* Repair works initially estimated to take 2-3 mths

* Refinery running at near maximum capacity - source

* Smaller petrochemical units most damaged - source (Adds details)

By Jessica Jaganathan and Ron Bousso

SINGAPORE/LONDON, April 3 (Reuters) - Abu Dhabi National Oil Company (ADNOC) is expected to restart a gasoline-making secondary unit at its Ruwais refinery in the first quarter of 2018, following a fire at the refinery earlier this year, three industry sources said on Monday.

The state-owned oil company shut a residue fluid catalytic cracker (RFCC) capable of processing 127,000 barrels per day (bpd) in the western section of the 800,000 bpd refinery in January after a fire at a linked petrochemical unit.

Repair works were initially estimated at two to three months, but this will likely be prolonged until next year, two of the sources said.

"We are in the midst of a diligent process of damage assessment and repair to ensure that operations can resume as swiftly as possible without making any compromise on safety or asset integrity," an ADNOC spokesman said.

It is unclear if the RFCC will have to be replaced.

Smaller units which produce petrochemicals such as ethylene and propylene were damaged the most and are completely down, one of the sources said.

The extended shutdown of the RFCC unit will likely mean that the company will continue to buy gasoline in the spot market, the sources said. This could, in turn, boost profit margins for the transport fuel.

ADNOC was in the spot market recently seeking a total of 316,000 tonnes of gasoline for May and June, following earlier purchases of over 200,000 tonnes for March and April.

The refinery is currently running near maximum capacity and there are no immediate plans to lower the run rates despite a prolonged shutdown of the RFCC, a second source said.

This will mean continued exports of fuel oil in the spot market as well, the source added.

A RFCC unit converts residue fuel oil into higher-value fuel products, typically gasoline and some gasoil.

ADNOC has been exporting about 450,000 to 650,000 tonnes a month of fuel oil since the fire including 90,000 to 135,00 tonnes of cracked fuel oil and 340,000 to 510,000 tonnes of straight-run fuel oil, a Singapore-based fuel oil trader said. (Reporting by Jessica Jaganathan in SINGAPORE and Ron Bousso in LONDON; Additional reporting by Rania El Gamal in DUBAI; Editing by Tom Hogue and Christian Schmollinger)

© Reuters News 2017