Seabed Trench Induced By Mooring Line Dynamics And Anchor Foundation Bearing Capacity

At present,marine oil and gas resource exploitation,wind energy development and the island construction in the South China Sea hold significant places in China’s marine resource strategy.In the development process from offshore to far-reaching sea,the floating platform always plays an important role.The floating platforms need mooring systems to be positioned,which are composed of mooring lines and anchors.The mooring line transfers the load from the upper platform to the lower anchor,and its static and dynamic characteristics affect the service performance of the floating platform.The anchor resists the upper load by seabed soil resistance,and its bearing capacity signficantly influences the floating platform safety.Therefore,in order to ensure the service performance of floating platform,it is necessary to study the anchor bearing capacity and the mooring line-seabed interaction.At present,the main engineering problems are as follows: first,for the deep-water tension mooring system,the mooring line repeatedly cuts the seabed and trenches forms in the bottom contact area.The shallow soil loss will affect the failure mode of the anchor foundation and significantly reduce its bearing capacity;Secondly,the bearing capacity of anchor foundation is not only related to the foundation type and soil properties,but also affected by the interaction between foundation and soil interface,especially its interface shear characteristics.However,there is still a lack of evaluation method for the predicting the trench profile,and a lack of deep understanding of the interface physical parameter evolution characteristics and its influence during soilstructure interface shear.The current design code does not consider the influence of the dynamic trenching process of mooring line on the bearing capacity of anchor foundation,which may lead to the unsafe design of deep-water mooring system.Aiming at the above engineering problems,this paper makes a systematic investigation by integrating technical means such as element tests,centrifuge model tests and numerical simulations.The specific contents and innovations are summarized as follows:1.The large-scale interface ring shear device is developed to investigate the interface shear characteristics between marine foundation surface and soil.Based on the device,large-displacement monotonic and cyclic sand-steel interface shear tests are carried out,and the evolution law of interface physical parameters(including deformation band,interface roughness,particle size and shape)in the tests are quantitatively evaluated.The interface shear mechanism between sand and steel surface is revealed,and the particle breakage degree in the interface shear and its influence on the interface strenghth mobilization are quantitatively evaluated.Then,the relationship between sand friction angle and particle breakage is established.2.The chain-soil interaction apparatus with controllable confining pressure is firstly developed,which can directly obtain the chain-sand axial resistances in sand.Based on the test results,the calculation method of chain axial resistance in sand is proposed.Then,a case is provided based on the centrifuge test results to show how to use the method to calculate the chain profile in sand.Based on the field seabed trench shape data,the influence of mooring line dynamic contact on the trench profile is analyzed,and a two-dimensional trench profile prediction method considering seabed soil dynamic interaction is innovatively established;the mechanism of increasing the seabed trench width is revealed,and a three-dimensional shape prediction method of trench considering the transverse movement of mooring line and shallow trench wall collapse is established.3.The influence of shallow soil loss in sand on the failure mode and bearing capacity of suction anchor is evaluated for the first time.The displacement field mode of soil around suction anchor in the presence of trench is analyzed,and the internal mechanism of failure mode change and weakening of bearing capacity of suction anchor is revealed.The optimization strategy of gravity anchor bearing efficiency is proposed,and a new type of anchor called caisson-plate gravity anchor(CPGA)is developed.Based on centrifuge model test and numerical simulation,the bearing performance and failure mode of caisson-plate gravity anchor are studied,the internal mechanism of its bearing capacity improvement is revealed,and the rapid calculation of CPGA bearing capacity and configuration optimization design method for construction scene are established.