A Direct Design Procedure for FPSO Side Structures Against Large Impact Loads

Abstract The performance and consequence of FPSOs subjected to large impact loads such as collisions from supply vessels or merchant vessels are of great concern in the offshore industry, notably when they are located close to heavy traffic lanes. Due to the lack of operation experience for ship-shaped FPSOs, direct design procedures are needed to rationalize the structural design of FPSOs, which can mitigate the consequence of collision accident and avoid possible contaminated compartment flooding. In this paper, three collision scenarios between a FPSO and a bulbous supply vessel are analyzed through explicit nonlinear finite element analysis code LS-DYNA. Thereafter, a direct design procedure is proposed for ship-shaped FPSO side structure against accidental collision forces, which follows the principle of accidental limit state. The procedure comprises the determination of the impact forces, shell plating, and stiffener framing design, and the consideration of the acceptance criterion. The proposed method is especially useful in the preliminary design phase because the design procedure for plating and stiffener is based on analytical formulas derived from plastic method and appropriate collapse mechanism. The side structure decided by the proposed design procedure also complies with the strength design principle that has been adopted in NORSOK standard. The proposed approach is demonstrated by the design of the FPSO side structure against impact loads from a 7500 tons supply vessel and verified by means of integrated collision analysis. The procedure could also be served to estimate the damage due to accidental loads.