API 650 Add. 4, App. E Annular Plate Width Calc

[TankDude]

I would greatly appreciate if anyone would comment or provide their opinion regarding the following matter: should the annular plate inside projection calculated in appendix E of API650 Add. 4 be taken as the controlling required projection when greater than that taken from section 3.5.2? I am specifically referring to section E.6.2.1.1.2 and the "L" calculation in subsection (c). Also, as part of this calculation, do you use the actual annular plate thickness for the value of "ta" or the minimum required thickness per 3.5.2 (exclusive or corrosion allowance)? Thanks in advance for your input.

 

[JStephen]

The way I read the requirement, the width of the annular plate has to meet the L equation, using the corroded thickness of the annular ring, even if that thickness is greater than required for seismic loading.

The intent there is that a certain amount of water near the shell is going to be used to furnish overturn resistance, and the annular ring width has to be wide enough to develop that amount of force.

I guess the issue you have is that if for some reason, the annular ring is thicker than it needs to be for seismic, do you also have to make it correspondingly wider? I suppose this could be where you have a customer-mandated minimum thickness, or where some intermediate decimal thickness is adequate, but you're rounding up to next thickness, or where Table 3-1 or maybe App. M requires a thicker annular ring than required by seismic.

I think logic dictates that you could set the width based on some lesser thickness that was also adequate. But that isn't what it actually says.

This would be a good question for an API interpretation, using each of those cases.

 

[JStephen]

A related question is if radiographs per 6.1.2.9 are required if the annular ring is only required by App. E. I think API answered that in the negative, although I can't find that interpretation at the moment. But it tends towards the idea of separating the App. F and the 3.5.2 requirements.