The engineering task
Survey of a shell intended for vacuum service finds greater out-of-roundness than assumed in the base design. Under external pressure, a small geometric imperfection can sharply reduce buckling resistance. Internal-pressure thickness alone gives no assurance for this case.
The measurement must first be converted into the imperfection measure required by the code. Allowable external pressure, effective shell length and stiffening rings are then assessed together. If fabrication tolerance is exceeded, a special Annex F check, repair or additional stiffening may be required.
Required design data
- Survey record with diameters and measurement positions
- Shell geometry, thickness and stiffener spacing
- Maximum external pressure and temperature
- Material, fabrication tolerance and weld arrangement
Technical solution approach
Evaluate survey data
Determine ovality, local denting and measurement uncertainty separately in accordance with the code.
Calculate baseline stability
First check the unstiffened shell or bay between rings using permitted imperfection.
Include actual out-of-roundness
Where limits are exceeded, determine reduced allowable external pressure using the special imperfection method.
Vary stiffening
Optimise ring spacing, ring section or shell thickness for adequate buckling margin.
Document acceptance
Define acceptance, repair, repeat survey or operating restriction with explicit limits.
Expected deliverables
Suitable SOL ALPHA modules
The modules form a technically plausible toolchain. Their final selection and sequence will be confirmed during the later calculation against the applicable code and project conditions.