MULTI-HEAD SYSTEM PREVENTS HYDRATE FORMATION IN ZINC FIELD FLOW LINES

A high-pressure methanol multihead metering system will prevent hydrate formation on flow lines from the Zinc field subsea template in the Gulf of Mexico to the nearby Alabaster platform. Exxon Co. USA operates both fields. The Zinc field is produced through the largest multiwell satellite subsea template in the Gulf of Mexico. Installed in 1993, the template sits in 1,460 ft of water on Mississippi Canyon Block 354, about 50 miles south of Grand Isle, La. The $1-30 million project will

Dec. 13, 1993

3 min read

A high-pressure methanol multihead metering system will prevent hydrate formation on flow lines from the Zinc field subsea template in the Gulf of Mexico to the nearby Alabaster platform.

Exxon Co. USA operates both fields. The Zinc field is produced through the largest multiwell satellite subsea template in the Gulf of Mexico. Installed in 1993, the template sits in 1,460 ft of water on Mississippi Canyon Block 354, about 50 miles south of Grand Isle, La. The $1-30 million project will produce primarily gas.

The Zinc template is connected by umbilicals and flow lines to the Alabaster platform, 6.5 miles away. Alabaster sits in 468 ft of water on Mississippi Canyon Block 397. The methanol metering system was installed on the Alabaster platform.

Exxon estimates a peak production from the Zinc/Alabaster complex of 5,000 b/d of oil/condensate and 125 MMcfd of gas by end of 1994. Field life is expected to be 20 years.

HYDRATE CONTROL

To prevent gas hydrate formation at producing temperatures and pressure, Exxon specified the installation of methanol injection from the Alabaster platform to the Zinc template. The original idea employed a single-head injection system with one metering head on Alabaster and one injection point on Zinc. However, the flow range (and redundancy requirements) were too great for a single-head injection system to function properly.

Therefore, Exxon and Bran + Luebbe Inc. developed a custom-designed, multiheaded metering system for high-pressure methanol injection.

MULTITASK METERING

The chemical injection unit (CIU) designed by Bran + Luebbe consists of two three-headed metering pumps placed on the Alabaster platform (see photograph). These pumps meter methanol to seven injection points on the Zinc template, all operating independently of one another.

A closed loop control system automatically adjusts metering valves to regulate the exact flow-rate amount programmed by the operator. The system controls each of the seven injection points.

The two metering pumps have double-diaphragm pump heads designed for high-pressure applications of 5,000 psi.

A constant pressure system is included to increase or decrease the methanol pump speed, affecting throughput to maintain sufficient but not excessive pressure in the supply header. In addition, the double-diaphragm pumps have rupture monitoring devices that detect pressure changes in the event of a diaphragm failure.

The design allows each pump to have three more heads added, doubling metering capabilities for possible future needs.

NAS (National Aeronautical Society) Class 8 cleanliness testing was required on the CIU piping to ensure that contaminants would not adversely affect the downstream control system. Because of the NAS Class 8 requirements, Bran + Lubbe manufactured the CIU in a clean room.

A corrosion inhibition system was also designed into the CIU to prevent corrosion downstream of the wellhead. The eight-head corrosion inhibitor pump operates separately from the methanol metering system to prevent corrosion downstream of the subsea injection point.