ROHR2fesu

ROHR2fesu is the finite element module of the ROHR2 program system; the abbreviation stands for finite element substructure. A single component – a nozzle, a branch, a trunnion – is modelled with shell elements in its actual geometry and inserted as a substructure into the beam model of the piping.

The rest of the system remains a frame. Detailed analysis is thus applied only where the beam model reaches its limit, and the effort is confined to the location that justifies it.

The substructure is part of the overall model. This has two advantages over a separate finite element calculation. First, the loading of the component for every load case results from the system itself – forces and moments need not be transferred by hand, and signs and coordinate systems cannot be mixed up.

Second, the stiffness of the component enters the analysis of the system. A nozzle in a thin vessel wall is considerably more flexible than the rigid connection often assumed in the beam model. With the actual flexibility, the loads distribute more realistically.

The module is used for individual locations where the blanket factors of the code do not apply or are not sufficient.

The shell model delivers the stress distribution over the surface of the component, separated into membrane and bending. Assessment follows the stress categories of the applicable code – primary, secondary, peak – each against its own limit.

Care is needed where two shells meet. There the model shows numerical peaks that depend on mesh density. How they are to be assessed is a matter of the code and of experience, not of the software.

Shell elements presuppose thin-walled components. Thick-walled components, the exact geometry of a weld, contact between parts or fracture mechanics are tasks for a general-purpose finite element program with solid elements.

Effort sets a limit as well: a finite element substructure at every branch of a plant is neither necessary nor sensible. It is the tool for the few locations where the simple check does not work out.