How to fix a model of an underwater riser piping with a section of piping resting on the seabed floor that will not converge?
---------------------------Confirm---------------------------The load case did not converge. Do you wish to continue iterating?---------------------------Yes No Cancel ---------------------------
Buried pipe have a lot more nonlinear supports since every soil point has 4 nonlinear springs. On one hand, having more soil points would increase the number of nonlinear equations. On the other hand, having less soil points, especially were needed, like near pipe change in direction, can also cause convergence problems. Consider increasing the soil stiffness (if feasible) and/or decreasing your Maximum Spacing may alleviate the non-convergence issues. Other issues that may cause converging issues,
a. if the soil bearing capacity P1 is too low, this can cause very large downward displacements and hence convergence issues under gravity load case.
b. If upward displacements was becoming very large, this would indicate that the buoyancy load was too great to hold the pipe down.
c. Soil properties set to 0.00.
Using nonzero K2 values (0.001 - .0.1) in order to avoid unlimited displacements. Using larger K2 values temporarily (K2=K1/1000 or so) can help you see if P1 values are sufficient by checking displacements. You may need to add concentrated or distributed weight to submerged pipe to prevent it from uplifting (in practice concrete mats are added in some cases).
d. Modeling soil properties
Instead of modeling using soil, suggest using V-Stop at a reasonable spacing (that will make it pass sustained stress check), may be a way to get around convergence issues. You may use large friction factors (0.5 to 1.0) for these supports.
e. Also please note that AutoPIPE does not apply hydrodynamic factors to soil points and hence these factors, especially CL (under Xtra data/Hydrodynamic) should be increased to compensate for not applying hydro lift loads to soil points. Hydro factors under wave loading (drag and inertia) are applied to intermediate soil points. Lift factors can only be applied to actual nodes. Using V-STOP instead of soil can avoid this problem with lift factors
What to do when "load case did not converge"
Bentley AutoPIPE