Foundation Grouting Of Concrete Gravity Structures
Authors: Kennedy Jon G. , Sverdvik Truls , Domone Peter L. , Adams Mike A.J. 
Published: December 2007

North Sea platforms, whether of concrete construction, are typified by very high load capacity to accommodate both large deck loadings and storm forces. These high loads are transferred from structure to seabed through extensive foundations. Foundations for both structure types are illustrated for comparison and the requirements of concrete gravity-structure grouted foundations are presented in detail. In particular, grout materials development and operational aspects are discussed. Experience has been drawn from the installations of the first five CONDEEP structures installed in 1975, 1976, and 1977

Introduction

Structures installed in the North Sea are so large that it has become commonplace to compare them with familiar landmarks. This is sometimes forgotten since once emplaced on the seabed a structure, like the iceberg, presents only its deck, or tip, to view. Often not considered are the foundations, which have equally impressive dimensions. For example, pile lengths can be 200 meters. Gravity structures have foundation areas up to 15,000 m2 (4 acres).Grouting is used in the foundations of all types of structures to ensure efficient transfer of load from the structure to the seabed, and grout volumes of almost 14000 m3 (0.5 million cu. ft.) have been employed in the foundation grouting of a single platform. There are two types of grouting, depending on the structure type -steel jacket structures, which are generally pile to the seabed, utilise a grout which is normally an annular ring providing a shear-link between the jacket leg (or sleeve) and the pile (Figure 1.) High strengths are required and grouts of water/ cement ratios in the range 0.3–0.6 are used.concrete gravity platforms, where the space between the platform base and the seabed is filled with grout (Figure 2.) Low strength grouts with properties much nearer to the soil conditions are required. Innovative equipment, techniques and materials have been required for all grouted foundations in the North Sea. This paper will present in detail the operational and material development aspects of the grouting of concrete gravity platform installations.

GROUT FUNCTIONS

Within their base design most concrete structures incorporate skirts of steel and/or concrete which penetrate the seabed to provide lateral restraint penetrate the seabed to provide lateral restraint against current and storm loadings. These skirts form a perimeter to the underbase of the structure and in addition sub-divide the underbase into a number of compartments. Figure 2. illustrates a typical skirt system.

The penetration of the platform into the soil is controlled to ensure no excessive stress buildup within the base due to a sloping and/or uneven seabed or localised hard-spots in the soil. This limitation of penetration will result in pockets of water between penetration will result in pockets of water between the structure and the seabed. It is usually necessary to replace this water with grout for one or acre of the following principal reasonsto obtain continuity between the structure and the seabed soil over the total base area, thereby achieving uniform loading and avoiding build-up of excessive local contact pressures.to eliminate the risk of soil displacement by seawater pumping between the compartments as a result of dynamic loading during storm conditions. These effects are most likely to occur in the outer compartments.to improve the lateral stability of the platform.to reduce the overall settlement of the platform and to minimise uneven settlement which may otherwise result from variations in the seabed formations.

CRITERIA FOR SELECTION OF GROUT

In the theoretical case of total uniform penetration into a soft homogeneous seabed, no grouting penetration into a soft homogeneous seabed, no grouting would be required. Therefore, in practice, where grouting is required for the above reasons, the grout when placed need only have properties equivalent to the seabed soil.

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