UPDATE 2-Goldman global commods COO, oil traders depart after rough 2017

(Adds details of additional departure, background)

NEW YORK, Jan 31 (Reuters) - Goldman Sachs Chief Operating Officer of Global Commodities Don Casturo will retire from the firm after 20 years, according to a memo seen by Reuters on Wednesday, along with at least two other commodities traders after a sharp decline in earnings in 2017.

Casturo first joined Goldman in 1998 and was a key executive at the bank's commodities unit. The company's fixed income, commodities and currency business posted a 30 percent year-over-year fall in revenues in 2017 to $5.3 billion. One-third of that decline was due to the commodity business, Chief Financial Officer Marty Chavez said on a conference call earlier this month.

In addition, Rahul Dhir, who trades refined oil products, has also left the company, according to three sources familiar with the matter, and Saad Usmani, a London-based managing director in oil products, has also left, according to two of those sources.

Goldman did not immediately comment on Dhir and Usmani.

During Casturo's 20 years at the bank, he was head of oil derivatives trading in London, investor products trading in New York and metals trading.

"Don has played a leading role in helping our business to adapt to significant regulatory changes, and has advocated effectively on an array of important issues," the memo said.

The news of Casturo's departure was first reported by Bloomberg.

(Additional reporting by Marcy Nicholson; writing by David Gaffen and Renita D. Young; Editing by Bernadette Baum and Nick Zieminski)