Removing hydrogen sulfide from Alpine gas
North Slope Borough to install new skid to further process natural gas going to Nuiqsut; has applied for amendment to right of way
Nuiqsut uses natural gas from the Alpine field and the North Slope Borough is proposing to install additional natural gas processing at Alpine to remove hydrogen sulfide, H2S, from the gas going into the Nuiqsut Natural Gas Pipeline. The borough has applied to the state for a right-of-way lease amendment for 1.5 acres at the Alpine pad for equipment installation.
A Dec. 15 letter from the borough mayor’s office to the State Pipeline Coordinator’s Section in the Department of Natural Resources says a natural gas processing skid would be installed on the main Alpine pad north of Nuiqsut, increasing safety of the gas delivered by Nuiqsut by removing H2S from the gas stream.
The borough said coordination between Alpine field operator ConocoPhillips, Kuukpik Corp. and the borough Planning Department will continue.
The right-of-way amendment request, prepared by Coffman Engineers, says the equipment to be installed would be an H2S gas scrubber.
Original designIn the amendment request Coffman says the original Nuiqsut gas conditioning module assumed 0 parts per million H2S in the natural gas, but subsequent data obtained by the Nuiqsut Gas Utility indicates H2S levels in the gas supplied to the utility ranged from 0 to 4 ppm from 2003-2006 with a sporadic range from 0 ppm to 10 ppm from 2007-2008.
In 2010 H2S levels ranged from 0-15 ppm.
Coffman says an increase in H2S levels in waterflooded oil fields “can occur under certain circumstances and is expected to continue to increase in the Alpine Field.”
Detrimental effects from H2S in natural gas can include “hazards to the human respiratory system which in sufficient quantities may be fatal”; accelerating corrosion of piping system and associated components; and hydrogen stress cracking in piping system and associated system. Coffman does not provide parts per million at which such detrimental effects can occur, but says, “As a precautionary measure, it is beneficial to limit the content of H2S in utility gas systems.”
Borough responsibleThe North Slope Borough is responsible for equipment and operating costs to condition gas for pipeline transport and residential consumption in Nuiqsut and, Coffman says, “current equipment is not capable of supplying the existing gas in an efficient or reliable manner.”
A new skid will be installed with increased H2S removal capacity and the original equipment will be returned to use for dehydration of gas only.
The new conditioning skid is an improvement to Nuiqsut and not to the Alpine facility, so “the permit required for the work will require an extension of the existing right of way for construction and final operation of the new skid,” Coffman said.
The new module will be shipped as two units which will be attached and installed on the foundation at a site which is currently used to store connexes.
Coffman said modifications to the Nuiqsut gas line will be required and those are planned for a scheduled Alpine turnover. “If the edge of pad valve on the Nuiqsut line does not hold pressure, the entire Nuiqsut line will need to be de-inventoried for the piping modification.”