* Huntington oilfield producing at 40 pct capacity-partner
* Field partner cites constraints with BP's CATS pipeline
* BP says investigating fluid composition issue
OSLO/LONDON, Oct 21 (Reuters) - The North Sea Huntington oilfield in waters off Britain is producing at only about 40 percent of capacity due to problems in using a pipeline operated by energy major BP, a partner in the field said on Monday.
Norwegian oil firm Noreco said it was unclear when output at the small field, which is operated by German utility E.ON, could be ramped up.
Noreco's share of Huntington's production was 3,391 barrels of oil equivalent per day in September. This would bring Huntington's total output to some 17,000 boe per day last month.
But the field has the potential to produce 40,000 boe per day, Noreco said last month.
E.ON, which also pumps some gas out of the field, must ship some of that via the CATS gas pipeline to be able to produce oil. The capacity to do so is limited, however, by constraints on the pipeline, Noreco said.
Noreco holds a 20 percent stake in the field, E.ON 25 percent. Britain's Premier Oil has a 40 percent stake and Canada's Iona Energy 15 percent.
"It still is CATS that is the problem. We expect further information from the operator in the coming days," Noreco Chief Executive Svein Arild Killingland told Reuters.
The Central Area Transmission System (CATS) is a 404 km (251 mile) pipeline that transports gas from the North Sea to Teesside in northern England. A major gas pipeline for Britain, it can handle more than 48 million cubic metres of gas per day.
A BP spokeswoman said CATS was operating as normal while it dealt with an issue relating to fluid composition that had caused a full system shutdown in September. "The impact to the CATS pipeline is currently only around 5 percent," she said.
The September shutdown was caused by gas with a high liquid content going into the pipeline. "That is still being investigated and only once we are happy that the fluid composition is right will we lift the restriction," she said.
"Hopefully it will be resolved shortly but we are balancing keeping the pipeline up and protecting the throughput of all our customers with meeting the export criteria for the National Grid."