Research On Dynamic Analysis And Plow Surface Optimization Of Submarine Plowing Trencher

The safety and reliability of subsea pipeline are world-widely concerned since the subsea pipelines are connected to offshore oil platform,mooring oil tanker and onshore fuel storage.Burying the subsea pipeline below the seabed in a trench plays an important role in keeping pipeline stable and preventing damages from ocean natural environment,fishing anchor,net,thermal effects,and external movement of pipeline,etc.Submarine plowing trencher is cost-effective special equipment in offshore trenching engineering designed for making deep trench on the seabed while laying subsea pipeline.It is therefore necessary to investigate on the submarine plowing trencher with the domestic intellectual property right in order to promote domestic competitiveness of subsea pipeline trenching technology in the world.This thesis is aimed to research on the mathematical model method of plow surface and plow trenching soil force,the optimization of plow surface and calculation and simulation of submarine plowing trencher towing force.In the area of offshore trenching operation system analysis,the towing path and dynamic response of the submarine plowing trencher are obtained based on the vessel control system and the towline mechanics calculation.This thesis firstly investigates the plow surface design method.The three dimensional(3D)model and mathematical equations for the blade and moldboard are established by applying horizontal contour line design method.The soil trenching process is analyzed with a focus on the plow surface.According to soil element and soil flow principle on the plow surface,the soil flow equation is obtained by considering minimum energy direction of the soil turning-throwing path.The soil trenching and turning-throwing dynamic equations are established to obtain the plow draft force theoretical calculation method.It discusses that the curvature of moldboard influences the normal and tangential force of soil on the moldboard and the impact of internal friction angle and cohesion of soil on the blade draft force.A multi-objective genetic algorithm is used to specify the optimum draft force and area of the blade and moldboard for the plow surface design.The objective function is to minimize the draft force and surface area of the plow surface in a particular soil condition.The plow blade penetration angle and two surface angles of the blade are identified as the optimal parameters in blade optimization design.Parameters such as the moldboard side angle of deviation,moldboard lift angle,angular variation of the tangent line,and the spanning length are also optimized with respect to the force of the moldboard along soil flow direction and its design area.The optimal results for design parameters can be chosen from the Pareto solutions depending on the design requirement.The standard deviation of Gaussian curvature is used to evaluate the smoothness of the optimized moldboard surface.The aim for selecting optimal results is to decline the consuming energy for the towing and save the amount of manufacture materials.This thesis also investigates on the soil trenching process by blade and moldboard and towing force calculation of the submarine plowing trencher.Based on the linear Drucker-Prager plastic yield criterion and the seabed soil dynamic pressure equation with the first-order wave pressure equation,the dynamic equations and three-phase interaction simulation model for plow,seabed and seawater are developed.This simulation model is calculated numerically using the smoothed particle hydrodynamics method and coupled Eulerian-Lagrangian method in order to ensure the calculation convergence.Then,the towing force of submarine plowing trencher and skid force are calculated by the dynamic equations of submarine plowing trencher using the simulation results of plow draft force as the force input.This dynamic model and simulation method mentioned can be validated by comparison with towing force test results.It also discusses the influence of the artificial viscosity in smoothed particle hydrodynamic method on the simulation results.According to the simulation results,the impact of the fore cutter assembling position on the trench profile is discussed.Additionally,it discusses the influence of subsea environment on the plow draft force,skid force,plowing trencher towing force.The analyze conditions includes fluid field(the wave height,current speed and direction,and water depth),seabed soil physical parameters(soil elasticity modulus,undrained shear force and cohesion),impact loads from grave soil,and the complex seabed topography(the cosine waveform seabed and sloped seabed).The offshore trenching path control method of submarine plowing trencher is investigated in this thesis.It presents the dynamic equations of the 3 degrees of freedom vessel,towline,submarine plowing trencher using mass-spring-damp mechanical system modeling method.In addition,the curved path of the submarine plowing trencher is fitted by cubic spline interpolation method.Based on this method,the dynamic response of submarine plowing trencher(the desired displacement,velocity,acceleration)and reference path of vessel can be mathematically predicted.In order to keep the plowing trencher following the subsea pipeline,the sliding-mode control method and PID control method are applied to control the rudder angle and path of vessel.In the vessel control system,the dynamic responses of submarine plowing trencher and the elongation and force of the towline element are obtained using the analysis of mechanics performance of the towline under the vessel driven force.The test prototype of submarine plowing trencher is designed and manufactured for the land test.The land test system is designed.The land soil physical parameters are obtained by the test site soil mechanics experiments,which is used to calculate the towing force.The joint tests include plow trencher opening and closing test,pipeline guidance test,steering test,and trenching depth adjustment test.The test results prove validity of plow design method and towing force calculation and simulation method.This research results provide theoretical and technical references for further design of submarine plowing trencher and its sea trial.It also broadens and deepens the research directions for domestic submarine trenching technology.