Table 3.5 Prediction by two theoretical models.
| Mode1 | Theoretical mode 1 | Theoretical mode 2 |
|---|---|---|
| A1 | 43 MPa | 38 MPa |
| A2 | 55 MPa | 53 MPa |
| B1 | 134 MPa | 99 MPa |
| B2 | 38 MPa | 30 MPa |
In Table 3.5, it shows the prediction of theoretical model 2. It is always smaller than theoretical model 1’s, the main reason is that the theoretical model 2 doesn’t take the contribution of the cylindrical layers into consideration, so it is underestimated in Tables 3.4 and 3.5, the procedure of design can be concluded as below.
By given small radius and pressure, take 25 mm and 30 MPa, for example, the four layers SSRTP could meet this requirement enough. While the internal pressure increases to about 50 MPa, it’s necessary to adjust the winding angle or add more layers to satisfy the requirement. When the given radius is more than 50 mm or pressure is more than 60 MPa, the SSRTP might not satisfy in this condition. In addition, the pressure armor and tensile armor are often used in big radius pipe to subjected to high pressure. If the requirement of radius or pressure keeps increasing, adjust the winding angle or thickness of pressure armor and tensile armor. Based on these conclusions, it is easily to design a software to design structure of pipes when the internal pressure increases. The flowchart is shown in Figure 3.14.
Figure 3.14 Computer design flowchart of pipe section.
3.6 Conclusions
Within this chapter, the burst behavior of the flexible pipe was investigated by both theoretical model and numerical simulation. The accuracy and reliability of the theoretical model is verified by the good agreement between the two sets of results. But it is worth noting that this chapter is limited to prediction of pressure armor stresses during pipe operation only, and that the residual wires stresses from manufacturing are disregarded and not taken in consideration. Also, a simplified software to design structure section with the given radius and internal pressure is presented, which can provide some references for the factory engineers. From the research, we can learn that:
1 (1) The results of the theoretical model show good linearity of flexible pipes under internal pressure in the elastic phase. Theoretical model adopted here is basically valid in calculating the physical quantities stated above according to the comparison with the FEM. But the theoretical model doesn’t take the self-locking of pressure armor into condition, so there are certain errors in the comparison.
2 (2) The result of theoretical model and numerical model shows when the pressure armor yields, the axial displacement and radial displacement begin to increase sharply, which indicates that the pressure armor is the main internal pressure resistant structure. For safety, it is acceptable to consider when pressure armor fails, the pipeline will soon fail.
3 (3) In the process of internal pressure loading, the stress of Z-shaped section increases gradually from inside to outside, and the inner part near the end yield first.
4 (4) This chapter uses two theoretical models to predict the burst pressure of pipe and put up a software to design structure of pipes with given radius and pressure. When the radius and pressure are small, the steel strip reinforced thermoplastic pipe is useful to resistant certain pressure. While the radius or pressure is big, it is necessary to add pressure armor and tensile armor to subject to pressure.
5 (5) As in future work, a validation against test data is recommended for both the ABAQUS and Analytical models.
References
1. Fernando, U. S., Sheldrake, T., Tan, Z., and Clements, R., 2004, “The Stress Analysis and Residual Stress Evaluation of Pressure Armor Layers in Flexible Pipes Using 3D Finite Element Models,” Proceedings of ASME 23rd International Conference on Offshore Mechanics and Arctic Engineering.
2. Neto, A. G., de Arruda Martins, C., Pesce, C. P., Meirelles, C. O. C., Malta, E. R., Neto, T. F. B., and Godinho, C. A. F., 2013, “Prediction of Burst in Flexible Pipes”, Journal of Offshore Mechanics and Arctic Engineering, 135, 1, 011401.
3. De Oliveira, J. G., Goto, Y., and Okamoto, T., 1985, “Theoretical and methodological approaches to flexible pipe design and application”, In Proceedings. Offshore Technology Conference. New York NY[PROC. OFFSHORE TECHNOL. CONF.]. Vol. 3, pp. 517.
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8. Knapp R H. Derivation of a new stiffness matrix for helically armoured cables considering tension and torsion[J]. International Journal for Numerical Methods in Engineering, 1979, 14(4): 515–529.
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10. Witz, J. A. (1996). A case study in the cross-section analysis of flexible risers. Marine Structures, 9(9), 885–904.
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4
Tensile Behavior of Flexible Pipes
4.1 Introduction
Flexible pipelines must be designed considering the extreme situation for which additional components are needed against severe loading and environmental conditions. Tensile armor is employed in case of high tensile forces, which increase with water depth; and massive components used against high internal
