Research On In-situ Inspection And Hydrodynamic Analysis Of Seabed-pipeline System

The rapid development of offshore oil fields has increased the construction of submarine pipeline for transport of crude oil to onshore refineries, and the submarine pipeline has become one of the lifeline of marine resources. Failure of submarine pipeline will inevitably lead to financial losses and adverse environmental problems, so people pay more and more attentions on the pipeline securities. According to the program supported by NSFC, testing research on the engineering properties of submarine silty clay under wave-induced load and its applications (No. 10372089), this thesis analyses the methods of pipeline inspection, responses of seabed-pipeline system under wave-induced load based on the Hangzhou Bay Pipeline Crossing Project.The survey equipment used in pipeline inspection, such as single beam echo sounder, multi-beam echo sounder, side scan sonar and sub-bottom profiler, are analyzed. According to the features of micro-geomorphology of seabed and pipeline, a method based on inverse distance weighted (IDW) algorithm to construct seafloor digital terrain model (DTM) using multi-beam soundings, which can improve the ability of multi-beam echo sounder on pipeline inspecting. Restricted by the principle of operation, single survey equipment can just quantify one aspect of pipeline burial state. An integrating technology of multi-equipment pipeline inspecting is developed for improving the pipeline inspecting ability and efficiency, which has been verified in several pipeline inspecting projects.Based on the analysis of the mechanism of the self-burial, several key burial states are discussed using numerical modeling method. The in situ states of submarine pipeline in Hangzhou Bay in the processing of self-burial are inspected base on the methods discussed in this thesis. The processing self-burial of pipeline with spoiler is verified by the pipeline inspecting methods discussed in this thesis firstly and the mechanism of the self-burial processing is complemented and verified.A segregated solution algorithm of based on finite volume is applied to analyze the elastic response of the seabed under wave-induced load. Compared with the analytical solution, it is concluded that the numerical results are near consistent with analytical solution. The influences of the characters of wave and soil are discussed based on the numerical results.The cyclic triaxial testing system, HX-100, was carried out on undisturbed soil samples which are interbedded with clay and silt drilled from Hangzhou Bay. Low cyclic shear stress (CSS) and a great deal of numbers of cyclic are adopted in the testing to simulate the wave-induced load, and the pore water pressure buildup curve can be gained from analysis of the testing data. Development characteristics of the pore pressure can be evaluated based on the consolidation equation combined the model of the pore pressure gained from the testing data which can represent the process of generating and dissipating of the pore water in the seabed. An analytical solution of above equation of one dimension is gained. The effects of the soil and wave characteristics on the pore pressure buildup are analyzed under one dimension and it can be drawn that the buildup of pore water pressure displays a maximum value in the upper soils, and the position and the value of it are variable with the storm duration and drain conditions of the soil.The accumulation of the pore pressure with a small range fluctuations can be simulated based on the consolidation equation which can represent the process of generating and dissipating of the pore water in the seabed. The comparison of results of equations derived from the Biot consolidation theory and Terzaghi consolidation theory respectively suggested the residual pore pressure is nearly same after long wave duration. But the latter is convenient for engineering purpose. The characteristics of the residual pore pressure and the possibility of the liquefaction in the layered seabed, infilled channel and infilled pipe trench are analyzed based on the latter equation. The residual pore pressure accumulates quickly and even liquefaction can occur if the drainage is prevented by a clay layer on top of the sand or sand layer embedding clay layers. On the contrast, the dissipating of the pore water in the marine cohesive soil is slow than that in the sandy soils after the end of storm, and which is verified by CPT conducted in the Hangzhou Bay. The development of the pore pressure in the infilled channels and the infilled pipe trench is quite different from that in the homogeneous seabed. If the original seabed is composed of marine cohesive soil and the natural infilled sediment is sand, the zone around the pipeline trench is not easy to undergo liquefaction, while the original seabed is composed of sand and the natural infilled sediment is marine cohesive soil, the soils under the pipeline trench is easy to undergo liquefaction.