An Improved Design of the Subsea Atmospheric System
Authors: Coleman Dwayne M. , Chatas Angelos T. , Richardson Emmett M. 
Published: December 2007

The subsea atmospheric system (SAS) is part of a deep-water production system designed, built, and tested onshore and at sea. Recently, an effort was made to improve this earlier prototype version of the SAS and to extend its application into deeper waters. This paper discusses the resulting improved design. It describes the updated major hardware components of the SAS, its principal subsystems, and installation procedures.

Introduction

The first SAS was designed in 1969, constituting the basic component of a complete deepwater production system intended to handle field production from wellhead through offloading. A full-scale prototype of the SAS was fabricated and tested onshore in 1971. The following year it was installed in the Gulf of Mexico; it operated intermittently subsea with live oil production, and was retrieved in 1976. production, and was retrieved in 1976. The complete system (Fig. 1) contains five basic components:(1)the SAS;(2)the subsea atmospheric riser manifold, designed to manifold production from several SAS units and to provide a base for connecting with production from several SAS units and to provide a base for connecting with the production riser;(3)flowline bundles, consisting of as many as a dozen lines, to connect each SAS to the riser manifold;(4)a compliant production riser system; and(5)a floating production facility, which production riser system; and(5)a floating production facility, which contains equipment for oil/gas separation, optional gas and water injection, and provision for oil storage and offloading to tankers.

The SAS consists of four major hardware elements (Fig. 2). A subsea template serves as a base through which peripheral wells are directionally drilled to selected subsurface targets. Drilling is conducted with floater rigs; then master valve and wellhead connector assemblies (MVA's and WCA's) are installed sequentially in a wet hyperbaric environment. These wet split Christmas trees connect the wells to the subsea work enclosure (SWE), which is centrally attached to the base template. In contrast to the wet elements, the SWE provides a dry environment at atmospheric pressure on the seafloor, housing the control valves, chokes, piping, and instruments required for automated producing operations. It contains the necessary mechanical internals and supporting subsystems to function as a subsea mani folding station, while providing for accessible crew entry by means of submersibles for system maintenance.

Major SAS Hardware Components

Base Template

The base of the SAS serves as a multiwell template for closely spaced directional drilling, and provides both support and alignment for the elements connected to it. Circular geometry was selected (Fig. 2) where nine wells are accommodated, equally spaced around the periphery of the template on a common radius. The basic size of the periphery of the template on a common radius. The basic size of the template is governed by the number of wells, the minimum required spacing between wells, and the size of the work enclosure. An interwell spacing of about 15 ft [4.6 m] has been adopted, slightly more than the minimum required for no overlap of the blowout preventer (BOP) envelope. The base template is constructed with structural pipe (Fig. 2). It consists of two main sections-a lower support structure and an upper bumper structure. The lower section provides the predominant structural support for the SWE, BOP stack, MVA, WCA, and subsea flowline connections. It contains 30-in. [76.2-cm] horizontal tubulars for seafloor support, and vertical tubulars that serve as well-conductor guides. The upper section of the base template contains 20-in. [50.8-cm] pipe, and extends more than 30 ft [9.1 m] above the lower section. It pipe, and extends more than 30 ft [9.1 m] above the lower section. It serves two essential purposes:to protect the well assemblies and a major portion of the installed work enclosure andto aid the alignment and guidance of the SWE during its installation.

This upper bumper structure divides the base template into wedge-shape segments or bays. One bay functions as an alignment slot for the flowline connector system on the subsea enclosure, and the others serve as locations for subsea wells. Design provisions also were incorporated in the base template to assist in the maintenance and workover of other components.

SWE Hull

The SWE hull projects features that are distinctive and peculiar to the SAS. Its geometric configuration, a vertically oriented peculiar to the SAS. Its geometric configuration, a vertically oriented stepped cylinder, has long been identified with the SAS prototype and was basically preserved in this improved design. Fig.2 shows that the main chamber, called the "service section," is essentially a large-diameter cylindrical band capped by hemispherical ends. The lower hemisphere is attached to a skirt structure that helps support the hull and its internal contents. A smaller-diameter cylinder, likewise capped by a hemisphere, attaches integrally to the top of the main chamber to form the "control section."

JPT

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