This Section defines the design criteria that are specifically applied to pipe-in-pipe systems. Relevant failure modes for pipelines, are to be considered in the design of pipe-in-pipe systems.
Strength Criteria
The design of pipe-in-pipe systems is, in general, to follow the strength criteria. For high temperature/pressure systems, an equivalent strain criterion may be applied as:
The maximum allowable accumulation of plastic strain is to be based on refined fracture calculations and is to be submitted for approval by the Bureau. The inner pipe burst capacity of the pipe-in-pipe system is determined based on the internal pressure, and local buckling capacity is evaluated based on the outer pipe subjected to the full external pressure.
Global Buckling
In terms of structural behavior, a pipe-in-pipe system may be categorized as being either compliant or non-compliant, depending on the method of load transfer between the inner and outer pipes. In compliant systems, the load transfer between the inner and outer pipes is continuous along the length of the pipeline, and no relative displacement occurs between the pipes, whereas in non-compliant systems, force transfer occurs at discrete locations. Due to effective axial force and the presence of out-of-straightness (vertically and horizontally) in the seabed profile, pipe-in-pipe systems are subjected to global buckling, namely, upheaval buckling and lateral buckling. In contrast to upheaval buckling, lateral buckling may be accepted if it does not result in unacceptable stresses and strains. In case global buckling may occur, a detailed finite element analysis is recommended to further investigate the buckling behavior.
Pipe-in-Pipe Appurtenances Design
Design of pipe-in-pipe appurtenances is to be based on their functional and installation requirements:
- Field joint design
- Bulkhead design
- Spacers
- Water stops/intermediate bulkheads to prevent water intrusion at the field joints
- Insulation
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