A plan to refurbish the Navy’s government-run shipyards could cost billions of dollars more than expected, a recent report from the Government Accountability Office concluded. Navy experts say the results show how some important public infrastructure projects have been neglected for decades, increasing the long-term cost of maintaining them.

The Navy’s 20-year Shipyard Infrastructure Optimization Plan is meant to rebuild and modernize government-run shipyards at four U.S. ports where nuclear-powered submarines and aircraft carriers are repaired and maintained: in Norfolk; in Kittery, Maine; in the Puget Sound in Washington; and at Pearl Harbor in Hawaii.

The cost of the project was initially estimated at $21 billion. But researchers at the GAO concluded the Navy underestimated the costs and risks associated with the shipyard recapitalization plan, something that could leave the Navy with inadequate resources to carry it out. The issue is likely to come up at a Wednesday hearing before the Senate Armed Services Committee, when top Navy officials will take questions on fleet maintenance issues.

“The Navy’s initial cost estimate for the plan did not use certain best practices in developing the estimate, such as documenting key assumptions, accounting for inflation, and addressing risks that together could add billions to the ultimate cost,” the GAO wrote in its report.

The GAO said poorly defined cost estimates threaten to lead the project astray.

“Without high-quality estimates, agencies are at risk of experiencing cost overruns, missed deadlines, and performance shortfalls,” the GAO added.

The shipyard challenges could present an obstacle to the Pentagon’s stated national security strategy, which emphasizes competition with Russia and China for global military influence. Navy budget requests from recent years describe aging maintenance facilities that lack the necessary equipment to meet expectations, in some cases meaning fewer ships and submarines are available for their assigned missions.

Donald Trump said on the campaign trail that he would increase the Navy’s ship count to 350 if elected president. Although that goal is likely to be decades away, the Navy has closed out major acquisitions designed to build out its weapons capabilities. It recently finalized a $15.2 billion deal with Virginia-based shipbuilder Huntington Ingalls for two Ford-class nuclear-powered aircraft carriers, for example.

But the health of the United States’ public shipyard infrastructure has often been neglected in favor of other spending priorities, said naval infrastructure experts contacted by The Washington Post. And the Trump administration has in some cases diverted that funding to support other priorities: Shipyards at Bangor, Wash., and Norfolk were among those defunded to pay for President Trump’s border wall.

“If the U.S. wants to have the world’s best navy, it needs to have the world’s best navy infrastructure,” said Diana Maurer, defense capabilities and management director for the GAO.

“The Navy has been late in maintaining ships and submarines in part because of the poor conditions at Navy shipyards,” she added.

Bryan Clark, a Navy analyst with the Center for Strategic and Budgetary Assessments, said the Navy has done a good job of identifying what needs to be fixed. Finding the money for it is another problem entirely, he said.

Modernizing the country’s shipyards “will take more money and sustained effort than the Navy has currently laid in,” Clark said. “The Navy will soon have to make a decision in the next budget to take money out of force structure — new ships, new airplanes, new capabilities — and put that money into shipyard maintenance.”

A separate report from 2017 found that the average piece of shipyard capital equipment ― the large machinery used to repair large vessels ― had already surpassed its useful lifetime. That report concluded that the poor condition of Navy shipyards has contributed to maintenance delays and caused the Navy to lose more than 1,300 “operational days,” or days when ships were available for missions, over a 16-year period.

“These are very old industrial facilities … some of the oldest in the nation. … Many of them have been in continuous use,” said Craig Hooper, a defense consultant. “When you start digging into these facilities, you’re going to discover things that are going to add complexity and potentially add cost.”