A decade into the NFL's London experiment, the league believes the United Kingdom is nearly ready for a full-time franchise.

"In the 2021-2022 time frame," said Mark Waller, the NFL's executive vice president of international. "I've always felt the logical timing would be around the new CBA (collective bargaining agreement) and new media broadcast agreement."

Many major hurdles need to be cleared before the NFL would vote to place a team on another continent, but the league's push in the United Kingdom has been steadily increasing. When the Vikings and Cleveland Browns kick off Sunday, it will be the NFL's 21st regular season-game in London since it first dipped a toe across the pond in 2007.

The NFL's global expansion is a priority under Commissioner Roger Goodell, who continues to ramp up the number of games in London each year, peaking this season at four — or an experimental half-season for a would-be London team.

Some teams set up in London for a week. Some only show up for a couple of days. But unlike the other nine annual road trips for each NFL team, these London excursions require months of planning, from logistics to cultural adjustments and everything in between.

"There are a lot of (headaches)," Vikings quarterback Case Keenum said. "You can see those things as problems. They're inconveniences at most. We're getting to play a game on another continent in a really cool stadium. If you got to tell me as a kid I got to play American football overseas, I'd be like, 'What are you talking about?' "

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Before an NFL team moves to London, an owner needs to submit a request and have the relocation approved by the other owners. That can happen "very quickly," Waller said, as was the case in Los Angeles after years of planning resulted in two quick moves, by the Rams and Chargers, in the past two seasons.

However, the NFL has noticed the lukewarm reception by fans of those clubs in Los Angeles, where the Chargers failed to sell out a 27,000-seat soccer stadium for their home opener this season. The league doesn't want the same result in London, where it has been encouraged by regular sellouts, including all four games this year.

But how much of the NFL's popularity in London is caused by novelty?

The league is trying to answer that question by replicating a homestand of a London team by holding three games in October at two stadiums.

"Our job is to make sure the market is ready," Waller said, "the fan base is big enough, we've got stadiums to play in, and we've figured out the logistics if and when an owner wants to take a look at (London)."

What has Waller "incredibly optimistic" of an NFL team moving to London are the repeat customers. The league has tracked about 40,000 fans who have purchased a ticket to each game this year.

"Which for us would be a mini-season-ticket (base) if you think about a future franchise," Waller said.

There still are logistical headaches, big and small, starting with the NFL's tattered relationship with its players union, the NFLPA, and an expiring CBA that needs to be renegotiated in 2021. Could the two sides come together on the many issues for a London team? If so, how would it achieve something as simple as flying in free agents for Tuesday workouts like other teams?

"It's a great experience, but right now I couldn't imagine (a London franchise)," said Vikings defensive tackle Tom Johnson, who played in NFL Europe in 2007. "The travel and all the different things involved, it's a big knee-jerk in your daily routine. It's kind of a burden."

The NFL continues to push games out of the United States, passing two rules recently requiring teams to play an international game if they're either transitioning stadiums or hosting the Super Bowl. For the second consecutive season, an NFL game will be played in Mexico City when the New England Patriots and Oakland Raiders kick off Nov. 19. The league also expects to play a game in China in 2019.

"Mexico, Canada and the U.K., those are all really high-priority markets," Waller said. "I'd like to think as an organization we can manage all three of those relatively equally."

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Making the overseas trip feel routine is another hurdle. So far, 26 of 32 NFL teams have played in London at least once, and the Vikings' 3 1/2-day trip this past weekend was anything but normal.

Planning began in March, when Luther Hippie and Paul Martin, the Vikings' director and manager of operations and team travel, made an advance scouting trip.

Where would they practice? Three locations were needed. A roughly 50-yard-long field at Syon Park near the River Thames — with cows in the distance — made do for a walk-through Thursday. The team shifted 8 miles downriver for Friday's practice at the London Irish rugby team's training ground, and afterward players took part in an NFL-sponsored flag football event with kids. For Saturday, Hippie and Martin needed to plot out an early trek to Twickenham Stadium for a morning walk-through.

"I think the people who put the trip together realize it consumes you," Hippie said. "It's the equivalent of five trips to Chicago. I know it's good marketing to the world, but it's taxing on the equipment and operations, training staff."

A multi-day stay means bringing along practice and weight room equipment, burdens not needed for normal NFL road trips. Itemizing thousands of pounds of weights, playing gear, medical materials and more is needed for customs purposes. The process is tedious enough that the Vikings will ship back an estimated 90 percent of their belongings after they leave London, using a third-party freighter, so their roughly 170-person travel party can return to Minnesota as quickly as possible.

The return flight is smoother after a win, which is why the Vikings and coach Mike Zimmer surveyed other teams' success rates in London based on when they arrived, how long they stayed.

Zimmer chose for the Vikings to arrive a day earlier, on Thursday morning, to try to stave off any lethargy caused by the nine-hour flight.

So as the NFL beat goes on for global expansion, teams are left to adjust to the rhythm.

"No question we're branching off in those areas more and more," Browns coach Hue Jackson said. "There are more teams going over there, more games being played. I would not be surprised if there's a team playing there year-round."