Predicting The Drivability Of Large Diameter Offshore Piling

Abstract

ABSTRACT Large diameter piling are increasingly being used in offshore structures to house the well conductors. In order to predict the drivability of these large diameter members, it is desirable to utilize the wave equation theory developed by Smith and others, which has been used for a number of years to examine the drivability of conventional piling. This paper examines the questions:"Does the wave equation theory accurately predict the drivability of large diameter piling?" and"If so, what variations from the standard input data are required?" Three recently installed structures were analyzed and the predicted blow counts and driving stresses were compared to those measured in the field. The first of the structures was a 96- to l68-inch diameter, tapered, self-supporting caisson, and the other two were mudslide platforms with l25-inch diameter piling. Two variations from the standard Smith input data were required to predict the drivability of these piles. The first variation was a result of the fact that large diameter piling will usually require a tapered drive head because of the smaller diameter of the standard pile cap. In addition, large diameter caisson well protectors are usually tapered above the mudline to make more efficient use of the steel. Formulas were derived to simplify calculation of the stiffness and weight of a tapered, non-prismatic pipe segment. Then a study was made comparing the use of tapered drive heads with standard pile caps to the use of a special large diameter pile cap. The second variation from the standard input data is related to the problem of soil skin frictional "set-up" and end bearing resistance. The wave equation theory was found to accurately predict the drivability of large diameter piling with only a slight variation from the standard Smith method of analysis. INTRODUCTION Special precautions are sometimes taken to protect the well conductors in an offshore drilling platform. One way of protecting the wells is by housing them inside a large diameter member. In a mudslide structure, the piling may serve the dual purpose of supporting the platform and housing the wells to protect them from unstable soil movements. In some situations a self supporting caisson well protector can be used in lieu of a platform to support the well cluster when dynamic or fatigue problems exclude the use of self-supporting well conductors. The caisson may be a constant diameter pipe or the more efficient tapered member. The design engineer must determine the answer to two questions. He must decide whether or not the pile can be driven to the design penetration or if drilling or jetting will be required. Secondly, he must be able to predict the driving stresses induced in the pile. New massive hammers are available and the engineer needs to know whether a large hammer is required or if a smaller and cheaper conventional hammer will suffice. Naturally, the engineer would like to utilize the one dimensional wave equation for the evaluation of the drivability of large diameter piling.