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App.B: Structural Analyses - Modelling (Guidelines) [Table of Contents] App.C: Structural Analyses (Guidelines)

DNV-OS-C502 Offshore Concrete Structures

[-] App.B: Structural Analyses - Modelling (Guidelines)
[-] A: General

App.B
A. General

App.B
A 100   Physical representation

App.B A
101   Dimensions used in structural analysis calculations shall represent the structure as accurately as necessary to produce reliable estimates of load effects. Changes in significant dimensions as a result of design changes shall be monitored both during and after the completion of an analysis. Where this impacts on the accuracy of the analysis, the changes shall be incorporated by reanalysis of the structure under investigation.

App.B A
102   It is acceptable to consider nominal sizes and dimensions of the concrete cross-section in structural analysis, provided that tolerances are within the limits set out for the construction and appropriate material partial safety factors are used.

App.B A
103   Where as-built dimensions differ from nominal sizes by more than the permissible tolerances, the effect of this dimensional mismatch shall be incorporated into the analysis. The effect of tolerances shall also be incorporated into the analysis where load effects and hence the structural design are particularly susceptible to their magnitude (imperfection bending in walls, implosion of shafts, etc.).

App.B A
104   Concrete cover to nominal reinforcement and positioning of prestressing cables may be provided where these are defined explicitly in detailed local analysis. Again, this is subject to construction tolerances being within the specified limits and appropriate material partial safety factors being applied to component material properties.

App.B A
105   The effects of wear and corrosion shall be accounted for in the analysis where significant and where adequate measures are not provided to limit such effects.

App.B A
106   It will normally be sufficient to consider centre-line dimensions as the support spacing for beams, panels, etc. Under certain circumstances, however, face-to-face dimensions may be permitted with suitable justification. The effect of eccentricities at connections shall be considered when evaluating local bending moments and stability of the supporting structure.

App.B A
107   Material properties used in the analyses of a new design shall reflect the materials specified for construction. For existing structures, material properties may be based on statistical observations of material strength taken during construction or derived from core samples extracted from the concrete.

App.B A
108   It is normally acceptable to simulate the concrete by equivalent linear elastic properties in most limit states. Unless a different value can be justified, the secant modulus of plain concrete at zero strain may be used as the modulus of reinforced concrete in such an analysis. The value used shall be in accordance with the concrete design rules in use. For loads that result in very high strain rates, the increase in concrete modulus of elasticity should be considered in the analyses of the corresponding load effects.

App.B A
109   Age effects on the concrete may be included, if sufficiently documented by applicable tests. Effects of load duration and resultant creep of the concrete shall also be considered, where significant. Where loads may occur over a significant period in the life of the structure, the least favourable instance shall be considered in determining age effects.

App.B A
110   Accurate evaluation of concrete stiffness is particularly important for natural frequency or dynamic analysis, and for simulations that incorporate significant steel components, such as the topsides or conductor framing. Consideration shall be given to possible extreme values of concrete stiffness in such analyses. The aggregate type may influence the stiffness of the concrete and this effect shall be allocated for in the analyses.

App.B A
111   Non-linear analysis techniques are often applied to local components of the structure. It is typical to discretely model concrete, reinforcement and prestressing tendons in such simulations. Where this is the case, each material shall be represented by appropriate stress-strain behaviour, using recognized constitutive models.

App.B A
112   The density of reinforced concrete shall be calculated based on nominal sizes using the specified aggregate density, mix design and level of reinforcement, with due allowance for design growth. For existing structures, such densities shall be adjusted on the basis of detailed weight reports, if available. Variation in effective density through the structure shall be considered, if significant.

App.B A
113   Unless another value is shown to be more appropriate, a Poisson's ratio of v= 0.2 shall be assumed for uncracked concrete. For cracked concrete, a value of v = 0, may be used. A coefficient of thermal expansion of 1.0 x 10-5 /°C shall also be used for concrete and steel in lieu of other information. Where the design of the concrete structure is particularly sensitive to these parameters, they shall be specifically determined by the materials in use. Special considerations are required for concrete exposed to cryogenic temperature.

App.B A
114   The representation of a fixed structure foundation will differ depending on the type of analysis being undertaken. For static analysis, reactive pressures applied to soil contact surfaces shall be sufficient, but for dynamic analysis or where soil/structure interaction is significant, an elastic or inelastic representation of foundation will normally be required to provide suitable stiffness. Seismic analysis is typically very dependent on soil properties, particularly at the ductility level earthquake. Further details of foundation modelling requirements for specific analysis types may be found in Appendix D.

App.B A
115   Reactions on the structure from its foundation/anchorage shall be based on general principles of soil mechanics in accordance with DNV-OS-C101 Sec.11. Sufficient reactive loads shall be applied to resist each direction of motion of the structure (settlement, rocking, sliding, etc.). The development of hydraulic pressures in the soil that act in all directions should be considered where appropriate. Consideration shall be given to potential variation of support pressures across the base of a fixed concrete structure.

App.B A
116   The calculations used shall reflect the uncertainties inherent in foundation engineering. Upper and lower bounds and varied patterns of foundation reaction shall be incorporated and an appropriate range of reactive loads shall be assessed. In particular, the sensitivity of structural response to different assumptions concerning the distribution of reaction between the base and any skirts shall be determined.

App.B A
117   Consideration shall also be given to the unevenness of the seabed, which can potentially cause high local reactions. Foundation unevenness may be considered as a deformational load in subsequent design checks in accordance with Sec.5 B900. Other than this, foundation pressures shall be considered as reactive loads, their magnitude being sufficient to react all other factored loads.

App.B A
118   Upper limits of soil resistance should be considered during analysis of platform removal.

App.B A
119   The analyses shall include intermediate conditions, such as skirt penetration and initial contact as well as the fully grouted condition, if significant. Disturbance of the seabed due to the installation procedure should be considered in calculating subsequent foundation pressures.

App.B A
120   Where it significantly affects the design of components, soil interaction on conductors shall also be incorporated in the analysis, particularly with regard to local analysis of conductor support structures.

App.B A
121   Other than direct support from foundation soils, a component may be supported by:
—external water pressure, while floating
—other components of the structure
—anchore supports
—any combination of the above, and foundation soils.


App.B A
122   The load of water pressure in support of a fixed concrete structure while floating or a floating concrete structure shall be evaluated by suitable hydrostatic or hydrodynamic analysis and shall be applied to appropriate external surfaces of the structure.

App.B A
123   Representative boundary conditions shall be applied to the analysis of a component extracted from the global structure. These boundary conditions shall include possible settlement or movement of these supports, based on a previous analysis of the surrounding structure.

App.B A
124   In the absence of such data, suitable idealized restraints should be applied to the boundary of the component to represent the behaviour of surrounding structure. Where there is uncertainty about the effective stiffness at the boundaries of the component, a range of possible values shall be considered.

App.B A
125   Force, stiffness or displacement boundary conditions may be applied as supports to a component. Where there is uncertainty as to which will produce the most realistic stresses, a range of different boundary conditions shall be adopted and the worst load effects chosen for design.

App.B A
126   Where components of the structure are not fully restrained in all directions, such as conductors within guides and bearing surfaces for deck and bridge structures, allowance shall be made in the analysis for movement at such interfaces.

App.B
A 200   Loads

App.B A
201   Loads shall be determined by recognized methods, taking into account the variation of loads in time and space. Such loads shall be included in the structural analysis in a realistic manner representing the magnitude, direction and time variance of such loads.

App.B A
202   Permanent and live loads shall be based on the most likely anticipated values at the time of the analysis. Consideration shall be given to minimum anticipated values as well as maximum loading. The former governs some aspects of the design of gravity based structures.

App.B A
203   Hydrostatic pressures shall be based on the specified range of fluid surface elevations and densities. Hydrostatic pressures on floating structures during operation, transportation, installation and removal stages shall include the effects of pitch and roll of the structure due to intentional trim, wind heel, wave load or damage instability. The above also apply to fixed structures under transportation, installation and removal phases.

App.B A
204   Prestressing effects shall be applied to the model as external forces at anchorages and bends, or as internal strain compatible effects. In both cases, due allowance shall be made for all likely losses in prestressing force. Where approximated by external reactions, relaxation in tendon forces due to the effect of other loads on the state of strain in the concrete shall be considered.

App.B A
205   Thermal effects are normally simulated by temperatures applied to the surface and through the thickness of the structure. Sufficient temperature conditions shall be considered to produce maximum temperature differentials across individual sections and between adjacent components. The temperatures shall be determined with due regard to thermal boundary conditions and material conductivity. Thermal insulation effects due to insulating concrete or drill cuttings shall be considered, if present.

App.B A
206   Wave, current and wind loads shall include the influence of such loads on the motion of the structure while floating. In cases where dynamic response of the structure may be of importance, such response shall be considered in determining extreme load effects. Pseudo-static or dynamic analyses shall be used.

App.B A
207   Uncertainties in topsides centre of gravity, built-in forces and deformations from transfer of topsides from barges to the concrete structure shall be represented by a range of likely values, the structure being checked for the most critical extreme value.

App.B A
208   Structures designed to contain cryogenic gas (LNG) shall additionally be designed in accordance with the provisions made in DNV-OS-C503.

App.B
A 300   Mass simulation

App.B A
301   A suitable representation of the mass of the structure shall be prepared for the dynamic analysis, motion prediction and mass-acceleration loads while floating. The mass simulation shall include relevant quantities from at least the following list:
—all structural components, both steel and concrete, primary and secondary
—the mass of all intended equipment, consistent with the stage being considered
—the estimated mass of temporary items, such as storage, lay-down, etc
—masses of any fluids contained within the structure, including equipment and piping contents, oil storage, LNG storage, flooding, etc
—the mass of solid ballast within the structure
—snow and ice accumulation on the structure, if significant
—drill cuttings or other deposits on the structure
—the mass of marine growth and external water moving with the structure
—added water mass
—added soil mass.


App.B A
302   The magnitudes of masses within the structure shall be distributed as accurately as necessary to determine all significant modes of vibration (including torsional modes) (when required) or mass-acceleration effects for the structural analysis being performed. Particular attention shall be paid to the height of topsides equipment or modules above the structural steelwork.

App.B A
303   It is normally necessary to consider only the maximum mass associated with a given analysis condition for the structure. For dynamic analyses, however, this may not produce the worst response in particular with respect to torsional modes and a range of values of mass and centre of gravity may have to be considered. For fatigue analysis, the variation in load history shall be considered. If appropriate, an average value over the life of the structure may be used. In such cases, it is reasonable to consider a practical level of supply and operation of the platform.

NOTE Calculation of the added mass of external or entrained water moving with the structure shall be based on best available published information or suitable hydrodynamic analysis. In lieu of such analysis, this mass may be taken as the fullmass of displaced water by small-submerged members, reducing to 40% of the mass of displaced water by larger structural members. Added mass effects may be ignored along the axial length of prismatic members, such as the shafts.

App.B
A 400   Damping

App.B A
401   Damping arises from a number of sources including structural damping, material damping, radiation damping, hydrodynamic damping and frictional damping between moving parts. Its magnitude is dependent on the type of analysis being performed. In the absence of substantiating values obtained from existing platform measurements or other reliable sources, a value not greater than 3% of critical damping may be used.
App.B: Structural Analyses - Modelling (Guidelines) [Table of Contents] App.C: Structural Analyses (Guidelines)