OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.3 Design input – Page 28 8 Exposure from the surroundings 8.1 General 8.1.1 Surroundings shall be understood in this standard in a general sense. It designates the natural, functional and human phenomena to which the product is exposed to during its lifetime. 8.1.2 It shall be determined to which surroundings the product or parts of the product are exposed to in each phase. Guidance note: Surroundings can be divided into: - Natural surroundings, which covers natural phenomena such as wind, wave and currents - Functional surroundings, which cover phenomena due to the functional surrounding of the structure such as chemicals, fire, temperature or weight of content - Human surroundings, which cover events due to human activity such as dropped objects or weld spatter. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 8.1.3 This document does not specify the surroundings, since they are dependent on the applications. However, a list is provided in App.A to ensure that at least the most frequently encountered surroundings are considered in the design. 8.2 Loads and environment 8.2.1 A distinction is made in this standard between: — loads — environment. 8.2.2 The term loads designates in this standard the direct and indirect loads on the product, e.g. wave load on a structure, or thermal expansion loads. Both direct and indirect loads impose load effects, like stresses or strains on the product. Loads can be different in nature: functional loads, environmental loads or accidental loads. The loads on the product shall be specified according to Sec.3 [9]. 8.2.3 Calculations of the load effects on the product to the various environmental phenomena, i.e. environmental loads, are made by a transfer function. Specific transfer functions are not described in this standard (e.g. calculation of the load effect on a structure due to wind, with a specified wind speed). The load effects should be determined according to relevant standards or guidelines. As guidance for calculation of characteristic environmental loads on a structure the principles given in DNV-RP-C205 “Environmental conditions and environmental loads” may be used. 8.2.4 The term environment designates in this standard the surroundings that impose no direct load on the product, e.g. ambient temperature or moisture. The environment shall be specified according to the requirements of Sec.3 [10]. 8.2.5 The environment is generally considered for its effect on the degradation of material strength (see Sec.4 [5] and Sec.5 [5]). 8.2.6 The environment may also impose indirect loads on the structure, e.g. thermal stresses or swelling due to moisture uptake. This effect should be considered as a load, according to Sec.3 [9] and Sec.9 [8] and Sec.9 [9] (structural analysis). 8.2.7 Material properties may be influenced in the long-term not only by the environment but also by the loads, or by the combination of environment with loads, e.g. creep and stress rupture. The combination of loads and environment to be considered when assessing the degradation of material properties is detailed in Sec.3 [11]. 8.3 Obtaining loads from the exposure from the surroundings 8.3.1 The surrounding environment can often not be described as a direct load acting on a structure. In such a case a transfer function shall be established that transforms the surrounding environment into a load. Any uncertainties in the transfer function shall be included in the load model factor described in Sec.8 and Sec.9. 8.3.2 It is recommended to use a conservative transfer function. In that case it is not necessary to consider the model uncertainties of the transfer function in the load model factor. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.3 Design input – Page 29 Guidance note: The wind load is a typical example where the speed of the wind is transformed to a load on a structure. The load depends not only on the speed but also on the exposed surface and the aerodynamic profile of the structure. The transfer function is the mathematical model that transforms wind speed to a load on the structure. 9 Loads 9.1 General ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.1.1 This standard does not specify specific load conditions and characteristic load effects, since these are dependent on the applications. Guidance note: A non-exhaustive list of the most common loads to be considered in design is given as guidance in App.A of this section. This list is organised according to a classical classification into functional, environmental and accidental loads. This classical classification is only used in this standard as a checklist. The load factors are dependent on the probabilistic representation of the loads. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.1.2 A load is defined as an assembly of one or more concentrated or distributed forces acting on the structure (direct loads), or the cause of imposed or constrained deformations in the structure (indirect loads). Guidance note: The environment may impose indirect loads on the structure, e.g. thermal stresses or swelling due to moisture uptake. This should be considered as a load effect, and calculated according to the relevant parts of Sec.9. However, the environment is generally considered for its effect on the degradation of material strength. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.1.3 All the load cases shall be described separately for each phase during the design life of the structure. 9.1.4 All loads have to be represented as appropriate with due consideration of: — type of load and load effect: global, regional or local load, or response — direction of load — variation with time. 9.1.5 A representative time history of all loads should be documented for the entire life of the structure. This includes a probabilistic representation as specified in Sec.3 [9.2]. 9.1.6 Different load values are defined in this standard: — the characteristic value (Sec.3 [9.4]) is defined based on the probabilistic representation of the load — the sustained value (Sec.3 [9.5]) is defined based on the time representation of the load — the fatigue value (Sec.3 [9.6]) is defined based on the time representation of the load. Guidance note: The definition of the different load values is summarised in the Table 3-1. The detailed definition presented in the relevant chapters shall be used. Table 3-1 Definition of load values Designation Definition To be used for Characteristic value Extreme value with return period of 100 years Check of Ultimate Limit States Sustained value Average value over a long period Long-term degradation of material properties Fatigue value Mean and amplitude of variations Check of Fatigue Limit States Accidental value Same as characteristic value ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 9.1.7 The notion of accidental value is not used in this standard. It shall be decided whether the product should be designed for a given accidental event (e.g. fire, impact) or whether it should not be designed for it and instead protected against it by other means (e.g. impact protection structure around the product). 9.1.8 Different types of loads and environments shall be combined. Depending on which load and environment values are combined, different load and environmental conditions are defined. These different load and DET NORSKE VERITAS AS
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OS-C501

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