The United Kingdom Offshore Operators Association (UKOOA) Specification and Recommended Practice for the Use of GRP Piping Offshore is similar in many respects to the BS 7159, except that it simplifies the calculation requirements in exchange for imposing more conservatism on the piping operating conditions. Rather than explicitly calculating a combined stress, the specification defines an idealized envelope of combinations of axial and hoop stresses which cause the equivalent stress to reach failure. This curve represents the plot of:
(sx / s-all)2 + shoop / shoop-all)2 - [sx shoop / (sx-all shoop-all)] ³ 1.0
Where:
sx-all = Allowable Stress Axial
shoop-all = Allowable Stress Hoop
The specification conservatively limits you to that part of the curve falling under the line between sx-all also known as sa(0:1) and the intersection point on the curve where shoop is twice sx a natural condition for a pipe loaded only with pressure. An implicit modification to this requirement is the fact that pressure stresses are given a factor of safety typically equal to 2/3 while other stresses are not. This gives an explicit requirement of:
Pdes ³ f1 f2 f3 LTHP
Where:
Pdes = Allowable Design Pressure
f1 = Factor of Safety for 97.5% Lower Confidence Limit Usually 0.85
f2 = System Factor of Safety Usually 0.67
f3 = Ratio of Residual Allowable After Mechanical Loads
= 1 - (2 sab) / (r f1 LTHS)
sab = Axial Bending Stress Due to Mechanical Loads
r = sa(0:1) / sa(2:1)
sa(0:1) = Long Term Axial Tensile Strength In Absence Of Pressure Load
sa(2:1) = Long Term Axial Tensile Strength Under Pressure Loading Only
LTHS = Long Term Hydrostatic Strength Hoop Stress Allowable
LTHP = Long Term Hydrostatic Pressure Allowable
This is implemented in the CAESAR II using the following equations:
Code Stress |
Code Allowable |
|
sab (f2 /r) + PDm / (4t) |
³ |
(f1 f2 LTHS) / 2.0 |
Where:
P = Design Pressure
Dm = Pipe Mean Diameter
t = Pipe Wall Thickness
On the Allowable auxiliary dialog box, the product of f1 and LTHS is entered in the SH1, SH2, SH3 fields; r is entered in the F1, F2, F3 fields; f2 is entered in the Eff field; and the temperature reduction factor K (described for BS 7159 above) is entered in the Fac field if omitted, it defaults to 1.0. K- and i-factors for bends and tees, and bending and pressure stresses are calculated as described for the BS 7159.
Calculate SIFs using UKOOA
UKOOA refers to BS 7159 for SIF calculations.