OS-C501

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.
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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.
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