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Title: Designing offshore pipeline safety systems utilising flow and pressure in multi-design-pressure pipeline systems
Category: Technical papers from the Journal of Pipeline Engineering
Downloadable: Yes 
Catalog No.: 2186s
Date of Publication: Dec 1 2009 12:00AM
Price: $25.00 US
Authors: Gjertrud Elisabeth Hausken, Jørn Yngve Stokke, and Steinar Berland
Abstract: THE NORWEGIAN Continental Shelf (NCS) has been a main arena for development of subsea pipeline technology over the last 25 years. The pipeline infrastructure in the North Sea is well developed and new field developments are often tied -in to existing pipeline systems [3]. Codes traditionally require a pipeline system to be designed with a uniform design pressure. However, due to the pressure drop when transporting gas in a very long pipeline, it is possible to operate multi-design-pressure systems. The pipeline integrity is ensured by limiting the inventory and local maximum allowable pressure in the pipeline using inlet and outlet pressure measurements in a safety-instrumented system (SIS). Any blockage in the pipeline could represent a demand on the safety system.

This concept was planned to be used in the new Gjøa development when connecting the 130-km long rich gas pipeline to the existing 450-km long FLAGS pipeline system. However, a risk assessment detected a new risk parameter: the formation of a hydrate and subsequent blockage of the pipeline. In theory, the hydrate could form in any part of the pipeline. Therefore, the pipeline outlet pressure could not be used in an SIS to control pipeline inventory. The export pressure at Gjøa would therefore be limited to the FLAGS’ pipeline code. Available pressure drop over the Gjøa pipeline was hence limited and a large diameter was necessary.

Various alternatives were investigated: using signals from neighbouring installations, subsea remote-operated valves, subsea pressure sensors, and even a riser platform. These solutions gave high risk, reduced availability, or high operating and/or capital expense. A new idea of introducing flow measurement in the SIS was proposed. Hydraulic simulations showed that when the parameters of flow, temperature, and pressure – all located at the offshore installation – were used, a downstream blockage could be detected early. This enabled the topside export pressure to be increased, and thereby reduced the pipeline diameter required. Flow measurement in SISs has not been used previously on the NCS.

This paper describes the principles of designing a pipeline safety system, including flow measurement, with a focus on the hydraulic simulation and designing the safety system. Emphasis will be put on improvements in transportation efficiency, cost reductions, and operational issues.

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