UPDATE 1-Trans Alaska Pipeline to shut Sat. for planned work

(Adds details, background)

ANCHORAGE, Aug 14 (Reuters) - The Trans Alaska Pipeline, which carries crude oil from the Alaska North Slope to an export terminal at Valdez, will shut for planned maintenance on Saturday, a spokesman for the pipeline operator said.

The shutdown is expected to begin at 6:00 am Alaska Time (1400 GMT) on Saturday and will last 36 hours.

"This is the second of two major shutdowns that we have for this summer," said Matt Carle, the external affairs manager for Alyeska Pipeline Service Co. The TAPS was also shut on June 28 and 29.

Alyeska plans to install about 1,700 feet of new mainline pipe to bypass a pump station that is no longer needed and replace a valve at the southern end of the pipeline in Valdez during the shutdown, Carle said.

The bypass line will be the longest stretch of new mainline line installed in the last 20 years and will bypass Pump Station 2, a facility located 58 miles south of Prudhoe Bay that was put on standby in 1997.

Average daily throughput in the Trans Alaska Pipeline System has been 717,042 barrels per day so far this year, according to Alyeska's website.

Alaskan oil production has fallen sharply since peaking at 2 million bpd in 1988 due to natural decline at major fields such as Prudhoe Bay.

Alyeska's main owners are BP Plc (BP.L), ConocoPhillips Corp (COP.N) and Exxon Mobil Corp (XOM.N). Units of Chevron Corp (CVX.N) and privately held Koch Industries own minor stakes. (Reporting by Yereth Rosen; Editing by Christian Wiessner)