Since WTI-priced stockpiles at Cushing could not easily be transported to the Gulf Coast, WTI crude was unable to be arbitraged in bringing the two prices back to parity. Oil prices at coastal areas of the United States were closer to Brent than to WTI. In June 2012, the Seaway Pipeline, which had been transporting oil from the Gulf Coast to Cushing, reversed its flow direction, to transport WTI-priced crude to the Gulf Coast, where it received Brent prices. The price difference persisted, however, and was large enough that some oil producers in North Dakota put their oil on tanker cars, and shipped it by rail to the Gulf and East Coast, where it received Brent prices. Brent continued to trade $10–20 higher than WTI for two years, until June 2013. Freight rate factors To the extent that price difference between WTI and Brent crude entice traders to ship WTI to North Sea refineries or ship Brent to US Gulf Cost refineries, the premium/discount between WTI and Brent crude must reflect oil tanker freight costs. Oil tanker freight cost rates could be highly volatile, due to circular dependence on fuel oil prices and ultimately crude prices, to demand for oil tankers to serve non-USGC non-North-Sea trade routes (especially to China), and to demand for using oil tankers as floating storage for crude oil. From 2000 to 2009, oil tanker freight rates represented a substantial contributor to the WTI premium over Brent crude. Brent crude production and trading factors Premiums/discounts between WTI and Brent crude also reflects declining crude oil production priced under Brent crude prices. As crude production declines from depletion in Brent crude associated North Sea oilfields (Brent, Forties, Oseberg, Ekofisk, and Troll; also referred to as BFOET), a higher proportion of that oil production is consumed by local European refiners, and both a lesser proportion and a lesser absolute amount of that oil production can be exported to the US. During periods when WTI trades at a premium to Brent crude, declining Brent crude production pushes up that premium as traders cannot source supplies to sell into the US for a profit. As well, participants in the Brent crude market compensates for declining North Sea production by associating additional oilfields and different quality crude production into the definition of Brent crude, which affects the quality differential between Brent and WTI crude oils. Forties and Oseberg crude oil was