The Stability Analysis For Chunxiao Gas Complex Submarine Pipeline

With the exploitation of marine oil and gas, It has been found that submarine pipelines have been increasing every year and their losing stability frequently occurred. So, engineers must consider the stability of pipeline when building. Stability of pipelines relates directly to the environment loading and seabed instability phenomena. Many scholars have given sufficient research to the areas of Chunxiao gas complex submarine pipeline, but the stability of pipeline is little studied. So, based on a great deal of reports, papers and data, according to dynamic condition and sediment characteristics in areas of submarine pipeline, the influencing factors such as liquefaction of sand layer, seabed scour stability, seabed instability (shallow gas and burial channel) and human activity (shipping and fishing) are discussed in the paper, the maximum scour depth and limit erosion length of pipeline exposed on seabed caused by current also are calculated by correlative formula. The research contents and conclusions in the paper are important to the programming and construction of future pipeline.In light of the geotechnical testing result of cores, the soil along the areas of pipelines can be classified into three types: clay, silt and sand. The shallow sediment within10m below seabed can generally be divided into three zones: Zone I in which sediment is clay, Zone II in which is silt, Zone III in which is sand.Through an initial assessment of liquefaction potential using cone penetration test result, the sand layers within 2.5m of zone III are susceptible to liquefy under the assumed earthquake magnitude M=7.0, and a part of sand layer in zone II is to liquefy under extreme wave of 100y return period. The possible liquefaction effect on pipeline are analyzed and the proper protecting measures are suggested in this paper.Based on seafloor stability analysis of tidal channel and comb-shaped ridge, the shallow water region of bay mouth is generally slowly accumulated while Niubishan channel is slightly eroded because of artificial impact, comb-shape ridge is in stability under the fair weather condition, but during the typhoon weather, bottom sediments can be stirred by wave. The maximum scour depth in tidal channel is greater than that in platform because of greatercurrent velocity. Because the pipeline diameter in platform is one third of that in tidal channel, so the limit scour length is less.In addition, shallow gases distribut in inshore and platform area, which in inshore may affect the stability of pipeline because depth below the seafloor is shallow. Burial channels distribut in the east of 12310'E, the infilled sediments within burial channel is clay-silt-fine sand while that around channel is fine sand, differential settlement may happen because of the different engineering properties between the sediments in and around the channels, which can result in the instability of pipeline. Human activity such as anchor and fishing net can also affect the safety of Chunxiao submarine pipeline.