OS-C501
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Offshore Standard DNV-<strong>OS</strong>-<strong>C501</strong>, November 2013<br />
Sec.14 Calculation example: two pressure vessels – Page 196<br />
5.3.2 The following values are selected for the gas vessel with liner:<br />
Table 14-20 Short term values used for gas vessel with liner<br />
Partial factor Value Explanation<br />
Characteristic value of the local<br />
response of the structure (strain) in the<br />
fibre direction n<br />
Guidance note:<br />
The characteristic strain to failure of 0.87% is the worst case in this example for short-term loads at the beginning of<br />
the life of the component and after exposure to cyclic and permanent loads.<br />
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---<br />
5.3.3 Evaluating the design criterion in [5.3.1] we find the maximum allowable strain in fibre direction ε nk<br />
after 25 years of service to be:<br />
This is more than the largest actual strain (in the fibre directions) ε 1 =0.65%.<br />
(Note that short term loads are not critical for the design, but long term loads as described below.)<br />
5.4 Fibre dominated ply failure due to static long-term loads (ref. Sec.6 [10])<br />
5.4.1 The characteristic stress rupture curve is given by:<br />
log[ σ ( t)<br />
] = log[ 348] − 0.0423log( t)<br />
(from [4.6.2]).<br />
5.4.2 The time to stress rupture shall be checked by the criterion given in Sec.6 [10.4.8]:<br />
5.4.3 The following values are selected for the gas vessel with liner:<br />
ε nk 0.65% Largest strain in fibre direction, see the table in<br />
[5.2.8]<br />
Characteristic fibre strain to failure ˆ ε fibre k<br />
0.87% See [4.1.4], [4.4.5] and [4.4.6]<br />
Partial load effect factor<br />
Partial resistance factor<br />
γ F x γ M 1.18 From Sec.8 [2.4]:<br />
Maximum load is known with 0 COV<br />
Strain to failure: COV < 5%<br />
Load-model factor γ Sd 1.05 Due to simplifications in the analytical model used,<br />
see [5.2.4] and [5.2.5].<br />
Partial resistance-model factor γ Rd 1.0 Degraded properties are used in the analysis<br />
abs<br />
γ<br />
fat<br />
γ<br />
Rd<br />
t<br />
N<br />
∑<br />
ε nk<br />
y<br />
j = 1<br />
t<br />
0.87%<br />
<<br />
= 0.70%<br />
1.18⋅1.05<br />
⋅1.0<br />
t<br />
charact<br />
actual<br />
{ σ applied }<br />
j<br />
Sd<br />
<<br />
j<br />
{ } 1<br />
γ σ applied<br />
Table 14-21 Long term values used for gas vessel with liner<br />
Factors Value Explanation<br />
Design life t y 25 years Design life of 25 years.<br />
The total number of load conditions N 1 Only one load condition.<br />
Actual time at one permanent static load<br />
condition per year<br />
t actual 1 year The vessel is basically loaded all year, except for<br />
the short unloading times that are ignored here.<br />
Local response of the structure to the σ applied<br />
Calculated below.<br />
permanent static load conditions (max. stress)<br />
Characteristic time to failure under the<br />
permanent static load condition<br />
t charact<br />
Calculated below.<br />
Load-model factor γ Sd 1.05 Same as before, due to simplifications in the<br />
analytical model used, see [5.2.4]-[5.2.5].<br />
Resistance-model factor γ Rd 0.1 Only one load condition.<br />
Partial fatigue safety factor γ fat 50 See Sec.8 [5].<br />
5.4.4 The criterion in [5.4.2] can be evaluated to show that the characteristic time to failure should be:<br />
γ<br />
Sd<br />
t<br />
charact<br />
actual<br />
= t γ<br />
Rdγ<br />
fatt<br />
= 125years<br />
y<br />
DET NORSKE VERITAS AS