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 186<br />
2.6.3 A complete analysis should include all parts of the component and all phases.<br />
2.7 Failure modes (ref. Sec.3 [7])<br />
2.7.1 The minimum list of failure modes is evaluated in Table 14-6 for the laminate of the main body.<br />
Table 14-6 Minimum list of failure modes for the laminate<br />
Minimum list of failure Evaluation<br />
modes<br />
Fracture (local or global) Relevant<br />
Buckling (local or global) Not relevant, since we have no compressive loads. (The liner may see compressive loads<br />
after yielding. Such loads can potentially cause buckling)<br />
Burst<br />
Here same as fracture, since no high rate loads are applied<br />
Leakage<br />
Relevant<br />
Impact<br />
Relevant, if tools can be dropped on the vessel etc.<br />
Excessive deformation,<br />
Ovalisation,<br />
Excessive displacement<br />
Wear<br />
No other failure modes than the ones given in the table have been identified.<br />
2.7.2 The evaluation above should be carried out for all parts of the vessel, but this is not covered in the example.<br />
2.7.3 The relevant failure modes shall be linked to the functional requirements of each part of the component.<br />
In the present example this link is only considered for the main body of the vessel, as shown in Table 14-7.<br />
2.7.4 Discussions regarding the link between failure mode and limit state should take place between the<br />
designer and the client. All failures related to modes that lead to pressure loss or leakage are considered to be<br />
Ultimate Limit States (ULS) in this example, i.e. all failure modes are linked to an ULS condition.<br />
2.8 Loads (ref. Sec.3 [9])<br />
Relevant for vessel with liner, because the liner may have a failure strain that should not be<br />
exceeded. This strain may put a limit on the strain of the body of the vessel. Otherwise not<br />
relevant, since the vessel has no restrictions on deformation. Large deformations may be<br />
linked to some other failure modes, will be covered by analysing the other failure modes.<br />
Not relevant, since nothing slides over the vessel.<br />
Table 14-7 Link of failure modes and functional requirements<br />
Functional<br />
Failure mode<br />
Comments<br />
requirement<br />
Pressure containment Fracture, local fracture Shall always be checked.<br />
Impact<br />
Damage from impact may effect capacity to contain pressure.<br />
Excessive deformation Relevant if deformation is large enough to cause the liner to fail.<br />
Leakage<br />
Related to fracture, but often just a gradual release of fluid from a<br />
pressure vessel. Fracture will cause leakage, but other minor failure<br />
mechanisms may also cause leakage. Failure consequence is often less<br />
critical and related to normal safety class, but it depends on the fluid.<br />
Tightness/ Fluid<br />
containment<br />
Same as pressure<br />
containment<br />
2.8.1 For each load a characteristic value shall be established. A checklist of common loads is given in App.A<br />
in the standard. Only pressure load is considered in this example as shown in Table 14-8 and Table 14-9.<br />
Table 14-8 Characteristic pressure load for gas tank with liner<br />
Load Char. value COV Sustained value Fatigue value<br />
Pressure loads:<br />
Maximum peak pressure*,<br />
here also identical to assumed<br />
operating pressure<br />
46 bar 0 46 bar 0 – 46 bar (1300 times)<br />
Table 14-9 Characteristic pressure load for water tank without liner<br />
Load Char. value COV Sustained value Fatigue value<br />
Pressure loads:<br />
Maximum peak pressure*,<br />
here also identical to assumed<br />
operating pressure<br />
14.8 bar 0 14.8 bar 0 – 14.8 bar (1300 times)<br />
The peak pressure is the maximum pressure the system can reach.<br />
DET NORSKE VERITAS AS