(Repeats story first published earlier on Friday)

By Stephanie Kelly and Devika Krishna Kumar

NEW YORK, May 10 (Reuters) - U.S. refiners had a plan for 2020: use their complex operations to maximize profits by making products that would comply with new international laws capping sulfur content in shipping fuels.

But after a series of unexpected market moves, heavy, sour crude oil processed by U.S. refiners has become more expensive, eating up hoped-for profit windfalls before they even materialized, forcing refiners to rethink plans to invest more in heavy crude processing units.

New regulations by the International Maritime Organization (IMO) will require ships globally to use fuels with a sulfur content below 0.5% beginning in 2020. Current shipping fuel is much dirtier, with a higher sulfur content.

The move was expected to make heavy crude oil cheap as most refiners worldwide shifted to lighter crudes that yield compliant lower-sulfur fuels - and benefit complex U.S. refiners that possess greater capability to break down that heavy crude into high-margin products.

Instead, global heavy crude supplies have become scarce due to sanctions on Venezuela, one of the world's biggest heavy producers, pipeline bottlenecks in Canada and OPEC output cuts. The Organization of the Petroleum Exporting Countries' April output fell to 30.23 million barrels per day (bpd), the lowest since 2015.

"The biggest single factor is the big loss of heavy crude," said Todd Fredin, executive vice president of supply, trading and logistics at Motiva Enterprises, which operates a 603,000 bpd operation in Port Arthur, Texas, the largest U.S. refinery. The benefit to complex refiners from the regulatory change "is going to be less than people thought," he said.

Heavy crude once fetched a big discount compared with light crude, but it has narrowed after sanctions on Iran and Venezuela. That weighed on first-quarter earnings for major independent refiners Valero Energy Corp and Phillips 66 .

Marathon Petroleum Corp this week halted plans to add a coking unit to its Garyville, Louisiana refinery that would have processed more heavy crude. Marathon said the coker, which was expected to come online in 2021, was no longer financially viable due to narrowed spreads.

U.S. refiners rely on cokers to break down residual oils into other refined products, including gasoil and naphtha. Refineries without that capability typically process more light crude, produced in abundance by the United States, versus heavy crude, which U.S. refiners have to import.

The price difference between U.S. Gulf Coast grades Louisiana Light Sweet (LLS) and Mars, the best proxy for comparing light, sweet and heavy, sour crude in the U.S. Gulf, has narrowed in the last few months.