Installation Of The Mobile Bay Offshore Pipeline Systems
Authors: Arthur T.T. , Cook E.L. , Chow J.K. 
Published: October 2008

This paper describes the planning and installation of approximately 200 miles of high pressure alloy and steel pipe in the environmentally sensitive waters of Mobile Bay. The implementation of this multifaceted pipelines project involved several unique aspects including:Field application of new welding technology.Construction of a shallow draft, multi-line Iaybarge.Installation of four directionally drilled, bundled, crossings.Installation of a 11,000 psi, nickel alloy, insulated pipein-pipe flowline system and associated power cable.

PIPELINES OVERVIEW

The Mobile Bay Offshore Pipeline System is shown in Figure 1 and consists of 200 miles of alloy and steel lines, interconnecting three major production platforms and five remote well templates (refer to OTC papers 007447 and 007448), Corrosive, high temperature, high pressure (10,600 psi) gas is transported from the templates via insulated, pipe-in-pipe, cold-worked nickel alloy flowlines. Also included in the flowline corridor are two utility lines which provide power and dilution water/diesel to the remote well locations.

A five line carbon steel gathering system connects the onshore treating plant to the three production platforms and is used to transport diesel, fuel gas, produced gas, produced liquids and dilution water. A subsea tie-in at the Bon Secour Bay platform connects a short "jumper line" section of small diameter, alloy lines, to the primary gathering system.

The pipelines are located in both the shallow, protected, inshore waters of Mobile Bay and the high current, open waters outside the bay. Water depths along the pipeline right-of-way range from 2 to 50 feet and soils range from an extremely soft clay inside the bay to a very hard sand on the east side of the Mobile Bay Ship Channel. Project environmental requirements included "no discharging" from anchored vessels and minimization of excavation volumes and turbidity inside the bay.

BACKGROUND

Starting in early 1986 a comprehensive design, planning and technology development effort was undertaken. Thirty contractors were involved with various aspects of the work, with the owner assuming an overall coordination role. The specific phases of development were:Preliminary EngineeringWelding Technology DevelopmentDetailed Engineering and Procurement

PRELIMINARY ENGINEERING

The main purpose of the preliminary design phase was to involve multiple specialized contractors to resolve the technical issues associated with the flowlines. In addition, construction experts were contracted to develop optimum execution scenarios and provide budgetary estimates. Major activities included:Developing a thorough understanding of industry experience in installing insulated linesDeveloping design criteria for flowlines, as current codes did not always provide appropriate safety factorsDeveloping specialized insulated pipeline components such as waterstops and structural bulkheadsDeveloping the optimum insulating method and installation scenarioSelecting materials, line sizes, and testing requirementsPerforming pipeline stability, expansion and other analyses that are unique to high pressure/temperature flowlines