On the basis of the experiences gained through structural analysis and survey of design, construction and in-service performance of all together 16 offshore concrete platforms certain design problems typical for these structures have been identified. The paper gives practical recommendations for global structural analysis and interpretation of their results, design against temperature problems, structural instability and fatigue as well as design recommendations based on observed performance of the structure.
During the last years a number of huge concrete drilling and production gravity platforms are placed in the North Sea. As for all kinds of offshore structures the requirements regarding safety and serviceability are severe. Thus, the highest quality of structural, analysis and design calculations for verification and approbation tasks is of paramount importance. Complex geometries, extreme and numerous loads on the structure, and irregular sea-bed-conditions make complete structural strength analysis prohibitive without applying numerical techniques as the finite element or the finite difference methods in connection with large computers. Stress fields, caused by local effects as well as global effects of the structure should be taken into account yielding detailed information in all essential parts of the structure. Thus, the engineer may be able to design and utilize the materials in an effective and economic way taking into account the special properties of reinforce concrete,
With experience from analysis of all together 16 concrete platforms for the North Sea and Brazilian waters, considerable knowledge is gathered regarding requirements which should be made on computer programs to be applied for structural analysis and on the design approach. Valuable feedback is gained through Det norske Veritas's in-service inspection of the structures after installation in field. These experiences have been crystallized in Det norske Veritas's rules (1) with appendices. In the following, essential parts of this experience will be discussed.
Typical for most kinds of offshore concrete gravity platforms is a high degree of repetition of some fundamental structural parts. E.q. for the CONDEEP and SEA TANK platforms the cell represents the basic member of the structure. (See fig. 1). The computer programs to be employed for the global analyses should definitely contain the ability of taking advantage of this repetitiveness, being justified for many reasons, e.g.:
Making a numerical model of a repetetive part only once, certainly involves a considerable saving of computer time. For instance by applying the supere1ement technique (sub-structuring) in the analysis of the CONDEEP platforms, a total of 300.000 degrees of freedom for the complete structure was reduced to approximately 20.000 by repeating one cell 19 times to establish the total model. Accordingly, the computer time and thus the computer costs were reduced from an astronomical amount to a level being acceptable regarding computer capacity as well as computer Expenses.
