Experimental Investigation of Green Water Loads on an Elevated Topside Module on an FPSO Hull

Green water loading on elevated topside modules was investigated through a series of physical experiments using obstacles lifted above a fixed prismatic model of an FPSO cross section. Focused wave groups incident from the beam were created from realistic spectra representative of the design seas to generate three characteristic green water events: plunging wave (PW), dam break (DB), and hammer fist (HF). Each was executed at three inundation levels by varying incident group amplitude. Tests were performed with and without a simplified, impermeable topside module; undisturbed reference conditions were also recorded. Results show that horizontal loads increase with inundation level across all event types and are approximately linear in inundation level at intermediate to high inundations. Vertical loading exhibits a two-stage signature: an initial downward (negative) force as the on-deck flow passes through the module deck gap, followed by an upward (positive) force; this emphasizes the potential importance of any structural or equipment elements beneath the module for load paths and warrants further assessment. Unlike the horizontal component, a simple quantitative scaling for vertical force with inundation is not yet evident. For all tested cases, the ratio of peak horizontal to vertical force lies between 0.2 and 2, indicating comparable magnitudes of the two components. The type of overtopping event type matters: DB cases follow the intuitive trend of larger undisturbed crest leads to larger loads, whereas HF behaviour can be influenced by the preceding deep wave trough, complicating the relationship between crest height, inundation, and load. These findings provide experimentally grounded load trends and signatures to inform simplified design models and layout decisions for elevated topside modules on FPSOs exposed to beam seas.