ANCHORAGE, Alaska, July 26 (Reuters) - The Trans Alaska Pipeline will shut down over the weekend to allow scheduled maintenance work, a company spokeswoman said on Thursday. The shutdown is scheduled for Saturday and expected to last for 18 hours, said Michelle Egan, corporate communications director for Alyeska Pipeline Service Co, the pipeline's operator. North Slope oil output will be curtailed during the shutdown, with the cutbacks spread among the producers, Egan said. "We are coordinating a pro-ration with the producers, so there'll be a slowdown in production for the 18-hour period," she said. Throughput in the pipeline averaged 517,573 barrels per day in June and 582,353 barrels per day for the first half of 2012, Alyeska reported. The focus of this weekend's shutdown -- the fourth of the summer -- is mainline valve testing. A final summer maintenance shutdown is set for Aug. 5, Egan said. In the past, Alyeska has conducted one or two shutdowns each summer to allow for major projects to be completed without oil-flow complications. Those shutdowns typically lasted 36 hours. This year, the company has opted for more frequent shutdowns of shorter duration, Egan said. Alyeska has been using temporary bypass lines that allow for maintenance tasks to continue even after throughput resumes, she said. Meanwhile, Alyeska's corporate ownership is undergoing a change. As of Aug. 1, minority owners Chevron and Koch Industries will no longer be shipping any oil through the system. The two companies account for just under 4.5 percent of Alyeska's ownership. Earlier this summer, the companies gave notice to Alyeska's major owners - BP, ConocoPhillips and Exxon Mobil - that they were withdrawing from the Trans Alaska Pipeline System consortium. The major owners will decide what to do with the Chevron and Koch shares, Egan said.