Concrete Offshore structures-structural Aspects
Authors: Gausel E. , Fluge F. , Hafskjold P.S. 
Published: October 2008

The utilization of concrete for sea structures is based on two-dimensional structural elements - plates and shells forming structures, which are statically indeterminate to a very high degree.

Correspondingly the structural analysis is complex and tedious, and the question has been raised:

How reliable are the results of these calculations, and the design which are based tests on minute specimens?

Only the structure can give the answer.

On the large units now being built in Norway, steps have been taken to approach this problem by direct measurements, and it is the sincere hope of the authors of this paper that corresponding information will be made available too from similar structures in other parts of the world.

INTRODUCTION

It is a fact that the dimension of the structural shell elements in the concrete platforms designed for the Nott Sea have a magnitude beyond our empirical knowledge. This is not specially related to the use of such elements in marine environments, which, on the other hand, evidently involves particular problems as to the durability of concrete and reinforcement.

By designing a prototype load assumptions caused by waves and wind are based on environmental data (E-data) gathered from the operation area.

Together with geotechnical data, structural analysis and design criteria it forms the basis for the detailed design of the structure. After the construction period and the platform having been installed to its final position it is exposed to the loads of the operating conditions. A simultaneous registration of performance data (P-data) and E-data now makes it possible to verify the load and design assumptions (see Fig. 1.)

SCOPE OF THE PRESENT STUDY.

At the Cement and Concrete Research Institute at the Norwegian Institute of Technology a full scale study of the strain development in reinforced concrete shells exposed to hydrostatic loads has been initiated. The gathering of response-data from full scale structures is important in the further development of offshore concrete structures. The present research constitutes a part of the evaluation of concrete design criteria in force. This report is based on measurements obtained from the first Condeep oil production platform.

The method used for constructing these platforms makes it possible to study the response of clearly defined loads acting on the concrete structure before towing to its final site. During the installation of the steel-made top structure, the maximum 'hydrostatic pressure will act on the floating structure, Fig. 2.

If we look at a desired verification from a concrete-geological/design point of view, the following summary may be made:Before sinking to final position.Comparison between computed and measured strains in concrete and reinforcement.Comparison between the elastic response of the structure and the modulus of elasticity of concrete determined from standard tests.The influence of shrinkage, creep and temperature gradients during the construction period.After sinking to final position.Comparison between computed and measured strains.The response of the structure to external loads.Redistribution of internal stresses.