Steady energy demand in the world’s second-largest oil-consuming country has been a saving grace for the oil-tanker market, with much of the credit going to one Chinese oil trader: China International United Petroleum & Chemicals Co., better known as Unipec.

Unipec was the No. 1 charterer of oil tankers for the second year in a row in 2013, beating global oil giant Royal Dutch Shell PLC in both years, according to consulting firm Poten & Partners. Unipec accounted for 791 fixtures—or agreements to hire tankers for specific routes—last year and also accounted for the largest volume of cargo transported, at 171 million metric tons, or 11% of the total, the data show.

Shell trailed with 752 fixtures and less than half of Unipec’s volume, at 83 million tons.

A decade ago, Unipec didn’t even figure among the top-20 charterers of oil tankers. Since then, trade flows have dramatically changed and the tanker-chartering business has seen some ups and downs, with the 2008 financial crisis having dealt the most serious blow, part of a broader decline in shipping as demand growth for commodities slowed.

Although China’s economic growth has diminished since the early post-financial-crisis period, when massive government stimulus made the country the last great hope of exporting nations, its gross domestic product growth has maintained a level above 7.5% that is enviable in most developed countries. It imported 255 million barrels of oil in the first 11 months of 2013, or 5.6 million barrels a day, up 3.2% on year. Its steady demand combined with the U.S.’s declining need for imports thanks to its energy boom have skewed the global flow of both oil and products.

China’s move up the rankings—PetroChina Co. is also increasing its presence among tanker charterers, joining the top-10 group last year—reflects its growing control over elements of its oil supply chain and underscores the rising influence of national oil companies at the expense of private oil majors as the center of global oil demand growth shifts toward developing Asia.

In the segment for very large crude carriers, or VLCCs, which sail long distances from the Middle East to China, Unipec —a unit of China Petroleum & Chemical Corp.600028.SH -0.23%, or Sinopec—accounted for 556 fixtures, more than the next four charterers combined.

That contributed to a 5.5% on-year increase in overall chartered volume last year, according to Poten & Partners. ”Chinese chartering activity was the driving force behind the total increase in volumes this year as many charterers…reported declines of 20% or greater when compared to 2012,” the firm said.