Required verifications

3.3.2.5.1 Verification of the stability of the seabed

It shall be demonstrated that the subsoil system on which the verifications for the foundation and load calculations for the support structure are based is stable.

The following items are to be considered in particular:

• Potential reduction in stability of the seabed due to external influences, such as current and wave action, influenced by the substructure, effect of earthquake, offshore operations such as pile driving or dredging etc.

• Potential change in the geometry of the seabed due to influences such as erosion (scouring) and suffosion

• Potential change in the arrangement of foundation elements due to influences such as cyc-lical creep of the subsoil, accumulated deformation (hysteresis) and

• Potential change in the mechanical properties of the subsoil due to influences such as pore water pressure accumulation (liquefaction), consolidation, deconsolidation.

It shall be demonstrated that

• either these mechanisms do not occur

• or that these mechanisms have been adequately taken into account in the design

• or that these mechanisms will be adequately observed during installation and operation and that suitable measures will be taken to compensate for them if necessary.

3.3.2.5.2 Verifications for foundation elements

As a minimum requirement, the following shall be investigated, documented and given due consideration in all respective verifications:

• The process engineering of the installation and effects of the installation process

• The description and modelling of the significant interactions between the subsoil and the structure as the basis and input parameters

– for the verifications pertaining to the external load capacity of the foundation and its ele-ments

– for the verifications pertaining to the internal load capacity of the foundation elements and

– for the natural frequency analysis of the offshore structure

• The influence of stability properties of the seabed on the stability against collapse and on the serviceability of the foundation, the entire structure and its components in accordance with B3.3.2.5.1

• The effects of cyclic and dynamic loads (degradation and liquefaction)

• The constraint stress on structural elements and add-on components due to permanent subsoil deformation

• The safety against shifting of ballast material.

For all verifications pertaining to foundation elements, the lower and upper limits for the rele-vant subsoil parameters shall be varied in an unfavourable combination. Where the ductility of the subsoil and the structure is adequate, verifications based on characteristic parameters are sufficient; the respective ductility shall be demonstrated. As a minimum requirement, the lower and upper limits for the stability of the foundation elements shall be determined and varied in an unfavourable combination for the purpose of analysing the natural frequency of the overall systems.

Special verifications which are required for pile foundations:

• The axial pile load capacity and safety

• The lateral pile load capacity and safety

• Axial deformation and shifting estimates (settling, inclination and rotation)

• Lateral deformation and shifting estimates and

• Pile group effects and interactions between individual foundation elements (e. g. in tripile, tripod and jacket structures).

The axial pile load capacity shall be demonstrated through conducting a sufficient number of dynamic pile load tests. When these tests are to be carried out depends on the forecasting method used. For further information, please refer to Appendix 3-III.

Special verifications which are required for heavyweight foundations:

• External stability against collapse (safety against tilting, sliding and bearing capacity fai-lure), if necessary under consideration of a potential pore water excess pressure build-up

• Subsoil loads through normal subsoil stress and limitation of the open gap and

• Settling estimates under full load conditions and accumulated settling when subjected to cyclic loads.

For special types of foundation (e. g. suction anchors, suction caissons), verifications shall be obtained in a similar way to that specified for pile and heavyweight foundations, in consulta-tion with the BSH. In this case, the BSH may engage the services of suitable assessors.

Suitable structural design criteria shall be defined for the deformation and shifting verifications based on the potential static and dynamic effects on the components and on the structure, e. g. max. permitted deflection and torsion at the pile head at the level of the seabed and at the pile foot, max. exploitation of the lateral bedding reaction of the subsoil and minimum or maximum values for subsoil stiffness.

An expert assessment shall be conducted with regard to the effects of the following parame-ters and how they can be practically considered in the verifications:

• Installation of the foundation elements on the subsoil and foundation system, e. g. piling;

driving aids

• Positional shifts of the foundation elements

• The exceeding of manufacturing tolerances

• Deviations in piling depths.

3.3.2.5.3 Verifications pertaining to the ability of the foundation elements to interact with the support structure

For the support structure, in addition to the inclination forecast required in the serviceability verification, it shall be demonstrated that constraint stress exerted on the support structure as a result of potential differences in the shift of the individual foundation elements can be relia-bly and sufficiently absorbed. Any resulting changes in the loads exerted on the foundation elements by the support structure shall be taken into account in the verifications for the foun-dation elements.

3.3.2.5.4 Geotechnical component tests

In the case of deficient subsoil conditions, or where the respective foundation and design methods have not yet been sufficiently tested, the BSH reserves the right to demand that comparable (with regard to the subsoil, pile geometry and installation procedure) foundation elements be manufactured in advance so that they may be tested for their suitability with re-spect to their intended purpose.

These examinations shall be conducted well in advance so that the results can be used in the design phase. Accordingly, the verified results are generally required to be submitted for the 2^nd release.

3.3.2.5.5 Additional geotechnical information and verifications

All presumptions made with respect to the devised foundation elements and the precondi-tions for realising the construction project shall be accordingly specified.

All geotechnical verifications which will have to be presented for the devised foundation ele-ments during the installation and operating phases (please refer to chapters B3.4.4 and B3.5.4) shall be accordingly specified.

3.3.3 Geotechnical documents to be submitted