Risk-based ultimate strength design criteria for very large floating structure

Very large floating structures (VLFSs) are novel offshore structures without direct reliability design criteria available currently. Target reliability, as a critical safety indicator, could be used to evaluate the ultimate strength reliability of VLFSs. Therefore, it is necessary to establish acceptable risk criteria to determine the structural design target reliability of VLFSs and to calculate partial safety factors (PSFs) for use as ultimate strength design criteria. This paper developed a reliability assessment process in order to achieve the ultimate strength design criteria of VLFS. A reasonable target reliability range for VLFS was determined by risk assessment methods and principles considering the structure failure probability and consequences. The longitudinal ultimate strength for the typical sections of VLFS was calculated by applying the simplified progressive failure method. The vertical extreme wave bending moments and their uncertainties for the corresponding typical sections of VLFS were analysed based on wave scatter diagram of the South China Sea. Finally, the ultimate strength reliability was assessed based on the reasonable target reliability range established within this paper. Results have shown that the reasonable target reliability range is found to be applicable for identifying the weak structural components of VLFS and some typical sections of VLFS should be redesigned to satisfy the criteria. This paper can provide a reference for the safe design and compilation of relevant guidelines for VLFS.