Penetration And Pullout Characteristics Of Torpedo Anchors In Cohesive Or Calcareous Sandy Seabed

At present,the exploitation of marine resources has become the national development strategy of many maritime powers,which is developing towards the goal of diversification,wide range,and high efficiency.With the ocean development from the coasts to the deep sea,the marine facilities should have the ability in deep sea.These marine facilities would face vast challenges of its stability to their safety.Therefore,new anchorage structure is neccesary for deep-sea area.Torpedo anchor is a new type of deep-sea anchorage structure,which is found to have the advantages of low manufacturing cost,simple and rapid installation in deep-sea area.Previous studies show that the anchorage force of the torpedo anchor is closely related to the characteristics of the seabed,load angle,anchor shape and penetration depth.However,many scholars have focused on their penetration and vertical pullout characteristics in the homogeneous cohesive seabed,but in engineering practices,torpedo anchors were generally installed in layered cohesive seabeds or calcareous sandy seabeds,and the anchorage lines were mostly 30 °-45 ° with the horizontal direction.Undoubtedly,the differences of seabed structure would significantly affect the penetration depth,so does the load angle on the anchorage force of torpedo anchors,which would then affect the design and application of torpedo anchors.Therefore,it is of great significance for the design and application of torpedo anchors in ocean engineering to explore the penetration or pullout characteristics in stratified cohesive seabed and calcareous sandy seabed,analyze the influence of different load angles on the pullout characteristics,and then establish the calculation formulae of the penetration depth and the anchorage force.In this paper,cohesive soil and calcareous sand are used as the experimental bed materials.According to the sediment rheology,soil mechanics and finite element theory,the laboratory experiment,theoretical analysis and finite element numerical simulation are performed.Based on the previous studies on the characteristics of torpedo anchor in homogeneous cohesive seabeds and calcareous sandy seabeds by scholars at home and abroad,the inclined anchorage force of torpedo anchors in homogeneous cohesive seabed,the penetration depth and the inclined anchorage force in two-layered cohesive seabeds,as well as the anchorage force of the anchor in calcareous sandy seabeds are respectively investigated by experiments and numerical simulation.The main research contents and achievements are as follows:(1)The penetration characteristics of torpedo anchors in two-layered cohesive seabeds.A series of penetration tests of several different torpedo anchors in two-layered cohesive seabeds are performed.The coupled Eulerian-Lagrangian numerical method is used to explore the flow characteristics and stress distribution of the soil around the torpedo anchor during the penetration.It is found that the penetration depth of the torpedo anchor in two-layered cohesive seabeds is affected by the top soil thickness and the strength ratio of the soil layer.Based on theoretical analysis and experimental data,a formula for calculating the penetration depth of torpedo anchors in two-layered cohesive seabeds is established.Through the sensitivity analysis of the parameters in the formula,it is found that the bearing capacity coefficient of the anchor tip,shear thinning index,soil sensitivity and soil undrained shear strength have significant influence on the calculated penetration depth.Finally,the accuracy of the formula is verified by using the existing data of other scholars.(2)The pullout characteristics of torpedo anchors in homogeneous cohesive seabeds.Several different torpedo anchors are used for the inclined pullout tests in the homogeneous cohesive seabed.The half-anchor test,Particle Image Velocimetry method and CEL method are used to explore the flow characteristics and stress distribution of the soil around the anchor.It is found that there is a cavity and suction force blow the anchor during the inclined pullout in the homogeneous cohesive seabed.The formation of the cavity and the collapse of the cavity wall surface would cause the fluctuations in the load curve.Based on the force analysis and experiment data,a simple,fast and accurate formula for calculating the anchorage force of torpedo anchors in homogeneous cohesive seabeds is established.The accuracy of the formula is verified by the existing data of other scholars.Finally,the sensitivity analysis of the main parameters in the formula is performed.(3)The pullout characteristics of torpedo anchors in two-layered cohesive seabeds.Intensive pullout tests are conducted by using several different torpedo anchors in twolayered cohesive seabeds.The CEL method is used to study the flow characteristics and stress distribution of the soil around the anchor during the pullout process.It is found that there are two load peaks in the load curve of torpedo anchor on the stiff-over-soft cohesive seabed,which is different from the homogeneous cohesive seabeds.The anchorage force of torpedo anchor in two-layered cohesive seabeds is closely related to the thickness of top soil,undrained shear strength of soil and load angle.Based on the soil flow characteristics,test data and the formula of anchorage force in homogeneous cohesive seabeds,a formula for calculating the inclined anchorage anchorage force of torpedo anchors in two-layered cohesive seabeds is established,and the accuracy of the formula is verified by the existing data of other scholars.(4)The pullout characteristics of torpedo anchors in calcareous sandy seabeds.Intensive pullout tests are conducted by using several different torpedo anchors in the calcareous sandy seabeds.The flow characteristics of the calcareous sand around the anchor during the pullout process are observed by using the half-anchor test and PIV method.It is found that there is no suction force below the anchor in the calcareous sandy bed,and the torpedo anchor is only subject to the vertical anchorage force,buoyant weight of the anchor,side friction resistance and end bearing resistance.During the pullout of torpedo anchor,the failure of the bed around the anchor is caused by the compression of anchor tail and the friction of anchor side wall.The shape of bed failure area is related to the embedment depth of the torpedo anchor,and the shape of bed failure area caused by torpedo anchor tail is similar to that caused by the plate anchor.Based on the experiment results and the existing studies,a formula for calculating the anchorage force of torpedo anchors in the calcareous sandy seabeds is established,and the accuracy of the formula is verified by the existing data of other scholars.