Parametric Study and Dynamic Analysis of Compliant Piled Towers
Authors: Moog K.H. , Lou J.Y.K. 
Published: October 2008

A Compliant Piled Tower (CPT) consists of two main structural components, the piles and the tower. Different configurations of CPT were studied in the present paper. The mechanical system of restoring forces was based on the coupling that provides dynamic balance between the piles and the tower. A parametric analysis was conducted to determine the variation of the first and second natural periods with parameters such as pile length, base width, deck payload, water depth, and pile area. The dynamic behavior of the CPT was studied for various prototypes. Response characteristics of the structure to a random wave derived from a characteristic wave spectrum were studied. Envelopes of overturning moment and shear force for different sea states were studied as well as the derivation of such envelopes from corresponding profiles at various instances.

INTRODUCTION

During the last several years different compliant structures have been proposed to overcome the drawbacks of fixed offshore structures in deeper water. Among them, the Compliant Piled Tower (CPT) appears to be a suitable alternative.

Several mechanisms such as guidelines, water traps and buoyancy tanks have also been suggested to provide the restoring forces. A system of piles acts as a restoring mechanism for the CPT. The piles allow considerable displacement but at the same time provide enough stiffness to the structure.

The behavior of the structure is nonlinear in nature, and therefore special features have to be taken into account in order to model the performance of the system. For instance, large displacements can generate a P-? effect that induces changes in the stiffness of the structure. The input force given by the action of environmental forces is also nonlinear.

The forces are obtained from Morison's equation. The relative displacement of the tower with respect to the water is accounted for in the hydrodynamic model.

The Compliant Piled Tower is a relatively new concept for deep water structures. Parametric analysis is an important part of the preliminary study process. Determination of the influence of several specific parameters on the dynamic characteristics of the structure leads to a more precise design analysis.

The objective of the present study is to investigate the effect of various physical parameters such as the dimensions of the base of the structure, pile length, pile area, water depth and deck payload on the CPT natural periods. The study of the dynamic behavior of the structure is aimed at obtaining the distribution of overturning moment and shear force. It is expected that the results of this study will provide useful information for future design of Compliant Piled Towers. Although beyond the scope of this work, a more detailed structural analysis should include design considerations of paramount importance such as fabrication and installation.

PLATFORM CONFIGURATION AND MECHANICAL SYSTEM

According to Maus et al. (1986) the Compliant Piled Tower is a structure that has been proven adequate for water depths between 300 m. (?1000 ft.) and 800 m. (?2600 ft.) for characteristic sea states of the Gulf of Mexico.