Wave Run-ups On Surface-piercing Square Column And The Mitigation Method Using Multi-layer Porous Barrier

Wave run-up on surface-piercing columns is of significance in the design and operation of large offshore structures.Platforms with columns of large dimensions,such as semisubmersibles,TLPs,and Spars,usually experience strong nonlinearity during the interaction with ocean waves.In addition to the increase of wave elevation around the column due to the wave scattering and radiating,wave upwelling or even free jet along the column surface often occurs and intensifies the risk of the lower deck slamming or green water,threatening the safety of structures and staff.Recently,the relatively frequent occurrence of facility damages and even fatal accidents caused by critical wave run-up events under severe sea states has aroused significant concerns about the wave run-up and air gap problem in both the industry and academia.Conventional measures to improve the air gap performance of an offshore platform,including increasing the deck clearance and/or altering the structure configuration,are constrained by the platform weight,stability,construction cost,etc.The present study focused on the wave run-up on a semi-submersible square column and sought a more detailed insight into the flow field in the vicinity of the column,the evolution of run-ups,and the free jet occurrence mechanism.Thereafter,a concept to mitigate critical wave run-ups and reduce its height,as well as the potential risk of the lower deck slamming or green water on a semi-submersible,using attached multi-layer porous barrier on the column surface was developed based on the investigation on the wave-dissipating performance of multi-layer porous structures.Firstly,the methods of wave flume experiment for the fixed surface-piercing square column and measurement of instantaneous run-up profiles based on the computer vision technique were established,and wave run-ups under the Stocks waves of different periods and steepnesses were investigated,combined with CFD simulation.It is shown that the run-up height ratio increases with the wave steepness.The relative increment is more significant under long waves than that under short waves,and different types of nonlinear wave scattering can be observed.Under short waves,scattering wave crest which arises on the back surface of the column propagates upstream and superposes the next incident wave crest just near the front surface,increasing the wave run-up height;under long waves,however,the relatively high-velocity flow on the wave crest is retarded,resulting in a localized but notable water mound driven in front of the column,and increasing the wave run-up height.Such difference also explains the different variation trends of the horizontal wave force coefficients under short and long waves as the incident wave steepness increases.Moreover,the increase of incident wave steepness intensifies the disturbance of column corners to the wave field,causing notable localized wave crests in the neighboring region of the column.This may result in strong inter-column interference in the case of multi-column structures and increase the potential of critical wave run-ups.Subsequently,experiments were carried out to further investigate the wave run-ups under focused waves of different peak periods and maximum focused crest heights,and a set of the typical cases was simulated numerically to obtain concerned flow details,including free surface profiles,velocity and pressure fields in the neighboring region of the column.The results reveal the significant increase of the run-up height ratio with the focused wave steepness.In particular,extreme wave run-ups may occur and reach significant heights under focused wave crests nearly or just breaking.This is mainly because the increase of wave steepness intensifies the variation of the velocity field in front of the column,resulting in the increase of dynamic pressure peak value and gradient and hence the accelerating effect of uprush flow.For nearly or just breaking focused wave crests,the fluid on the wave crest,of which the front is almost vertical,changes the velocity in a sudden when it impacts on the water mound driven in front of the column,resulting in an extremely high-pressure peak value and gradient in this very localized region.Consequently,critical slamming on the column occurs and strong free jet bursts out.Comparing with the focused wave steepness,the variation of peak period has limited influence on the run-up height ratio,except under some extreme conditions,relatively large peak period increases the volume(or thickness)and vertical velocity of the uprush,partly increasing the run-up height ratio.Porous structures are highly effective in dissipating wave energy and therefore are widely applied in the port and coastal engineering.The present study investigated the wave-dissipating performance of a submerged multi-layer horizontal porous-plate structure under regular waves.The effect of flow field disturbance and energy dissipation with the seepage was investigated,and the influence of porosity,number of layers,and layer spacing on the overall wave-dissipating performance was examined.The results suggest that a properly designed number of layers,layer spacing,and plate porosity can make the structure block and dissipate wave energy efficiently,and in the meanwhile limit the wave loads on it.On this basis,the concept of porous structure was applied to the platform column.The interaction between a simplified two-dimensional uprush jet and the attached porous structure was investigated based on the numerical model for porous media using VA-RANS method,and the evolution of uprush flow and localized velocity field and the vertical forces on the structure under different porosity and initial vertical uprush velocity and thickness were compared.The porous barrier can block and dissipate the uprush jet efficiently of which the thickness does not exceed the barrier,while the performance is limited if the uprush jet is notably thicker.In addition,because the porosity of the barrier exhibits a significant influence on the uprush energy dissipation and slamming force damping,a relatively high porosity is recommended in the engineering practice with the consideration of water splashes of high-velocity uprush jets.The mounting of a multi-layer porous barrier on the column surface to dissipate and obstruct the uprush and improve the air gap performance was proposed.Experiments were performed using a truncated square column to examine the performances of three versions of the barrier,namely,solid-plate,porous-plate,and intermittent-plate types,under four different focused waves which are close to or just breaking.And the uprush-dissipating performance was compared.All the barrier types were found to obstruct and deflect uprush flow under most storm conditions.However,the solid-plate type tended to experience considerable wave forces,with its impermeability also rendering the higher layers ineffective.The intermittent-plate type dissipated the uprush flow and decreased the wave impact,although it exhibited relatively strong flow disengaging,which decreased the efficiency under large wave run-ups.Conversely,the porous-plate type exhibited adequate performance,with a larger plate porosity and moderately high mounting elevation tending to improve the uprush obstruction performance and further decrease the wave slamming loads.A barrier with an appropriately designed plate porosity,number of layers,and mount elevation is expected to perform efficiently under severe sea states,providing protection for the lower deck against extreme wave run-ups while avoiding critical slamming forces.On the basis of foregoing studies,a multi-layer porous-plate attached structure was advanced to mitigate the wave run-up on semi-submersibles under severe sea states and improve the air gap performance.Experiments were carried out to demonstrate various flow regime of wave run-ups along the platform column,compare the air gap distributions at concerned locations with and without the designed barrier in place,and reveal the interaction between the uprush jet and barrier under extreme wave environments.It is shown that wave run-ups on the relative fore-column(s)under each wave directions usually exhibits relatively high uprush velocity but small thickness,and under such cases,the barrier can reduce the probability of negative air gap efficiently.Although the uprush is much thicker on the relative rear-column(s)and may not be deflected by the barrier completely,the vertical velocity is reduced significantly,and hence the potential slamming damage.