ONEOK will use a real-time transient-model leak detection system on 45 miles of a natural gas gathering line it hopes to convert to an NGL transmission line. The finished line has three water crossings, including one under the Missouri River, just 6 miles upstream from the Western Area Water Supply’s water intake, which supplies WIlliston with water..
ONEOK offered testimony about its proposal to convert the line to an NGL transmission line during a Public Service Commission hearing Thursday in Williston.
Gathering lines don’t generally fall under the jurisdiction of the Public Service commission, but now that they are proposing to convert the lines to transmission, they do.
The completed transmission line would begin just south of Rawson at ONEOK’s newly constructed Lonesome Creek plant, and travel north, passing under the Missouri River before passing between Trenton and Williston. It ends at ONEOK’s Stateline plant, just north of Highway 2.
At Stateline, the y-grade NGLs — about 50,000 barrels per day — can enter the company’s Bakken pipeline, which has a capacity of about 135,000 barrels per day.
Commissioners asked the company a range of questions about the pipeline during the three-hour hearing, but, just as with the Hess conversion project hearing in Watford City on Tuesday, particular attention was given to leak detection, particularly as it crosses the Missouri RIver so near to the water intake for the Williston and surrounding region’s water supply.
The crossing there is 118 feet below the river. The gathering pipeline, already in place, included an anti-abrasion coating at that point to protect it during installation.
ONEOK Field Engineer, Andrew J. McVey, testified that the company’s electronic monitoring system would take readings on the density, temperature and pressure for the pipeline starting at the Lonesome Creek inlet, as well as additional pressure and temperature readings at each block valve along the way to Stateline.
The real-time data would go into a 24-7 Supervisory Control and Data Acquisition system, that constantly analyzes the line for potential leaks. It is a sophisticated system, and should be able to quickly detect leaks, McVey indicated.
If any leaks are detected, the system can remotely shut the line down immediately, as well as close the two remotely operated actuator valves that lie before and after the line’s Missouri River crossing, preventing any further material leaking in that area.
Commissioner Julie Fedorchak wanted to know about the company’s spill history, as well as its worst-case spill scenario for the line.
Travis Ashby, ONEOK’s manager of regulatory compliance, said the company used a conservative scenario to estimate what the worst case would be. It assumes the company’s electronic monitoring system would take as long as 15 minutes to detect a leak and take an additional two minutes to shut the line down. The times used in the scenario are longer, he indicated, than they are likely to actually be.
In that case, the most that would be released is 2,800 barrels.
“A smaller leak would take more time to detect,” Ashby acknowledged. “But it’s also leaking at a much smaller rate. You’d see a smaller total release.”
Fedorchak asked if the product would be able to migrate up from the buried pipe, past 118 feet of dirt and rock layers, to the river.
“I have difficulty speaking to the geology and how it would work its way through all that,” he said. “But our model assumes it gets to the water, and we model based on that.”
In the case of a spill, most of the material would rise to the surface of the water and evaporate, he testified.
“We modeled two scenarios,” he added. “One was June with high water flow and high temperatures. The other was February with a low flow and low temperature.”
In the high-flow model in June, the time to complete evaporation was just under an hour and the distance traveled was about 1 mile. For February, the evaporation time was two hours, and the distance traveled was about half a mile.
Any NGLs that hit the water would bubble up to the top as gases, Ashby said. Even the heavier hydrocarbons would do that, since they are lighter than water. They would evaporate quickly, but the heavier hydrocarbon gas components could hover just above the surface for a time, appearing a bit like fog.
Commissioner Randy Christmann asked whether there will be warning signs along the pipeline route. He is concerned about a scenario where a landowner sees a fog hanging over his fields, but thinks it’s just some kind of weird weather.
“How do you educate the landowners so that if they see that they know to stay away?” he asked Ashby.
Ashby said the company does have warning signs and its emergency number posted periodically along the route. They will also mail a brochure once a year to landowners that explain what an NGL leak would look like and what to do about it.
The proposed NGL transmission line also crosses under the Trenton and Lewis and Clark Wildlife Management areas. The infrastructure in place there now as a gathering line was horizontally drilled at the time of installation, to avoid disturbing either wetland.
Those two crossings and the river crossings required additional permitting processes, mostly from federal agencies.
Likewise, the conversion will require additional review by those agencies. The line will need a modified easement from the U.S. Army Corps of Engineers and an updated Sovereign Lands permit from the state’s engineer. These are still being pursued, but the company believes they will be complete sometime in May.