(Adds company's plans to divest stake, background on project)...

(Adds company's plans to divest stake, background on project)

Dec 18 (Reuters) - Australia-listed Oil Search (OSH) said on Wednesday it would start the preliminary engineering work at its Pikka unit in Alaska in early 2020 and has begun search for buyers for up to 15% of its 51% stake in the unit and nearby exploration leases.

The progress into front-end engineering and design phase at the oilfields is subject to approval from joint venture partners and the finalization of third-party agreements, the Papua New Guinea-based oil and gas producer said.

Starting the preliminary work will help preserve the joint venture's target for early production in 2022 and full production from a new central processing facility in 2024, the company said.

Oil Search raised its interest in the Pikka project in June to 51% from 25.5%. Spanish energy firm Repsol SA REP.MC holds the remaining stake in the project.

Meanwhile, it expects to complete the divestment by around mid-2020 and said it will continue to retain operational control over all areas.

The Nanushuk field, situated within the Pikka unit, is one of the largest conventional onshore oil discoveries made in the United States in the past 30 years, according to Oil Search's website.

A final investment decision for the Pikka field is expected in the third quarter of fiscal 2020, the company said.