Scope of verification activity

7.8.1 Planning of verification

The present section outlines the plans for some major activities within verification, the objective being to come up with major issues in the verification of documents from engineering. It is, however, to be emphasized that the activity schemes below do not represent the complete set of verification activities for a structure design project.

7.8.2Verification of global analysis model

The control of the global model for stress analysis has its main objective to verify that the finite element mesh represents the load carrying behaviour of the structure. The verification is also to identify areas where the model is not representative for the real structure behaviour and where extra effort on design needs to be taken.

The control of the model follows the level 2 scheme of verification. The document on global model by engineering is to have plots on section stress resultants and verification ought to check these with simplified techniques. Tests are to be made on the accuracy of the model in areas with high degrees of element distortions, ending up in a documentation of the feasibility of the analysis model to handle certain complicated areas.

As a supplement to the document control of the global analysis model, an independent analysis of stress results is to be performed by verification, applying an independent computer program. The modelling should include an evaluation of areas where separate analyses are needed (D-regions). design calculations are representative for the load effects and the response characteristics of the structure.

The engineering evaluation of design waves is checked, involving the hydrodynamic model, unit amplitude waves, stochastic response analysis with extreme value estimates, as well as choice of wave directions and range of wave periods.

For the hydrodynamic model, panel sizes are related to the shortest wave. The selection of wave periods is related to the Design Basis document on environmental data, and the natural periods of the structure system.

The models for stress analysis are considered with major effort placed on element types that are related to geometry, stiffness, mass and damping.

For the global rigid-body model on hydrodynamics, effort is to be placed on the control of wave directions and periods. The wavelengths should also be related to the geometry of the structure. Critical waves for essential load effects are sorted out by simplified frame models.

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For the stochastic analysis, the choice of wave input parameters is considered in parallel with the Design Basis document and the NPD regulations (NPD, 1997). Extreme value estimates from the stochastic analysis are compared with the hand-calculated responses, see Chapter 3. In general, the design wave period should be close to the predefined critical sea state peak period. Scaling of the amplitude of the design wave should also be given special attention, The document control of verification, as described above, is a level 2 control with simple supplementary calculations. Special attention is to be placed on the selection of design waves, as these are fictive regular waves, representing the critical sea states for different responses in the structure.

7.8.4 Verification of loads in global model

The purpose of the present activity on load modelling verification is to check the application of unit load cases, together with the scaling of responses into design combinations.

The basic load cases in the engineering analysis model are related to the different stages of construction, installation and operation. Wave loading, together with current, wave drift and wind are checked against the Design Basis document, supplemented by estimates on forces.

For the dynamic effects from waves, special care ought to be taken for the modelling of inertia forces due to rigid-body motion of the structure.

Load effects from pretensioning are to be checked against the actual routing of cables.

Evaluation is also to be made on possible modifications of the pretensioning system during fabrication in heavily loaded areas to cover possible redesign.

The independent global model by verification also includes the load modelling referred to above in Section 7.8.2.

7.8.5 Verification of design loads combinations

The activity on verification of design load combinations is essential in the way that it covers the latest stage of calculations before fabrication drawings. Time co-ordination is important for the present verification activity.

For different areas of the structure, design load combinations are taken out of the automatic process of capacity control. Plots of section forces are part of this information from engineering. Then there are also stress resultants in the form of shell forces as well as global stress resultants for columns and shafts. Often, verification is given direct access to the engineering files.

Based upon critical load combinations, estimates of stress resultants are made by simplified calculations. The structure system depends on the stage of fabrication, installation or operation.

For cases where first order waves come into a design load combination, relevance to Section 7.8.3 on design wave selection is appropriate. However, in combination with other load types, correlation between the different load effects is to be taken into account, and thus the critical single wave does not necessarily come out as the worst wave for the combined load effect.

The selection of design waves is based upon scaling of regular wave responses to fit the

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extremes of a stochastic analysis. The process of design wave generation is made for a certain number of structure areas. The control of design waves from engineering should also include a check of the correct design wave being applied for the structure part for which it is valid. Based on the independent analysis control of stress resultants, a separate verification is made of capacity and reinforcement amounts.

References

Norwegian Petroleum Directorate (NPD), (1997) Acts, Regulations and Provisions for the Petroleum Activity. Norwegian Petroleum Directorate, Stavanger, Norway.

Statoil (1991) Structural Design for Offshore Installations, Specification for Design. N-SD-001, Statoil, Stavanger, Norway.

Appendices

Appendices 193