A Normalized Wave-Forcing Model for Gas-Lift Design Assessment near a Fixed Semisubmersible Platform

The hydraulic design of offshore gas-lift systems requires joint assessment of mean lifting capacity and wave-induced delivery fluctuations. This study develops a normalized wave-forcing model coupled to a transient drift-flux formulation to compare candidate intake locations and operating conditions near a fixed semisubmersible platform. Model tests provide 168 local wave-response ratios from 12 gauges (PT1–PT12), seven regular-wave conditions (W1–W7), and two platform drafts at a wave heading of 315°. The measured ratios are converted into equivalent harmonic amplitudes and transferred to the intake pressure of a fixed vertical reference pipe. Normalized pressure and outlet-flow amplitudes provide a common basis for comparing wave conditions, gauge locations, and platform drafts. The measured response ratios range from 0.61 to 2.10 under operating draft and from 0.70 to 2.32 under survival draft. Pressure-penetration depth dominates the cross-period transport response: at the 193 m intake, the pressure-transfer factor increases by approximately 1.64 × 104 between the shortest and longest tested periods. Consequently, W7 produces the largest outlet-flow fluctuation although the maximum surface-response ratio occurs in W3; local amplification determines the spatial ordering within each wave case. For the survival-draft W7/PT11 case, the outlet carrier-flow amplitude is 0.3751 kg/s, corresponding to 0.304% of the mean delivery, with a phase lag of 59.1°. Flow fluctuations scale approximately linearly with imposed pressure amplitude, while cycle-mean delivery remains nearly unchanged over the tested forcing range. Increasing gas injection raises mean delivery with diminishing incremental gains. The model provides a common basis for identifying the wave conditions governing hydraulic variability, comparing candidate intake locations, and assessing mean delivery against gas-supply requirements.