Figure 8. Observed flowline lateral buckling on seabed (plan view)


The flowline is modeled with PIPE31H elements. Twelve or sixteen sections points are used to capture the circumferential variation of stress and strain in the flowline. This increase in computational cost is needed to model unpredictable bending deformations in the lateral buckles.


The response of PLET anchor system is represented with a nonlinear spring element attached to the ends of the PIPE31H mesh. During the installation steps of the simulation, these spring elements representing the PLET anchor system are removed from the model. After the flowline has been installed on the seafloor they are added back into the model before the hydrotest and operational loads are applied. The hydrotest of the pipeline system is one of the last qualification activities before starting up the system. The test applies a very large internal pressure, using water, to ensure system integrity. The hydrotest pressure is much higher than the maximum allowable operating pressure (MAOP). Considerable axial and lateral deformations can occur during the test as large effective axial forces are created in the flowline. The operational loads, temperature and internal pressure profiles along the entire flowline, are determined using flow assurance simulation tools. The profile data are then used as loading conditions in the Abaqus simulation.


Obtaining convergence in a deepwater flowline simulation can be challenging because of the highly nonlinear interactions between the flowline and seafloor and the general instability of the flowline itself (the structure may be less than 35 cm across and yet is 20 km long). Experience has demonstrated that judicious modifications to the HCRIT value, to the convergence controls on the rotational degrees-of-freedom, and to the time incrementation parameters can improve the convergence rate with minimal impact to the solution's accuracy. In addition, the automatic static stabilization algorithm is used to help control the unstable lateral buckling displacements.


4.3 Results


The deformed configuration of a typical deepwater flowline is shown in Figure 9. The center image in the figure is a “global” view of one segment of the flowline mesh. The edges of the seafloor elements are displayed only when the “feature angle” is greater than 1 degree. Thus edges in the image indicate significant discontinuities in seafloor slope. The close up views in top


2009 SIMULIA Customer Conference 9


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