Research On The Kinematic Behavior Of Towing And Falling Ship Anchor And The Protection Of Submarine Pipeline

The marine pipeline is a key issue in the ocean engineering with high risks.The damage of pipeline will cause tremendous economic losses and environmental pollution.In recent years,the distribution of submarine pipelines in the coastal waters has become more and more intensive;and at the same time,there are increasing ship activities in this area,which greatly increase the frequency of falling and dragging anchor accident.Marine pipelines are generally shallowly buried or even not buried.The anchor weight of commercial ships is usually dozens of tons.When the ship is berthing,the falling of anchors may hit the pipeline and the impact energy may cause serious damage.During the extreme weather,such as the strong storms,the ships without enough mooring force will be running freely,towing the anchors in the seabed to move along.If the track of the anchor being towed intersects with the pipeline,it will cause damage to the pipeline.Therefore,it is very important to study the failure modes and protection mechanism of pipeline damage caused by the falling and/or the towed anchors.Compared with other research fields related to pipeline safety(such as buckling and scour of Marine pipelines),there are relatively fewer researches on the influence of dropping and/or towed anchor on marine pipeline safety in the literatures at home and abroad.Based on the results of model tests,theoretical derivation and numerical analyses,this paper systematically studies the kinematic and dynamic characteristics of the falling and/or towed anchors in different type of soils,and its influence on the safety of submarine pipelines,as well as the mechanism and effects of protecting structures.The involved researches and key points are as follows:(1)For non-cohesive seabed: Based on the limit equilibrium theory,the threedimensional failure mechanism of soil wedge before anchor is proposed,in which the dilation angle and failure angle of soil wedges are introduced.Through the analysis of soil and anchor mechanics system,a theoretical method is established to predict the track of the towed anchor in the non-cohesive soil.Combined with the Coupled EulerLagrangian large deformation finite element method,the effects of the anchor structure,the unit weight and the internal friction angle of soil,dragging speed and other factors on anchor dragging motion in the non-cohesive sea bed are analyzed.At the same time,the model tests are carried out in a large model tank,and the influences of soil internal friction angle,anchor type(such as Hall anchor and AC-14 anchor),anchor size,initial buried depth,dragging speed,underwater environment and other factors on the dragging process are considered.The research results agree well with the theoretical approach.(Corresponding to chapter 2 of this paper)(2)For cohesive seabed: considering the forces of soil wedge and anchor body,the force analysis of the embedded section of the chain is introduced to establish a prediction model of anchor track in clay seabed,while the effects of soil strength are investigated.The model is used to analyze the influences of heterogeneous soil strength,anchor type,anchor size and chain on the process of dragging anchor.Based on the Coupled Eulerian-Lagrangian method,the model of anchor dragging movement in cohesive soil seabed is determined,in which the influence of horizontal anchor towing speed is considered.Model tests are performed to detect the movement of the towed anchor for different anchor types in the cohesive soil seabed.It is shown that they are identical to the theoretical and numerical results.Chapter 2 and 3 mainly study the track of dragging anchor in ordinary sea-bed soils,which can be used to guide the planning and design of anchorage area,shipping route and submarine pipe and cable routing area,so as to minimize the influence of towed anchor on the pipeline.(Corresponding to chapter 3 of this paper)(3)In order to prevent the possible damage to the marine pipeline when the towed anchor track crosses with the pipeline,it is necessary to take protective measures,among which a layer of crushed stone protection is mostly adopted.The mechanism of protecting submarine pipeline by gravel structure is studied by large scale model test,and a sensitivity analysis of the effects of anchor type,model anchor size,drag anchor speed,cross-section shape and slope ratio of gravel structure,gravel strength parameters and thickness on the protection effect is conducted.Based on the analytical model of the dragging anchor in the crushed stone protection,and the lower limit solution of the functional equation of the dragging anchor by using the principle of variational method,a design method of the structure of the crushed stone protection is put forward,considering the type of sea bed soil,anchor type and anchor size,physical and mechanical parameters of rock,horizontal towing speed and other parameters.In addition,the numerical model of anchor dragging in rock layer is established by discrete element method.The influence of dragging velocity and the depth of anchor before entering rock protection is studied.The research results can be used in design of crushed stone protective structure.(Corresponding to chapter 4 of this paper)(4)Through a force analysis of the falling anchor,the kinematic differential equation of the falling anchor in water and the seabed is established,which can predict the relationship between the velocity and depth in the whole falling process after the anchor is dropped from the ship.Therefore,it can be applied to the damage assessment of the riser,naked or buried pipelines subjected to the falling anchor.In addition,based on the Coupled Euler-Lagrangian large deformation finite element method,a numerical calculation model of the penetration of anchor in the sea bed is established.Large scale model tests of falling anchor in cohesive and cohesiveless soils are carried out,and the meausred results agree well with the numerical and theoretical ones.Model tests of falling anchors in crushed stone protection are also conducted,indicating that the rock layer can effectively reduce the penetration depth of the anchor into the seabed.The research results can provide reference to the determination of the reasonable buried depth and protective structure design of submarine pipelines.(Corresponding to chapter5 of this paper)...