| China is rich in offshore wind power resources and has a broad prospect for development.It will be in high-speed development and construction,which has higher requirements on the safety and operation efficiency of the wind power construction and installation platform.Because of a shallow mud penetration depth,the mat-supported jack-up platform can be quickly positioned,installed and moved in operation,thus has good stability and high operating efficiency.However,in the operation of jack-up platform,under the action of waves and flows in the marine environment,the changes of pore pressure and surface scour around the mat foundation causes pore pressure increase and basal erosion of the seabed,which reduces the bearing capacity and stability of the mat foundation and bring operation risks.In this paper,flume model experiments and numerical models are combined to study the pore response and scour mechanism around mat foundation.Based on the design of a multi-physical field flume model experiment of wave-flow-mat-sandy seabed,the experimental study of seabed response under wave action and local scour under current was carried out.On the basis of the measurement results of pore pressure in flume experiment,the one-way coupled seabed numerical model with mat foundation was established,and the pore pressure response and liquefaction depth distribution of seabed were analyzed.Combined with the results of scour flume test and the numerical model,the local scouring law of seabed around the foundation were studied.The main research contents and conclusions are as follows:1.The model experiment of wave flume with mat foundation was designed,the model size and experimental wave parameters were determined by using variable scale,and the experimental apparatus and site were arranged.According to the experimental procedure after the specification,the wave flume experiment under 11 working conditions was operated.In the experiments,the wave height before and after the mat foundation,the pore pressure response of different positions and depths inside the seabed under the mat were monitored.The results show that the wave is reflected by the mat foundation,and the wave height in front of the mat increases.The maximum value of pore pressure amplitude appears before the model,and the minimum value is located directly below the mat.The pore pressure attenuation and phase lag appear along the direction of depth.After reaching a certain depth,the cumulative pore pressure appears.The amplitude of the cumulative pore pressure does not change when the wave height and period increases,but it can accelerate the accumulation rate of the pore pressure.The maximum scour depth appears at the two corners of the front side,and develops towards the bottom and middle of the mat model.When the long side of mat meets the current,the sediment accumulates at the rear two corners.The increase of flow rate will aggravate the scour and accumulation of sediment.When the short side of the mat meets the current,there is no obvious sediment accumulation,but strip scour pits appear along the long side.2.A three-dimensional one-way coupling model of wave-mat foundation-seabed was established,and the wave form changes around the mat foundation and the seabed response were numerically studied.The feasibility and accuracy of the numerical model were verified by comparing with the wave height and pore pressure measured in the experiment.The results show that the wave pressure is distributed crisscrossing in the whole seabed,but the numerical value increases obviously in the vicinity of the foundation.The reduction of pore pressure along the depth direction is faster in the shallow seabed.When the maximum positive and negative pore pressure appear before the foundation,the pore pressure state of the seabed under the mat foundation is the same,but the pore pressure value becomes higher,and the value of the seabed pore pressure under the mat is close to 0at two equilibrium moments.When the parameters of wave and seabed are changed,the variation trend of pore pressure along the direction of depth is the same,but it will affect the reduction rate of pore pressure at different depths.For the depth of liquefaction study with extreme wave height cases,when the wave trough has just arrived at the mat,the liquefaction depth of the Y direction is symmetrical distribution,and the deepest position is at the edge of the mat foundation.Within the scope of the mat foundation,the average depth of liquefaction is greater than that of the seabed edge,and the length of liquefaction increased depth is 2.3 times that of the mat foundation.3.A numerical scouring model of flow-mat foundation-sandy seabed was built to study the local scouring law around the mat foundation.The feasibility of the scour model was verified by the comparison with scour pit morphology and maximum scour depth measured in the scour flume experiment.The numerical results show that when the current meets the mat foundation,it will flow around from the top and side of the mat,and form a vortex at the tail,which will cause local scouring around the mat.The scour pits appear at the front two corners of the mat foundation and develop towards the front 45° direction and the bottom of the mat foundation,resulting in local erosion under the mat and accumulation of sediment in the wake vortex.The increase of velocity,the thickness of mat foundation and the decrease of water depth will aggravate the scour around the mat.When the depth of water increases to 6 times of the thickness of the mat foundation,the influence of the water depth on the balance scour result can be ignored.For the special rectangular waterless form of mat foundation,based on the result of numerical scour models,the prediction formula for the maximum scour depth is determined considering the ratio of mat thickness to the length of the edge of the flow.By comparing the empirical formula of scour depth development with the results of numerical model,the formula of maximum scour depth around mat foundation with time is determined. |
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