Compensation for Thickness of Shell Plates

Traditionally ships have had only one shaped hull and this hull has had an inner surface coinciding with the moulded surface. In such projects there is no (or at least little) need to consider the thickness of the shell plates when modelling parts against the shell.

However, the situation is different for vessels with double skins where both surfaces are non-planar or in vessels with a "planar" outer hull. In these cases it is essential that the plate thickness can be considered, preferably automatically. Such a feature should take into regard both the actual thickness and its "distortion" because of the (varying) angle along the section curve between the part and the surface.

Hull has a feature that considers both of these aspects. It has the following characteristics:

• Hull curves and traces of shell profiles will be intersected and stored in the model as intersections with the moulded surface.

• When a hull curve is used as a boundary of a panel it will be transformed to consider the shell plate thickness (and the intersection angle). This will also be done for the "implicit" curves, created when reference is made directly to the surface.

• Likewise, when a shell profile section is referred to, it will be moved to the correct side of the shell plate against which it is welded. This means that cutouts for it and stiffeners or brackets attached to it will get a correct size. (The movement from the moulded surface will occur in the plane of the trace curve, that means, not necessarily in the direction of the web of the profile).

This feature is activated by setting the environment variable SBH_ENABLE_SHPLACOMP to any value. This should be done only in projects where the feature is relevant since it will have some implication on the system performance.

AVEVA Marine can graphically represent shell plates in two ways, as a faceted or as a solid primitive. The accuracy of the shell plate compensation is higher, especially in complicated areas, when using the solid representation.

Remark: In simple cases and with simple geometry it is possible to consider the shell plate thickness by "manually" compensating for the plate thickness when modelling parts against the shell. However, as soon as references to shell profiles are involved the automatic feature must be activated.

The figures below show the result when a panel has been generated against the shell in a ship with the plate thickness inwards. In the left figure a try has been made to compensate for the plate thickness by a parallel movement of the "moulded" curve. In the right figure the new feature has been activated and the designer has not had to bother about the plate thickness.

Figure 8:1. Panel against shell without and with automatic plate thickness compensation.