Saturday, September 10, 2011

Going both ways; Marathon files for bidirectional gas flow through Cook Inlet CIGGS line

Alan Bailey
Petroleum News

Marathon Oil Co. has asked the Regulatory Commission of Alaska for approval of an agreement with Chugach Electric Association for the bidirectional flow of natural gas through the Cook Inlet Gas Gathering System, known as CIGGS, under Cook Inlet. And in a parallel RCA filing Marathon has asked for approval of pipeline facility modifications on the Kenai Peninsula to enable the bidirectional flow to operate.

Flexibility needed

Currently CIGGS only carries gas west to east under Cook Inlet from oil and gas fields on the west side of the Inlet. However, faced with ever tightening gas deliverability during cold winter weather, Southcentral Alaska power and gas utilities have been anxious to enable bidirectional flow through CIGGS, to increase the flexibility of arrangements for flowing gas from wells to the places where the gas is needed. Under the agreements filed with RCA, Chugach Electric will fund the CIGGS bidirectional flow project, including the required changes to the pipeline infrastructure, with the state providing a grant for the capital costs of the project.

Chugach Electric is concerned about winter supplies of gas to its gas-fired Beluga power station on the west side of the Inlet. The plant is built over the large Beluga gas field, the primary source of fuel for the plant. However, as production from the aging Beluga field declines, Chugach Electric needs flexibility to source gas from other fields around the Cook Inlet basin. With east to west flow in a bidirectional CIGGS, gas could flow from gas fields on the east side of the Inlet through CIGGS to the west side, and thence through another line, the Marathon-operated Beluga pipeline, to the Beluga power station. In August businesses involved in the Cook Inlet gas industry filed with RCA a tariff settlement for the Beluga line, which already accommodates bidirectional gas flow.

Without bidirectional flow through CIGGS, if Chugach Electric purchases gas on the east side of the Inlet, the utility is dependent on flowing an equivalent volume of gas into its power station from gas utility Enstar Natural Gas Co.’s pipeline system on the west side. Essentially, rather than flowing gas molecules from the Kenai Peninsula to Beluga, east-side gas is simply exchanged for west-side gas.

But Enstar cannot guarantee the availability of gas by for delivery by this mechanism, Marathon said in its CIGGS bidirectional flow RCA filing.

Increasingly important

“Chugach has advised CIGGS that, as gas sourced from the east side has become an increasingly important part of Chugach’s supply portfolio, Chugach has become concerned with the adequacy, availability and security of pipeline transportation from east to west across the Inlet,” wrote Marathon Commercial Manager Craig Chambers in the RCA filing. “Chugach seeks to protect and enhance the reliability of its gas transportation by having access to direct cross-Inlet pipeline capacity to protect against the possibility of cold weather capacity constraints, bottlenecks and outages on the … Enstar system.”

And, with Cook Inlet Natural Gas Storage Alaska building a new gas storage facility on the east side of the Inlet for use by Southcentral power and gas utilities, flexibility in moving gas between different sides of the Inlet would be of more general benefit than just the support of Chugach Electric’s power generation capacity.

However, to meet its own needs, Chugach Electric wants CIGGS bidirectional flow to be available in the winter of 2011-12, a timeframe that requires one of the necessary CIGGS modifications, the installation of piping to bypass some equipment at East Foreland on the Kenai Peninsula, to be started by Oct. 1, Marathon says. Marathon says that it took the opportunity of a scheduled pipeline shutdown in August to carry out another required modification, the removal from CIGGS of some check valves, originally intended to prevent the back flow of gas in the line. The gas flow meter on the Kenai Peninsula, at the point where CIGGS connects with the Kenai Nikiski gas pipeline, will also need to be modified to allow the metering of gas flowing in either direction through the pipeline junction, Marathon says.

Differential pressure

The direction of gas flow through a line such as CIGGS depends on the differential gas pressure across the length of the line — gas flows from high pressure to low pressure. So, to achieve bidirectional flow through CIGGS Marathon plans to install a new gas compressor in the Kenai Nikiski line near the junction with CIGGS, to elevate the pressure of gas from the Kenai Nikiski line above that in the CIGGS line whenever east to west flow through CIGGS is needed. However, given concerns about the future need for flexibility in compression arrangements in the Kenai Nikiski line, Marathon says that it plans to rent, rather than buy, the new compressor.

Marathon is also proposing changes to its CIGGS tariff, to take account of the bidirectional flow arrangement, but the company says that it has closely modeled its new tariff terms on the terms for interconnections with CIGGS in its existing tariff.

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Republished with the permission of the Petroleum News