Local non-linear ovalisation
Local non-linear ovalisation, known as the ‘OVAL’ option means that the ovalisation of the cross-section due to bending of the pipe or bend is taken into account in the bending stiffness properties of that cross-section. But only if the OVAL option (Section model: Ovalising) has been activated.
In case of in-plane in-bending the cross-section bending stiffness decreases with increasing bending and thus increasing ovalisation and in case of in-plane out-bending the opposite is the case.
The effect on a 60 degree R=3*D bend with D/t = 55 is shown in the diagram below.
The iteration process on the non-linear ovalisation starts as soon as the geometrical non-linear process on the pipeline has reached the “near stability” status.
The iteration process proceeds in such a way, that the stiffness reduction or increase is calculated based on the ovalisation (end iteration n) that results from the calculated curvature (end iteration n) and the internal stressing of the cross-section is calculated from this ovalisation). From this stressing the internal forces are calculated (end iteration n). The un-equilibrium between external forces and internal forces (end iteration n ) is a test whether the iteration process is completed or not. If not, the next iteration (n+1) is done with the new stiffnesses calculated from the previous iteration (iteration n).
The stiffness of the cross section is based on the enlarged diameter (D+ovalisation) in the linear bend stiffness formulas and the new shape of the cross section:
where yφ is the distance of the fraction dAφ = t*r*dφ to the neutral axis.
NonLinearOvalisation, last changed: 14/09/2016