Admissibility of External Cracks in a Pipeline API X60 Using the SINTAP Procedure

In this paper we tried to apply the failure assessment diagram method on an API X60 pipeline under two pressures 70 and 90 bar, this work will be divided into two parts; the first part will be devoted to modeling and simulation of a pipeline under pressure 70/90 bar. With abaqus software to determine the stress intensity factor of several ratios, The second part will focus on the exploitation of these results in order to draw the diagram of evaluation of the failure (FAD), once finished, We can pronounce on the vulnerability of the cracks which can cause the ruin of the pipeline to study, on mode of ruin and proposed safety factors.


Introduction
Breakdown is a problem that man will have to face as long as he builds buildings or builds structures. This problem is currently more crucial with the development of complex structures linked to technological progress. Advances in the knowledge of the mechanics of rupture now make it possible to better prevent the risk of rupture [1].
One of the most used and most answered methods in the field of diagnosis of cracks is the SINTAP because it offers several levels of studies [2,3], in our case we have three information, yield strength, ultimate tensile strength and The critical intensity factor, we will in the following tried to apply this method on a pipeline API X60 under pressure 70 bar and 90 bar and to make a comparison between these two cases.
The choice of these two pressures is judicious; indeed the pressure of 70 bar is the working pressure, and the pressure of 90 bar the minimum test pressure at the factory. We will in some way tried to have if a few Cracks may not be detected during factory testing.

Failure assessment diagram
To determine whether a crack can cause structural failure, the FAD method uses two ratios: fragile fracture and plastic collapse. The brittle fracture ratio is computed from the crack front stress intensity, obtained by an elastic Abaqus analysis [4,5] -Brittle fracture: The plastic collapse ratio is calculated using the reference stress, which is calculated as a function of the size of the cracks.
-Plastic ruin: Where, A dimensioning based on the rupture integrity diagram ensures. That the operating point (Kr, Sr) is found within the diagram delimited by the interpolation curve Kr = f (Sr), the structure retains its integrity. If the calculated point is outside this area, the structure breaks down. .
Where is associated with the manufactured pressure.

SINTAP equations
The failure assessment line is defined by the following equations: For the evaluation of the failure assessment line only the tensile proprieties and young's modulus are required.

J Integral and stress intensity factor
In order to evaluate the stress intensity factor. We tried to calculate it from the value of an integral independent of the integration contour proposed by Rice (Fig. 2). Is defined by the relation (I-12)  The J integral is independent of the integration path chosen for a material having. An elastic nonlinear behavior, In the case of elastic linear behavior, the integral J is identical to G, the energy released by stress relaxation. We have the following relation between KI and J: Stress plane 1 ν Deformation plane

Modeling and boundary conditions
For reasons of symmetry. A structured mesh in quadratic elements with 8 nodes is used and refinement to crack front to avoided the problem of singularity

Pipeline study
In the case studied, it is a pipeline API X60 with an outside diameter D = 528 mm and a thickness t = 7.1 mm. (4)

Stress intensity factor
From the Fig. 6, noticing an increase in the stress intensity factor with the increase in pressure, this increase is increasing with the increase in the size of the crack. This is logical because the reduction in the thickness of the tube caused by the increase of the crack greatly increases the stress in the crack front. It is also noted that for ratios (a/t) 0.05,0.1,0.2,0.3 and 0.4 for both pressures. It is found above the critical value of the stress intensity factor Factor Sr is calculated by Eq. (13)      Taking a safety factor Fs (a) = 2, it is noted that it covers that the ratios 0.05 and 0.1 at pressure 70 bar, given the almost linear arrangement of the ratios, one can predict the point of exit of the crack of the Zone of plastic ruin to the elasto-plastic rupture zone, and a safety factor Fs (a) = 2.37 can be proposed by a simple calculation which covers all the ratios which is in the plastic ruin zone.

Conclusion
At the end of this modest work, we can conclude that: