ABSTRACT

If large enough, a gravity structure in reinforced concrete will have no problems in overall safety.

Reduction of the dimensions, however, may change this picture. This became evident to Ingenior F. SELMER A/S when modifications of the company's large offshore platform 'design - of 1970 - to more moderate units began. An extensive test program was therefore initiated at the River and Harbour Laboratory (VHL) at the Norwegian Institute of Technology, Trondheim.

The research work was mainly concentrated on wave forces, but other hydraulic aspects relate to bottom mounted concrete structures - such as the effect of shock waves, wave damping devices and scour -are also discussed.

1. INTRODUCTION

Based upon long experience and great confidence in reinforced concrete, a very large platform project for the North Sea was forwarded to the Norwegian authorities in August 1970 by Ingenior F. SELMER A/S. Reference is made to [1] for the background of the design and a ore detailed description than is given here:

Figure 1 shows the basic principles - these include a monolithic raft foundation of circular cells - diameter 20 m - with a system of cutting edges protruding 4 to 5 meters below the bottom slab. From the caisson top - 15 meters below sea level - columns carry the platform and serve as wave dampers as well. The platform would have sides of 300 meters length, giving an area of more than 10 acres.

The intention was to collect many activities on one large, safe base to replace structures scattered over a wider area, thus being more hazardous to navigation as well as more vulnerable individually to the impact.

On a gravity structure sufficiently large in relation to the wave length, the wave loads will the of relatively little significance; but as dimensions decrease, wave loads grow more and more important.

Sea bottoms to which gravity structures are applicable are often also vulnerable to scour. At large structures local scour can usually be counter acted in time, at smaller ones the situation may be more serious.

Problems and risks usually increase as the dimensions of gravity structures decrease. For ordinary steel piling structures the opposite is the case.

Consequently the competition between steel and concrete structures may not lead to the optimum field design if the general lay-out has been- based too rigorously on one type or the other.

The advantage of combining two or more ordinary platforms on a single, large concrete raft should especially be Kept in mind since it opens the possibility of a very inexpensive extention of platform area.

2. WAVE FORCES ON LARGE VOLUME STRUCTURES

The calculation. of wave forces. on large volume structures, e.g. structures where drag effects are of no importance has followed two approaches. The first approach is to use a quasistatic approach introducing force coefficients This approach requires measurement of forces on a model or a prototype-structure to determine these coefficients [1], [2].

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