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HOUSTON, July 21 (Reuters) - U.S. medium sour coastal oil grades continued to firm on Tuesday, dealers said, on worries that energy supply disruptions could worsen in the Middle East due to more attacks between the U.S. and Iran and as a tropical storm threatened production from U.S. offshore energy platforms.

Two oil tankers carrying Saudi crude to Asia reversed coursein the Red Sea on Tuesday after Yemen's Iran-aligned Houthis said they would impose a naval blockadeon Saudi Arabia, while vessel crossings via the Strait of Hormuz dropped further at the start of the week. Asian refiners were seeking to ship crude oil from Saudi Arabia's Red Sea port through the Suez Canal and around Africa, and the tight supplies could push the companies to look to the United States for crude.

The spread between the globally-traded Brent benchmark and U.S. West Texas Intermediate was little changed at minus $6.73 but remained wider than the minus $4 required by traders to ship the oil economically across the Atlantic.

Tuesday also marked the expiry of the WTI futures contract and kickstarts the roll period that traders use to square positions, adjust their slates and net out any exposure.

Prices to roll positions from August to September were bid and offered at 70 cents and 85 cents respectively in the first half-day of roll trading. The roll period will continue for three full days to end of Friday.

Meanwhile, Tropical Storm Bertha was located about 170 miles (275 km) southeast of Mobile Alabama and threatened production from Gulf Of Mexico oil platforms. BP BP.L said on Monday that it had removed some non-essential personnel from its Thunder Horse and Na Kika platforms in the U.S. Gulf of Mexico as a precaution ahead of an approaching storm.

In refining news, CITGO's 165,000-barrels-per-day Corpus Christi, Texas refinery's west plant experienced a coker wet compressor trip on June 20, according to a Texas Commission on Environmental Quality filing on Tuesday.

• Light Louisiana Sweet for August delivery was unchanged at a midpoint of a $2.50 premium and was seen bid and offered between a $2.40 and $2.60 a barrel premium to U.S. crude futures CLc1 ​

• Mars Sour firmed 75 cents to a midpoint of a $1.25 premium and was seen bid and offered between a $1.00 and $1.5 a barrel premium to U.S. crude futures CLc1 ​

• WTI Midland firmed 5 cents to a midpoint of a 5-cent discount and was seen bid and offered between a discount of 25 cents​​ and premium of 15 cents a barrel to U.S. crude futures CLc1 ​

• West Texas Sour firmed over $1 to a midpoint of a $3.45 discount and was seen bid and offered between a $3.65 and $3.25 a barrel discount to U.S. crude futures CLc1 ​

• WTI at East Houston , also known as MEH, traded between a 20-cent and a 60-cent​ a barrel premium to U.S. crude futures CLc1

• ICE Brent September futures LCOc1 rose $1.79 to settle at $91.01 a barrel on Tuesday​.

• WTI August crude CLc1 futures rose $1.68 to settle at $84.91 a barrel on Tuesday​.

• The Brent/WTI spread narrowed 1 cent to to last trade at minus $6.73, after hitting a high of minus $6.31 and a low of minus $7.00.