LONDONLONDON (Reuters) - The head of Royal Dutch Shell's (RDSa.AS) global refining operations Lori Ryerkerk will step down at the end of the month after five years in a job where she oversaw a vast overhaul of the business, according to an internal memo seen by Reuters.

Ryerkerk will be succeeded by Robin Mooldijk, who currently serves as Vice President Manufacturing Americas, responsible for the Anglo-Dutch company's refining and chemical plants in the United States, Canada and Argentina.

Ryerkerk played a key part in the transformation of Shell's refining operations in recent years into a major cash generator, particularly during the 2014-2016 market downturn.

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That included the dissolution of the Motiva joint venture with Saudi Aramco last year, which saw Shell sell its stake in the Port Arthur, Texas, refinery to become sole owner of the Norco and Convent refineries in Louisiana.

Shell sold other refineries and invested billions in upgrading its operations in recent years to create three major hubs in the U.S. Gulf Coast, Rotterdam and Singapore.

Over the past 15 years, Shell has cut refining operations from 51 sites with a capacity of 4.4 million barrel per day (bpd) to 18 sites with 2.9 million bpd, Ryerkerk said in a presentation in March.

"The consolidation of our footprint has focused on retaining increasingly complex sites, which offer crude grade flexibility, proximity to trading centers for full product-side flexibility, connectivity to material marketing demands, and strong integration with Chemicals manufacturing assets," she said.

The company will invest $2 to $3 billion a year by 2020 in its refining and trading business out of a total spending budget of $25 to $30 billion.

Before joining Shell in 2010, Ryerkerk worked in refining for Exxon Mobil for 24 years, and at Hess Corporation.

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Mooldijk was appointed Vice President Manufacturing Americas on July 1, 2016. He joined Shell in 1991 and has since worked in the Netherlands, Britain and South Africa in the refining and chemicals business.

(Reporting by Ron Bousso; Editing by Jan Harvey)