http://www.iso.org/iso/catalogue_detail.htm?csnumber=27507

From a 2001 edition of this code

13.2 Tubular members subjected to tension, compression, bending, shear or hydrostatic pressure

Ch = 0,44t / D for μ ≥ 1,6D/ t
= 0, 44t/D + 0, 21(D/t)^3/μ^4 for 0,825D/ t ≤ μ < 1,6D/ t
= 0,737 (μ − 0,579) for 1,5 ≤ μ < 0,825D/ t
= 0,80 for μ < 1,5

μ= L/D*SQRT(2·D/t)

elastic hoop buckling stress, Fhe
Fhe = 2Ch·E ⋅t / D

L = length of tubular between stiffening rings, diaphragms, or end connections
For (L / D) ≥ 1,13·SQRT( D/ t )(i.e. μ ≥ 1,6D/t ), the elastic buckling stress is approximately equal to that of a long unstiffened tubular. Hence, stiffening rings, if required, should be spaced such that
L / D < 1.13·SQRT(D/ t) (i.e. μ ≤ 1,6D t )

13.2.6.2 Ring stiffener design.
The circumferential stiffening ring size may be selected on the following approximate basis:

Ic=Fhe·t·Lr*D^2/(8·E)

where
Ic = required moment of inertia for ring composite section.
Lr = ring spacing
D = diameter. See Note 3 for external rings.

NOTE 1: Equation 13.2-24 assumes that the yield strength of the stiffening ring is equal to or greater than that of
the tubular.
NOTE 2: An effective width of the shell equal to 1,1·SQRT(D·t) may be assumed as the flange for the composite ring
section.
NOTE 3: For external rings, D in Equation 13.2-24 should be taken to the centroid of the composite ring.
NOTE 4: Where out-of-roundness in excess of clause 21 tolerance limits is permitted, larger stiffeners may be
required. The bending due to out-of-roundness should be specifically investigated.
NOTE 5: Local buckling of ring stiffeners with flanges may be excluded as possible failure mode provided that the
following requirements are fulfilled:

h/tw ≤ 1.1·SQRT(E/Fy)

and

b/tf ≤ 0.3·SQRT(E/Fy)

h = web height, tw = web thickness, b = half the width of flange of T stiffeners or the full flange width for angle
stiffeners, and tf = thickness of flange.

Local buckling of ring stiffeners without flanges may be excluded as a possible failure mode provided that:

h/tw ≤ 0.3·SQRT(E/Fy)

NOTE 6: For flat bar ring stiffeners, whether internal or external, the minimum thickness should be 10 mm.


13.6.5 Ring design

Gives further info on design (calculations) too long to copy. Better check the current code.


 

Give also a look to chapter 14th of Galambos, 5th Ed. Circular Tubes and Shells, have much info on the effect of stiffening.