The Ok Tedi Mining Operations consists of the mining of copper orebody at Mount Fubilan in the northwest corner of the Western Province of Papua New Guinea, and the treatment of the ore to the form of copper concentrate.
The Ok Tedi porphyry copper deposits form the core of Mount Fubilan, in a remote area of dense rain forest approximately 18 km east of the international border with the Indonesian territory of Irian Jaya.
The Ok Tedi Mining Operations area lies on the south side of the central mountain ranges, and is an area of high relief and dense vegetation. The highest peak in the vicinity has an elevation of 3,138 m, and there are many peaks in excess of 2,000 m. The catchment area in this region is drained by the Ok Tedi River, which is one of the principal tributaries of the Fly River flowing south into the Gulf of Papua.
Accessing the site
Mobilisation of the HDD equipment including the Coe Drill 180 HDD rig from Brisbane to the site was completed using the Kiunga Chief, an Ok Tedi Mining vessel, via the Fly River, which is a major waterway and is navigable from the Gulf of Papua from 865 km to Kiunga.
Kiunga also has an unsealed airstrip where the crew flew in and out of, permitting direct access to Port Moresby and other centres by air. From Kiunga to Bige road transport was used to transport the HDD spread comprising 14 trailer loads. Camp facilities were provided at the HDD site by the client.
Soggy conditions
The dominant characteristic of the mining operations area is its high rainfall. Annual values in excess of 14,000 mm have been recorded and the mean annual rainfall varies from 4,700 mm at Kiunga, increasing to 7,000 mm at Ningerum, to 8,200 mm at Mount Fubilan and 11,900 mm at Lake Wangbin, 5 km northeast of Tabubil.
The distribution of rainfall throughout the year is relatively uniform with the driest month being September and March as the wettest month with a mean of 819 mm. Records show that the site can expect rain in the range of 312 days to 349 days per year.
Rocky river
The geology in the vicinity of the river crossing was dominated by the presence of river gravels over cobbles and boulders to a depth of 25 m, below which, siltstones and sandstones dominate.
The presence of the river gravels over cobbles and boulders presented difficulties for standard HDD construction methods and required an innovate approach in order to successfully complete the crossing.
The HDD process
Coe Drilling planned and successfully installed steel conductor casings on both the east and west banks of the Ok Tedi River using a Terra pipe ramming hammer to install the steel casing and flight augers to remove the spoil form within the casings. 75 m of 508 mm 20 mm wt steel casing was installed at each side of the crossing.
The entry side casing was installed down to bedrock, and after being cleaned out by the augers the pilot hole operation commenced using a 9 inch TCI Rock Bit driven by a 6 ¾ inch Mud Motor. Downhole survey control was provided using a Tensor Magnetic Guidance System. The pilot hole was completed through the gravel/cobble and boulder overburden material and once the pilot hole was completed the downhole drilling assembly was removed and sections of the 508 mm 20 mm wt steel casing was driven into the borehole guided by a stinger attached to the drill pipe. Once the casing was installed an auger was attached to the drill pipe and the HDD rig was used to rotate the augers and remove the spoil ready for the hole opening. The borehole was completed by back reaming using a standard 18 inch split bit hole opener.
In order to ensure the integrity of the pipeline coating system during the pipeline installation HDPE casing liners were inserted into both entry and exit conductor casings to protect the coating, which was subject to a cathodic integrity test after installation of the pipeline.
The project was installed successfully to the satisfaction of the Ok Tedi Mining engineering staff and is evident of good planning, procedures and execution of a difficult HDD in a remote location.
