Lateral Load Capacity of Piles In Offshore Structures
Authors: Wolters J.G. 
Published: December 2007

In evaluating lateral load capacities of piles, three types of interaction are taken into account: between pile and soil, between pile and jacket structure, and between wave and structure. These interactions are normally analyzed individually. The trick is to piece them together. Here is an explanation of how that can be done and an example involving a fixed drilling platform in 350 feet of water.

Introduction

Offshore structures are installed today in areas of deeper and deeper water. As the platforms become higher and structure masses increase, more accurate and reliable calculations are necessary. The pile-capacity problem is commonly approached by pile-capacity problem is commonly approached by considering the structure in two parts: a linear part (structure above the jacket base) and a nonlinear part (piles and conductors below the jacket base and supported by the soil). The description of interaction between the two parts is often based on poor and incomplete parts is often based on poor and incomplete compatibility conditions. In pile analysis the boundary condition at the cut-off level is a pile-moment-rotation ratio (clockspring). However, neglecting the jacket deformation will influence the results of the pile analysis considerably. In general, for limited water depths this neglect will give conservative results. The bending pile moment occurring at jacket-base level will increase where a stiffer pile-jacket connection is assumed. In these shallower depths the moment governs at the jacket-base level. In deeper waters, however, the maximum moment will occur below the mud line owing to the release of the pile-jacket connection for higher structures, and the maximum moment will decrease when a stiffer pile-jacket connection is assumed. Moreover, the lateral pile deflections will decrease and this may influence the dynamic behavior as well. Another point is that in principle the platform analysis is often deterministic and based on a single storm wave load. This approach, however, is satisfactory only if the natural frequencies of the structure are out of the frequency range of wave energy. For platforms in deeper water this condition is no longer platforms in deeper water this condition is no longer fulfilled. In this paper we shall emphasize the two aspects mentioned above - i.e., random-wave-load description instead of single wave load, and a proper specification of the compatibility conditions between piles and jacket. Fig. 1 shows the relationship of the piles and jacket. Fig. 1 shows the relationship of the components involved.

Description of the Analysis

The structural analysis of the entire structure can be symbolized by the models in Fig. 2. The structure is split in two main parts:

- The piles and conductors below the base of the jacket

- The jacket including the piles and conductorsabove the base of the jacket (piles connected to the jacket top).

Each of these parts is influenced by its environment. The piles and conductors below the mud line are surrounded by soil, which acts as a supporting medium laterally and axially. The jacket is surrounded by the sea, which may cause wave and current action. Now step by step the interaction between piles and soil, between piles and jacket, and finally between structure and waves piles and jacket, and finally between structure and waves will be analyzed.

JPT

P. 487