* Refinery planned maintenance in March and April
* Forties pipeline to limit plant's crude supplies (Adds details throughout)
LONDON, Dec 13 (Reuters) - Ineos may bring forward maintenance work at its Grangemouth refinery in Scotland due to the unplanned outage at its Forties North Sea crude pipeline, which threatens to choke the plant's feedstock, industry sources said on Wednesday.
The Forties pipeline which carries about 450,000 barrels per day (bpd) of Forties crude, roughly a quarter of the North Sea's total output, was shut early this week after a crack was found, sending global crude prices soaring to a two-year high.
The original plan was to perform maintenance on the Grangemouth diesel-making hydrocracker throughout March 2018, and to shut down a crude distillation unit and a diesel hydrotreater from April 1 to May 13, 2018, the sources said.
Regional refinery margins, which move inversely to crude oil prices, declined to their lowest since March 2016 after the Forties pipeline was shut down, according to Reuters data.
Ineos said on Wednesday that it was still considering repair options on the pipeline and reiterated that any repairs would take several weeks.
"Ineos could keep the Grangemouth refinery running but at a fair cost," one of the trading sources said.
Moving maintenance forward now appears like the most likely option but nothing has been decided so far. For one thing, it was unclear if maintenance crews were available on short notice, he said.
Another trading sources said the refinery was considering shutting down crude sections because importing crude into the site was complicated and costly.
"Imports will kill a pretty much non-existent (refining) margin," he said.
Amrita Sen, chief oil analyst at consultancy Energy Aspect, said the 200,000 barrel-per-day refinery imported a maximum of 120,000 bpd of crude oil, mostly from West Africa, this year from the nearby Finnart oil terminal northwest of Glasgow. The rest of its crude supplies came through the Forties Pipeline System.
"The issue for Grangemouth is to make up for lost FPS volumes via imports. That faces both infrastructure constraints but also long transit times of around 20 days which may mean the refinery has to reduce runs if the FPS outage is prolonged," Sen said.
Ineos did not immediately reply to a request for comment.
(Additioanl reporting by Libby George; Editing by Edmund Blair and Susan Fenton)