The latest tit-for-tat fees on port callings in the U.S.-China trade spat threaten to create additional vortexes in global oil flows. Shipowners and charterers are scrambling for clarity after China imposed this week a fee on U.S.-owned, operated, built, or flagged vessels, in retaliation for a similar U.S. move on Chinese ships. China-built ships are exempted from the new Chinese fee, but the impact on oil trade flows would still be significant, at least until owners and charterers find a way to move forward more smoothly in the choppy waters…