HONOLULU — The Navy has estimated that the least expensive method of double-walling its fuel tanks that in 2014 spilled 27,000 gallons (102,000 liters) of fuel could cost between $500 million and $2 billion.

The costs are part of an agreement entered into by the Navy, Defense Logistics Agency, U.S. Environmental Protection Agency and state Department of Health to upgrade the Red Hill Bulk Fuel Storage Facility, the Honolulu Star-Advertiser reported Monday.

The facility, with 20 giant underground tanks that cumulatively hold 250 million gallons (946 million liters) of fuel, is deemed by the military to be an essential facility for operations. In December, the Navy submitted a tank upgrade study that examines three single-wall options and includes improved repair and restoration as the most basic option, as well as two double-wall designs and one tank-within-a-tank design.

The most expensive tank-within-a-tank option, with carbon steel and full interior and exterior coating, is estimated at between $2 billion and $5 billion. The cheapest single-wall option is projected by the Navy to cost between $180 million and $450 million, the Star-Advertiser reported.

At a Red Hill public information meeting last week, Waianae resident Kapua Keliikoa-Kamai said, “If we can spend billions of dollars in a war in somebody else’s country, we need to spend billions of dollars for national defense here in my home.”

Keliikoa-Kamai favors secondary containment — double-walling the tanks — in five years. The Sierra Club of Hawaii has a similar stance. The consent agreement requires that whatever upgrade is selected needs to be completed by September 2037.

The Navy last month also completed a Red Hill study that looks at 12 potential relocation sites for what it said would be an “extraordinary project” to duplicate in 40 tanks the same 250 million gallon capacity that exists now.

The replacement facility would need to be hardened to the same level as Red Hill — which is buried 100 feet underground — to withstand missile or electronic attack, the Navy report said. That would mean similarly burying the new tanks 100 feet (30 meters) below ground or installing them at a shallower depth and covering them with the equivalent thickness of concrete.