Designing Offshore Pipeline Safety Systems Utilising Flow and Pressure in Multi Design Pressure Pipeline Systems

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 Gjo̸a 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 a Safety Instrumented System to control pipeline inventory. The export pressure at Gjo̸a would therefore be limited to FLAGS pipeline code. Available pressure drop over the Gjo̸a pipeline was hence limited and a large diameter was necessary. Various alternatives were investigated; using signals from neighbour installations, subsea remote operated valves, subsea pressure sensors and even a riser platform. These solutions gave high risk, reduced availability, high operating and/or capital expenses. 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 Safety Instrumented Systems has not been used previously on the NCS. This paper describes the principles of designing a pipeline safety system including flow measurement with focus on the hydraulic simulations and designing the safety system. Emphasis will be put on improvements in transportation efficiency, cost reductions and operational issues.