Experimental Study of Slam Forces on External Turrets With Different Top Angles

The external turret is widely used for mooring FPSOs. However, external turrets can be exposed to slamming and it leads to generate FPSO’s whipping. To analyze the fatigue strength of the FPSOs, probability distributions of slam forces acting when slamming happens are necessary. In this study, we focus on slam force filtered by 10Hz low-pass filter. Then, we examined the probability distribution of peak slam forces, the effect of the top angle on slam forces, and the difference of two types (fixed and oscillator-mounted) tests. For three turret models with different top angle (180°, 160°, and 140°), two types of model tests were carried out. To confirm the experiment’s validity and reliability, we compared these model tests with the previous study. The slam forces measured on a conical (140°) turret were consistent with the estimated curve in the previous study. The comparison of two types of tests indicated that fixed turret tests were an easy and valid way of measuring the slam forces on external turrets. 3-hour return values calculated by five methods were used to examine the characteristic of each turret. And these values were compared with 3-hour mean maxima to examine the difference between the five methods and experimental results. As a result, the use of Generalized Extreme Values and Generalized Pareto distributions is more suitable than using Rayleigh distribution in estimating return values of slam forces on external turrets.