Research On Hazardous Geological Factors In The Outer Shelf Of Northern South China Sea And Influence On The Submarine Pipelines

With the development of petroleum industry, more and more submarine pipelines are laid on the seabed. Development and migration of sand waves, collapses, slides, uplift ridges, trenches, pock marks and other potential geological hazards may cause spanning and fatigue damage of submarine pipelines, which aroused the concern of many researchers. In order to research the distribution and evolution rules of potential geological hazards in the oil and gas zone of Pearl River Mouth Basin, outer continental shelf of northern South China Sea, sand waves, slides, collapses, uplift ridges, trenches and pock marks were identified and analyzed. On this basis, distribution, causes and evolution of these potential geological hazards were systematically researched. This research helps to analyze the control factors of pipeline spanning and make the maintenance of pipeline targeted. Ultimately, the fatigue damage of pipelines will be reduced.Study area locates 160-240km to the south east of Hong Kong,100-200m depth zone of Pearl River Mouth Basin on the outer continental shelf of northern South China Sea.18 surface sediment samples were collected and analyzed in order to probe into the distribution characteristics of sediment patterns and grain size. Using the SONIC2024 multi-beam echo sounder system, water depth in the study area was surveyed with high resolution and all coverage, In virtue of geological plotting software such as Surfer and Global Mapper, topography in the study area was analyzed, shape parameters of multiform micro-relief were measured, and the distribution discipline of sand waves, slides, collapses, uplift ridges, trenches and pock marks was acquired. Through the combined analysis method of critical flow velocity of flow and analysis of spatial and temporal equilibrium-range spectra, forming age of sand waves was estimated qualitatively. On the basis of grain size parameter, shape parameters of sand waves and flow velocities near the seabed, the movement velocities of sand waves were calculated using the formulas from Rubin and modified Hardisty. By comparing the distribution of hazardous relief and pipeline spanning condition obtained by ROV, causes of the formation of pipeline spanning of different areas were discussed.The results indicate that the surface sediments in the study area can be divided into seven types, including gravel, sandy gravel, gravely sand, gravely muddy sand, sand with gravel, muddy sand with gravel, and mud with gavel. Average particle size of sediment is in the range of -1.1Φto 2.2Φ. From outer continental shelf to inner continental shelf, the changing trends of grain size parameter were conspicuous, average grain size becomes smaller, sorting coefficient becomes larger, skewness becomes more positive, and kurtosis becomes larger, diversity of provenance of surface sediment is strengthened. In the outer continental shelf, residual sediment formed in the last glacial period is the main source, which is mixed up with debris from Philippine Islands. To the inner continental shelf which is closed to mainland, sediment from transported from South China by Pearl River and sediment from Taiwan Island transported by Penghu channel become important in the composition of surface sediment. Sand waves in the study area are mostly ripples with straight crest line, and the strike of the crest line is NE-SW, the lee slopes have two different trends, which are NW and SE respectively. Sand waves with wave height lower than 0.5m are widely distributed in the area. With the increase of the distance with the mainland, heights of the sand waves become larger, up to 5.67m. Wavelength, inclination of lee slope and symmetry index are all increase with the increase of wave height. Through the comparison of maximum velocity with the critical flow velocity forming sand waves and analysis of spatial and temporal equilibrium-range spectra, we concluded that sand waves in the study area formed in the modern hydrodynamic condition, and and waves were in the state of active. Migration of sand waves occurred during episodic storm events, but was not normalization events. Migration rate of sand waves was slow. Small sand waves in the inner shelf with wave height of 0.5m migrated 2.27m per year, and those in the outer shelf with wave height of 2.5m migrated only 0.31m per year. All sand waves were in the category of weak activity. There are 6 submarine slides and collapse in the study area, which were close with the shelf break line, front of ancient delta and continental slope toe line. Obliquities of the incline on which the collapses and slides distributed were in the range of 0.4°to 1.2°, and strikes were mainly NE-SW.5 uplift ridges developed in the southern part of the study area, which were approximately 4m to 12m high. Crest lines of slides and collapses have two different strikes, which were NE-SW and E-W. Uplift ridges and trenches caused by encroachment or movement of faults. There was a great deal of pock marks in neighborhood of the KP268-KP273 of the pipeline. Diameters of the pock marks were 30m to 100m, and depths were 1.0m to 3.0m. According to the configuration of the pock marks, it was speculated that the pock marks were formed by shallow gas. After the shallow gas escaped from the stratum, the stratum subsided. Amount of pipeline spinning caused by sand waves was large, and that spanning caused by pock marks was relatively fewer, and the extension of distribution was relatively smaller. Slides and collapses did not jeopardize the security of pipelines. Pipeline spanning caused by uplift ridges and trenches was less, but spanning length and height were larger than those caused by other geological hazards, so the harm was more serious.