Page 271 - Pipelines and Risers

244 Chapter 14

The model uncertainty is calibrated from the 86 tests results in the AGA database (Kiefner
and Vieth (1989)). A Hemit model is applied to simulate the four lower moments. The mean
bias and COV for the existing and new criteria is listed in Table 14.1.

Bias for Normalized Pressure, xP
The characteristic value of the normalized pressure X, is obtained by substituting safety
factors, characteristic values of the other parameters into the design equation. In general, the
annual maximum operating pressure is higher than the nominal operating pressure. This is
reflected by the mean bias in x,. Sotberg and Leira (1994) assumed that the ratio of the
annual maximum operating pressure to the design pressure followed a Gumble distribution.
Its mean and COV is 1.07 and 1.5%. By further analyzing the data @ai, 1994), a Gumbel
distribution with a mean of 1.05 and a COV of 2% is used in this development.

Bias for Normalized flow stress, Xf
The Xf mainly reflects the material property. Uncertainty of Xf is largely dependent of the
material grade. A log-normal distribution is assumed to fit the data in the existing database.
From the data analysis, the mean value and COV are selected as 1.14 and 6%, respectively.

Bias for Normalized Area X,
The normalized defect area Xa is the ratio of metal loss area and its original area. Two kinds
of inaccuracy are possible:

0 inaccuracy due to the calculation method for the area of metal loss.
0 inaccuracy due to use of measurement instruments.

A log-normal distribution with mean 0.8 and COV 0.08 is recommended for X, .

Bias for Normalized Depth Xd
The uncertainty in the corrosion depth is the combination of the uncertainties associated with
pit separation, inspection, and future corrosion prediction. A log-normal distribution is thus
assumed for Xd , and the mean value and COV are taken as 0.8 and 8% respectively based on
the experimental data and expert judgement.

Normalized Length xL

Similar to the discussion on corrosion depth, the uncertainty in normalized defect length XL
is the combination of the uncertainties associated with pit separation, inspections, and future
corrosion. However, corrosion length is easier to measure in inspections. Normal distribution
is used to fit the XL . Its mean value and COV are taken as 0.9 and 5% respectively.