Page 59 - Pipelines and Risers

32 Chapter 2

(2.16)

t2 = t-km
ApPd = design differential overpressure
z, = the characteristic tangential shear stress
SMYS(T) = SMYS at temperature T
qe = usage factor for equivalent stress

Variations in the code requirements for combined stress criterion are evident not only in terms
of the usage factor, but also with respect to applicability of the criterion. While this design
format may be suitable for predominantly longitudinal stresses, it becomes irrelevant when
localized stress concentration, caused by e.g. impact loads, is of concern. No explicit design
criteria are currently available for design against impact loads.

For deepwater pipeline, the wall-thickness should be designed such that sufficient bending
momentktrain are reserved for free-spans and external loads as discussed in Chapter 4. In
addition, it is necessary to design buckle arrests to stop possible buckle propagation.

2.5 Hydrostatic Collapse

The limit external pressure ' p, ' is equal to the pipe collapse pressure and is to be calculated
based on (BSSOlO (1993), ABS (2000) and DNV (2000)):

(2.17)

where:

(2.18)

2. t
pP = qfd.SMYS(T).- (2.19)
D
fo = Initial out-of-roundness I), (Dmx-Dmin)/D
D = Average diameter
SMYS(T) = Specified Minimum Yield Strength in hoop direction
E = Young's Module
v = Poisson's ratio
qfab = Fabrication derating factor

Note:
1) Out-of-roundness caused during the construction phase is to be included, but not flattening
due to external water pressure or bending in as-laid position. Increased out-of-roundness