HOUSTON (REUTERS) - Leading Gulf of Mexico oil producer Shell Oil Co, the US arm of Royal Dutch Shell Plc , said it was removing non-essential workers from offshore platforms as a precautionary measure ahead of a low-pressure storm system given an 80 per ent chance of becoming a tropical cyclone in the next 48 hours.

Shell said there would be no impact to offshore production due to the removal of non-essential workers ahead of the storm system's approach to the US Gulf Coast.

The US National Hurricane Centre has warned that tropical storm conditions could appear along the coasts of Texas and Louisiana as early as Tuesday.

Earlier in the day, Shell said it was only monitoring the storm system as did BP Plc. But by Sunday night, Shell said it was withdrawing workers who primarily provide support services to workers who operate off-shore platforms.

"Shell has initiated efforts to reduce non-essential personnel on some offshore assets as a precautionary measure in addition to normal preparations for heavy weather," said Shell spokesman Ray Fisher.

Removal of non-essentials can also be the first step taken prior to temporarily closing production, which happens when production workers are removed to safer areas on-shore.

US oil companies have been closely watching the large tropical disturbance. The system does not yet have a closed circulation needed to classify it as a tropical storm or hurricane, but was producing winds near gale force or about 61km per hour, or about half the strength of winds generated in the weakest hurricane.

The Hurricane Centre as well as academic and private weather forecasters are expecting a below-average number of storms during this year's Atlantic Ocean hurricane season.

The season began on June 1 and lasts through Nov 30. Some forecasters have warned that the best chance for a tropical storm to develop in the oil production areas off the US Coast could come early in the season.

A strong El Nino is expected to form in the eastern Pacific later this summer and would probably send high winds across the southern United States, disrupting tropical storm development.

The Gulf of Mexico produces 17 per cent of US crude oil production and 5 per cent of dry national gas output, according to the U.S. Energy Information Administration.

Gulf Coast states of Texas, Louisiana and Mississippi are also home to more than 45 per cent of the nation's crude oil refining capacity, according to the EIA.