N_h calculation Clarification - API 650 1

[CaliEng]

Referring to an old closed thread here:

https://www.eng-tips.com/viewthread.cfm?qid=178312

Interesting N_h isnt not explicity covered in an equation, though its in list of Notaiton (E.2.2)

In the previous thread @JStephen states "It's the hydrostatic stress times the thickness. .. Nh= 2.6(H-1)DG". But thickness is not in that formula. Would it rather be Nh=2.6(H-1)DGt?

On the contrary @jcoots assumed it might be "(Head Press/shell thk.)" which seems to jive more with the equation JStephen was refering to.

Thanks for the clarification!

ich actually seems to align more with the equation

 

[JStephen]

First a clarification.
"hydrostatic, adjective- relating to or denoting the equilibrium of liquids and the pressure exerted by liquid at rest..."
IE, hydrostatic pressure does not imply pressure due to water, but pressure due to liquid head.
So the use of "hydrostatic" in the definition of St is unfortunate, and the seismic design does not involve the hydrotest condition.

In the equation, td=2.6D(H-1)G/Sd + CA, neglect CA for the moment and td=2.6D(H-1)G/Sd
Or Sd = 2.6D(H-1)G/td
Or, Sd=(2.6D(H-1)G) / td
That 2.6D(H-1)G is Nh.
So with no seismic, you design for td=Nh/Sd, or td = Nh/Sd+CA if CA is required.
Usually, t is greater than td, so actual stress will be somewhat lower than the allowable Sd.

 

[CaliEng]

@JStephen, Ah, thank you for that clear explanation!