Numerical investigation on critical length of impermeable plate below underwater pipeline under steady current

This study proposes a new approach in which an impermeable plate is placed under the pipeline to prevent the local scour around the pipeline.In order to understand the performance of this approach,the finite volume method(FVM) and volume of fluid(VOF) method are adopted to simulate the flow field around the pipeline.The pressure distribution along the sandy bed surface is obtained by considering the variation of water surface.Furthermore,the effects of water depth,unidirectional and bidirectional impermeable plates on pressure difference are discussed.The seepage flow field of sandy bed near underwater pipeline is numerically simulated using the laminar and porous media model.On this basis,the effect of the impermeable plate length on hydraulic gradient is investigated and the critical length of impermeable plate is obtained.The simulated results show that when the water depth is smaller than 5.00D(D is the diameter of pipeline),the effect of the water depth on the pressure difference is remarkable.The pressure differences between two endpoints of both the unidirectional and bidirectional plates decrease with the increase of the plate length.The variations of the pressure differences for both the unidirectional and bidirectional plates are similar.With the increase of plate length,the hydraulic gradient decreases and the piping at the seepage exit is avoided effectively as long as it reaches a certain length.Such a critical length of the plate decreases with the increase of the water depth.When water depth is larger than 4.00D,the effect of the water depth on the critical length is small.For the same water depth,the critical length of impermeable plate increases with the increase of the dimensionless flow parameter.Numerical simulation results are in good agreement with the available experimental measurements.

Semi-physical simulation of AUV pipeline tracking

Before the task of autonomous underwater vehicle(AUV) was implemented actually,its semi-physical simulation system of pipeline tracking had been designed.This semi-physical simulation system was used to test the software logic,hardware architecture,data interface and reliability of the control system.To implement this system,the whole system plan,including interface computer and the methods of pipeline tracking,was described.Compared to numerical simulation,the semi-physical simulation was used to test the real software and hardware more veritably.In the semi-physical simulation system,tracking experiments of both straight lines and polygonal lines were carried out,considering the influence of ocean current and the situation of buried pipeline.The experimental results indicate that the AUV can do pipeline tracking task,when angles of pipeline are 15°,30°,45° and 60°.In the ocean current of 2 knots,AUV could track buried pipeline.

Design of motion control system of pipeline detection AUV

A great number of pipelines in China are in unsatisfactory condition and faced with problems of corrosion and cracking,but there are very few approaches for underwater pipeline detection.Pipeline detection autonomous underwater vehicle(PDAUV) is hereby designed to solve these problems when working with advanced optical,acoustical and electrical sensors for underwater pipeline detection.PDAUV is a test bed that not only examines the logical rationality of the program,effectiveness of the hardware architecture,accuracy of the software interface protocol as well as the reliability and stability of the control system but also verifies the effectiveness of the control system in tank experiments and sea trials.The motion control system of PDAUV,including both the hardware and software architectures,is introduced in this work.The software module and information flow of the motion control system of PDAUV and a novel neural network-based control(NNC) are also covered.Besides,a real-time identification method based on neural network is used to realize system identification.The tank experiments and sea trials are carried out to verify the feasibility and capability of PDAUV control system to complete underwater pipeline detection task.