A key indicator in measuring the health of the South Korean shipbuilding industry has fallen to its lowest level in 13 and a half years.

The so-called "order backlog" measures the amount of shipping capacity customers around the world have ordered South Korean shipyards to build. 

As of December, South Korea's order backlog was just under 20 million compensated gross tons --- a bit less than a fourth of global shipbuilding demand. That's the lowest since October 2004.

South Korea was once the top shipbuilding nation in the world, but has fallen to third place, with China in first place and trailing Japan in terms of order backlog for the first time in 17 years.