Environmental Method Controls Corrosion/Cracking in Mobile Bay
Authors: Alexander Richard A. 
Published: December 2007

This paper overviews the system installed to dehydrate produced sour gas onoffshore platforms in the lower Mobile Bay Mary Ann field. The system, asilica-gel bed-dehydration system, is the first such installation forproduction of offshore sour gas in our U.S. domestic operations. Discusses thedecisions made in developing the system and describes the system as it wasinstalled.

Introduction

The discovery of the lower Mobile Bay Mary Ann field in 1979 put into motionan enormous design and development effort to build the facilities required toproduce the newly found reserves. Many aspects of this project required newtechnology or new applications of existing technology. This paper presents onlyone of these cases: it defines the approach and outcome paper presents only oneof these cases: it defines the approach and outcome of a portion of the designeffort to protect the pipeline that transports the produced sour gas from theoffshore platforms to the onshore treating facility.

The gas, as produced at the wellhead, is potentially very corrosive to thefacilities and the pipeline. The gas contains all the ingredients necessary forenvironmental cracking and severe metal-loss corrosion-H2S and CO2 atrelatively high partial pressures in contact with free water. The problemrequired an analysis of various techniques of corrosion/cracking control. Thesetechniques can be divided into three major approaches: metallurgical, mechanical, and environmental. One or a combination of these approaches wasused in all components of the production facilities. All facilities weredesigned for a 40-year life with production facilities. All facilities weredesigned for a 40-year life with zero catastrophic failures.

Field Operations

The lower Mobile Bay Mary Ann field, at the mouth of Mobile Bay, is the site of the discovery well for the offshore Alabama Norphlet formation. The firstcommercial production of natural gas from the Norphlet formation offshoreAlabama began in May 1988. Six Norphlet wells are drilled in the field. Theaverage depth of the wells is about 21,000 ft [6400 m] true vertical depth. Each well is capable of producing 10 to 20 MMscf/D [283x103 to 566x103 stdm3/d] of natural gas. The gas is dry, with significant concentrations of H2Sand CO2. Table 1 shows the produced-gas composition used to design theproduction facilities. The gas is produced offshore at the productionplatforms. Initial startup involved two wells on Platform 77B and one well onPlatform 76A. Fig. 1 is a general map of the field. Each platform is designedfor two sour-gas wells with a platform design capacity of 30 MMscf/D [850 stdm3/d]. A future platform, Platform 95E, will be designed for 40 MMscf/D[1.13x106 std m3/d]. Platform 76AUX is a central gathering/distributionplatform bridge connected to Platform 76A. Produced liquids, primarilycondensed water, are separated at the platforms Produced liquids, primarilycondensed water, are separated at the platforms and pumped to shore fordisposal. After separation, the produced gas is dehydrated on the offshoreplatforms and sent to the onshore plant for treating before sale. The onshoreplant consists of a treating unit, a dehydration unit [triethylene glycol(TEG)], a sulfur recovery unit, a tail-gas cleaning unit, and associatedutilities. The plant is designed to process 80 MMscf/D [2.27x 106 std m3/d] ofnatural gas. Fig. 2 illustrates process 80 MMscf/D [2.27x 106 std m3/d] ofnatural gas. Fig. 2 illustrates the overall field production process.

Corrosion/Cracking Control: Methods Selection

This discussion of the various corrosion control methods provides somebackground for the selection of dehydration as the primary means ofcorrosion/cracking protection for the sour-gas pipeline. With the metallurgicalapproach, setting rigid specifications in the design and construction of thepipelines and process facilities was the primary method of protection againstenvironmental cracking. To ensure conformance with these specifications, quality control, from the mill through construction, was tight. Among thespecifications used were NACE Standards MR-01-75, Sulfide Stress CrackingResistant Metallic Materials for oil Field Equipment, and TM-02-84, Evaluationof Pipeline Steels for Resistance to Step wise Cracking. Beyond thespecifications in these standards, increased chemistry control of theequivalent carbon content to reduce hardenability was enforced in manufacturingthe pipe used in the sour pipelines. pipelines. To combat corrosion, theprimary metallurgical approach was to select a pipe-wall thickness withsufficient corrosion allowance. pipe-wall thickness with sufficient corrosionallowance. JPT

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