Wave-in-Deck Forces and Response of Semi-Submersibles
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- Institutt for marin teknikk 
In critical sea states, semi-submersibles may experience large negative air gaps in which incoming waves cause large slamming loads that may lead to severe structural deformations. Wave loads due to slamming are difficult to predict using analytical approaches as there are many uncertainties involved. Older rules and guidelines base the loading estimates on the particle velocities in the waves whereas newer guidelines have based the loading on the relative upwelling on the platform. Model tests are among the most accurate ways of estimating wave loads on offshore platforms and are recommended by classification societies. However, model tests are expensive and difficult to execute as it is complicated to recreate the critical sea states that offshore platforms may experience. In this thesis, it is therefore proposed to investigate if drop tests at equivalent velocities as the particle velocities in incoming waves may be used to recreate the same loading conditions. Drop tests were simulated using the arbitrary Lagrangian Eulerian, denoted ALE, approach. To verify both the parameters and the ALE-approach, a drop test described in (Faltinsen, 2005) was recreated in the finite element software, LS-Dyna. The results showed good agreement with the measured strains, pressures and natural periods adding credibility to the subsequent results obtained in the thesis. The structural components may undergo large deformations which in turn affects the pressure distribution on the structure. The interaction between the fluid pressure and structural deformation is known as hydroelasticity. In general, not accounting for hydroelastic structure response is conservative. Pressure distributions for rigid and deformable panels, where hydroelastic effects become increasingly prominent, were compared for the same parameters as in (Faltinsen, 2005). The initial pressure impulses observed at impact were smaller for the deformable plate than the rigid plate at the same impact velocity, but the initial pressure peak was equal. For the deformable plate, a pressure approximately proportional to the acceleration of the plate is observed after the initial pressure pulse during the deformation phase and may be viewed as an added mass pressure. A larger deformation was seen when solving for an equivalent beam with the rigid pressure pulse. Drop tests where simulated for different impact velocities. At an angle of impact of 0 degrees, the initial pressure peak appears to be linearly dependent upon the impact velocity. It was also observed that the larger loads associated with higher impact velocities led to larger deformations where membrane forces became prominent, hence decreasing both the wet and dry natural period of the structure. For the two-dimensional drop tests, the wet natural period was found to be approximately the same for both the elastic and elasto-plastic drop tests. For the three dimensional drop tests, the elasto-plastic plate had a longer period than the elastic plate. The peak of the second pressure surge was larger for the elastic plate compared to the elasto-plastic plate. By increasing the velocity, the difference increased. The pressure-time curves for the simulated drop tests of deformable plates display some different properties compared to the pressure time curves for wave impact. The peak pressures are seen to be larger for the drop tests where the duration of the load is shorter. It is however seen that the structural deformations obtained in the deformable drop tests are in the same order of magnitude as those obtained using the loading model from (GL,2016b). About the same levels of structural response was observed for a 3[m] by 3[m] deformable panel dropped at a velocity of 11.19[m/s] as when loaded by an OTG-pressure-time curve with a pressure peak of 1200[kPa]. Moreover, the same structural response was observed for a column dropped at a velocity of 20[m/s] into water as when loaded with an OTG-pressure-time curve with a pressure peak of 2300[kPa]. Relative velocity seems to be the driving factor in structural deformations in the drop tests of deformable plates with the narrow pressure peaks of limited importance. Two main benefits are seen with response calculations where drop tests of deformable panels are simulated. The first is that the spatial pressure distribution is captured. The second is that added mass is included in the analysis, reducing the uncertainty in establishing added mass. The simulated drop tests of rigid plates yield pressure curves that are unrealistic in terms of both magnitude and duration compared to the loading seen from deformable panels and in the technical guidelines. Overall, the results for simulating wave impact loading by drop tests were promising. However, there are much that should be explained before the method can be applied in practice. For instance, the differences in pressure-time-curves should be explained. In addition, more precise methods for establishing the impact velocity and angle of impact should be developed.