Mobile Bay Fairway Field Flowline Project

Abstract

ABSTRACT Development of the Fairway Field offshore Mobile Bay, Alabama required installing four flowlines to transport high pressure, hot, sour, and highly corrosive natural gas from wellhead platforms to the Central Facility Platform. New technologies applied on these flowlines included: use of bimetallic pipe to control internal corrosion, installing the pipe in a Zig-Zag configuration to control upheaval buckling, and using continuous steel coiled tubing for utility line bundles. Design, fabrication and installation of the flowlines and utility lines are described. INTRODUCTION The Fairway Field is located in the Gulf of Mexico, approximately 4 miles outs ide the entrance of Mobile Bay, Alabama. Offshore facilities are in State waters in Mobile Bay Blocks 113 and 132 (Figure 1). The field is operated by Shell and produces natural gas from the deep Norphlet formation at production rates presently averaging 40 MMcfd per well. In addition to having high wellhead pressures, the gas is hot, sour, highly corrosive and posed special design requirements for the flowlines. Gas produced from 4 outlying single well platforms is transported through the flowlines to a Central Facility Platform (CFP). There it is dehydrated and piped onshore to the Yellowhammer Gas Plant for treatment prior to sale. Four trunkline between the CFP and the gas plant (16 in. primary gas, 12 in. secondary gas, 8 in. produced water and 6 in. fuel gas line) were laid simultaneously from a single barge during Summer 1990. FLOWLINE REQUIREMENTS Lengths of the 4 flowlines connecting the satellite wellhead platforms to the Central Facility Platform are summarized below. A fifth wellhead platform (JC) is connected to the CFP by a bridge.(available in full paper) Water depths in the Fairway Field range from 39 feet at the deepest wellhead platform to 15 ft at the shallowest point along the flowline routes. Flowline Design Criteria Under normal operating conditions, the gas pressure will be reduced at the wellhead to less than 2000 psig. The maximum design operating pressure for the flowlines, however, is defined by the maximum shut-in tubing pressure of 10,600 psig. The gas is produced at temperatures up to 280 deg. F and cooled on the wellhead platforms to a maximum of 230 deg. F before entering the flowlines. The flowlines arenot insulated and are trenched to a minimum depth of 6 ft. The design natural gas composition includes 2.1 percent hydrogen sulfide, 6.0 percent carbon dioxide, and 4.9 percent water. Utility Line Requirements A Utility line bundle was also instilled between each of the 4 satellite wellhead platforms and the wellhead platform (JC) next to the CFP. The utility line bundles each consist of six pipes (air, diesel and water lines at 1.500 in. OD; and methanol, fuel gas and spare lines at 1.000 in. OD) and a 5 kV electrical power/communications cable. Design maximum operating pressures for the utility lines ranged up to 6,000 psig.