Nonlinear Properties |
 
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Nonlinear Properties |
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It is important to remember that Flexcom adopts a tangent approach for modelling most non-linear relationships. One exception is non-linear soil structure interaction, where the P-y relationships are modelled using a secant stiffness approach. If your model includes any non-linear properties (e.g. non-linear materials, non-linear flex joints, non-linear spring elements etc.), the following simple guidelines should generally be adhered to:
•Avoid abrupt changes in slope. The non-linear characteristic relationship should appear as a smooth curve, without any sharp changes or “corners”.
•Increasing the number of data points may help to provide smooth curves.
•If do wish to model an instantaneous change in slope in reality (for example, to simulate contact between an inner and outer pipe in a pipe-in-pipe configuration), then it may be advisable to gradually increase the slope over a finite range of displacement. This approach should ensure a reasonable compromise between solution accuracy and robustness.
•The resistance provided by a non-linear curve should either monotonically increase or monotonically decrease with increasing displacement. If this is not the case, there may be more than one location along the curve which results in the same restoring force, and this may contribute to solution instability.
•*MOMENT-CURVATURE is used to define moment-curvature curves for non-linear materials (Flexible Riser format).
•*FORCE-STRAIN is used to define force-strain curves for non-linear materials (Flexible Riser format).
•*TORQUE-TWIST is used to define torque-twist curves for non-linear materials (Flexible Riser format).
•*STRESS/STRAIN DIRECT is used to define stress-strain curves for non-linear materials (Rigid Riser format).
•*PIP STIFFNESS is used to define force-deflection curves for non-linear pipe-in-pipe connection stiffnesses. Specifically, the TYPE=POWER LAW input is used to create a non-linear contact relationship based on a power law approach.