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Black & Veatch study recommends stubs Natural gas off-take stubs would be built as line from Alaska North Slope is built; activated when commercial agreements reached Kristen Nelson Petroleum News
A study by Black & Veatch for the Alaska Gas Pipeline Project Office, GPPO, is recommending that the best way to provide local off-take from a large-diameter natural gas pipeline would be to install stubs during construction.
Kurt Gibson, GPPO director, said in a Nov. 28 press release that installing stubs as the line is built would provide gas off-takes that “are both reasonable and adaptable to community needs.”
The focus of the Alaska Pipeline Project continues to be a line to commercialize Alaska North Slope natural gas, GPPO said.
Gibson said the Black & Veatch “study identified the possibility of installing stubs at strategic locations along the route that could be activated — ‘hot tapped’ — at some point in time after completion of a big gas line.”
He said that “approach provides flexibility for communities, utilities and other parties interested in accessing natural gas to enter into commercial agreements for obtaining gas on their own schedule.”
Capital costs for a community gas off-take system — not including the local distribution system — were in the $150,000 to $200,000 range, per location, with an estimate of $50,000 to $75,000 per year in operation and maintenance costs per location.
Two options considered The Black & Veatch report said GPPO identified two potential options to facilitate delivery of natural gas to small communities and industry: compressor station side stream and stub gas delivery.
There will be eight compressor stations along the line to maintain gas pressure and they require natural gas at reduced pressures to fuel compressor turbines and other utilities, typically 600 psi compared to the 2,500 psi mainline operating pressure.
Drawing off gas at compressor stations would take advantage of the reduced pressure, but Black & Veatch said it found that “business and regulatory concerns” were likely to make such delivery points unfeasible. Also, such gas would be available only to communities “within a feasible distance to a particular compressor station.”
The other option studied, the use of stubs, would include installation of stubs at points on the line identified for off-take during mainline construction.
A small diameter stub piece of pipe “would be welded on and tested during construction of the pipeline. The stub would not have live gas in it and its end location would be marked with a standard pipeline marker for future reference once a commercial agreement has been reached for the community the stub would serve,” the report said.
Hot tapping Once a local community or industry reached a commercial agreement to buy gas, the pipeline would need to be tapped.
“The hot tapping procedure would involve removing the stub cap and securing an isolation valve to the end of the stub. Hot tapping equipment would then be connected to the isolation valve, the valve would be opened, and the pipeline would be tapped whilst in operation. With the isolation of the valve and removal of the hot tapping equipment, the gas delivery location would be ready for service,” the Black & Veatch report said.
The stub would have to be extended and a metering/regulating station installed, consisting of three sections where the pressure would be reduced in three steps from the 2,500 psi on the mainline to an outlet range of 125-300 psi.
Four stages of gas heating would also be required.
Black & Veatch said the metering/regulating sections would be prefabricated offsite and installed once the site has been prepared.
The estimated cost for the metering/regulating station of $150,000 to $200,000 is based on discussions with equipment suppliers, prefabricators and contractors who build equipment, the report said. The estimate is also based on the assumption that several metering/regulating stations are built at the same time, “and does not include any line items for the stub, hot tapping operations or the distribution system downstream of the M/R station,” Black & Veatch said.
Heating value issue The report said that in addition to the difference between the mainline operating pressure and gas pressures needed for local distribution, the mainline gas “will likely have a heating value higher than what is typically delivered to residential customers.”
Based on available gas analysis provided under the Alaska Gasline Inducement Act, Black & Veatch said “The gas specification proposed for transmission in the pipeline is relatively uncommon in a number of its characteristics, namely the high calorific value of the gas and its low water content.” AGIA included “rich” and “lean” gas cases, with the rich gas having a heating value of 1,118 British thermal units per cubic foot and the lean case having a heating value of 1,067 Btu.
Black & Veatch said parts of gas systems in Alberta, Canada, and in the eastern United States, have Btu content ranging from 1,000 to 1,110 Btu per cubic foot without “significant issues” related to the high Btu content.
Black & Veatch also looked at volumes of potential gas usage by communities along the pipeline in Northern Economics’ “In State Gas Demand Study,” and estimated that the majority of communities along the pipeline (Wiseman, Coldfoot, Stevens Village, Harding-Birch Lakes, Dot Lake, Tok/Tanacross/Tetlin, Northway Junction/Northway Village, Paxson, Gakona, Gulkana, Glennallen, Copper Center, Willow Creek and Tonsina) would have average usage of less than 1 million cubic feet per day. Big Delta, Delta Junction and Deltana are estimated at 1 million cubic feet per day. Valdez is estimated at 7 million cubic feet per day and Livengood at 9 million cubic feet per day.
Black & Veatch said it anticipates “that the small diameter stub size will allow for sufficient gas supply volumes for all potential delivery point sites except for Fairbanks or Anchorage.”
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