Research On The Hazards Of Engineering Geology About The Route Of Ledong22-1/15-1 Proposed Pipeline

The 21 century is the most development time of the exploitation of the ocean. The focus of the construct has been transferred on the ocean, going with the development and advancement of the society. More and more project in the ocean to be explored need research on the engineering geology and hazardous geology environments, in order to prevent all kinds of the ocean hazards.The proposed pipeline of the LD22-1/15-1 lies on the shelf in the Beibu Gulf. In the Yinggehai, oil and gas resource development area were made. Many scholars have carried through lots of the scientific research in this area. But the research is lack, which combine the hazardous geology with the stability of the pipeline. Based on lots of engineering reports paper and data, according to dynamic condition and sediment characteristics, the influencing factors such as transferred sand waves, submarine landslide, burial channel and shallow gas, are discussed in the paper. At the same time, the forecast is made about the erosion and suspended span of the pipeline. The research contents and conclusions in the paper are important to the programming and construction of future pipeline.Based on the side scan sonar records, the Export Pipeline route could be divided into three sections, from land to sea they are offshore flat section; eroded section of the middle route; flat section near platform. In the second section, the area which is 12km-15km off shore is called sand wave zone, where sand dunes and sand waves are bestrewed. The area which is 40km-42km and 68km-75km off shore is called hard seabed zone, where stiff ridges are bestrewed. According to the age dating, the stiff ridges are remnant of the clay soil with river facies which deposit in the Late Pleistocene epoch. The landform is complex on the sand wave and hard seabed zone, which is bad engineering. The pipeline will traverse the hard seabed zone because it is a small area, and the pipeline can't traverse the sand wave zone because the dimension is lager. So the emphases are made on the sand wave zone.Basis on the configuration and the shallow stratum layer character, the conclusion which the activity of the sand waves is intensive is made. In the usual state, the motion of the little sand wave is dominated, and the migration rate is about 0.2m/a. In typhoon state, the motion of the loose sand is strengthen, and the movement speed increased fast, which will be from 1m/h to 5m/h. the movement of the loose sand will induce the form of the erosion rut and trough. The rut and trough ,and also the transferred sand waves will induce the suspended span of the pipeline. By calculating with the equation and analogy analysis, the depth of the erosion is about 0.4m, and will be 1.0m when the bad weather influences the area. the length of the suspended span will be 15m～35m, that less than the length which the pipeline can endure . In order to recede the influence of the bad weather, the suggested pipeline routing needs to choose the sand dune trench, on the other hand it is suitable to choose the steep slope side in the sand dune. The deeper the trench is the higher safety of the pipeline. As long as the depth of the trench is less than the height of the sand wave, it will not be able to produce the pipeline free span in the near future.In study area which is 10km-12km off shore, There is deposited loose-saturated mucilage fine sand in the steep slope belt it also exist the possibility of the landslide of unconsolidated sediment. The investigation reveals that the soil body is stability, but we can't eliminate the possibility of the submarine landslide. In addition, the burial channels and shallow gases distribute near the platform area, which may affect the stability of pipeline. Sediments infill within burial channel is clay-silt-fine sand while that around channel is stiff clay, differential settlement may happen because of the different engineering properties between the sediments in and around the channels. Because the depth of burial is deeper, they can't influence the pipeline in the near future. But the series-detection should be made in the area where the burial channels and shallow gases distribute in coincidence.