EXCLUSIVE-Shell global refining boss Ryerkerk to step down - memo

* Lori Ryerkerk to leave company at end of Aug

* To be succeeded by Robin Mooldijk, head of Americas refining

* Ryerkerk oversaw major refining overhaul

LONDON, Aug 13 (Reuters) - The head of Royal Dutch Shell's 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.

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 in recent years to create three major hubs in the U.S. Gulf Coast, Rotterdam and Singapore.

Shell today operates 14 refineries in eight different countries with a capacity of 2.9 million barrels per day.

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

(Reporting by Ron Bousso)