Use of a Membrane Air Separation System for Nitrogen Generation on an Offshore Platform
Authors: Gwin J.L. , Milne R.T. , Helm R.H. , Pogers R.J. 
Published: October 2008

This paper describes the consideration and selection of a nitrogen supply system for an offshore platform in the Santa Barbara Channel, including a discussion of the design features and platform operating exper1ence. The system selected employs hollow fiber membrane technology to provide a continuous supply of nitrogen for routine platform purging and blanketing applications.

BACKGROUND

The Point Arguello Field is located in the Santa Maria basin, 10 miles offshore and about 55 miles northwest of Santa Barbara in the Santa Barbara Channel. The Point Arguello field lies 6000 to 8000 feet beneath the seabed in a water depth of 400 to 1100 feet. Three production platformstap the reservoir: Hermosa and Hidalgo, operated by Chevron, and Harvest, operated by Texaco. Production in the environmentally sensitive region currently awaits final permit, and is estimated to peak at about 93,000 barrels of oil and 60million cubic feet of gas daily.

Platform Hermosa, installed in late 1985, stands 750 feet tall in 602 feet of water. Platform Hidalgo, installed in mid-1987, is almost identical to its sister, standing 600 feet tall in 430 feet of water. Both platforms have living accommodations for up to 93 people. Hermosa is the central platform for the field, collecting oil and gas from Hidalgo and Harvest, and combining the flows to gas and oil pipelines that run 10 miles to shore and 17 miles through on-shore lines to consolidated processing plant at Gav1ota. The Gaviota facility strips water, sulfur and impurities from the oil and gas, and is designed to produce 100,000 barrels per day of clean oil and 48 million cubic feet per day of clean gas, recovering 20 tons of sulfur and more than 5000 barrels of natural gas liquids.

NITROGEN USES AND REQUIREMENTS

Early in 1987, Chevron evaluated options for supplying nitrogen on Platform Hermosa. The primary requirement is a supply of clean dry nitrogen for purging the S1X p1g rece1vers and launchers to reduce the hydrogen sulfide concentration of the residual gas and oil. Other routine uses for the nitrogen include blanketing of deionized water storage tanks, general purging of lines and vessels, and continuous purging of sour gas compressor seals. These applications requ1re a supply of about 4000 SCFH (96 MSCFD). Since space is limited on the existing decks, weight and space were 1mportant factors in the evaluation of nitrogen supply options, as was reliability of supply for safe operation of the platform.

General blanketing and purging applications on a platform require a clean and dry 1nert gas that will suppress flammability by maintaining oxygen concentrations below the minimum level that will sustain combustion. Table 1 shows the maximum safe percentage of oxygen (MSPO) for several hydrocarbons. Mixtures containing oxygen in concentrations greater than the MSPO are flammable, but mixtures containing less oxygen will not ignite. Subramanian and cangelosi1 provide a procedure for predicting the maximum safe percentage of oxygen for combustible mixtures of gases with oxygen and nitrogen.