HIPPS-Based No-Burst Design of Flowlines and Risers

Abstract

Abstract An approach is presented for design of production flowlines and risers infields that rely on subsea High Integrity Pressure Protection System (HIPPS)because of high shut-in tubing pressure (SITP). The approach enables aninherently safer design using a design pressure lower than the SITP byselecting a wall thickness that provides high reliability against pipe failurein the unlikey event of HIPPS failure and hence exposure to SITP. The paper isintended to generate industry discussion on the feasibility and merits of thepresented approach particularly in the absence of API design codes forHIPPS-based subsea risers and flowlines. The presented design methodology issupported by a combination of analytical and experimental results. Ahypothetical deepwater field is used to demonstrate the methodology. Introduction Background As the oil & gas industry moves to high pressure reservoirs in deepwaterwith SITP in excess of 15ksi, the design of fully rated flowlines and risersbecomes extremely challenged due to increased wall thickness, difficulty ofwelding and inspection, and weight of the line pipe. For such fields, the useof HIPPS becomes an enabler, i.e. by allowing a reduced design pressure for thecomponents downstream of HIPPS and hence reducing the wall thickness offlowlines and risers. By definition, HIPPS is a high integrity system with a low probability offailure on demand. A possible design approach is to focus on the unlikely eventof the HIPPS valves failing to close upon demand and set the following designobjectives:In case of HIPPS failure and exposure of the system to SITP, flowlines andrisers should have an adequate margin of safety against failure. This objectiveis set by introducing reliability-based acceptance criteria for flowlines andrisers.Risers should be stronger than flowlines hence keeping any potential failureaway from the facility and avoiding harm to people.In case of HIPPS failure, there should be little or no damage to theflowlines and risers in order to minimize any follow-up replacement andrepair. In addition, in case of HIPPS valves successfully closing upon demand, asection of the flowline immediately downstream of the HIPPS may be subjected topressures exceeding the design pressure hence requiring it to be fortified (asrequired by HIPPS design guidelines [4]). It is proposed that this fortifiedsection be designed with the same reliability objective as that adopted forrisers. In order to meet the aforementioned objectives, the flowlines and risersdownstream of HIPPS must be checked against burst assuming that the system issubjected to the SITP. This design state is designated as the " accidental limitstate" or ALS if the frequency of this event is less than 1E-2 per year asdefined in [5]. The design requirement for the ALS is a low probability ofburst failure and a second requirement is little or no damage to the pipe andequipment downstream of HIPPS.