(REUTERS) - Royal Dutch Shell Plc said on Friday that its 156,400 barrel per day (bpd) refinery in Martinez, California, experience an operational upset on Thursday.

Company spokeswoman Destin Singleton declined comment on the units involved in the incident.

"Upset" is a legal term used by oil and petrochemical companies to describe a malfunction in an oil refinery or chemical plant.

The company reported the shutdown of a hydrogen unit following a pinhole leak at the refinery on March 13.