Efficient Conceptual Design of an Offshore Gas-Gathering Network

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 116593, "Efficient Conceptual Design of an Offshore Gas-Gathering Network," by M.J. Watson, SPE, N.J. Hawkes, P.F. Pickering, SPE, Feesa, and L.D. Brown, ConocoPhillips, prepared for the 2008 SPE Asia Pacific Oil & Gas Conference and Exhibition, Perth, Australia, 20-22 October. The paper has not been peer reviewed. Offshore gas-gathering networks require large capital investments in wells, subsea equipment, pipelines, and compression systems. A thermal/hydraulic integrated-production model (IPM) was used to evaluate many design options for such systems. IPM calculations were performed to establish the phasing of drilling, field development, and compression necessary to sustain required gas-delivery rates. Introduction Conceptual design in the upstream industry can confirm if a project concept is economically feasible and then directs later design stages toward an optimal design. Many options must be analyzed to find workable solutions and highlight the potential of new technologies. With new developments in harsher environments and with fluid properties becoming more difficult, the possible use of a traditional tieback solution decreases and the need for new technologies increases. Consequently, a multidisciplinary approach to concept design is beneficial because disciplines such as drilling, flow-assurance, process, and corrosion engineering may influence feasibility as much as traditional subsurface factors. The difficulty is keeping the disciplines aligned with each other. This is particularly difficult during conceptual design when the rate of change is usually at its greatest. The system of interest is a large gas-gathering network with multiple drill centers. Although common, these fields are in a region with extreme ambient-temperature conditions where the annual window for drilling is comparatively short. A key objective was to investigate the development plan for offshore facilities within which the drilling schedule could be "stretched out" while maintaining the required gas-production rates. The model for this study is shown in Fig. 1. It consists of eight small gas reservoirs (DC-1 through DC-8), individually tied back to an offshore processing and compression facility on a larger reservoir (the Hub), which is approximately 150 km from an onshore processing facility. Production commences with wells at the Hub. When production from the Hub field declines below the capacity of the processing facility, satellite fields are brought on successively. When compression is required, the wells are switched from a high-pressure (HP) to a low-pressure (LP) manifold in a manner to meet production targets and minimize compression horsepower.