The largest-ever iteration of the biennial Exercise Rim of the Pacific came to an end last week as ships from the 26 participating countries began either making their way home or moving on to overseas deployments around the Indo-Pacific region.

The exercise came under intense scrutiny this year from Hawaii residents as the Navy inches toward defueling its underground Red Hill fuel storage facility, which holds the U.S. military’s war reserve for the region and sits above a critical aquifer that most of Honolulu relies on for clean drinking water. In November a fuel leak in the facility contaminated the Navy’s Oahu water system, which serves some 93,000 people.

During previous RIMPAC exercises, the Red Hill fuel reserve was tapped. In 2018 the facility provided upward of 19 million gallons of fuel to participating U.S. and foreign ships and aircraft, according to a Navy news release at the time. This year participating troops had to make due without Red Hill fuel, instead relying on military oilers and commercially contracted tankers.

“The silver lining is it forced us to hone … at-sea skills,” said Vice Adm. Michael Boyle, commander of the Navy’s San Diego-based 3rd Fleet and the officer who led Combined Task Force RIMPAC during this year’s exercise.

When Defense Secretary Lloyd Austin ordered the closure of the Red Hill facility in March, he cited both health risks and a need to build a more “resilient” military fuel infrastructure in the region to replace the aging facilities the Navy has relied on. Part of that is moving fuel stores “afloat” to tankers that will serve ships around the region.

This year RIMPAC participants relied on “consolidated cargo operations,” better known as CONSOLs. During a CONSOL specially outfitted commercial tankers, under military supervision, transfer cargo through connected lines while underway. Essentially, the tanker ships serve as gas station at sea for ships. It’s not a new concept, but while simple in principle, it can be complicated in practice.

“It’s actually a remarkably difficult activity to do for two ships to come alongside, in any weather condition at very close proximity, maintain that specified distance alongside of each other and transfer fuel,” said Royal Canadian Navy Rear Adm. Christopher Robinson, who served as deputy commander of Combined Task Force RIMPAC for the exercise. “I wouldn’t say that we learned new things as much as we perfected the skills that each nation brought to do that.”

In 2015, U.S. Military Sealift Command reintroduced conducting tanker-to-oiler CONSOLS at sea. According to a Navy news release, RIMPAC 2022 marked the first time this method has been used during a major exercise in the Pacific area of operation.

All fuel transferred conducted during the exercise came from “what the oilers and the commercial CONSOL tankers brought with them,” said Navy 3rd Fleet spokesman Cmdr. Sean Robertson in an email statement.

According to numbers released to the Honolulu Star-Advertiser by the Navy’s 3rd Fleet, participants in the exercise used 20.7 million gallons of fuel. Tankers transferred 16.1 million gallons of marine diesel fuel to ships and 2.57 million gallons of JP5 (a kerosene­-based fuel used in military aircraft). An additional 2 million gallons was also transferred to the aircraft carrier USS Abraham Lincoln before its departure from the exercise.

The civilian tankers participating were the Maersk Perry, a South Korean-built and American-flagged tanker, and Schuyler Line Navigation Co.’s Chinese-­built and American-flagged SLNC Goodwill. Both are operating under charters with Military Sealift Command.

“That kind of takes away some of the requirements for fixed-site logistics of like Red Hill, because we’re able to do that from a mobile platform, a more survivable platform at sea,” said Boyle. “We take whatever parts were dealt, and we look for the silver lining, and in this case we really did move our capability and our proficiency at refueling at sea for all of our partner nations to new heights.”

However, while the Navy increasingly looks to commercial tankers to address its fueling needs across the vast Pacific, analysts have raised concern about how many tankers — as well as land-based storage — will actually be available as the Navy moves toward shutting down Red Hill.

Following Austin’s announcement of Red Hill’s closure, a senior Pentagon official told the Star-Advertiser that one way to offset the loss of Red Hill could involve “additional hulls in the water,” and that the military branch is “at the beginning of working through the process of determining exactly how many hulls and how fast can you get them.”

The Navy’s Red Hill defuelment plan, released in June, calls for a commercial tanker to be involved in the process but also notes that planners anticipate storage space needed to move the Red Hill fuel won’t be available until sometime in 2023.

Congress, in 2020, authorized creation of the Tanker Security Program, which provides stipends for a fleet of 10 U.S.-flagged tanker vessels crewed by civilian mariners in exchange for a commitment that the vessels would be available to the military during a conflict or national emergency, though the program has yet to be funded. A 2016 study by U.S. Transportation Command recommended wartime requirement of 86 tankers.