Page 276 - Pipelines and Risers

Remaining Strength of Corroded Pipes 249

Corrosion Rate
The corrosion rate, or the annual degree of grooving corrosion, is estimated based on the
empirical "deWaard & Milliams" formula that the influence of the operating pressure and
temperature on the corrosion rate is defined:

(14.50)

where T is the temperature in Kelvin, nco2 is the mole fraction of C02 in the gas phase and
Apoper(0 is the operating pressure (bar).

The estimated degree of corrosion over a time period, t, can be derived by integrating the
corrosion rate over the time period:
t
dcorr(t) = Xco, J a(t)v(t)dt (14.51)
0

where parameter a(t) expresses the influence of inhibitors and Xcom defines the model
uncertainty associated with the empirical corrosion rate.

Basic Variables
The uncertainty defined in the Table 14.2 is introduced in the model, where the symbols N,
LN, Beta and Gumbel indicated a Normal, Log-normal, Beta or Gumbel distribution. The first
parameter is the mean value, the second is the COV, the third and fourth parameters are the
lower and upper limits of the distribution.

14.6.1 Condition Assessment
The first stage of the requalification process is an evaluation of the present state of the system.
If the system satisfies the specified constraints, the system will continue to operate as initially
planned prior to the corrosion observation.

The specified constraints are summarized as:

Acceptable level of safety within the remaining service, or at least until next scheduled
inspection;
The annual bursting failure probability is less than within the next 5 years.

Three level analyses are conducted:

1. simplified analysis,
2. deterministic analysis