Implosion Strength Of Cylindrical Concrete Shells: A Comparison Of Theoretical And Experimental Results
Authors: Leick R.D. , Bode J.H. 
Published: October 2008

Concrete has been found to be a suitable construction material for production and storage facilities employed in developing offshore oil reservoirs such as those in the North Sea. A critical load condition for the base or caisson of these concrete structures is the maximum hydrostatic pressure which occurs during the submergence, sequence for deck installation. The failure mode associated with this load condition is termed implosion.

This paper describes an analytical technique for determining the implosion strength of concrete cylindrical shells. Nonlinear material behavior is included through a theoretically derived plasticity reduction factor. The influence of structural end closures is investigated using the finite element method. Reinforcing steel and construction imperfections in the form of out-of-roundness are examined using an iterative technique incorporating second order geometry effects. In addition, comparison of theoretical and experimental results is presented.

INTRODUCTION

Large concrete structures, have found application as drilling and/or production platforms (see Fig. 1) in the North Sea. These structures are called gravity platforms because they are maintained on location by the weight of the structure and entrained ballast material. The platform base or caisson typically has a cellular configuration which serves to provide buoyancy for adequate payload capacity during tow and may also serve to provide oil storage. The critical design load condition for the caisson occurs during the submergence sequence for installation of the deck when the caisson cells are subjected to maximum hydrostatic pressure. The potential failure mode for this condition is termed implosion and may result due to structural instability, inadequate shear capacity at discontinuous junctures of shell segments (i.e., structural end closures on cells), or general material failure. This paper deals only with the instability and general material failure mechanisms. The calculation methods used pertain to a single cell and do not directly address the "grouping effect" of the multi-cellular caisson. This approach does, however, yield insight into basic failure mechanisms which may be extended to more complex structures.

The paper summarizes an historical review of solutions for the stability of cylindrical shells under various load conditions. Gerard'sll,12 development of a theoretical plasticity reduction factor based upon a modified form of Donnell's equation is described. The applicability of a theoretically derived plasticity reduction factor in stability calculations for simply supported, unreinforced concrete cylinders is established by comparison with experimental data. The influence of the end constraints on the cylinder is examined using the MARC finite element program. Finally an analytical technique incorporating second order geometry effects is described whereby the influence of out-of-roundness on the implosion strength of plain and reinforced concrete shells may be evaluated.

LITERATURE SURVEY

The stability of cylindrical shells subjected to hydrostatic pressure has been a subject of considerable interest since the turn of the century. As early as 1911, Lorenz1 presented solutions for cylinders subjected to axial compression.