Abstract

The popular and traditional system, which has been used as a foundation in many offshore structures, is the system of piles. In some conditions this system has not sufficient efficiency. Some conditions such as existence of the thick liquid layers mud mat, the depth of water and also required free board in some offshore projects cause some problems. In these conditions there isn't any sufficient accuracy in pile driving and because of long length of piles in lateral buckling it is necessary to use stronger piles to avoid buckling during operation and it leads to use the uneconomical profiles as pile. This paper describes a new combinational foundation system, using caisson and pile, and explains the construction and installation stages. In addition, the paper explains the advantages of this method in compare with the traditional method of pile foundation system.

Introduction

The pile foundation system is one of the popular methods, which is used as the foundation in offshore structures. In this method piles are designed for bearing vertical and also lateral loads. The design loads for pile are not only the loads that exert during the life of main structure but also the loads, which apply during pile driving. The quantities of these loads aredependent on the type and the thickness of soil layers and also the properties of selected pile are dependent on the length of pile in free buckling.

The accuracy of driving operation is another problem that occurs during pile driving. Guaranty of operation accuracy is not possible in traditional method completely and it is clear that providing a high accuracy during pile driving, especially in the batter piles, is very important in some offshore projects. New system, which this paper proposes, can provide high accuracy for pile driving and also the caisson buoyancy's can reduce the vertical applied loads to piles and caisson can protect piles against negative surface friction, which happens between the surface of piles and bed soil. Therefore the designed piles in the new system can be more economical than designed piles in traditional foundation system that is constructed with piles only. Also in this new system, piles can work in a higher efficiency.

The state of structure

The main goal of this project is design and construction an efficient and economical foundation for an offshore structure. Especial conditions in installation location of foundation make this foundation to be difference from other foundation. Therefore a new method is needed for construction in these conditions. The new foundation system transfers loads from main structures to the bed soil. The water depth is about 10 meters and free board is 12 meters. The upper layers of bed soils are 15 to 20 meters of liquid mud and under these layers there are layers with considerable strength. Therefore the free length of pile during pile driving is about 40 meters.

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