OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.14 Calculation example: two pressure vessels – Page 188 Table 14-12 Link between failure modes and failure mechanisms Failure modes of Failure mechanisms Comments this example Fracture Fibre failure Is assumed to cause fracture. Shall always be checked. (local/global) Matrix cracking Causes leakage if no liner is present, but no fracture. Matrix crack growth Not critical since matrix cracking is not critical. Delamination Not critical because no compressive in-plane stresses are present (no danger of buckling) and no through thickness tensile stresses (thin shell) appear. Yielding Not relevant Buckling Not relevant since no compressive and/or significant in-plane shear loads are present. Unacceptably large No requirements to displacements or deformations are formulated. displacement Stress rupture Effect shall be checked for all critical failure mechanisms mentioned above. Fatigue In this case it is only fibre failure. Impact Shall be checked Wear Not relevant in this application Fire Not relevant in this application Explosive decompression Shall be checked Chemical Shall be checked decomposition Leakage Fibre failure Shall be checked. Matrix cracking Check this for vessel without liner. Acceptable for vessel with liner. Matrix crack growth If data exist that show leakage will only occur after a certain crack density has been reached, this failure mechanism may be used instead of simple matrix cracking. In this example we use first matrix cracking as a conservative leak condition. Delamination Not critical if liner is present. For vessel without liner matrix cracking happens prior to delamination. Since we check for matrix cracking we do not need to consider delamination. Yielding Not relevant for the laminate. Buckling Not relevant. See the comments for fracture. Unacceptably large displacement Displacement should be limited to prevent liner failure for gas vessel. No restriction for water vessel without liner. Stress rupture Fibre failure: Same as for fracture. Matrix cracking – vessel with liner: Not Fatigue relevant. Matrix cracking – vessel without liner: Check. Impact Shall be checked Wear Not relevant in this application Fire Not relevant in this application Explosive decompression Shall be checked Chemical Shall be checked decomposition Impact Impact Shall be checked Excessive deformation Unacceptably large displacement Shall be checked for gas vessel to ensure that liner does not yield. 3.1.3 Based on these tables we shall check the following failure mechanisms: — fibre failure: — short-term static — long-term static — long-term fatigue — matrix cracking (for vessel without liner): — short term static — long-term static — long-term fatigue DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.14 Calculation example: two pressure vessels – Page 189 — unacceptably large displacement (for vessel with liner) — impact resistance — explosive decompression — chemical decomposition. 3.2 Classification of failure mechanisms by failure types (ref. Sec.6 [1]) 3.2.1 The critical failure mechanisms shall be linked to a failure type. This is described for laminates in Sec.6 [1.2] and also indicated in Table 14-13. The failure type must be known to find the right safety factors in the failure criteria. Table 14-13 Link between failure mechanisms and failure type Failure mechanisms Failure Type Comments relevant for laminates Fibre failure Brittle From Table Sec.6 [1.2.1] Stress rupture Brittle From section Sec.6 [1.2.2]. (The component cannot tolerate local fibre Fatigue Brittle failure without burst.) Matrix cracking Ductile From section Sec.6 [1.2.2]. Stress rupture Ductile From section Sec.6 [1.2.2]. (The component can tolerate initiation of Fatigue Ductile local matrix cracks without instant leakage.) Unacceptably large displacements Ductile From Sec.6 [9.1.3]. It is assumed here that the liner may be used up to the yield point. Since this is a ductile failure mode the failure type for the related displacement criterion can also be ductile.) Explosive decompression* Not applicable Impact* Not applicable Chemical decomposition Not applicable * these items are load conditions, but are treated here as failure mechanisms to simplify the approach in the standard. 3.3 Failure mechanisms and target reliabilities (ref. Sec.2 [3.5]) 3.3.1 The target reliability level shall be determined for each relevant failure mechanism to obtain the right partial safety factors for the failure criteria. 3.3.2 Target reliability levels are given in Sec.2 [3.5] for different safety classes and failure types. Safety classes are related to components and phases. In this example we have only one component (the vessel) and we consider two phases, testing and operation (see [2.5]): — testing phase Low safety class — operational phase High safety class. 3.3.3 All failure modes were identified as Ultimate Limit State conditions. Therefore, all associated failure mechanisms are also related to ULS and can be found in Sec.2 [3.5]. Table 14-14 Failure mechanisms and failure types Failure mechanisms relevant for laminates Failure Type Target reliability levels Testing Phase Target reliability levels Operation Phase Fibre failure Brittle (C) D Stress rupture Brittle (C) D Fatigue Brittle (C) D Matrix cracking Ductile (A) C Stress rupture Ductile (A) C Fatigue Ductile (A) C Unacceptably large displacements Ductile (A) C Explosive decompression* NA - - Impact* NA - - Chemical decomposition NA - - NA: Not applicable (…): not considered, see [3.3.4]. Note: Matrix cracking is only relevant for the vessel without liner (water) * these items are load conditions, but are treated here as failure mechanisms to simplify the approach in the standard. Displacements should only be checked for the vessel with liner (gas) DET NORSKE VERITAS AS
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