Analysis of Offshore Structure Including Coupled Structure, Pile and Inelastic Soil Properties
Authors: Feibusch Robert J. , Keith Edward J. 
Published: October 2008

This paper presents a procedure for the analysis of pile supported offshore structures that couples, in one calculation, the combined effects of the elastic structure and piles with the inelastic properties of the supporting soil. The results of an analysis of a 400' drilling tower are used to illustrate the application and advantages of this procedure. Comparisons of these results with those obtained by other analytical methods are also presented.

INTRODUCTION

Because of the ever increasing demand for petroleum products, offshore leases are now held in water depths up to 1800'. (1) As a result of this increasing depth, more accurate and reliable methods of structural analysis of offshore towers are desirable, particularly in areas of high seas or large earthquakes.

In the past, the analysis of offshore towers has often been performed in two basic steps. (2) (3) (4) First, the restraining influence of the pile-soil foundation was determined by treating the structure above the pile as a simplified equivalent system. Second, the structure itself was analysed by idealizing the pile foundation as an equivalent elastic system. The method of analysis described in this paper combines these two steps into one, taking into account both the elastic structural members and the inelastic soil restraint. Furthermore, this method includes the beam-column effect caused by eccentricity of the axial force on the deflected piles.

A 400' tower which is representative of a large class of offshore structures was analysed for the controlling loading condition as a means of comparing the effects of several assumptions relating to fixity and eccentricity of the structure and piles at the mudline. This comparison is used to illustrate the value of the analytical procedure described herein in terms of the accuracy of results obtained and the amount of computational effort involved.

DESCRIPTION OF STRUCTURE ANALYSED

A typical four legged drilling tower was analysed for combined dead and maximum lateral static wave load. The tower, located in 400 feet of water, consists of 14' diameter legs with 42 and 36 inch diagonal bracing. The tower is supported by six piles per leg which extend 250' below the mudline. All six piles are grouted inside the tower leg. Figure 1 shows a complete elevation of one frame of the sample tower.

Mathematical Model Representation

Figure 2 shows the mathematical model representation of the sample frame including both elastic and inelastic members. The foundation soil is represented by a set of inelastic springs which are spaced at 5' intervals near the surface and at 10' intervals 80' and more below the mudline. Only 150' of pile is used in the model since this length adequately represents the lateral stiffness of the soil. The soil springs are spaced sufficiently close to represent the continuous support of the surrounding soil.

The properties of the inelastic springs are determined from non-linear curves which represent the force deflection properties of the soil. These "p-y" curves are typically available for offshore structures and vary with depth below the mudline. As an example the "p-y" curves used in this analysis for the inelastic springs at elevations -5.0, -15.0 and -25.0 are shown in Figure 3.