OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.14 Calculation example: two pressure vessels – Page 202 6.5 Matrix cracking under long-term cyclic fatigue loads (ref. Sec.4 [3.9]) 6.5.1 Matrix cracking under long term static loads can be ignored if the stresses are below the level to initiate matrix cracking, and if the vessel can carry the loads with a fully cracked matrix according to Sec.4 [3.9.5]. This is shown in [6.6]. In addition, the total number of fatigue cycles shall be less than 1500. This is the case in this example. 6.6 Fibre failure - short term (ref. Sec.6 [3]) 6.6.1 Matrix cracking will occur before fibre failure. Fibre failure can be analysed by modelling the laminate as a laminate full of matrix crack. This is basically the same way as for the gas vessel, except that elastic properties shall be degraded for the possible presence of water when analysing the component. The ply strains are given in [6.2.6]. 6.6.2 The short-term static design criterion for fibre failure on the ply level is given by: 6.6.3 The following values are selected for the water vessel without liner: Table 14-27 Short term values used for vessel for water without liner Partial factor Value Explanation Characteristic fibre strain to failure ˆ ε fibre 1.69% For the new vessel k 0.87% For the vessel after 25 years See [4.1.4] and [4.4.5], [4.4.6] Partial load effect factor Partial resistance factor γ . γ . ε ∧ fiber k γ F × γ M 1.18 From Sec.8 [2.4]: Maximum load is known with 0 COV Strain to failure COV ≤ 5% Target reliability level E Load-model factor γ Sd 1.05 Due to simplifications in analytical model, see [5.2.4]- [5.2.5] Partial resistance-model factor γ Rd 1 Degraded properties are used in the analysis 6.6.4 Evaluation of the criterion above (see [6.6.2]) shows that the maximum allowable strain in fibre direction ε nk after 25 years of service is 0.70%. This is much more than the actual strain ε 1 =0.23%. Guidance note: This vessel is designed against cracking of the matrix. Usually the margin against fibre failure is large in such a case. Calculating the margin against failure with a new stress analysis using fully degraded properties may appear as an unnecessary effort in such a situation. It is, however, the proper way of calculating for fibre failure. Using the right method may be more critical in more complicated structures. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 6.7 Fibre dominated ply failure due to static long term loads (ref. Sec.6 [10]) 6.7.1 The analysis method is the same as for the gas vessel in [5.4]. 6.7.2 The stress level corresponding to a characteristic life of 125 years is 155 MPa according to [5.4.5]. This value should be reduced by 10% due to the possible presence of water, see App.F ([F.5]). Therefore: j =139.5 MPa, and σ applied ε j applied = This is more than the actual strain ε 1 =0.17% for an internal pressure of 1.48 MPa. σ F Sd j applied fibre E 1 nk ε < γ . γ Guidance note: The strain level for a laminate without cracks is used for the applied strain. This value represents the actual condition of the laminate and should be used when applying the design criterion. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 6.7.3 Stress rupture data do not have to be confirmed by testing if the factor g Rd can be multiplied by 20 (see [5.4] and Sec.6 [10.4.10]). In this case the strain for a life of 125 x 20 = 2500 years should be found. In this M Rd 139 .5 MPa = 23 .7 GPa = 0 .58 % DET NORSKE VERITAS AS
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
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OS-C501

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