OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.3 Design input – Page 30 environmental conditions define the different design cases to be considered. These design cases are described in Sec.3 [11]. 9.2 Probabilistic representation of load effects 9.2.1 The response of the structure to applied loads shall be calculated on a global or a local level depending on the failure mechanism being checked and its associated design rule. See Sec.9 [1.4]. 9.2.2 A probabilistic representation should be established for the effect of each load on the structure in every relevant location. The load effect is obtained by from the basic load by structural analysis. The basic load is obtained directly or by a transfer function (e.g. converting wind speed to a basic load on the surface). 9.2.3 The probability distribution function representative for each load process should be determined. The probability distribution function representative for the response of the structure associated with each load process, the load effect, should also be determined in every relevant location. In particular, the type of distribution should be determined for each distribution function. 9.2.4 The arbitrary value distribution over the lifetime of the structure and annual extreme value distribution shall be determined for all loads and the corresponding response of the structure. 9.2.5 A recognised procedure for determination of the distribution type shall be used. The procedure required in the DNV Classification Note No.30.6 “Structural reliability analysis of marine structures” may be used. 9.2.6 If two types of distributions give equally good fits to data, the distribution with most probability content in the upper tail should be chosen, unless one of the distributions fits possible data observations in the tail better than the other. 9.2.7 If no satisfactory distribution fit can be obtained or if insufficient data are available, then the simplified probabilistic representation of load effects presented in Sec.3 [9.3] shall be used. Guidance note: The partial safety factors specified for the simplified probabilistic representation of load effects are conservative. A precise determination of the extreme-value distribution for load would normally lead to lower requirements to the values of the partial safety factors. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.2.8 For some load variables, sufficient knowledge and or experience is available to provide a basis for recommendation of distribution types. A list of variables with their recommended distribution types is given in App.A. 9.2.9 The coefficient of variation of each load variable and the corresponding load effect shall be specified. If insufficient data are available the simplified probabilistic representation of load effects presented in I300 should be used. 9.3 Simplified representation of load effects 9.3.1 A simplified set of partial safety factors is given for use whenever a satisfactory probabilistic representation of the load effects, as required in Sec.3 [9.2], is not available. 9.3.2 The characteristic load effect shall be defined as specified in Sec.3 [9.4]. 9.3.3 The simplified set of partial safety factors given in Sec.8 [2.3] was determined assuming that the coefficient of variation of load effects were not larger than 20%. These partial safety factors shall not be used for load effects with a COV larger than 20%. 9.3.4 The simplified set of partial safety factors given in Sec.8 [2.4] was determined assuming that the coefficient of variation of load effects is 0%, i.e. the load effects are exactly known and they do not have any statistical variation. This is usually based on a very conservative description of the load effect. 9.3.5 The simplified set of partial safety factors shall be used when the characteristic strength is defined as the 2.5% quantile, as generally required in this standard. 9.3.6 Loads may also be defined as combinations of functional loads and environmental loads according to offshore standards like DNV-OS-F201 “Dynamic risers” or DNV-OS-F101 “Submarine pipeline systems”. Partial safety factors for this choice are given in Sec.8 [2.6]. 9.4 Characteristic load effect 9.4.1 The characteristic load effect value, S k , is a value that will only rarely be exceeded. For time-dependent processes, it is generally given in terms of a return value for occurrence, i.e. a load effect which on average is exceeded once in a specified reference time period (denoted return period or recurrence period). DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.3 Design input – Page 31 9.4.2 A unique definition of the characteristic load effect is prescribed and used throughout this standard. It shall be used both in case of single and multiple load processes. 9.4.3 In principle, the characteristic load effect shall be determined as the characteristic value of the local response of the structure to the applied load. It shall be based on a probabilistic representation of the variability in the local response, as defined in Sec.3 [9.2]. Guidance note: The partial safety factors specified in this standard and calibrated against specified probabilities of failure apply on the characteristic values of the load effect, i.e., the local response of the structure. Simplifications in the transfer function (from loads to local response) lead to uncertainties. These uncertainties are accounted for by the load model factors. When the transfer function from applied loads to local response is linear, the probabilistic representation of the variability in the local response is identical to the probabilistic representation of the variability of the loads. In that case, partial safety factors can be applied directly on the characteristic values of the applied loads. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.4.4 The characteristic load effect can be determined as the characteristic value of the externally applied global load in the following cases: — when the design rule is expressed in terms of the global response of the structure — when the transfer function from global to local response and the analysis is linear. 9.4.5 The characteristic load effect is defined as the 99% quantile in the distribution of the annual extreme value of the local response of the structure, or of the applied global load when relevant. 9.4.6 The 99% quantile in the distribution of the annual maximum load effect is used as the characteristic load value throughout this document. The 99% quantile may be interpreted as the 100-year load, i.e., the load value which has a return period of 100 years. This is the load effect value, which on average will be exceeded once every 100 years. 9.4.7 Extreme values driving the design can be maximum as well as minimum values. Should minimum values of load effects be driving the design, the characteristic load effect shall be defined as the 1% quantile in the distribution of the annual minimum load. For example, the pressure on the wall of a submerged pressure vessel is function of the differential between internal pressure and external hydrostatic pressure and increases when the external pressure decreases (i.e. when the depth decreases). 9.5 The sustained load effect 9.5.1 The sustained load effect value should be used for the determination of time-dependent material properties as described in Sec.4 [3] (for laminates) and Sec.5 [3] (for sandwich structures). Guidance note: In general, it would be very conservative to determine the time dependent degradation of material properties under long-term loads by using the characteristic load effect value (i.e. extreme load effect value). The sustained value is defined in this standard as a kind of average load effect value over the lifetime of the product. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.5.2 Sustained load values are defined over an observation period, which can correspond to the entire design life of the product or to a part of that design life. This observation period shall be divided into several time intervals. Time intervals should not be chosen shorter than 1 hour. The maximum length of a time interval depends on the load variations. Variations in magnitude of the load within a time interval shall not be larger than half the absolute load amplitude during the total observation period. 9.5.3 Load effects are divided, according to their variation with time, into: — permanent load effects; effects which are likely to act or be sustained throughout the design life and for which variations in magnitude with time are negligible relative to their mean values; or load effects which are monotonically in- or decreasing until they attain some limiting values — variable load effects; effects which are unlikely to act throughout the specified design life or whose variations in magnitude with time are random rather than monotonic and not negligible relative to their mean values. 9.5.4 The sustained value of permanent load effects shall correspond to their characteristic value, as defined in Sec.3 [9.4]. 9.5.5 The sustained value of variable load effects is defined as the mean value of the effects over the time interval. The sustained value S s during the time interval t o is determined such that the corresponding total duration above S s is a portion µ = 0,5 of the exposure period t s . See Figure 3-1: DET NORSKE VERITAS AS
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