OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.9 Structural analysis – Page 150 Guidance note: Some of these difficulties can be detected by error tests in the coding, such as a test for the condition number of the structure stiffness matrix or a test for diagonal decay during equation solving. Such tests are usually made posterior rather than prior. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 5.2.11 Need for mesh refinement is usually indicated by visual inspection of stress discontinuities in the stress bands. Analogous numerical indices are also coded. 5.2.12 For local analysis, a local mesh refinement shall be used. In such an analysis, the original mesh is stiffer than the refined mesh. When the portion of the mesh that contains the refined mesh is analysed separately, a correction shall be made so the boundary displacements to be imposed on the local mesh are consistent with the mesh refinement. 5.2.13 For non-linear problems, the following special considerations shall be taken into account: — the analyst shall make several trial runs in order to discover and remove any mistake — solution strategy shall be guided by what is learned from the previous attempts — the analyst shall start with a simple model, possibly the linear form of the problem, and then add the nonlinearity one by one. 5.2.14 Computed results shall be checked for self-consistency and compared with, for example, approximate analytical results, experimental data, text-book and handbook cases, preceding numerical analysis of similar problems and results predicted for the same problem by another program. If disagreements appear, then the reason for the discrepancy shall be sought, and the amount of disagreement adequately clarified. 5.2.15 The analyst shall beware the following aspects: — for vibrations, buckling or non-linear analysis, symmetric geometry and loads shall be used with care since in such problems symmetric response is not guaranteed. Unless symmetry is known to prevail, it shall not be imposed by choice of boundary conditions — for crack analysis, a quarter point element can be too large or too small, thereby possibly making results from mesh refinement worse — the wrong choice of elements may display a dependence on Poison’s ratio in problems that shall be independent of Poisson’s ratio — if plane elements are warped, so that the nodes of the elements are not co-planar, results may be erratic and very sensitive to changes in mesh — imperfections of load, geometry, supports and mesh may be far more important in a buckling problem than in problems involving only linear response. 5.2.16 In the context of finite element analysis (FEA) of laminate structures (one of) the following element types should be applied: — layered shell elements with orthotropic material properties for each layer (for in-plane 2-D analysis, see [1.3.4]) — solid elements with orthotropic material properties (for 3-D and through thickness 2-D analysis, see [1.3.4]). Guidance note: There are two options for the solid elements: The modelling may be performed with (at least) two solid elements through the thickness of each ply. Alternatively, one may apply layered solid elements where the thickness of a single element includes two or more plies. 5.3 Software requirements ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 5.3.1 Selection of finite element software package shall be based on the followings: — software availability — availability of qualified personnel having experience with the software and type of analysis to be carried out — necessary model size — analysis options required — validated software for intended analysis. 5.3.2 Useful options for the analysis of composite structures include: — layered solid elements with orthotropic and anisotropic material behaviour — layered shell elements — solid elements with suitable core shear deformation models (for analysis of sandwich structures) DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.9 Structural analysis – Page 151 — solid elements with correct material models or appropriate interface elements allowing for debond (for analysis of bonded and laminated joints) — interface elements allowing for large aspect ratio (for analysis of thin layer bonds) — the possibility to select different co-ordinate systems in a clear and unambiguous way. 5.3.3 Depending on the area of application, additional analysis options should be available e.g.: — appropriate solver with stable and reliable analysis procedures — options characterising large displacements and large strains (for geometrically non-linear analysis) — material models describing the behaviour of, e.g., laminates beyond first failure as well as ductile sandwich cores (for materially non-linear analysis) — robust incremental procedures (for non-linear analysis in general) — tools for frequency domain analysis and/or options such as time integration procedures (for dynamic analyses) — appropriate post-processing functionality — database options — sub-structuring or sub-modelling. 5.4 Execution of analysis 5.4.1 FEA tasks shall be carried out by qualified engineers under the supervision of an experienced senior engineer. 5.4.2 Analysis shall be performed according to a plan, which has been defined prior to the analysis. 5.4.3 Extreme care shall be taken when working with different relevant co-ordinate systems, i.e. global, ply based, laminate based, element based and stiffener based systems. 5.4.4 The approach shall be documented. 5.5 Evaluation of results 5.5.1 Analysis results shall be presented in a clear and concise way using appropriate post-processing options. The use of graphics is highly recommended, i.e. contour plots, (amplified) displacement plots, time histories, stress and strain distributions etc. 5.5.2 The results shall be documented in a way to help the designer in assessing the adequacy of the structure, identifying weaknesses and ways of correcting them and, where desired, optimising the structure. 5.6 Validation and verification 5.6.1 FE programs shall be validated against analytical solutions, test results, or shall be benchmarked against a number of finite element programs. 5.6.2 Analysis designer shall check whether the envisaged combination of options has been validated by suppliers. If this is not the case, he shall perform the necessary validation analysis himself. 5.6.3 FEA results shall be verified by comparing against relevant analytical results, experimental data and/or results from previous similar analysis. 5.6.4 Analysis and model assumptions shall be verified. 5.6.5 Results shall be checked against the objectives of the analysis. 5.6.6 Verification whether the many different relevant co-ordinate systems have been applied correctly shall be considered. 6 Dynamic response analysis 6.1 General 6.1.1 Dynamic analysis should generally be performed when loads are time-dependent and/or when other effects such as inertia (and added mass) and damping forces are significant. 6.1.2 In a dynamic analysis one may be interested in the transient response of a structure due to prescribed, time-dependent loads or the 'eigenvalues' (natural or resonance frequencies) of the structure. 6.1.3 In order to obtain an accurate transient analysis a detailed structural model and small time steps should be used, in particular for rapid varying loads. 6.1.4 For slowly varying loads a quasi-static analysis may be applied. In such an analysis inertia and damping forces are neglected, and the corresponding static problem is solved for a series of time steps. DET NORSKE VERITAS AS
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