Page 291 - Pipelines and Risers

264 Chapter 15

The condition of pipe structure with respect to failure can be described by a Limit State
Function (LSF) which is the boundary between safe and failure states. The limit state
considered in this study is the fracture ultimate limit state. Then, reliability-based assessment
can be performed.

A safety factor y to be applied with the proposed fracture criterion is calibrated towards a
selected target safety level. Calibration can be defined as the process of assigning values of
the safety factor to be employed in the given design formats. The objective of the calibration
is to ensure that the predicted failure probability is close to the target safety level.
15.4.1 Design Formats vs. LSF
Design format
If only internal pressure is considered, the partial safety factor approach given by Equation
(15.6) leads to the design format as:
Pc ?y.P, (15.29)

where:
Pc= characteristic strength of the pipe according to a criterion
PL= characteristic load (internal pressure)
y = safety factor.

The new design equation for dented pipes with cracks in operation with respect to fracture
criterion can be formulated by substituting Equations. (15.19) and (15.18) into Equation
(15.29) as:

(15.30)

All the parameters in the new design format can be referred to the aforementioned sections. It
should be noted that characteristic values of those parameters will be used to estimate the
design pressure.

Limit state function
LSP can be formed based on failure criteria for the specified case. For seam-welded pipes,
there is a great possibility that weld defects or crack-likes exist along the seam. With a
combination of expectantly large defect and low fracture toughness, the fracture failure mode
may become critical for pipes.

Fracture is defined as the exceedance of the material toughness, this criterion has been used
for determining bursting strength criterion. In this sense, the bursting and fracture limit states
considered in this chapter are consistent.