In this paper, we propose a hydroelastic design contour (HDC) that can be practically used for the preliminary
design of pontoon-type rectangular very large floating structures (VLFSs). Using the design contour, we can
easily predict the maximum bending moment of VLFSs in irregular waves. To develop the design contour, we
first construct the hydroelastic response contours (HRCs) by extensively carrying out hydroelastic analyses
considering various structural and wave conditions, namely, the bending stiffness and aspect ratio of VLFSs,
incident wave length and angle, as well as the sea state. Based on the pre-calculated HRCs, we develop the
HDC considering irregular waves. We then propose a preliminary design procedure for VLFSs using the HDC
and demonstrate it by designing a simple pontoon-type rectangular VLFS. The HDC can significantly reduce
time and effort for the design of VLFSs.