Nozzleloads
Superposition of nozzle loads
Basics:
Problem:
According to AD2000-Merkblatt S3/0 Annex 2, not only the local effect of nozzle/pipe connection loads on the vessel shell must be taken into account, but also the global effect on the shell and supporting elements must be analyzed. For this purpose, nozzle loads from nozzle tables are to be superimposed according to the SRSS method presented in AD2000 leaflet S3/0 Annex 2.
In the case of several nozzles, the following problem arises:
One force and one moment act on each nozzle of a vessel per spatial axis.
This results in three forces and three moments per nozzle. For one nozzle, these are already the following 64 combinations for the effective direction of all six loads.
| Fx | Fy | Fz | Mx | My | Mz | Fx | Fy | Fz | Mx | My | Mz | |
| + | + | + | + | + | + | – | + | + | + | + | + | |
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| + | + | + | – | – | – | – | + | + | – | – | – | |
| + | + | – | + | + | + | – | + | – | + | + | + | |
| + | + | – | + | + | – | – | + | – | + | + | – | |
| + | + | – | + | – | + | – | + | – | + | – | + | |
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| + | + | – | – | + | + | – | + | – | – | + | + | |
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| + | + | – | – | – | + | – | + | – | – | – | + | |
| + | + | – | – | – | – | – | + | – | – | – | – | |
| + | – | + | + | + | + | – | – | + | + | + | + | |
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| + | – | + | – | + | – | – | – | + | – | + | – | |
| + | – | + | – | – | + | – | – | + | – | – | + | |
| + | – | + | – | – | – | – | – | + | – | – | – | |
| + | – | – | + | + | + | – | – | – | + | + | + | |
| + | – | – | + | + | – | – | – | – | + | + | – | |
| + | – | – | + | – | + | – | – | – | + | – | + | |
| + | – | – | + | – | – | – | – | – | + | – | – | |
| + | – | – | – | + | + | – | – | – | – | + | + | |
| + | – | – | – | + | – | – | – | – | – | + | – | |
| + | – | – | – | – | + | – | – | – | – | – | + | |
| + | – | – | – | – | – | – | – | – | – | – | – |
The 64 possible combinations are quickly calculated for a single nozzle with today’s computing power of a PC.
However, the vessels usually have more than one nozzle.
The loads acting on these nozzles can load or unload the vessel shell to a greater or lesser extent. The number of possible combinations Knz is calculated for n=6 nozzle forces and moments for z nozzles according to the formula
X = 2n*z
For z=10 nozzles, there are already 2^(106) = 1.15310^18 possible (load case) combinations. In many calculation programs there are programmable evaluation and superposition procedures, but these procedures are only applicable for linear calculations. LES GmbH, for example, has programmed such superpositions for post-processing with the programming language for “vessel models” integrated in the FE program Ansys, called “APDL”, but has found out in practice that the input and the subsequent calculation effort is already quite high with only a few nozzles.
Solution:
With the program for the transformation of forces and moments created by LES GmbH at www.pvp-software, which is also available online, the section loads resulting from the pipe connection/spigot loads can be determined very quickly for a very large number of nozzles.
Since the nozzle coordinates and nozzle loads can be copied very quickly via the Windows clipboard, e.g. from Word, Excel, Access and HTML tables, the resulting cutting forces and moments are determined within a few minutes even for very complex vessels with more than 100 nozzles (see following screen snapshots).
Within the program, the nozzles can be entered both in the Cartesian coordinate system and in the so-called nozzle coordinates described in WRC Bulletins 107 and 297, which are used by default, and transformed between the coordinate systems taking into account the nozzle position in the vessel shell. The user can select a superposition method but is also shown the resulting section loads according to other superposition methods.
Transformation of forces and moments
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