Summary
A methodology is proposed for design of subsea flowlines and risers coupled with a subsea high-integrity pressure protection system (HIPPS) for fields with high shut-in tubing pressure (SITP). The proposed approach uses a design pressure that is lower than the SITP while maintaining a high reliability against burst failure. This approach enables an inherently safer design and ensures that the system integrity is not compromised in the unlikely event that HIPPS valves fail to close upon demand. The proposed design methodology is supported by a combination of analytical and experimental results. Further, an example is provided for demonstration purposes.
Introduction
Background. As the oil and gas industry moves to high pressure reservoirs in deepwater with SITP in excess of 15 ksi, the design of fully rated flowlines and risers becomes extremely challenged because of increased wall thickness, difficulty of welding and inspection, and weight of the line pipe. For such fields, the use of HIPPS becomes an enabler (i.e., by allowing a reduced design pressure for the components downstream of HIPPS and hence reducing the wall thickness of flowlines and risers).
By definition, HIPPS is a high-integrity system with a low probability of failure on demand. The proposed design philosophy focuses on the unlikely event of the HIPPS valves failing to close upon demand and sets the following design objectives:
- In case of HIPPS failure and exposure of the system to SITP, flowlines and risers should have an adequate margin of safety against failure. This objective is set by introducing reliability-based acceptance criteria for flowlines and risers.
- Risers should be stronger than flowlines, thus keeping any potential failure away from the facility and avoiding harm to people.
- In case of HIPPS failure, there should be little or no damage to the flowlines and risers in order to minimize any follow-up replacement and repair.
In addition, in case of HIPPS valves successfully closing upon demand, a section of the flowline immediately downstream of the HIPPS may be subjected to pressures exceeding the design pressure and thus requiring to be fortified [as required by HIPPS design guidelines (Collberg 2010)]. It is proposed that this fortified section be designed with the same reliability objective as that adopted for risers.