Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 510

the affected member(s) from the model. A less conservative strength estimate is
obtained by modeling the damage in the nonlinear analysis. Some nonlinear
analysis software has special formulations for modeling dented or distorted mem-
bers. Alternatively, the damage can be modeled explicitly by shell elements or a
reduced cross-sectional area can be specified in the damage zone.
The same philosophy can be applied in the case of cracked joints to eliminate
the member from the model. The less conservative approach is to reduce the
strength of the affected joint by some fraction. Note that the presence of a
crack will also limit the ductility and the cyclic capacity of the cracked joint.
In the structure analysis the cracked joint will be simulated near to its strength
capacity. In some cases, for special joints, one can use the finite element method
to define the joint capacity. A crack will incur a reduction factor, F JR :

m q

A C
A

1
Q β

F JR = 1 −

where A C is the cracked area and A is the weld lengthmultiplied by the thickness, T.
Q β is known as the geometrical modifier, usually used in design codes to account
for the increasing capacity of uncracked tubular joints at high

β (where β is the

brace diameter/chord diameter) and the maximum strength occurs at β values

equal to 0.6:

Q β = 1for β ≤ 0 : 6

Q β = 0 : 3/ βð 1 − 0 : 833 βÞ for β > 0 : 6

m q is the power allocated to Q β and depends on the approach used to estimate the
capacity of the uncracked joint. In case of tubular joints containing part-thickness
flaws, m q =0.
For tubular joints containing through-thickness flaws, validated correction
factors giving lower-bound estimates of the collapse load are at present limited
to joints with b ratios less than 0.8 and the following configurations:

K joints with a through-thickness crack at the crown subjected to balanced
axial loading

●

Axially loaded T and DT joints with a through-thickness crack at the saddle. ●

7.9.3 Determining the Probability of Structural Failure
It is necessary to understand the probability of structural failure for the range of
offshore platforms that exist in the region in question. This is achieved by carry-
ing out a structural reliability analysis of the platforms selected as representative
of the fleet. As mentioned already, statistical sensitivity studies may be required
to establish whether certain characteristics are significant or not.
The probability of failure is estimated using reliability analysis to account
for uncertainties in the derivation of both the loading and the response. The
analysis procedure is outlined in the flow chart in Figure 7.22 .