Estimation of the influence of a defect on the operational suitability of a pipeline

A finite-element model of a pipe header with a defect in the metal wall was created in the ANSYS software complex. The boundary conditions consider the hinging of the ends of the section from linear displacements. The calculation is based on the procedure of the allowable conditional elastic stresses. Values of equivalent stresses arising in the defective pipeline from the operational loads are obtained. Analysis of the obtained stresses in comparison with the allowable values is given based on the calculated resistance of steel. It is established that a dent in the pipeline causes a local stress perturbation. Thus, an increase in equivalent stresses in the region of the maximum depth of the dent amounted to 1.3% in comparison to the vicinity of the defect. Maximum values of membrane, tangential, and equivalent stresses do not exceed the design resistance. A conclusion is drawn on the influence of the dent-type defect on the operational suitability of the pipeline under study.


Introduction
When carrying out technical inspection of process pipelines at oil transportation facilities, there is often a problem of assessing their technical condition and the possibility of further operation [1][2][3]. As a result of the diagnostics of the large tank RVSPK-100000, a defect of the dent type was found in the near-weld zone of the process pipeline ( Figure 1). According to preliminary estimates, there is no combination of external operational loads that can cause the onset of the limiting state in the tank pipe header. According to [4][5], it is allowed to perform specifying calculations based on the results of diagnostics of metal structures. In addition, the delivery time of the pipe header elements for its complete replacement did not allow timely fulfillment of the planned repair of the tank. It was decided to confirm by calculations the possibility of operating a pipe header with a dent, and coincide its replacement with the next major repairs.
The purpose of this calculation is to determine the possibility of further operation of a pipeline with a defect of this type by establishing the values of the internal forces acting in the defective area and comparing the obtained values of the maximum stresses with the design resistances of the pipeline material. The state at which internal changes in the metal lead to its destruction is called the limit stress state. Conclusion on the reliability of the structure should be made on the basis of a comparison of the maximum stresses that can occur at the most dangerous point, with the maximum allowable values for a given material. The limit stress state of a structure is the boundary beyond which its operation is unacceptable. The reliability of operation is the higher, the further from the limit state the level of actual stresses is inside the material of the part.

Methods
The calculation is based on the procedure [9,10,11] of the allowable conditional elastic stresses. The stress-strain state of a defect of the dent type in the cylindrical shell of a pipe is determined by the spatial work of the calculation model under the action of combinations of operational loads.
Since the purpose of the work is to examine the stresses that arise in the region of the pipeline defect, the calculation model can be limited to the pipeline section containing the defect. Internal overpressure is the working load on the pipeline. Also, for calculations, you need the own weight of the pipe header, wind and snow load. Thus, to ensure the static definability of the model, the boundary conditions consider the hinging of the ends of the section from linear displacements.
The length of the shell should be chosen so that to eliminate the possibility of the influence of the endings on the defective area. An inward dent defect is modeled in accordance with the data of measurements during the diagnostics. The loads distributed on the shell area are applied to the shell section. Weld seams are neglected. When analyzing membrane stresses in ANSYS, SHELL181-type finite elements from the ANSYS library are used to model the shell [12][13][14][15].
To do this, the imported 3D model from the Autodesk AutoCAD software package is processed and optimized in the DesignModeller preprocessor in order to obtain surface elements. The finite element model consists of five shell objects, linked together by a bonded contact of the bonded type, i.e. a complete dependence of displacements in all nodes is ensured. The coordinate system is Cartesian.
The imposition of a finite-element grid is assumed to be free, repeating the curvature of the surface. The grid is superimposed automatically. The minimum size of the grid face of 0.6 mm was obtained during condensing in the vicinity of the defect. For other constructive elements, the grid size is assumed equal to 10 mm (Figure 2).  Based on the results of calculations, the following stresses were obtained: the maximum value of the membrane stresses in the defect region (σm) is 14.3 MPa, the maximum value of the membrane stresses outside the defect area (σm) is 8.1 MPa, the maximum value of the equivalent von Mises stresses in the defect region (σe) is 12.3 MPa, the maximum value of the equivalent von Mises stresses outside the defect area (σe) is 9.6 MPa ( Table 1).

Conclusions
The article presents a final-element model of the PRP-900 pipe header with a dent-type defect in the metal wall developed by the authors using the ANSYS software package. Equivalent stresses arising in the defective pipeline from the operational loads are 4% of the calculated resistance of steel. The dent in the pipeline causes a local stress perturbation. Thus, an increase in equivalent stresses in the region of the maximum depth of the dent amounted to 1.3% in comparison to the vicinity of the defect. Maximum values of membrane, tangential, and equivalent stresses do not exceed the design resistance of 189.58 MPa according to [16,17]. The condition of static strength is satisfied. Thus, at a design pressure of 155 kPa within the pipe header pipeline with a wall thickness corresponding to the actual value, the strength of the pipeline is ensured and it can be said that there is no significant effect of the dent-type defect on the operational suitability of the pipeline under study.
Safe operation of the PRP-900 pipeline with a defect of the dent type is possible without limiting the operational loads, for a period until the next, complete diagnostics of the tank.