Implosion Strength of Cylindrical Concrete Shells: A Comparison of Theoretical and Experimental Results

Abstract

Offshore concrete production and storage facilities sustain maximum hydrostatic pressure during submergence for deck installation. The failure mode associated with this critical load condition is termed implosion. This paper describes an analytical technique for determining the implosion strength of concrete cylindrical shells. Introduction Large concrete structures are being used as drilling and/or production platforms in the North Sea (Fig. 1). 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 that serves to provide buoyancy for adequate payload capacity during tow and also may serve o 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 from 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 address directly the "grouping effect" of the multicellular caisson. This, approach does, however, yield insight into basic failure mechanisms that 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's development of a theoretical plasticity reduction factor based on 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, Lorenz presented solutions for cylinders subjected to axial compression. Southwest in 1913 and Von Mises in 1914 published solutions for buckling under uniform lateral pressure. Von Mises followed in 1929 with a solution for the combined load case of axial compression and uniform lateral pressure (simple hydrostatic pressure). Flugge in 1932 addressed stability under pressure). Flugge in 1932 addressed stability under combined loading and bending. Stability under torsional loading was investigated by Schwerin in 1925 and Donnell in 1933. JPT P. 27