By Yereth Rosen

ANCHORAGE, Alaska, Nov. 13 (Reuters) - A joint venture between BP (BP.L: Quote, Profile, Research) and ConocoPhillips (COP.N: Quote, Profile, Research) seeking to build a massive Alaska natural gas pipeline has started negotiations with a potential shipper that might feed gas into the project, the companies said in a report to the state legislature.

Denali, the BP-Conoco joint venture, plans to hold a formal open season next year to solicit commitments from potential shippers, said the report sent this week from the companies to lawmakers.

"We'll be holding our open season in 2010, but have not committed to a date," David MacDowell, a Denali spokesman, said Friday. "We're focused on conducting our own open season and creating commercial opportunities that we hope will be attractive to our customers."

The venture already has spent $120 million to date on its proposal for the pipeline, which aims to send natural gas from Alaska to the lower 48 U.S. states.

It is unclear whether the Denali season will come before, after or during the 90-day summer open season planned by TransCanada Corp (TRP.N: Quote, Profile, Research), which is pursuing a competing natural-gas pipeline proposal.

To be considered successful, an open season must result in solid pledges from shippers leading to project financing and sanction, according to MacDowell.

"A successful open season is one where shippers make the multibillion dollar financial commitments to the project so that Denali can move forward toward FERC certification, financing, and construction," he said.

Both the Denali project and the TransCanada project, in which Exxon Mobil Corp. (XOM.N: Quote, Profile, Research) has joined as a partner, propose a large-diameter pipeline running about 1,700 miles from Prudhoe Bay to an existing pipeline hub in Alberta.