Page 430 - Pipelines and Risers

Design Codes and Criteria for Risers 397

Table 21.2 Usage factors in API RP 2RD.

Load Combination Normal Extreme Survival
Operating (100 year) (1O00/10 OOO year)

Functional plus 0.67 0.8 1 .o
Environmental

21.2.3 Ultimate Limit State Design Checks
Based on the above discussion on pipe failure, the design of the pipe will be considered fit to
resist ultimate limit states when it has been checked that the following conditions will not
occur:
- Ductile bursting due to internal overpressure only, excluding any externally applied loads.
- Excessive axial yielding of cross-section (plastic mechanism) under the combined effect
of bending, axial force, and pressure differential.
- Collapse due to external overpressure only, excluding any externally applied loads.
- Local buckling due to the combined bending, axial force, and external overpressure.
- Buckle propagation due to external overpressure

Such design checks are performed with the design equations, which are based on the above
capacity formulations that have been rewritten in the LRFD format in terms of characteristic
values and safety factors. All relevant load cases that can be expected during the lifetime of
the riser, including installation conditions, have to be examined to ensure an acceptable
design.

21.3 Limit State Design Criteria

21.3.1 General
This chapter addresses the limit state design approaches applicable for marine risers. Different
loads and corresponding limit states are defined. The design procedure based on limit state
concept is outlined. Limit state design formats are given.

21.3.2 Failure Modes and Limit States
The following failure modes shall be considered:
- Yielding
- Bursting (Hoop)
- Buckling
- Fatigue
- Fracture

Besides, the following failure modes are critical for the design of deepwater risers: