LONDON, May 16 (Reuters) - Chevron is set to launch the sale of its remaining UK North Sea oil and gas assets, in a move that would mark the U.S. energy giant's exit from the ageing basin after more than 55 years.

The process comes as Chevron prepares for the $53 billion acquisition of rival Hess which it said will include up to $20 billion in assets sales around the world.

The planned exit is the latest step in a steady retreat of top oil and gas companies from the declining British basin which pioneered deepwater production in the 1970s, as they focus on newer assets around the world.

The sale process includes Chevron's 19.4% stake in the BP-operated Clair oilfield in the West of Shetland region, the largest in the British North Sea with production of 120,000 barrels per day, Chevron confirmed to Reuters in a statement.

Chevron is also seeking to sell its marginal interests in the Sullom Voe oil terminal, as well as the the Ninian pipeline SIRGE pipeline systems which are both linked to Sullom Voe, it said.

The sale could raise up to $1 billion excluding tax benefits, one industry source said. The process is expected to be formally launched in June, industry sources told Reuters.

The exit follows a review of Chevron's global portfolio as CEO Mike Wirth seeks to focus on the firm's most profitable assets, Chevron said.

The development follows Chevron's sale of many of its North Sea assets in 2019 to Ithaca Energy. Other major oil companies, including Exxon Mobil and Shell, have sold assets in the basin since the 2010s.

Chevron has said it would sell between $15 billion to $20 billion in assets as part of its planned acquisition of Hess, which has hit a stumbling block due to a legal conflict with rival Exxon over assets in Guyana.

Chevron said the North Sea sale process is not related to a 35% windfall tax the British government imposed on North Sea producers following the surge in energy prices in 2022.

"As part of Chevron's focus on maintaining capital discipline in both traditional and new energies, we regularly review our global portfolio to assess whether assets are strategic and competitive for future capital," it said.

The process is expected to take months, it added. (Reporting by Ron Bousso;Editing by Elaine Hardcastle)