Blast stress analysis

Dear members,

If i performed blast analysis for Ductility Level Blast (DLB) criteria and setting the allowable as per Norsok L-002 which is Min (2.4sh, 1.5sy). Does Caesar Ii still the correct tools for it? I have a doubt because in that range it already in non linear region. Altough it is an accidental even still the load is considered as primary.

Thank you.

Quote from NORSOK-002:
"The effect of blast loads shall be evaluated for piping which is required to maintain the installation integrity in an explosion event.
Normal working conditions with respect to temperature and pressure may be used for the blast calculations."

And:
"Maximum allowable stress in blast case shall be the minimum of 2,4 x S or 1,5 x SY , where S is the ASME B 31.3 allowable stress limit and SY is the pipe material yield stress."

And another:
"It shall be documented that the mechanical joints (e.g. flanges, hubs, couplings, etc.) on piping systems
selected for blast calculations are leak free after the explosion event. However, it is acceptable that the
mechanical joints leaks during the explosion event."


I think authors of these requirements are aware that there are too many unknown input factors that can significantly change results of analysis.



1. When linear strain/stress border is crossed CII will calculate higher stress compared to what would be in real situation.

2. Blast input overpressure/drag values are highly conservative if detailed blast analysis is not carried out for each module separately.

3. Pipe supports are commonly modeled with default CII stiffness.
With this approach output loadings will produce most stiff design of pipe supports.

4. Stiffness value can be entered at every pipe support location but that would lead to iterative calculations of piping and pipe supports, until satisfactory accuracy is achieved. This is time consuming with questionable results.
I would not normally do this, unless pipe supports are significantly different/bigger compared to existing structures.

5. Piping should not fall apart under blast, but it does not need to be able for continuous operation after the blast.

6. It would be interesting to see how would different design approach influence behavior of piping systems in real conditions under blast.
Fortunately explosions are not that often, and it is better if we never find out.


When designing blast proof piping systems two challenges are:
1. To design safe system and
2. Not to produce highly overdesigned pipe supports.


CII setup procedure would be:

Build your load cases as you would do for OCC loading as minimum.

Change the OCC load factor in the load case setup to match your calculated allowable stress Min(2,4xSh,1,5xSy).
Second is optional if you do not want to see red output values.

Hello Masic,

Thank you for the input. I am in-line with most of yours valuable input (point 1, 2,3, 4, 6).
For point 5, it is correct for DLB criteria. There are some system that was put in SLB criteria by the Company and safety people. For this, it need to be able to operate after inspection. Though it is a few of them. For this SLB criteria, the allowable is set to 1.33 Sh.

Once again I valued your valuable input.

Best Regards
NLB

Hi again,

If you do not have detailed blast over-pressure and drag pressure profile throughout installation, setting 1.33Sh would be overkill.

Blast is one time event and allowable set to 0.9Sy could be still OK even if piping needs to maintain post blast operating capabilities.
It all depends how accurate are input data.

Best regards.