A model test study on the effects of mooring truncation ratio on the response of a deepwater turret-moored FPSO

Deepwater platform model tests require truncated mooring systems, but the effect of the truncation ratio on the measured response has not been experimentally isolated. A turret-moored FPSO with a twelve-line chain–polyester–chain system designed for 2000 m water depth was tested at 1:60 in the KRISO Deep Ocean Engineering Basin. Two truncated systems with truncation ratios of 0.18 and 0.36 were realised at basin depths of 6 m and 12 m, together with a soft-mooring reference. The two designs share every component except the tube-sheathed wire length, removing component-to-component variability from the comparison. Both reproduce the prototype restoring curve, and heave and pitch transfer functions are unaffected by the mooring configuration. The linear surge damping ratio increases from 2.35% to 3.74% between the two configurations. In irregular seas the less severely truncated system reduces the standard deviation of line tension in the collinear head-sea conditions by 23.3% on average, and the standard deviation of the turret offset in all eleven conditions tested by 23.9% on average; maximum values from single 3-h realisations show no resolvable difference. The more severely truncated configuration therefore provides less mooring-induced damping, and overstates the variability of both line tension and turret excursion.