The Bosporus or Bosphorus is a strait that separates the European part (Rumeli) of Turkey from its Asian part (Anadolu), connecting the Sea of Marmara with the Black Sea. It is an important oil transit chokepoint, 30 km long, with a maximum width of 3,700 meters at the northern entrance and a minimum width of 750 meters between Anadoluhisari and Rumelihisan. The depth varies from 36 to 124 meters in midstream. It is a former river valley that was drowned by the sea at the end of the Tertiary period. The city of Istanbul straddles the Strait with a population of more than 11 million people.
Due to the importance of the strait for the defense of Istanbul, the Ottoman sultans constructed a fortification on each side of it, Anadoluhisari (1393) and Rumelihisari (1451). Its strategic importance remains high: several international treaties have governed vessels using the waters, including the Montreux Convention regarding the regime of the Turkish Straits, signed in 1936.
The Bosporus Straits are one of the world’s busiest (50,000 vessels annually, including 5,500 oil tankers), and most difficult-to-navigate waterways. Some of the export routes for crude oil production from the Caspian Sea region pass westwards through the Black Sea and the Bosporus Straits en route to the Mediterranean Sea and world markets. The largest expansion of transit volumes would come from the expansion of the CPC oil pipeline*. The ports of the Black Sea, along with those in the Baltic Sea, were the primary oil export routes of the former Soviet Union, and the Black Sea remains the largest outlet for Russian oil exports. An estimated 2.9 million bbl/d flowed through the Turkish Straits in 2010, almost all of which was crude oil. Oil shipments through the Turkish Straits decreased from over 3.4 million bbl/d at its peak in 2004 to 2.6 million bbl/d in 2006 as Russia shifted crude oil exports toward the Baltic ports. Traffic through the Straits increased again as crude production and exports from Azerbaijan and Kazakhstan rose in recent years.
Export through the Bosporus Straits have grown since the breakup of the Soviet Union in 1991, and there is growing concern that projected Caspian Sea export volumes exceed the ability of the Bosporus Straits to accommodate the tanker traffic. Under the Montreux Convention of 1936, commercial shipping has the right of free passage through the Straits in peacetime, although Turkey claims the right to impose regulations for safety and environmental purposes. While major spills can bring about immediate environmental consequences, the presence of large oil-and-gas carrying ships in the Straits causes others problems, such as the day to day release of contaminated water as the ships ballast their holds. Bottlenecks and heavy traffic also create problems for oil tankers in the Turkish Straits. While there are no current alternate routes for westward shipments from the Black and Caspian Sea region, there are several pipeline projects in various phases of development underway.
* The Caspian Pipeline Consortium’s crude link that runs from northwest Kazakhstan to the Black Sea will reach full capacity by 2015, according to OAO Transneft. The link’s output is to be increased to 67 million metric tons annually, or 1.4 million barrels a day. The CPC pipeline carries roughly half that volume of crude from Kazakh fields including Chevron Corp.-led Tengiz to a terminal close to the Russian port of Novorossiysk on the Black Sea. The first phase of the expansion to add 10 million metric tons, or about 210,000 barrels a day, will be finished this year or by early 2014.