By Marianna Parraga and Mircely Guanipa.- (Reuters) - In the scorching heat of the Caribbean Sea, workers in scuba suits scrub crude oil by hand from the hull of the Caspian Galaxy, a tanker so filthy it can't set sail in international waters.

The vessel is among many that are constantly contaminated at two major export terminals where they load crude from Venezuela's state-run oil company, PDVSA. The water here has an oily sheen from leaks in the rusty pipelines under the surface.

That means the tankers have to be cleaned before traveling to many foreign ports, which won't admit crude-stained ships for fear of environmental damage to their harbors, port facilities or other vessels.

The laborious hand-cleaning operation is one of many causes of chronic delays for dozens of tankers that deliver Venezuela's principle export to customers worldwide, according to three executives of the state-run firm, eight employees of maritime firms that contract with PDVSA and Thomson Reuters vessel-tracking data. Other reasons include delayed repairs and impoundments by service providers that are owed money by cash-strapped PDVSA.

Neither PDVSA nor Venezuela's Oil Ministry responded to requests for comment about the firm's maritime operations.

The tankers sidelined for cleaning provide a vivid example of the firm's downward spiral: Lacking the cash to properly maintain ships, refineries and production operations - or to pay business partners on time - PDVSA can't boost exports, which is its only option for raising more cash.

The lagging exports crimp the flow of cash back to the country's crippled socialist economy, as citizens struggle daily amid soaring inflation and shortages of food and medicine. Because Venezuela relies on oil for more than 90 percent of export revenues, the problems of its state-run oil company pose a national crisis.

Venezuela's crude exports declined 8 percent to 1.69 million barrels per day (bpd) in the first quarter versus the same period in 2016, according to Thomson Reuters data.

When oil prices were high, crude and fuel exports almost entirely financed an elaborate system of government price controls and social subsidies that maintained the popularity of late President Hugo Chavez, the socialist firebrand.

Although embattled Venezuelan President Nicolas Maduro insists the government has maintained social programs, he has publicly acknowledged that lower oil prices have left the government with less money to finance them.

Venezuela's Information Ministry did not respond to a request for comment about the nation's social spending.

THOUSANDS OF BRUSH STROKES

At oil export terminals around the world - where crude leaks like those in Venezuela are relatively rare - an oil-stained tanker would normally be taken out of the water and cleaned with industrial equipment in a dry dock.

But Venezuela has just one small dry dock and lacks the cash or the time to send its soiled tankers there for proper cleaning, according to the PDVSA executives, ship captains and two workers from tanker cleaning companies.

So workers on a small fishing boat clean the giant tanker with thousands of scrub-brush strokes. The work - which involves scouring ships above and below the water line - can take up to ten days per vessel, a worker involved in the cleaning said.

In a scene witnessed by Reuters in April, workers wearing scuba suits baked on the deck of a small boat as they reached out with brushes to scrub the Caspian Galaxy, a tanker leased for one trip by a PDVSA customer.

The workers labored just offshore from Amuay beach, near a tourist hub and PDVSA's largest refinery. The crews here have washed so many vessels in recent months that they have dubbed their operation "the boatwash".