Typical ship-like floating bodies, for certain operating conditions, may experience large slamming loads in the bow or aft parts of the vessel. In addition to the local structural damages induced by slamming, a significant global hull vibration might also be excited. This vibratory response, referred to as whipping phenomenon, is usually not considered in the offshore industry, in particular in the design of ship shaped FPSOs. However, in case of resonance with slender structures such as the flare tower, these effects can have a strong influence on the fatigue performance at the interface between the FPSO’s hull and those structures. The objective of the work presented in this paper is to assess the influence of these effects on the total fatigue damage.
Numerical modelling of whipping is extremely complex from several aspects and only approximate models exist today. For this study, a state of the art whipping model is used, which couples the hydrodynamic loads (linear diffraction and radiation, nonlinear incident and slamming) with dynamic structural response of the floating body. The hydrodynamic part of the model includes a weakly nonlinear diffraction-radiation tool which is supplemented by a modified Logvinovich model for slamming. These hydrodynamic models are fully coupled with the structural vessel dynamics which are represented by a simplified finite element model. Direct assessment of hot spot stresses are performed on refined fatigue models.
Typical FPSO’s design includes the flare tower installed in the fore part, where the accelerations induced by the hull girder global vibrations are the highest. If the natural frequency of the flare tower is close to the natural frequency of the hull girder, significant flare vibrations might occur. In addition, spread moored FPSOs receive wave loads from 360 degrees and since typical FPSOs’ hull shape is similar to oil tankers’ hull shape, its stern flare is relatively larger flare than its bow flare. If waves come from the stern and FPSO’s draft is shallow, hull vibration induced by stern slamming might be large. The present work evaluates the influence of these vibrations on the flare tower structural response. The deterministic results are used within a dedicated methodology allowing for the evaluation of fatigue life of the structural details.
This work proposes an innovative methodology for including the influence of highly nonlinear loads, such as slamming induced whipping, in the structural fatigue assessment of offshore structures.