Experimental Study On The Mechanical Regularity Of Submarine Pipeline In Wave-induced Liquefied Silty Soil

In the process of marine oil and gas resources development, it is essential to secure transportation pipeline.In the action of complex wave of neritic sea and other ocean dynamics, pipeline damage events have occurred from time to time. There are two layouts of arranging the undersea pipeline:buried and unburied. Generally, unburied pipelines were more susceptible to failure in practical engineering, so the researchers pay more attention to unburied pipelines’damage.Under wave loading, silt bed was prone liquefaction instability with cyclic loading pressure. While the pipelines on liquefied bed suffered more pressure than on unliquefied one, this made buried pipelines damaged. This paper investigated on the pressure conditions of pipeline buried under liquefied seabed. The investigation carried on indoor wave flume test.The test was based on bed made of mud from yellow river delta and used PVC pipe and steel pipe as submarine pipeline respectively. Set the pressure transducers installed on the two opposite directions (i.e., straight up and straight down) at the same cross-section of the pipeline to measure vertical pressure that straight up, straight down and resultant.Besides, there were small wave-height gauges above pressure transducers to compare the variety of pressure curves and wave curves while recording the variety of wave in time,and research is there a phase difference between two pressure curves.The test included two stages:liquefied bed and unliquefied bed. At the stage of unliquefied bed, seabed was set four different kinds of waves, at the same time collecting related test data with collector. After seabed was liquefied by artificial stir, the wave as same as the stage unliquefied was set to the seabed, recorded the depth of liquefaction where the transducers set. With related data we had researched the curves of current pressure values of the two transducers on the same pipeline’s up and down sides and the current pressure on vertical upward, vertical downward and vertical resultant.That is there a phase difference between two curves of transducers on the same pipeline is the key, and to study the influence of liquefied soil made to pipelines phase difference pressure and the resultant vertical pressure.Results showed at the stage of unliquefied, the curves of vertical upward and downward pressure of transducers overlap completely of the same pipeline and no phase difference. And when the bed is liquefied, the curves overlap normally, only when the weight of wave and the depth of liquefiaction were quite large would phase difference of the two curves accur. in some conditions the phase difference would be the period’s 0.4 times. Since the pressure measured by the two transducers are on the opposite direction, the pressure on the vertical direction is relatively small, vertical pressure of pipeline will increase in some range when the phase difference between two pressure curves increased, and the data from transducers further confirmed this assumption. While analyzing the data I found that the vertical pressure that the pipelines suffered when the seabed was liquedfied 1-2 times not in the same wave height and no phase difference. And when it came some phase difference, the pressure was 4～6 times not.Because of the difficulty in measuring the soil pressure, there would be some numerical error between the experimental results and the practical situation, but the trend of variety from result is credible. On this test model scale, the pressure of pipelines 4-6 times the unliquefied ones, would do damage to pipelines buried in liquefied soil. However, differences still existed between small model test and practical situation, for this reason, next step we need to do research on how pipelines suffered pressure when the pipelines were buried in liquefied silty soil on conceptual model.