Axial pdf

Translate PDF. The FEM was verified against results of full-scale burst tests. The developed FEM incorporated material nonlinearity, which allows for large strains and displacements. Furthermore, true ultimate tensile strength UTS instead of the typical engineering UTS was employed to determine the point of failure. The pipes used in the finite element analysis FEA were modeled based on API 5L X52 with a length of mm, a nomi- nal outer diameter of mm, and a nominal wall thickness of 10 mm.

The results obtained from the FEA were presented in a graph form and then used to determine the failure pressure trendlines of multiple longitudinally aligned interacting corrosion defects for varying defect depths and spacing between defects. In addition, the failure pressure trendlines of multiple longitudinally aligned interacting defects were compared to determine their differences.

As expected, 3 longitu- dinally aligned interacting defects showed lower failure pressure compared to 2 longitudinally aligned interacting defects. Corrosion is defined as a reduc- Pipelines are one of the facilities used both offshore and tion or destruction of materials due to electrochemical onshore to transport hydrocarbons from the well reser- reactions with the surrounding environment.

Although voir up to the topside facilities and further to onshore the pipelines have corrosion-preventive measures such facilities. It serves as arteries in oil and gas industry and as cathodic prevention methods during the installation, has been widely accepted as one of the most economi- the protection may be damaged due to the harsh working cal ways of transporting oil and gas over long distance environment.

In addition to corrosion damage, pipelines [1]. The strength of the pipeline is the main concern as it which are installed at regions with ever-changing land determines its service lifespan. However, the harsh work- conditions are exposed to axial compressive loadings ing environment can rapidly deteriorate the integrity of which when imposed on corrosion-affected pipelines can the pipelines.

In con- ential expansion caused by the increase in internal pres- trast, interacting defects are when corrosion defects are sure [6]. In addition, the increase in transport fluid temper- sufficiently close to each other causing stress and strain ature [6] as well as local geological events i. This work specially focuses on multiple 2 and axial compressive stress on pipelines [7—9].

A study con- 3 defects longitudinally aligned interacting corrosion ducted by Chauhan et al. Longitudinally aligned interacting corrosion to internal pressure and axial compressive stress revealed defects are characterized by full-wall thickness separation that the additional axial compressive stress does adversely in the longitudinal direction of the pipe.

Results from the In most cases, the failure pressure caused by interact- study also revealed that buckling failure was more promi- ing corrosion defects is far lower compared to the failure nent for shallow defects, where failure was governed by pressure caused by a single corrosion defect.

This is due to global instability instead of local instability failure due the complex interaction behavior when stress and strain to burst. Xu and Cheng [9] studied the effects of tensile disruptions occur on at least one of the corrosion defects. The Each corrosion defect imposes its own stress and strain FEA study revealed that the failure pressure was adversely disturbances.

These stress and strain disturbances expand affected, regardless of the type of external axial loading out from the corrosion defects. The regions of expansion of tensile or compressive. This finding is also supported by the stress and strain disturbances are known as regions of Cosham and Hopkins [10], Cosham et al.

Results obtained from the FEM were used to determine the differences in trends between There are many corrosion defect parameters that affect 2 and 3 longitudinally aligned interacting defects sub- the final failure pressures, such as defect depth, defect jected to internal pressure and axial compressive stress. However, this study focuses on two of the critical parameters which are the defect depth and defect spacing.

The study compared the failure pressure of 2. These In cases involving interacting corrosion defects, defect defects were then subjected to varying axial compressive spacing is an important parameter.

It was 2. First, the corrosion defects longitudinally aligned interacting defects. This limit value were assumed to be rectangular rectangular defect ideali- is also in line with the guideline set by DNV-RP-F [5]. For this Diameter of pipe, D mm study, Autodesk AutoCAD was used to design a symmetri- Length of pipe, L mm cal quarter model to represent the actual pipe geometry.

Since symmetrical quarter models were used, appro- priate boundary conditions were applied at the neces- sary regions so that the quarter model would be treated as a whole model. Figure 2 illustrates the region in red where symmetrical boundary conditions were applied. The DOF of the quarter models were constrained to pre- vent unwanted rigid body motions.

Referring to the yel- low region in Fig. SOLID elements allow aligned interacting corrosion defects for meshing of curved boundaries. It was recommended by British Standard Institute [15] that at when predicting failure pressure of corroded pipelines [3].

Thus, in this study, 3 layers of elements that internal corrosion and external corrosion in pipeline were meshed at the defect regions, while 6 layers of have no difference in failure behavior [11]. Next, a sym- elements were meshed at regions between the defects. Mesh bias was set away from the defect from the finite element analysis FEA.

The method of regions to reduce the total number of elements used, Fig. Based on Eq. Then, Eq. The out-of-balance load vector must be zero in order to fulfill the equilibrium conditions.

Therefore, a tolerance is introduced for the out- Fig. Based on the specified tolerance, thus, reducing the simulation time. As shown in Table 2, the convergence criterion for the out-of-balance load vector results of the convergence study revealed that 3 layers is checked and the trial solution is updated if the conver- were adequate as higher number of layers resulted in an gence criterion failed. Hence, the next incremental solution exponential increase in simulation time without much vector based on the calculated incremental displacement gain on the accuracy of the result.

Note that the conver- leading to the computation of the new out-of-balance load gence study was conducted on a pipe with a corrosion vector is identified based on Eqs. In order to improve the convergence behavior, To determine the failure pressure of corroded pipe, options such as time stepping, bisection method, and line an iterative method was performed by ANSYS.

The incre- search algorithm method can be employed as well. For for this static nonlinear analysis. A timestep of was set for both loads to allow for accurate FEA as well as to prevent elements Stress-based criterion was employed in this study to deter- undergoing high distortions, especially when high axial mine the onset of failure.

Failure is said to occur when the compressive stress loads are imposed. In this study, when the von Mises 2. For brevity, only 2 of the full-scale burst test results were used to verify the performance of the developed FEM. Before moving on to the methodology verification, an overview of the full- scale burst tests will be outlined. Table 3 lists the dimensions of the artificially machined corrosion defects, failure pressure of the full-scale burst tests, FEA failure pressure predictions, and the absolute percentage difference between both failure pressure val- ues.

The failure pressure predictions have a maximum of 3. Study conducted by Benjamin et al. Hoop Long. The failure pres- Similarly, for brevity, only 2 of the full-scale burst tests were used to verify the performance of the developed FEM. The fields of operation range from huge shopping centres, to underground car parks, up to For most applications in the ventilating or air conditioning sector as well as in a lot of industrial and commercial applications a Systemair fan can be selected.

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