OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.14 Calculation example: two pressure vessels – Page 203
case the acceptable characteristic stress level is 143 MPa, after applying the load model factor and the 10%
reduction for sea water we get 123 MPa. Therefore:
This is more than the actual strain ε 1 =0.17% for an internal pressure of 1.48 MPa. Stress rupture data do not
have to be confirmed by testing, provided the other similarity requirements from (Sec.6 [11.3.8])are fulfilled.
6.7.4 Short-term failure due to maximum loads after 25 years was already considered in [6.6].
6.8 Fibre dominated ply failure due to cyclic fatigue loads (ref. Sec.6 [11])
6.8.1 The analysis method is the same as for the gas vessel in [5.5].
6.8.2 The maximum strain corresponding to a characteristic life of 6500 cycles is: ε
j
applied = 0 .907 % .
This is more than the maximum actual strain: ε 1 = 0.17%.
6.8.3 The strain in the laminate: ε 1 =0.17% is also less than 0.67%. Therefore, the fatigue properties do not have
to be confirmed by testing, provided the similarity requirements from Sec.6 [11.3.8] are fulfilled (see also
[5.5.6]).
6.8.4 Short-term failure due to maximum loads after 25 years was already considered in [6.6].
6.9 Unacceptably large displacement (ref. Sec.6 [9])
6.9.1 No requirements to be checked.
6.10 Impact resistance (ref. Sec.6 [12])
6.10.1 Impact may be caused by dropped tools etc. The possible impact scenarios, if any, should be defined.
6.10.2 There is no good theoretical criterion to evaluate the resistance to impact. According to Sec.6 [12], the
resistance of a structure to impact shall be tested experimentally.
6.10.3 The critical failure mode in this example is leakage and burst, linked to the mechanisms fibre failure
and matrix cracking. It would have to be shown that the defined impact scenarios do not cause any fibre failure
or matrix cracking. This could be shown on full scale specimens or on representative laminates.
6.10.4 Some matrix cracks after impact may be acceptable as long as they do not cause leakage. This could be
shown on tests on pressurised pipes.
6.10.5 Alternatively, the vessel could be protected against impact by covers or other protection devices.
6.11 Explosive decompression (ref. Sec.6 [15])
6.11.1 Water can diffuse through the laminate at low rates. It is unlikely that water can accumulate in the
laminate and cause effects related to explosive decompression.
6.11.2 There is no interface in this design where water could accumulate.
6.12 Chemical decomposition (ref. Sec.6 [17])
6.12.1 It is proven by many applications that composite laminates do not chemically decompose in water
within 25 years.
6.13 Component testing (ref. Sec.10)
ε
j
applied
=
σ
j
applied
fibre
E 1
=
123 MPa
23 .7 GPa
0 .518
6.13.1 The results show that the design limiting factor is matrix cracking. Matrix cracking is assumed here as
the beginning of leakage. It is known however, that many more matrix cracks are needed before leakage starts.
6.13.2 Testing a component with this laminate can show when leakage really starts and would allow to utilise
the design much better than it is done here. Short term and long term performance with respect to leakage can
be tested according to Sec.10 [2.2]-[2.3]. Having obtained those data they can be used instead of the checks
made here for matrix cracking. This would allow a much better utilisation of the vessel.
6.13.3 Another alternative to utilise the component better is to use a liner, as shown in the example of the gas
vessel.
=
%
DET NORSKE VERITAS AS