By Liz Hampton

HOUSTON, Feb 19 (Reuters) - Stampede Energy, a privately-held midstream operator, is expected to default on penalty payments after failing to fulfill shipping commitments on a major Texas pipeline, according to sources.

The development is the latest sign of stress in the pipeline sector.

During results calls this month, Magellan Midstream Partners and Plains All American Pipeline spoke of an unnamed troubled client with 10 percent committed capacity on their jointly-owned 300,0000 barrel-per-day BridgeTex pipeline from the Permian Basin to the U.S. Gulf Coast.

Several well-placed sources told Reuters the unidentified shipper is Stampede. The company, started by a 30-year veteran of companies including Shell, Marathon, Gavilon, Plains and most recently JP Energy, did not respond to emails and did not comment when reached by phone.

Stampede has 30,000 bpd of capacity, according to the company website, or 10 percent of the total allotted on the BridgeTex line. Stampede formed as an LLC roughly a year ago, according to Texas regulators, just as oil prices entered a plunge.

Magellan and Plains did not comment when asked about Stampede.

Small midstream businesses such as Stampede proliferated during the shale boom as oil professionals-turned-entrepreneurs tapped contacts in their Rolodexes to capitalize on a rapidly growing industry. These companies, often with a handful of employees and private funding, carved out a niche by connecting oil producers to bigger pipeline and storage companies.

Now that oil prices have slid to around $30 a barrel, falling production by some clients is leaving these companies with fewer barrels to move, undermining their promises to ship on pipelines.

This in turn has pressured share prices of big pipeline companies, with investors skittish about declining volumes, and payments.

Big pipeline companies have stressed to investors in recent days that most shippers have investment grade ratings, a way of saying they expect to receive "deficiency" payments if volumes shrink.

Magellan has said its unnamed troubled client has not been shipping on the line, and that it does not expect to get a "deficiency" payment shippers normally must make for space they rent on a pipeline but do not use.

Asked whether that payment would arrive, Magellan Chief Executive Officer Mike Mears told investors this month "It's highly doubtful that we'll receive."

The number of oil rigs in the Permian Basin, the major focus of Stampede's business, has tumbled to 169 from 365 a year ago, according to Baker Hughes.

BridgeTex shippers with firm commitments of between 10,000 and 30,000 bpd have a required contract term of approximately seven years, with a tariff rate of roughly $4 a barrel, according to a federal regulatory filing.

Companies like Stampede represent a small portion of the pipeline business. Magellan said it did not expect its revenue to be affected. (Reporting by Liz Hampton; additional reporting by Catherine Ngai in New York; Editing by Terry Wade and David Gregorio)