tabubil, approximately 24 km from the border of West Papua, Indonesia. The main objectives of the Lower Ok Tedi Dredging Project are to minimize over bank flooding by reducing riverbed levels and alleviating floodplain dieback downstream of Bige along the Ok Tedi and Fly River through dredging a section of the river at Bige, approx. 100km from the mine. After a two year dredging trial that commences in 1997 to investigate the effectiveness of dredging as a measure to reduce sediment build up in the river system and alleviating die-back, the successful remediation dredging program has been incorporated into OTML’s operations.
The site is located in a very remote area with heavy rainfall, geotechnical instability & landslides, steep terrain and seismic activity resulting in challenges to be addressed and implementation of mitigation measures.
Since 1997, OTML has engaged the csd “Cap Martin” through DEME’s local entity Dredeco PNG, to capture the sand-sized sediments flowing down the river and storing it in engineered stockpiles on the East and West banks at Bige. To date, over 120 million m3 of sediments have been removed and pumped in stockpiles up to 30m high and up to 6km from the dredging area.
From the time dredging operations commenced, there has been a remarkable improvement in the river system downstream of Bige with reduced die-back and establishment of secondary growth along the river banks.
Due to unusual low water levels (El Nino drought) in the river system that heads up to Bige, it took five months for the cutter suction dredge ‘Cap Martin’ and the auxiliary vessels to steam up the 1.000 km Fly river and reach the final destination Bige. During that period, occasional dredging along the river and removal of timber logs was carried out by a clamshell barge forming part of the fleet.
Upon arrival excavation of the sand trap (“the slot”) started with the removal of 1,5 million m³ of gravel, cobbles and boulders followed by the deepening of the sand trap. 7,3 million m³ of sand were initially dredged and pumped ashore on the river banks.
Together with the client, the optimum balance was calculated between future rates of infill and dredge capacity, in order for dredging efficiency to match economics. The final slot dimensions were set at a length of 800 m, a width of 220 m, and a depth of -11 m. After the initial trial period, CSD Cap Martin has been extracting sand from the slot at a rate of 10 million m³ per year.
Due to the remoteness of the site, camp facilities to accommodate the workforce have been built as well as a fully equipped workshop and a spare part warehouse/yard in order to be self-supporting and work autonomous. Such remote site conditions require detailed planning well in advance in order to manage efficiently the operations and avoid discontinuity.