Wave Slamming on External Turrets of FPSOs

The paper discussed the possible slamming loads on the underside of the (chaintable of) external turret moored F(P)SOs. For this problem of turret slamming, a combined exceedance of the airgap between the underside of the chaintable and the relative wave is needed, as well as a certain relative wave velocity. A model test series was carried out to determine these loads on a typical external turret. The model test results provide insight into the complex loading process of external turret type structures. A clear quadratic relation between submergence velocity and impact load, as typically observed for slamming problems, has not been identified. This is probably a result of local effects, such as the strong disturbance of the free surface (giving a locally different deadrise angle) and air entrapment effects. The maximum pressure on the conical turret (chain table) is significantly reduced compared to the flat turret, which was expected based on the reduced rate of change of momentum with the conical turret. Based on linear 3D diffraction analysis and the empirical relations derived from the model tests, a method is presented to determine first design estimates of the pressures.