Issue 49

A. Guillalet alii, Frattura ed Integrità Strutturale, 49 (2019) 341-349; DOI: 10.3221/IGF-ESIS.49.34 343 parameters characterized by their uncertainties are involved and alter the three cited parameters such: fluctuating pressure, crack shape, constrain and other parameters. Stress intensity factor (SIF) is the most common parameter used to represent driving forces that allow surface crack to propagate. The early formula of Irwin for stress intensity factor is developed by many researchers. Closed form solution for SIF where derived for many simple configurations. Newman and Raju have proposed an empirical stress intensity factor equation for SIF estimation expressed by the following formula: ( ) I t b a K S S F Q      (1) where S t represent remote uniform-tension stress, S b is the remote bending stress on outer fiber, Q is the shape factor for elliptical crack and F is the stress-intensity boundary-correction factor. Figure 1 : Stress-intensity boundary-correction factor Uncertainties related to cracked surface in pipeline Several uncertainties are related to fatigue life estimation of onshore pipeline. Scatter in material proprieties, loading conditions, crack initial dimensions and pipeline geometrical configuration are input parameters for failure probability estimation, in which these parameters have large influence on calculation precision [12]. In probabilistic analysis, these parameters are presented as random variables. In practice, it is difficult to obtain an applicable representation of its randomness. When data are available from laboratory test or from service inspection, statistical representation can be derived easily. In other cases, the definition of scatter can be done by gathering variability of data from previous literature considering specification of the actual study. Uncertainties related to loading parameters The applied load on oil and gas pipeline in case of axial surface crack is a circumferential stress due to internal pressure which is calculated via Barlow equation presented in Eqn. (1): * 2 * b P D S t      (2) where: ∆σ: applied pipeline wall stress range (MPa). D: pipeline diameter (mm). ∆P: fluctuating pressure range (MPa). t: pipeline wall thickness (mm). The uncertainties considered for load parameters are related to variation of all parameters involved in stress estimation, typically the geometrical (D, t) parameters and service pressure (∆P). During assessment of pipeline subjected to fatigue crack growth, both maximum operating pressure and actual operational characteristics should be considered. The first indicate the maximum cyclic stress experienced by the pipeline. The second gives information about increase and drop in operating pressure (∆P) and the frequency of cyclic stress. The direct way to quantify the cyclic pressure experienced during service time is Pressure Time History recording. The information gathered