OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.10 Component testing – Page 162 3.1.9 If analysis and test results agree with each other based on the criteria given in [3.2] and [3.3] the analysis method is suitable for the application. (See also Sec.7 [2.2]). 3.2 Short term tests 3.2.1 The requirements here apply to one test aspect as determined in [3.1]. 3.2.2 The sequence of the failure modes in the test shall be the same as predicted in the design. If the sequence is different or if other failure modes are observed, the design shall be carefully re-evaluated. 3.2.3 The measured strength of each critical failure mode shall never be less than the predicted characteristic strength. Critical failure modes are failure modes that are linked to a limit state. The characteristic strength of the component shall be updated according to [3.3]. 3.2.4 In addition to the requirements above ([3.2.1] and [3.2.2]), one of the following requirements shall be met: — the test results fulfil the requirements for “confirmation testing for static data” given in Sec.4 [8.7]. Application of this criterion requires that at least three tests are carried out — the characteristic strength of the structure is updated by the test results as described in [3.4]. One test is sufficient to use this method, but more tests are recommended. 3.3 Long term testing 3.3.1 Whether cyclic load testing or long term static load testing or both is required depends on the evaluation of the test programme done in [3.1]. The approach for both testing types is similar and will be treated here in one part. 3.3.2 Fatigue testing for high safety class: at least two survival tests shall be carried out. The specimen should not fail during the survival test and it should not show unexpected damage. The requirements to the testing are: — tests should be carried out up to five times the maximum number of design cycles with realistic amplitudes and mean loads that the component will experience. If constant amplitude testing is carried out tests should be carried out up to 50 times the maximum number of design cycles to compensate for uncertainty in sequence effects. — if the anticipated lifetime exceeds 10 5 cycles testing up to 10 5 cycles may be sufficient. The load levels should be chosen such that testing of the two specimens is completed after at least 10 4 and 10 5 cycles respectively. The logarithms of the two test results shall fall within µ-σ of the logarithm of the anticipated number of cycles to failure, where µ is the mean of the logarithm of the predicted number of cycles to failure and σ is one standard deviation of the logarithm of the predicted number of cycles to failure, both interpreted from a log(stress)-log(lifetime) diagram for the anticipated number of cycles to failure. If more tests are made the requirements are given in DNV-OS-C501 Sec.4 [8.8.6]. 3.3.3 Fatigue testing for normal safety class: at least one survival test shall be carried out. The specimen should not fail during the survival test and it should not show unexpected damage. The requirements to the testing are: — tests should be carried out up to three times the maximum number of design cycles with realistic amplitudes and mean loads that the component will experience. If constant amplitude testing is carried out tests should be carried out up to 30 times the maximum number of design cycles to compensate for uncertainty in sequence effects. — if the anticipated lifetime exceeds 10 5 cycles testing up to 10 5 cycles may be sufficient. The load levels should be chosen such that testing of the two specimens is completed after at least 10 4 and 10 5 cycles respectively. The logarithms of the two test results shall fall within µ-2σ of the logarithm of the anticipated number of cycles to failure, where µ is the mean of the logarithm of the predicted number of cycles to failure and σ is one standard deviation of the logarithm of the predicted number of cycles to failure, both interpreted from a log(stress)-log(lifetime) diagram for the anticipated number of cycles to failure. If more tests are made the requirements are given in DNV-OS-C501 Sec.4 [8.8.6]. 3.3.4 Stress rupture testing for high safety class: at least two survival tests shall be carried out. The specimen should not fail during the survival test and it should not show unexpected damage. The requirements to the test results are: — tests should be carried out up to five times the maximum design life with realistic mean loads that the component will experience. If constant load testing is carried out tests should be carried out up to 50 times the design life to compensate for uncertainty in sequence effects. — if the anticipated lifetime exceeds 1000 hours testing up to 1000 hours may be sufficient. The load levels should be chosen such that testing is completed after 10 3 hours. The logarithms of the two test results shall fall within µ-σ of the logarithm of the anticipated lifetime, where µ is the mean of the logarithm of the predicted lifetime and σ is one standard deviation of the logarithm of the predicted lifetime, both interpreted DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.10 Component testing – Page 163 from a log(stress)-log(lifetime) diagram for the anticipated lifetime. If more tests are made the requirements are given in DNV-OS-C501 Sec.4 [8.8.6]. 3.3.5 Stress rupture testing for normal safety class: at least one survival test shall be carried out. The specimen should not fail during the survival test and it should not show unexpected damage. The requirements to the test results are: — tests should be carried out up to three times the maximum design life with realistic mean loads that the component will experience. If constant load testing is carried out tests should be carried out up to 30 times the design life to compensate for uncertainty in sequence effects. — if the anticipated lifetime exceeds 1000 hours testing up to 1000 hours may be sufficient. The load levels should be chosen such that testing is completed after 10 3 hours. The logarithms of the two test results shall fall within µ-2σ of the logarithm of the anticipated lifetime, where µ is the mean of the logarithm of the predicted lifetime and σ is one standard deviation of the logarithm of the predicted lifetime, both interpreted from a log(stress)-log(lifetime) diagram for the anticipated lifetime. If more tests are made the requirements are given in DNV-OS-C501 Sec.4 [8.8.6]. 3.3.6 For low safety class long term testing is not required. 3.3.7 The sequence of the failure modes in the test shall be the same as predicted in the design. If the sequence is different or if other failure modes are observed, the design shall be carefully re-evaluated. 3.3.8 The average of the measured number of cycles or time until occurrence of each critical failure shall never be less than the predicted characteristic lifetime or numbers of cycles. Critical failure modes are failure modes that are linked to a limit state. 3.3.9 Tests should be carried out with a typical load sequence or with constant load amplitude. If a clearly defined load sequence exists, load sequence testing should be preferred. 3.3.10 Whether reduced test times compared to the component's life are acceptable should be evaluated based on the anticipated failure modes and whether extrapolation of the data to longer lifetimes is possible. This will mainly depend on the confidence and previous knowledge one has about the failure modes that are tested. 3.3.11 In some cases high amplitude fatigue testing may introduce unrealistic failure modes in the structure. In other cases, the required number of test cycles may lead to unreasonable long test times. In these cases an individual evaluation of the test conditions should be made that fulfils the requirements of [3.3.2] or [3.3.3] as closely as possible. 3.3.12 The static strength of the structure after long term exposure shall be taken as the extrapolation of the long term test data of the fatigue or stress rupture tests. 3.3.13 Higher static strength values after long term exposure may be used if experimental or theoretical evidence can be provided. The same arguments as given in Sec.4 [3] may be used for matrix and fibre dominated properties. A procedure to obtain strength data after long term exposure is suggested in Sec.4 [3.4] and [3.9]. 3.3.14 Additional tests may be required if resistance to a failure mode cannot be shown by analysis with sufficient confidence and if this failure mode is not tested by the tests described above. 3.4 Procedure for updating the predicted resistance of a component 3.4.1 The resistance of the component is R and is assumed to be normally distributed: R∈N(µ R ,σ R 2 ) where, µ R = mean value of the resistance of the component (generally unknown). σ R = standard deviation of the resistance of the component, representing the natural variability in the material properties and the manufacturing/production process, and here assumed known. 3.4.2 The characteristic value of the resistance is specified as a specific quantile in the distribution of the resistance, here defined as: x C =µ R -2σ R However, because µ R is unknown, the true characteristic value x C is also unknown. 3.4.3 Estimates of µ R and x C prior to testing are sought. One way of obtaining such prior estimates is to carry out an analysis of the component by means of available analysis models. The estimate µ RA of µ R is obtained from a single analysis using mean values for the material properties. The uncertainty in the estimate µ RA should also be assessed, expressed in terms of a standard deviation σ m ’, and DET NORSKE VERITAS AS
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OS-C501

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