This paper presents an overview of the system presently installed to dehydrate, produced sour gas on offshore platforms in the Lower Mobile Bay Mary Ann Field. The system is a silica gel bed dehydration system and is the first such installation for production of offshore sour gas in Mobil's U.S. domestic operations. The paper discusses the decisions made in developing the system and presents the system as it is installed.
The discovery of the Lower Mobile Bay - Mary Ann Field in 1979 put into motion an enormous effort of design and development to build the facilities required to produce these newly found reserves. There were many aspects of this project which required new technology or new applications of existing technology. This paper presents only one of these cases. It defines the approach and outcome of a portion of the design effort taken to protect the pipeline that transports the produced sour gas from the offshore platforms to the onshore treating facility.
The gas, as produced at the wellhead, is potentially very corrosive to the facilities and the pipeline. The gas contains all the ingredients necessary for environmental cracking and severe metal loss corrosion; i.e., H2S and CO2 at relatively high partial pressures in contact with free water. The problem required an analysis of various techniques of corrosion/cracking control. These techniques can be divided into three major approaches - metallurgical, mechanical, and environmental. One or a combination of these approaches was used in all components of the production facilities. The design basis for the entire facilities was for a 40 year life with zero catastrophic failures.
The Lower Mobile Bay - Mary Ann Field is a section of the Norphlet formation in the mouth of Mobile Bay. This field is the site of the discovery well for the offshore Alabama Norphlet formation. The first commercial production of natural gas from the Norphlet formation in offshore Alabama will begin in early 1988. At present there are six Norphlet wells drilled in the field. The average depth of the wells is approximately 21000 feet true vertical depth (TVD). Each well is capable of producing from 10 MMscfd to 20 MMscfd of natural gas. The gas composition is dry with significant concentrations of H2S and CO2, Table 1 shows the produced gas composition used for the design of the production facilities.
The gas will be produced offshore at the production platforms. Initial start-up will involve two wells on the 77B platform and one well on the 76A platform. A general map of the field is shown in Figure 1. Each platform is designed for two sour gas wells with a platform design capacity of 30 MMscfd. The 95E, a future platform, will be designed for 40 MMscfd. The 76AUX platform is a central gathering/distribution platform that is bridge connected to the 76A platform. Produced liquids, primarily expected to be condensed water, will be separated at the platforms and pumped to shore for disposal.