Design Considerations in the Use of Pipe-In-Pipe Systems for Hp/Ht Subsea Pipelines

摘要

This paper discusses the design and use of pipe-in-pipe systems in the recovery of hydrocarbons from reservoirs with high temperatures and pressures. The superior insulation qualities of pipe-in-pipe systems make them well suited to the transportation of products at high pressures and temperatures, preventing hydrate formation and guaranteeing high arrival temperatures at the processing facility. Pipe-in-pipe systems can transport products at temperatures above 120°C in situations where conventional insulation material may face difficulties. This paper considers some of the more significant aspects associated with the design of high pressure / high temperature Pipe-in-Pipe systems. The integrity assessment of Pipe-in-Pipe systems is examined in the light of the key driving mechanisms of thermal expansion and pressure containment. The mechanism of force transfer between inner and outer pipes in both compliant and non-compliant systems is discussed. Both stress based and strain based design of HP/HT systems are evaluated, and limit state methods applied to pipe-in-pipe design are compared to the conventional design. Possible potential failure modes of bursting, fracture, fatigue and local buckling are highlighted. The susceptibility of Pipe-in-Pipe systems to lateral and upheaval buckling is addressed. Under the traditional method of allowable stress design, wall thickness design is based on allowable hoop and equivalent stress criteria. In the case of operating temperatures above 120°C, the limit on equivalent stress can place severe restrictions on the hoop stress capacity. An alternative is to use a limit state approach. The merits and limitations of stress based and strain based methods for the design of HP/HT subsea pipelines is discussed.