National Oilwell Varco Inc. (NYSE: NOV), the largest U.S. oilfield equipment provider, reported a loss for the fourth quarter, compared with a profit a year earlier, as it took a $1.63 billion charge.

Operating margins at the company's rig systems business, which accounted for nearly 40% of its total revenue, fell to 15.8% in the quarter ended Dec. 31, from 20% a year earlier.

National Oilwell said the charge it recorded in the quarter included inventory write-downs, severance and facility closure costs.

National Oilwell cut costs and 1,500 jobs in 2015 to offset a 70% decline in oil prices since June 2014 highs.

"Tumbling oil prices brought capital austerity and sharply lower oilfield activity, which is intensifying as we enter 2016," Chief Executive Clay Williams said in a statement.

Net loss attributable to the company was $1.52 billion, or $4.06 per share, compared with a profit of $595 million, or $1.39 per share, a year earlier.

The Houston-based company's revenue fell 52.3% to $2.72 billion.

National Oilwell's stock lost nearly 50% in value in 2015, while the Dow Jones Oil and Gas Titans 30 Index declined 21.5%.