(This Sept 3 story has been refiled to clarify that the tariff cuts cover shipments to U.S. Gulf Coast destinations, in paragraph 4 )

By Arathy Somasekhar

HOUSTON (Reuters) - Pipelines that historically carry Canadian crude to the U.S. are cutting rates and looking to ship different grades of crude oil due to rising competition from the newly expanded Trans Mountain pipeline.

The moves will temporarily cut the cost of transporting some of Canada's heavy crude to the U.S. Midwest and Gulf Coast next month. U.S. imports of Canadian crude hit a record in July as Trans Mountain expansion (TMX) volumes grew.

Shipments on TMX started in May, sending up to 890,000 barrels per day (bpd) to Canada's Pacific Coast. About 80% of the volumes are contracted, leaving 20% available for spot shipments.

With more oil moving on TMX, Canadian pipeline operator Enbridge ( ENB ) said in August it will cut its tariffs for September by 11% per barrel on heavy crude moving on its Mainline system to points on the U.S. Gulf Coast. The 3 million-bpd system ships the bulk of Canada's crude exports from Edmonton to the U.S. and is one of the main competitors to TMX.

The company is not rationing pipeline space for September for the first time in over a year, with sufficient capacity available to cover all nominated barrels.

Enbridge ( ENB ) said it anticipates Mainline will be well utilized for the remainder of the year, attributing the decrease in volumes to routine oil producer and refiner maintenance.

"We are starting to see the TMX impact play out for the Mainline, and therefore for systems that carry Canadian barrels to the U.S. Gulf Coast," said Dylan White, a North American crude markets analyst with researcher Wood Mackenzie.

Enbridge's ( ENB ) 190,000-bpd Spearhead and 720,000-bpd Flanagan South pipelines that deliver crude from the Mainline to Cushing storage hub in Oklahoma could likely lose volumes, analysts said. The 950,000-bpd Seaway, jointly owned by Enbridge ( ENB ) and Enterprise Products Partners ( EPD ), which ships oil from Cushing to the U.S. Gulf Coast, could also see lower flows.

Seaway and Flanagan pipelines remain well utilized, Enbridge ( ENB ) said.

Pipelines like MPLX's ( MPLX ) Capline, a key conduit for Canadian heavy crude, will likely transport more light crude from the Bakken oilfield in North Dakota to offset the loss of Canadian heavy grades, analysts said. The 1.5 million-bpd pipeline was once the largest crude oil pipeline in the U.S. before it was reversed in 2021 to carry crude oil from north to south. MPLX ( MPLX ) declined to comment on Capline product movements.

SHORT-LIVED IMPACT

Delays in TMX's completion provided ample time for Canadian producers to ramp up supply, and volumes on rival pipelines are likely to pick up as Canadian oil output is expected to grow rapidly.

"A combination of TMX coming online later than expected and Canadian supply ticking higher ... has elevated overall utilization on broader Canadian outbound pipelines, even as TMX has expanded overall capacity," Wood Mackenzie's White said.

Output will rise about 500,000 bpd in 2025 from 2023, offsetting the additional capacity added by TMX, according to analysts from energy infrastructure firm East Daley Analytics.

Excess pipeline space will be filled relatively soon, said Kristy Oleszek, director of energy analytics at East Daley.