A small group of shipowners is quietly adding energy to the Chinese coastal market in an unconventional backhaul trade, loading far out at sea in a small fleet of bulkers up to post-panamax size.

Few participating in the trade are willing to talk about it, and some technical experts are sceptical about its economic feasibility.

But dozens of vessel tracks point to a busy square on the map in the middle of the Taiwan Strait as the loading point for increasing volumes of an unreported cargo.