OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.9 Structural analysis – Page 140 1 General 1.1 Objective SECTION 9 STRUCTURAL ANALYSIS 1.1.1 The aim of the structural analysis is to obtain the stresses, strains and displacements (denoted load effects in the following) in the structure as a result of loads and environmental conditions. The load effects are subsequently evaluated against failure criteria, see Sec.6. The following procedures are typically involved in such an analysis: — procedure to calculate load effects in the structure based on the loads — procedure to check for global or local failure. 1.1.2 The objective of the present section is to provide methods to calculate the response, including evaluation of failure, of structures for specified loads, surrounding environments and boundary conditions. 1.2 Input data 1.2.1 The input data for the structural analysis should be established as described in the relevant parts of Sec.3. 1.2.2 Environmental conditions should be converted into loads based on well established physical principles. Guidance may be found in Sec.3 and in relevant standards or guidelines. 1.2.3 The boundary conditions should be selected carefully in order to represent the nature of the problem in the best possible way. It should be demonstrated that the chosen boundary conditions lead to a realistic or conservative analysis of the structure. 1.2.4 Thermal stresses that result from production process or in service loading should be considered in all analysis. 1.2.5 Stresses due to swelling from absorbed fluids should be included if relevant. 1.2.6 The elastic properties of the materials constituting the structure should be taken as described in Sec.4 for laminates and Sec.5 for sandwich structures. In particular, time-dependent stiffness properties based on the expected degradation due to environmental and loading conditions should be considered. Local variations of these conditions should also be considered. 1.2.7 Each ply should be described by 4 elastic constants (E 1 , E 2 , G 12 , ν 12 ) for in-plane 2-D analysis and by 9 elastic constants (E 1 , E 2 , G 12 , ν 12 , E 3 , G 13, G 23 , ν 13 , ν 23 ) in 3-D analysis. A nomenclature for the various elastic constants is defined in section 14. 1.2.8 As an alternative to elastic constants, the stiffness matrix for orthotropic plies may be used. 1.2.9 It should be shown that the estimated stiffness gives conservative results with respect to load effects. The choice of stiffness values may be different in the cases of strength and stiffness limited design. More details are given in the sections below. 1.3 Analysis types 1.3.1 Analytical and/or numerical calculations may be used in the structural analysis. The finite element (FE) method is presently the most commonly used numerical method for structural analysis, but other methods, such as finite difference or finite series methods may also be applied. Guidance note: While the FE method is applicable for a wide range of problems, analytical solutions and the finite series approach often put too many restrictions on laminate lay-up, geometry etc., and are thus insufficient in the design of most real world composite structures. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.3.2 Laminate analysis is an additional type of analysis that is applied to layered composites in order to derive the properties of a laminate from the properties of its constituent plies. 1.3.3 The structural analysis should be performed for all phases over the entire lifetime of the structure. Initial and degraded material properties should be considered if relevant. 1.3.4 A decision to use 2-D or 3-D analysis methods should generally be made depending on the level of significance of the through thickness stresses. If these stresses can be neglected, in-plane 2-D analysis may be applied. Additionally, the analysis of certain laminate and sandwich structures may be simplified by a through thickness (cross section) 2-D approach, in which plane strain condition is assumed to prevail. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.9 Structural analysis – Page 141 Guidance note: In-plane 2-D analysis is generally preferred when analysing relatively large and complex structures, in which through thickness stresses can be neglected. However, structural details with significant through thickness stresses, such as joints, require a more accurate analysis. In these cases 3-D or through thickness 2-D (for components possessing plane strain conditions) approaches should be applied. 1.4 Transfer function ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.4.1 The loads and environments described as input to the analysis in Sec.3, i.e. wind, wave and currents, are not always directly suitable in a stress analysis. A transfer function shall be defined that converts the specified loads to loads that can be used in the analysis. 1.4.2 Any non-linear effect in the transfer function may change the characteristics of the load distribution. These changes shall be taken into account when selecting the load-model factor under [12]. 1.5 Global and local analysis 1.5.1 The global response of the structure is defined as the response (displacement and stability) of the structure as a whole. 1.5.2 The local response of the structure is defined as the stresses and strains (and deformations) in every local part of the structure. 1.5.3 The response of the structure shall be calculated on a global or local level depending on the failure mechanism being checked and its associated failure criterion. Guidance note: The failure of the structure shall generally be checked on the basis of the local response of the structure by the use of failure criteria for each failure mechanism as described in Sec.6. Buckling is generally checked on larger parts of the structure and based on average stresses over large areas. Under such conditions a coarser analysis may be sufficient. However, if the FE method is used to calculate buckling stresses, a very local analysis of the structure may be needed. 1.6 Material levels ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.6.1 The local response of the structure can in principle be analysed at the following different material levels: — the “constituent level” corresponding to the fibre, matrix and core, separately — the “ply level” corresponding to the individual layers in a laminate or the faces of a sandwich structure — the “laminate level” corresponding to the whole laminate or sandwich structure. 1.6.2 Each failure mechanism can in principle be checked at any material level. However, due to the lack of theoretical knowledge or for practical reasons, it is not always possible to check a given failure mechanism at all material levels. 1.6.3 The local response of the structure shall be analysed at a material level consistent with the failure criteria used in the failure analysis as described in Sec.6. 1.6.4 This standard does not cover stress analysis on the level of the individual fibre or the matrix between the fibres, i.e. on the constituent level (except for sandwich core materials). 1.6.5 All failure criteria in this standard (except for buckling) require the stresses or strains to be accurately represented on the level of each ply. 1.7 Non-linear analysis 1.7.1 Non-linear analysis should be performed when geometrical and/or material non-linearity are present and when linear and non-linear analysis results are expected to differ. 1.7.2 Geometrical non-linearity are associated with, e.g., large displacements and/or large strains, boundary conditions varying according to deformations, non-symmetric geometry of structure and buckling. 1.7.3 Non-linear material behaviour is associated with the stress–strain relation. Following damage in the material, i.e. matrix cracking or yield, stress–strain relationships usually become non-linear. 1.7.4 Structures with non-linear materials should be checked either against early failure mechanisms, e.g. matrix cracking or yield, or against ultimate failure, or both. 1.7.5 A decision to use a progressive, non-linear failure analysis or a simplified (linear) failure analysis should be based on the failure modes of the structure/component and the failure mechanisms investigated, see [2] and [3]. DET NORSKE VERITAS AS
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