Several new platform types constructed and conceived recently are founded on three or more separate footings, e.g. as developed by Techno mare, (under construction for Maureen field) Chicago Bridge & Iron, MAN, Norwegian Contractors, DORIS etc. Also certain types of jack-ups designed to achieve a stiff foundation are of this type.
The system of footings and their structural interconnection constitutes a redundant system in which the forces will be distributed relative to their mutual stiffness. The soil stiffness is strongly dependent on several parameters such as location, duration of loading, strain, previous load history etc. This means that a corresponding redistribution of loads in the structure will occur. The problem is further complicated by the fact that the initial force distribution after installation will depend upon the local variation of soil stiffness and sea floor inclination. The magnitude of all the mentioned effects is difficult to quantify, and the final design should be based on an adequately wide range of variation of the relevant parameters. This range I will be different for the analysis of normal environmental conditions governing the fatigue strength compared to the extreme environmental conditions which are to be checked for the ultimate bearing capacity. For the latter check, a formal overload of an individual footing might be accepted, provided other footings have a sufficient extra capacity to support this footing. However, if this support is taken into account, the structure must be designed to transfer these extra forces. The practical implementation of these principles is elaborated in the paper.
The soil-structure interaction of current piled offshore structures is covered by recognized procedures. Even if these procedures and associated acceptance criteria might disregard significant strength reserves, their adequacy in practice is not questioned.
Multi-footing gravity platforms in which the individual footings are interconnected by 'steel or concrete frames or trusses constitute a static indeterminate system. The force distribution within such a system is governed by the stiffness ratio structure/soil. This ratio will often take values which do not permit simplifications such as assuming the ratio to zero or infinity. Uneven soil condition, consolidation and cyclic degradation of the soil have the consequence that the soil stiffness will vary in space and time.
Different assumptions on stiffness ratio structure/ soil may influence the force distribution in the structure and the soil reactions as exemplified in Figs. 1 and 2, respectively. Furthermore, significant restraining forces may be transferred from the footings to the superstructure due to uneven sea floor, soil stiffness and subsoil stress distribution. It is normally not obvious for all structural elements which soil stiffness condition will be governing for their design. Therefore an adequate range of soil stiffness?s will have to be considered in the structural design.
Under certain conditions restraining forces may be deleted in the evaluation of possible overload and fatigue. To avoid over design these possibilities will have to be thoroughly investigated.
