FACTBOX-The Keystone pipeline's history of spills

By Amanda Stephenson

TORONTO, April 9 (Reuters) - The approximately 600,000-bpd Keystone oil pipeline from Canada to the United States remained shut down Wednesday after an oil spill near Fort Ransom, North Dakota, on Tuesday released an estimated 3,500 barrels of oil.

The latest spill comes two years after the U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA) identified what it called a pattern of "increasingly frequent incidents" on the Keystone system.

* The Keystone pipeline spans more than 4,300 km and moves oil from Alberta to U.S. refining markets in Illinois, Oklahoma, and Texas. It has been owned by Canadian company South Bow SOBO.TO since last year, when former owner TC Energy TRP.TO spun off its crude pipelines business to focus on natural gas instead.

* A 2021 report from the U.S. Government Accountability Office found there had been 22 spills from the pipeline between 2010 and 2020. The report said while Keystone's accident history has been similar to other crude pipelines, the severity of spills has worsened in recent years.

The report also identified issues relating to the "original design, manufacturing of the pipe, or construction of the pipeline" as contributing to the largest incidents.

Here is a look at some recent Keystone spills:

* December 2022: The pipeline spilled about 14,000 barrels of oil into a creek in Washington County, Kansas, the biggest U.S. oil spill in nine years. The resulting third-party investigation concluded the pipeline ruptured due to a crack in a weld, which progressed due to pressure and temperature "fatigue."

* October 2019: The pipeline spilled an estimated 4,515 barrels near Edinburg, North Dakota.

* November 2017: The pipeline spilled an estimated 6,592 barrels near Amherst, South Dakota.

* The Keystone pipeline is the flagship asset for South Bow, contributing approximately 95% of the company's earnings in 2024, according to ATB Capital.

* The pipeline's physical integrity is one of the biggest risks to South Bow's investment thesis, RBC Capital said Tuesday.