| Structural safety under the extreme condition is the premise and foundation of the normal operation of Marine engineering equipment.In this regard,the structural design load conditions and the corresponding strength criteria are given in the marine platform specification of classification society.According to the code,the ultimate wave load of an offshore platform is calculated according to the design wave converted from the sea conditions and service life of the platform.The design wave adopts a wave model in which the wave surface maintains a certain integrity without rolling and breaking phenomenon.However,larger waves will roll and break in the actual Marine environment.The water particles at the front of the crest have a large horizontal velocity when the type of breaking wave is plunging breaker wave,which will produce a great impact force when it acts on the structure.The existing research and observation results show that the impacting load induced by plunging breaker wave is significantly larger than the wave load given by the no breaking wave model with the same wave height,which will threaten the safety of the structure.Therefore,it is necessary to study the calculation method of impact load caused by the plunging breaker wave and the structural response characteristics,which is of great significance to the structural design of offshore platform.Since cylindrical structures commonly used in marine engineering structures,this paper applies the open-source computational fluid dynamics software OpenFOAM to numerically simulate the interaction between the plunging breaker wave and the cylindrical structure.The main research contents are as follows.1.The k-ω SST turbulence model,buoyancy-modified k-ω SST turbulence model and stabilized k-ω SST turbulence model are used to numerically simulate the plunging breaker wave.The variations of free surface elevation,time-averaged horizontal velocity and timeaveraged turbulent kinetic energy calculated by different turbulence models are investigated.The k-ω SST turbulence model is the most accurate to predict the free surface elevation and the time-averaged horizontal velocity compared with the experimental data,but the turbulent kinetic energy is slightly overestimated.The buoyancy-modified k-ω SST turbulence model and the stable k-ω SST turbulence model can effectively reduce the turbulent kinetic energy.The turbulence characteristics,such as turbulent kinematic viscosity and Reynolds stress,during the wave run-up,rolling and breaking are also discussed.In addition,the stabilized k-ω SST turbulence model is modified.The time-averaged turbulent kinetic energy calculated by the stable-modified k-ω SST turbulence model is in better agreement with the experimental data.Based on the stable-modified k-ω SST turbulence model,the effect of the stress limitation coefficient λ2 on the fluid characteristics are investigated.2.The methods of simulating plunging breaker waves through vertical cylinder are studied.In the study of the grid division of the computational domain,it is found that the near wall area is gridded similarly to the surface shape of the structure,the intermediate transition area is drawn in a circle pattern,can greatly reduce the number of meshes and improve the calculation efficiency under the premise of ensuring results accuracy.No turbulence model,k-ω SST,buoyancy-modified k-ω SST,stabilized k-ω SST and realizable k-ε turbulence models are used to numerical simulate the interaction between plunging breaker waves and the vertical cylinder.The turbulent kinetic energy,turbulent kinematic viscosity and Reynold stress calculated by different turbulence models are analyzed.The k-ω SST turbulence model combined with VOF is the most accurate to predict the free surface elevation and the breaking wave impacting force by comparing with the experimental data.Finally,a set of numerical calculation scheme of the interaction between plunging breaker waves and cylindrical structures based on OpenFOAM is proposed.This scheme can ensure the accuracy of calculation results and avoid the waste of computing resources.3.Breaking waves past a single cylinder with different transverse inclined angles and two cylinders are numerically simulated,respectively.In the study of the interaction between breaking waves and single cylinder with different transverse inclined angles,the total horizontal breaking wave force is decomposed into low-order wave force and high-order wave force by Fast Fourier transform(FFT)filtering.In the case of the plunging breaker wave impacting the surface of a single cylinder with a vertical crest,both the free surface elevation in front of the cylinder and the normalized high-order wave force occur the minimum values when the transverse inclined angle of the cylinder is 45°.When the transverse inclined angles of the cylinder are 30° and 45°,the secondary load causing the high harmonic vibration of the structure disappears.For breaking wave passing a pair of cylinders,the effect of the distance between cylinders and the transverse inclined angles of the downstream cylinder on breaking wave forces and free surface elevations are analyzed.4.Stress analysis on the column structure of the ocean platform is performed.According to the wave parameters of design wave determined by the marine platform specification of classification society,the interaction between the platform pile leg and the breaking wave and the regular wave are numerically simulated,respectively.The wave forces obtained under different wave conditions are loaded onto the finite element model of the platform pile leg.Calculation results of wave loads show that the peak breaking wave load on the pile leg of the offshore platform is much larger than the peak non-breaking wave load under the same design wave parameters.The stress analysis results of pile leg finite element model show that the pile leg structure is safe under non-breaking wave load,but the stress of pile leg element under breaking wave load is greater than the limit of material and structure damage will occur.Therefore,it is recommended that the plunging breaker wave load in the extreme harsh environment should be calculated for the safety calibration standard to ensure the structures safe enough. |
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