Page 480 - Pipelines and Risers

Pipe-in-Pipe and Bundle Systems 447

The factors to be considered in material selection include adequate material toughness for
fracture and fatigue performance and practical weld defect acceptance criteria, and whether or
not sour service is required throughout the design life.

In situ stress conditions need to be assessed in wall thickness and material selection. Any
stress locked into the inner and outer pipes as a result of installation procedures needs to be
accounted for. Also, the tension or towing capacity of the installation vessel needs to be
checked for both normal lay and contingency conditions.

24.2.6 Failure Modes
Failure modes described for single pipeline in Chapter 4 are applicable for the pipe-in-pipe
system. Additional failure modes need to be considered for the design and assessment of the
pipe-in-pipe system. The complexity and the differing load carrying capacity of the system
add to the number of failure modes. In addition, the risk and consequence of each particular
failure mode, and the impact on the pipe-in-pipe system will differ from a single pipeline.

Bursting
The burst capacity of the pipe-in-pipe system is determined based on the inner pipe subjected
to the full internal pressure, and the outer pipe subjected to the full external pressure.

Fatigue and fracture
Pipe-in-pipe systems are subjected to both low cycle and high cycle fatigue due to daily
operational fluctuations and start-up/shut-down conditions. One area particularly prone to
fatigue is the weld joint. Typically, the weld joint for pipe-in-pipe systems comprise butt weld
on the internal pipe, and either split shells or some form of sleeve arrangement for the external
pipe connection. Special attention should be given to the fatigue assessment for the inner face
of internal pipe since it is subjected to corrosive environment, and the outer face of the
external pipe subjecting to seawater environment.

Global buckling
Due to effective axial force and the present 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. The upheaval buckling should be investigated if the pipe-in-
pipe system is intermittent rockdumped. Lateral buckling should be investigated for all the
case.

24.2.7 Design Criteria
Stress Based Design Criteria
Stress-based design criterion is that the hoop and equivalent stresses are limited to some
fraction of the SMYS depending upon the considered design cases.

The hoop stress criterion may be used with due consideration of material derating factor for
both internal and external pipes.