ISTANBUL — As war rages in Ukraine, to the south Turkey has been busy extending its grip in the Black Sea, stepping up exploration of natural gas resources and holding naval exercises. Earlier this month, Ankara dispatched the last of its three drilling ships — the Fatih, Kanuni and Yavuz — to the Sakarya gas field, which lies 185 kilometers (115 miles) north of the port of Zonguldak. The deployment is the latest stage in Turkey’s bid to harness 540 billion cubic meters of gas discovered two years ago, potentially loosening its reliance on gas from Russia.