Page 253 - Pipelines and Risers

226 Chapter 13

where & denotes the stochastic ultimate moment, Mp and ME are stochastic applied load
effects for the functional and environmental bending moment. Mc may be expressed as the
product of the model uncertainty parameter XM and the moment capacity given by Bai and
Hauch (1998) equations. A large amount of experimental and numerical tests are required to
quantify mean value, COV and distribution function of the model uncertainty XM. All of the
stochastic variables &, MF and ME shall be defined for the parameter range of interests (e.g.
hoop stress 0.8SMYS for operating conditions, 0.96SMYS for hydro test conditions).
Uncertainty measures may be found from the SUPERB project, e.g. Jiao et al. (1997).

13.5.5 Calibration of Safety Factors
In order to reduce the conservatism, the following shall be given considerations in the
calibration of safety factors:

1) Use capacity equation for corroded pipes. Section 5 B205 of DNV’96 states that the
wall-thickness used in bucklingkollapse calculation for pipelines in operation shall
exclude corrosion allowance. However, the width of corrosion defects is typically less
than a quarter of the circle. Neglecting the whole circle would likely lead to 10% less
capacity predicted.

2) Use SMTS (Specified Minimum Tensile Stress) as cry in Bai and Hauch equations.
Figure 3.13 to 3.16 shows that the moment capacity equation agrees well with the finite
element predictions (which have been validated against laboratory tests), if SMTS is used
as CFY in the equations. The reasoning of using SMTS is strain-hardening effects and the
strengthening due to outward deformation for highly pressurized pipes in collapse.
Unfortunately, in DNV’96, SMYS (Specified Minimum Yield Stress) is used as by. For
X65 material, the ratio of SMTS and SMYS is 1.17. Laboratory tests by Mohareb et al.
(1994) have confirmed that the mean bias for highly pressurized pipes is about 1.05 if
SMTS is used as by in the capacity equations.

3) Use strain-based design or a conditional load factor ‘yc (4.0) for displacement
dominant situations. For high pressure and high temperature (HP/I-IT) pipelines, pressure
and temperature induced axial stress and moment could be large. Up-lift and lateral
buckling behavior is typically displacement controlled. When a HP/HT pipeline is subject
to fishing gear pullover load, its response is load dominant for small diameter pipelines
and displacement dominant for large diameter pipelines. It is therefore suggested that a
conditional load factor yc is introduced to reflect the differences in the structural response
to fishing gear load.

In many practical situations, no adequate capacity equation is available for strength prediction
due to the complexity of the problem. Instead, numerical tests (using FEM) and laboratory
tests are conducted for strength design. It is then required to select partial safety factors that
may be applied together with a (numerical) structural laboratory (e.g. finite element analysis).
Selecting partial safety factors using reliability methods, FEM may be applied to strength
design as an alternative to direct use of code equations. The advantage of such an innovative
approach is that when information is lacking from the design codes for new materials,