Numerical Research on Elasto-Plastic Response of FPSO Topside Modules Subjected to Blast Loading Aiming at a Simplified Design Method

Abstract

Structural blast analysis is usually done with two load levels for different design stages; the Strength Level Blast, SLB, and the Ductility Level Blast, DLB. Strength Level Blast is the lower magnitude overpressure (with a higher frequency of occurrence and lower consequence) and Ductility Level Blast is the higher magnitude overpressure (with a lower frequency of occurrence and significant consequence). The objective of this thesis was to try to find an estimated relationship between the linear elastic analysis using Strength Level Blast pressure, and nonlinear plastic analysis using Ductility Level Blast pressure by making an analogy to the procedures of seismic design of structures. For that, four hypothetical methods were introduced and examined to find the one that can provide a better estimated relationship. Furthermore, four SBM topside modules with 22 blast directions were analyzed. Firstly, linear elastic analyses, by means of redundancy analysis, were performed to find the Strength Level Blast capacities. Secondly, nonlinear plastic analyses, by means of static pushover analysis, were performed to obtain the pushover curves. Subsequently, several important parameters that were required for finding the Ductility Level Blast capacities were obtained from the pushover curves, namely, ultimate limit capacities and their respective displacements, the reserve strength ratios and the ductility ratios. In the next stage, the four proposed methods were tested by means of introducing four hypothetical methods each with a different reserve capacity factor, Cr; next, the Strength Level Blast capacities were scaled with these factors to obtain the Ductility Level Blast pressures. These four methods were Displacement Ratios Method, a comparison of ISO 19902 for Earthquake Assessment of Fixed Offshore Platforms for Blast Assessments, the Energy Equivalence Method and the Behavior Parameters’ Method. • Displacement Ratios’ Method assumes the Cr value to be based on the ratio of the ultimate deflection of the structure to its deflection under the Strength Level Blast pressure. Thus, this method assumes that the structure will continue to behave linearly up until the ultimate capacity of the structure has been reached, which in turn will be the Ductility Level Blast capacity of the structure. • The method described by ISO 19902 for earthquake assessment of fixed offshore platforms assumes the Cr value to be based on the two scaling parameters; the first parameter was the same as the previous method, and the second parameter takes into account the energy absorbed by the structure after the ultimate capacity of the structure has been reached. • The Energy Equivalence Method, the Cr value was obtained by comparing the energy absorbed by the structure until the ultimate capacity has been reached with the energy that would be absorbed if the structure was assumed to behave linearly until the Ductility Level Blast capacity was reached. • The Behavior Parameters’ Method the Cr value was also obtained by equating the energy absorbed by the structure until the ultimate capacity has been reached; however, this method simplifies the area under the graph into two sections; first section in the elastic region, the second section in the nonlinear plastic region until the ultimate capacity of the structure has been reached. Next, this energy was expressed in terms of the Reserve Strength Ratios and Ductility Ratios. In this method the structure was also assumed to be behaving linearly until the Ductility Level Blast capacity has been reached. Finally, after finding the Ductility Level Blast pressures for every module and direction, the Cr values were arranged based on the direction of loading to longitudinal, transverse and diagonal directions, for the small and large modules, in order to find which method had the least scatter for the Cr value. The most desirable results, with the least deviations, were obtained were from the Energy Equivalence Method. In addition, the obtained Ductility Level Blast pressures were compared with the nonlinear dynamic capacities for every module and direction. The main criteria in this comparison was having the calculated pressure less than the calculated dynamic capacities. It was observed that the Energy Equivalence Method gave the most suitable results in this comparison as well. The minimum value of Cr from the Energy Equivalence Method that were concluded were 4.0, 4.0 and 3.6 for the longitudinal, transverse and diagonal directions of the large modules respectively; also, 5.2, 4.3 and 4.3 for the longitudinal, transverse and diagonal directions of the small modules respectively.