SAO PAULO, Jan 15 (Reuters) - Petrobras has obtained up to $1 billion in credit to buy equipment and services from Norwegian companies as the Brazilian oil giant struggles to improve efficiency and output. The Norwegian Guarantee Institute for Export Credits (GIEK) and Petrobras plan to cooperate over the course of the oil firms' five-year, $237-billion expansion plan, according to an e-mailed statement from Petrobras on Tuesday. Petrobras' output has fallen for eight straight months despite embarking on the world's largest corporate spending program. State-controlled Petrobras produces around 90 percent of Brazil's oil and natural gas but is struggling to achieve its goal of becoming one of the top four global producers by 2020. GIEK previously offered Petrobras a $1 billion line of credit in 2010.